Abstract

Europium doped β-PbF2 nano-particles with different doping concentration are prepared to investigate the site structure of Eu3+ dopants. It is concluded that the site symmetry of Eu3+ dopants in β-PbF2 nano-particles lowers from Oh to D4h with the increase of doping concentration. By X-ray diffraction analysis and photoluminescence spectroscopy study, a doping concentration induced phase transition from lowly doped cubic Pb3EuF9 to highly doped tetragonal PbEuF5 is detected. The intermediate phase of moderately doped nano-particles, which contains both phases mentioned above, is observed for the first time. Moreover, the temperature-dependent intermediate phase analysis suggests that the tetragonal phase is more stable than the cubic phase, which is also confirmed by the first-principle calculations. Our results suggest that the doping concentration induced phase transition in β-PbF2 nano-particles can be used for understanding other Lanthanide-doped nano-particle systems.

© 2013 Optical Society of America

1. Introduction

In recent decades, trivalent Lanthanide ions (Ln3+) doped materials are well-known due to their luminescent properties and have been utilized widely in display field and fluorescent lights. Compared to organic fluorophores and semi-conducting nanocrystals, Ln3+-doped inorganic nano-particles have high photochemical stability, sharp emission bandwidths and large anti-Stokes shifts, therefore can be applied as excellent luminescence materials [1,2]. More recently, they also attract great interests as a new class of bioprobes, because their long-lived and intense emissions offer promising applications to biosensing, biological labeling and imaging technology [39]. In general, the optical properties of Ln3+ ions are very sensitive to the local environment, and especially the emission intensity of Ln3+-doped materials are closely correlated with the surrounding crystal-field and the crystal structure [10,11]. Therefore, Ln3+ ions are normally used as probes to survey the local structure in luminescent materials [1216].

The family of Ln3+-doped inorganic nano-particles with fluorite structure, such as PbF2, CaF2, CdF2, SrF2, are widely applied in display devices, lasers and bioassays due to the low phonon frequencies [1720]. As luminescence center, fluorescence properties of Ln3+ dopants are determined by their site symmetries in fluorite structures. However, there is no common agreement on the site symmetry of the dopant Ln3+ ions in fluorite nano-particles has been reached, except the fact that the doping concentration can strongly affect the local environment of Ln3+ dopants. Different doping concentration can induce different site symmetries of Ln3+ ions in nano-particles. The X-ray diffraction (XRD) studies of β-PbF2:Er3+ nano-particles indicate that the segregation of Ln3+ ions induces the lattice parameters modification, i.e. the structural alteration of cubic fluorite phase PbF2 [21,22]. By using the upconversion spectroscopy, Wright et al. and Bouffard et al. have intensively studied the effect of doping concentration in system of MF2:Ln3+ (M = Sr, Ca, Cd, Pb, Ba; Ln = Eu, Tm), and concluded that cubic and single-pair are the main site types and the Ln3+-occupied site symmetry is correlated with the concentration of dopants [2326]. Méndez-Ramos et al. [27] have also found that the diluted Eu3+ ions occupy two different sites with high crystalline phase symmetry in moderately doped samples. Driesen et al. [28] reported that Eu3+ ions substitutes Pb2+ ions and the Eu3+ ions doping induces an orthorhombic distortion to D4h for the face-centered β-PbF2 structure. However, the experimental studies rely on fluorescence characterization techniques which can only obtain the structural information indirectly. The detailed site symmetry of Ln3+ ions or even the structure of nano-particles at different doping level are rarely studied. Only very recently, a tetragonal structure of PbREF5 in highly Er3+-Yb3+ co-doped β-PbF2 nanocrystals has been proposed by Hu et al. by using the energy dispersive X-ray spectroscopy (EDS) and XRD analysis [29]. However the site symmetry of Ln3+ ions at low doping level is still an open question.

By employing Eu3+ ions as an outstanding fluorescent probe [15], we have prepared fluoride nano-particles with different doping concentration to investigate the structure of crystal-field. Site symmetry of Eu3+ ions is identified with photoluminescence spectroscopy and XRD, and the phenomenon of doping concentration induced phase transition is described. Particularly the intermediate phase is observed at moderately doping level for the first time. Furthermore the direct and quantitative XRD characterization and the Rietveld full-pattern fitting algorithm [29,30] are employed to confirm our structure models. The detailed local structure and site symmetry are finally obtained, which help to perform the fluorescence modification of nano-particles and provide possibilities of further applications in optical field.

2. Experimental

Precursor oxyfluoride glasses with the composition (50-x)SiO2-40PbF2-10CdF2-xEu2O3 (mole fraction x = 0.05, 0.1, 0.5, 1.5), were prepared by traditional melting-quenching method. With the prepared precursor glasses, glass ceramics (GCs) were obtained by thermal treatment for 48 hours at certain temperature which is determined by differential thermal analysis (DTA). GCs are labeled with the mole fraction x such as 0.05GC. The thermal treatment temperature for 0.5GC was 400 °C, 405 °C and 410 °C, while it was 410 °C treated for the rest. With hydrofluoric acid etching, the corresponding fluoride nano-particles were obtained afterwards [31].

All the XRD measurements were performed with a Rigaku D/Max-2500 diffractometer (Rigaku Corporation, Tokyo, Japan) using CuKα as the radiation. Rietveld analysis of XRD patterns were carried out with the Fullprof program based on the profile function of pseudo-Voigt with axial divergence asymmetry [32,33]. The XRD data in the range from 10° to 135° were collected in step-scan mode with the step width 0.02° (2θ) at a counting time of 1 s per step. High-resolution transmission electron microscope (HRTEM) analysis was performed to observe the morphology of samples on a Philips TECNAI TEM (FEI Co., Netherlands) operating at 200 kV. The emission and site-selective excitation spectra and photoluminescence decays of nano-particles were recorded on an Edinburgh Instruments FLS920 spectrofluoremeter equipped with both continuous (450 W) and pulsed xenon lamps. All measurements were performed at room temperature.

3. Results and discussion

3.1 XRD analysis and concentration induced phase transition

XRD patterns of 0.05GC, 0.5GC, 1.5GC, and standard β-PbF2 are presented in Fig. 1(a). The inset shows the enlarged three labeled peaks with the corresponding indexes of the crystal facet. According to DTA studies in literatures [34,35], oxide phase PbSiO3 (JCPDS: 28-0540) appears inevitably with the formation of β-PbF2:Eu nano-particles in lowly doped GCs. As shown in the inset, XRD patterns of 1.5GC shift to the larger angle direction compared to those of 0.05GC, and both coexist in 0.5GC. These results indicate that Eu3+ ions are incorporated into β-PbF2 nano-particles, which are also supported directly by electron energy loss spectroscopy in previous studies [36,37]. Once the probe ions are doped into such fluoride nano-particles to substitute for Pb2+, the coordination field around the dopants will be changed due to the charge imbalance and the mismatch of ionic radius between Eu3+ (0.95 Å) and Pb2+ (1.29 Å). Given the fact that the number of substituting Eu3+ ions in a single β-PbF2 cell increases with the doping concentration, the structure of nano-particles in 1.5GC vary from that in 0.05GC significantly. Both structured patterns are observed in 0.5GC, indicating that the intermediate phase includes these two phase structures. To our best knowledge, this is the first time to observe the coexistence of both phases in such Ln3+ ions doped nano-particles by XRD as it can easily be hidden by the line broadening and scattering from the amorphous matrix. HRTEM images of 0.05GC and 1.5GC along with the fast-Fourier transformation (FFT) images are shown in Figs. 1(b) and 1(c). The mean size of nano-particles is about 18 nm, which is in consistent with the evaluated result by Scherrer equation from XRD. Based on the measured interplanar spacing, the FFT image of 0.05GC can be only matched with diffraction pattern of the crystal plane (112) of the cubic Pb3EuF9 and the image of 1.5GC can be only matched with the diffraction pattern of the tetragonal PbEuF5 crystal along the <110> zone axis.

 

Fig. 1 (a) XRD patterns of standard β-PbF2, lowly (0.05GC), moderately (0.5GC) and highly doped samples (1.5GC). The inset shows the enlarged three labeled diffraction peaks; HRTEM images of 0.05GC (b) and 1.5GC (c), and the corresponding fast-Fourier transform (FFT) of the diffraction patterns recorded from the area marked with the white circle.

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Leśniak suggested a tetragonal structure model with C4v (4mm) point group symmetry, in which the Ln3+ ions are surrounded by eight ligand F- ions and another interstitial F- ion for charge compensation [38,39]. Beggiora et al. have proved that F- interstitial mechanism is more favorable than Pb vacancy compensation mechanism with computer simulations [22]. The interstitial F- neighboring with Ln3+ dopant can form a tetragonal crystal-field symmetry, while the site symmetry becomes cubic when F- and Ln3+ are next-neighbor [24,4042]. Our group proposed that two Ln3+ ions are substituted for two Pb2+ sites in a β-PbF2 face-centered cell of highly doped nano-particles [29]. By XRD and TEM analysis, we present a concentration induced phase transition from cubic to tetragonal in Fig. 2.

 

Fig. 2 A complete description of the phase transition: the phase transformation from cubic to tetragonal phase in lowly and highly doped nano-particles, respectively.

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At low doping level, only one Ln3+ ion in a β-PbF2 face-centered cell substitutes for Pb2+ site and compensates the charge with one interstitial F- ion, and the cubic phase structured Pb3LnF9, whose space group corresponds to Pm-3m (NO. 221), is formed with Oh (m-3m) point group symmetry. The increase of doping concentration decreases the average distance between Ln3+ ions, thus in single cell two Ln3+ ions can substitute for the Pb2+ ions. The structure transforms from cubic phase Pb3LnF9 to tetragonal phase PbLnF5, which belongs to P4/mmm (NO. 123) space group and has D4h (4/mmm) point symmetry. Furthermore, this tetragonal phase structure model offers complete structural informations provided by all the previous models, and its corresponding D4h (4/mmm) symmetry operations include those of C4v (4mm) proposed in literatures [38,39].

3.2 Photoluminescence spectroscopy analysis

To probe the local structure around Eu3+ dopants in fluorite cell, four samples (0.05GC, 0.1GC, 0.5GC and 1.5GC) were studied with photoluminescence spectroscopy. Figure 3 displays the transitions of 5D0 to the 7Fj (j = 0, 1, 2) under 393 nm excitation. The small intensity ratio of electric-dipole (5D07F2) to magnetic-dipole (5D07F1) transition demonstrates that the local site symmetry of Eu3+ ions in crystalline phase is nearly centrosymmetric. Moreover, the magnetic-dipole transition contains a number of Stark components. As shown in Fig. 3, it is found that 1.5GC exhibits two strong Stark splits at 586.5 nm and 591.7 nm. With the decrease of doping concentration, another Stark component at 589.0 nm appears and its intensity increases rapidly. Relatively, the intensities of Stark components at 586.5 nm and 591.7 nm decrease gradually and they can be only observed as two small shoulders of the main 589.0 nm emission until decreasing Eu3+ doping content to be 0.05GC. The inset shows the photoluminescence decays of 5D07F1 transitions (By excitation at 393 nm and monitoring at 589.0 nm). The decays of 0.05GC and 1.5GC can be well fitted with a single exponential function (I(t)=I0exp(t/τ)), which indicates the nearly homogeneous crystal-field environment around Eu3+ ions in a single lattice site [43]. The photoluminescence lifetimes of 5D0 levels in 0.05GC and 1.5GC are 1.78 ms and 4.41 ms, respectively, suggesting that the distance between Eu3+ ions in 0.05GC nano-particles is much shorter than that in 1.5GC nano-particles. The decay of 0.5GC shows a double exponential behavior with two lifetimesτ1andτ2corresponding to 1.76 ms and 4.63 ms, respectively. τ1has the similar value with the lifetime in 0.05GC, whileτ2and the lifetime in 1.5GC are nearly the same. This result shows that Eu3+ ions occupy the both sites in 0.5GC nano-particles.

 

Fig. 3 Emission spectra of the 5D07Fj (j = 0, 1, 2) transitions with the excitation at 393 nm. The inset shows the photoluminescence decays of the 5D07F1 transitions in 0.05GC, 0.5GC and 1.5GC.

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In order to reveal the multiple sites of Eu3+ ions in nano-particles, site-selective excitation spectra of 7F05D1 transition of all the three Stark emission peaks (5D07F1) are presented in Fig. 4. According to branching rules and transition selection rules of the 32 point groups [43], no split of the magnetic-dipole moment bands denotes that the Eu3+ ions reside in the perfect cubic Oh symmetry field. When there are two strong Stark components of this transition appear, the site symmetry of Eu3+ ions lowers to the tetragonal D4h. In 0.05GC, the excitation spectra are nearly identical with only one peak at 523.8 nm implying single-site occupation by Eu3+ ions with Oh point group symmetry. The split peaks (at 523.8 nm and 524.6 nm) in 1.5GC suggest that the site symmetry of Eu3+ alters to tetragonal D4h. The cubic and tetragonal site symmetries are labeled with ‘C site’ and ‘T site’ in figures, respectively. In 0.1GC and 0.5GC, the excitation spectra include all features of 0.05GC and 1.5GC, indicating that both cubic and tetragonal structures exist in these moderately doped nano-particles. These results agree with the XRD analysis.

 

Fig. 4 Site-selective excitation spectra of the 7F05D1 transition monitoring at 586.5 nm, 589.0 nm and 591.7 nm.

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3.3 Theoretical simulations and quantitative XRD analysis

To confirm our proposed nano-particles structure models, Rietveld method with Fullprof program [32,33] was employed for XRD data refinement. The theoretical simulated line spectrum of each existed phase has been displayed for comparisons.

Figure 5(a) shows the observed diffraction patterns of 0.05GC. The XRD patterns of the Pb3EuF9 and PbSiO3 are distinguished and indexed separately. To exclude the effect of PbSiO3, only XRD data of the Pb3EuF9 are refined by the Rietveld method. The corresponding refinement factors Rp (= 6.64%) and Rwp (= 8.81%) indicate that the cubic Pb3EuF9 model is reasonable. The cell parameters of Pb3EuF9 (a = b = c = 5.9517 Å) are obtained from the refinement. The simulation results of 1.5GC are displayed in Fig. 5(b). The corresponding factors Rp (= 5.04%) and Rwp (= 6.49%) indicate a relatively good agreement between the experimental and calculated data. The cell parameters of tetragonal PbEuF5 are a = b = 4.1467 Å and c = 5.8625 Å. Moreover, the value of c/a is close to2, which indicates that the tetragonal PbEuF5 is taken out from a ‘pseudo-cubic’ cell [29]. Due to the fact that coexistence of cubic Pb3EuF9 and tetragonal PbEuF5 in 0.5GC, both structure models are adopted in the XRD refinements and their results are shown in Fig. 5(c). The corresponding Rietveld factors are Rp (= 5.17%) and Rwp (= 6.57%), Rb (= 7.35%) and Rf (= 5.13%) of cubic Pb3EuF9 as well as Rb (= 3.38%) and Rf (= 2.14%) of tetragonal PbEuF5, indicating a good agreement between the experimental data and calculated values by our models. The cell parameters of cubic Pb3EuF9 (a = b = c = 5.9461 Å) and that of tetragonal PbEuF5 (a = b = 4.1507 Å, c = 5.8601 Å) are obtained from the refinement of 0.5GC, which are close to those of 0.05GC and 1.5GC. The simulated diffraction line spectra of the oxide phase PbSiO3, the cubic Pb3EuF9 and tetragonal PbEuF5 are shown in Figs. 5(d)-5(f) for comparisons with the experimental data respectively. In 0.05GC, the average deviations of 2θ and relative intensity between theoretical and experimental values are only 4.2*10−3 degree and 2.5*10−2, respectively, and the corresponding values are 2.5*10−3 degree and 2.8*10−2 in 1.5GC. These results quantitatively support our proposed models. In conclusion, our proposed models are precise for analyzing the Eu3+ doped nano-particles and confirm the doping concentration induced phase transition from cubic to tetragonal.

 

Fig. 5 XRD Rietveld refinements and the corresponding error curve of 0.05GC (a), 1.5GC (b) and 0.5GC (c). The positions of the Bragg reflections are represented by vertical bars (|).The simulated diffraction line spectra of the oxide phase PbSiO3 (d), the cubic phase Pb3EuF9 (e) and tetragonal phase PbEuF5 (f) are presented for comparisons.

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3.4 Thermal stability of proposed structures

The XRD patterns of 0.5GC thermal treated at 400 °C, 405 °C, and 410 °C are shown in Fig. 6, with the enlarged diffraction peaks represented in its left inset. The diffraction intensity of the cubic phase increases with the thermal treatment temperature. The parameter R, the area of the cubic or tetragonal crystalline phase contrasting the total area of the XRD diagram calculated by fitting XRD peaks with a Gaussian line, could roughly evaluate the amount of crystalline phase in GCs [44]. As shown in the right inset, the tetragonal phase fraction RT grows slowly while the cubic phase fraction RC increases sharply from about 7% to 16% when the thermal treatment temperature varies from 400 °C to 410 °C. This indicates that tetragonal phase is more favorable at elevated temperatures other than cubic phase.

 

Fig. 6 XRD patterns of 0.5GC with thermal treatment temperature at 400 °C, 405 °C, and 410 °C. The left inset shows the enlarged diffraction peaks and the right inset presents the crystallized fraction of the cubic and tetragonal phase at different thermal treatment temperature.

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Density functional theory (DFT) [45,46] calculations were performed using DMol3 program (Accelyrs Inc.). The local density approximation (LDA) using formula of Perdew and Wang (PWC) exchange-correlation functional was employed. Double numeric basis sets supplemented with d-polarization functions (DND) and all-electron calculations were used [45,47]. The binding energy of the tetragonal phase (selecting the ‘pseudo-cubic’ cell for calculation) is 2.327 eV lower than that of the cubic phase, which means that the tetragonal phase is more stable and relatively easier to form at higher doping concentration. This theoretical calculation results are consistent with the experimental phenomenon and also give the reason of no cubic phase structures existing in highly doped nano-particles.

Overall, the substitution of Ln3+ ions for Pb2+ sites in β-PbF2:Ln3+ and the incorporation of Ln3+ ions into nano-particles are proved to induce the lattice parameters modification or structural alteration [21,48]. The similar experimental phenomenon was also observed in Ce3+-, Tb3+-, Ho3+-, Tm3+-, etc.-doped materials [49,50], as well as in Er3+ ions doped β-PbF2 solid solutions [22]. The breakdown of crystallographic site symmetry was also found in Gd3+ and Eu3+ ions doped NaREF4 structures [7,51]. Those indicate that phase transition analysis in this study can be used for investigating other Ln3+ ions doped systems. The optical properties of Ln3+ ion doped luminescent materials are closely correlated with the doping concentration. Wright et al. studied PbF2:Eu3+ system using site selective spectroscopy and achieved the same conclusions with our work in photoluminescence spectra [25,27,28,52]. These similar experimental phenomena from previous literatures strongly support our results, while the more detailed site symmetry of Ln3+ ions in nano-particles are provided in this paper. The dependence of optical properties on doping concentrations were observed in Pr3+-, Sm3+-, Tb3+-, Dy3+-, Ho3+-, Er3+-, etc.-doped materials and drawn great attentions from many researchers for achieving the certain fluorescence emission via adjusting the doping concentration [49,50,5357]. However the main issue involved in the luminescent variation is the structural changes of materials induced by the doping concentration. The conclusions drawn from this work are of great significance for the further optical researches, such as controlling the fluorescence via adjusting the structure of luminescence materials.

4. Conclusions

By using Eu3+ as a fluorescence probe in Lanthanide-doped β-PbF2 nano-particles, Ln3+ ions are substituted for Pb2+ sites and the doping concentration induces a site symmetry distortion from Oh to D4h. By photoluminescence and XRD study, we conclude that the structure of lowly doped nano-particles is cubic Pb3EuF9 (Oh (m-3m), Pm-3m (NO. 221)). With the increase of doping concentration, the cubic Pb3EuF9 transforms to tetragonal PbEuF5 (D4h (4/mmm), P4/mmm (NO. 123)). Particularly, the coexistence of both structures in moderately doped nano-particles is proposed and confirmed for the first time. The binding energy of the C and T structures differs with about 2.327 eV, which means that the T structure is more stable and easier to form in highly doped materials. Our work represents a significant advance towards a more comprehensive understanding of the site symmetry of Ln3+ ions in fluoride nano-particles, which would benefit the further research on the optical properties, such as fluorescence regulation and control of Ln3+ ions, and have great importance in the applications of this material in optical fields.

Acknowledgments

This work is supported by National Science Fund for Talent Training in Basic Sciences (No. J1103208). The authors are also grateful for the fruitful discussions with Bing Yang and Nan Hu.

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36. C. Liu and J. Heo, “Electron energy loss spectroscopy analysis on the preferential incorporation of Er3+ ions into fluoride nanocrystals in oxyfluoride glass-ceramics,” J. Am. Ceram. Soc. 95(7), 2100–2102 (2012). [CrossRef]  

37. C. Liu, X. J. Zhao, and J. Heo, “Direct imaging of inhomogeneous distribution of Er3 + ions in lead fluoride nanocrystals,” J. Non-Cryst. Solids 365, 1–5 (2013). [CrossRef]  

38. K. Leśniak, “Model simulation of the tetragonal symmetry centre of a rare-earth ion in a fluorite lattice,” J. Phys. C Solid State Phys. 19(15), 2721–2727 (1986). [CrossRef]  

39. K. Leśniak, “Crystal fields and dopant-ligand separations in cubic centres of rare-earth ions in fluorites,” J. Phys. Condens. Matter 2(25), 5563–5574 (1990). [CrossRef]  

40. J. M. Baker, W. Hayes, and D. A. Jones, “Paramagnetic resonance of impurities in CaF2,” Proc. Phys. Soc. 73(6), 942–945 (1959). [CrossRef]  

41. C. W. Rector, B. C. Pandey, and H. W. Moos, “Electron paramagnetic resonance and optical Zeeman spectra of type II CaF2:Er3+,” J. Chem. Phys. 45(1), 171–179 (1966). [CrossRef]  

42. M. J. Weber and R. W. Bierig, “Paramagnetic resonance and relaxation of trivalent rare-earth ions in calcium fluoride. I. resonance spectra and crystal fields,” Phys. Rev. 134(6A), A1492–A1503 (1964). [CrossRef]  

43. Q. Ju, Y. S. Liu, R. F. Li, L. Q. Liu, W. Q. Luo, and X. Y. Chen, “Optical spectroscopy of Eu3+-doped BaFCl nanocrystals,” J. Phys. Chem. C 113(6), 2309–2315 (2009). [CrossRef]  

44. G. Dantelle, M. Mortier, D. Vivien, and G. Patriarche, “Effect of CeF3 addition on the nucleation and up-conversion luminescence in transparent oxyfluoride glass-ceramics,” Chem. Mater. 17(8), 2216–2222 (2005). [CrossRef]  

45. B. Delley, “An all-electron numerical method for solving the local density functional for polyatomic molecules,” J. Chem. Phys. 92(1), 508–517 (1990). [CrossRef]  

46. B. Delley, “From molecules to solids with the DMol3 approach,” J. Chem. Phys. 113(18), 7756–7764 (2000). [CrossRef]  

47. J. P. Perdew and Y. Wang, “Accurate and simple analytic representation of the electron-gas correlation energy,” Phys. Rev. B Condens. Matter 45(23), 13244–13249 (1992). [CrossRef]   [PubMed]  

48. V. K. Tikhomirov, D. Furniss, A. B. Seddon, I. M. Reaney, M. Beggiora, M. Ferrari, M. Montagna, and R. Rolli, “Fabrication and characterization of nanoscale, Er3+-doped, ultratransparent oxy-fluoride glass ceramics,” Appl. Phys. Lett. 81(11), 1937–1939 (2002). [CrossRef]  

49. Q. Luo, X. S. Qiao, X. P. Fan, and X. H. Zhang, “Preparation and luminescence properties of Ce3+ and Tb3+ co-doped glasses and glass ceramics containing SrF2 nanocrystals,” J. Non-Cryst. Solids 356(50–51), 2875–2879 (2010). [CrossRef]  

50. J. J. Pan, R. R. Xu, M. Wang, G. J. Gao, J. M. Chen, L. L. Hu, and J. J. Zhang, “Enhanced 2.0 μm emission in Tm3+/Ho3+ codoped transparent oxyfluoride glass ceramics containing β-PbF2 nano-crystals,” Solid State Commun. 150(1–2), 78–80 (2010). [CrossRef]  

51. D. T. Tu, Y. S. Liu, H. M. Zhu, R. F. Li, L. Q. Liu, and X. Y. Chen, “Breakdown of crystallographic site symmetry in lanthanide-doped NaYF4 crystals,” Angew. Chem. Int. Ed. Engl. 52(4), 1128–1133 (2013). [CrossRef]   [PubMed]  

52. C. Bensalem, M. Mortier, D. Vivien, and M. Diaf, “Optical investigation of Eu3+:PbF2 ceramics and transparent glass-ceramics,” Opt. Mater. 33(6), 791–798 (2011). [CrossRef]  

53. M. Gu, Q. C. Gao, S. M. Huang, X. L. Liu, B. Liu, and C. Ni, “Luminescence properties of Pr3+-doped transparent oxyfluoride glass-ceramics containing BaYF5 nanocrystals,” J. Lumin. 132(10), 2531–2536 (2012). [CrossRef]  

54. B. C. Jamalaiah, M. V. Vijaya Kumar, and K. Rama Gopal, “Fluorescence properties and energy transfer mechanism of Sm3+ ion in lead telluroborate glasses,” Opt. Mater. 33(11), 1643–1647 (2011). [CrossRef]  

55. P. Babu, K. H. Jang, E. S. Kim, L. Shi, R. Vijaya, V. Lavín, C. K. Jayasankar, and H. J. Seo, “Optical properties and energy transfer of Dy3+-doped transparent oxyfluoride glasses and glass-ceramics,” J. Non-Cryst. Solids 356(4–5), 236–243 (2010). [CrossRef]  

56. Z. J. Hu, E. Ma, Y. S. Wang, and D. Q. Chen, “Fluorescence property investigations on Er3+-doped oxyfluoride glass ceramics containing LaF3 nanocrystals,” Mater. Chem. Phys. 100(2–3), 308–312 (2006). [CrossRef]  

57. W. J. Zhang, Q. Y. Zhang, Q. J. Chen, Q. Qian, Z. M. Yang, J. R. Qiu, P. Huang, and Y. S. Wang, “Enhanced 2.0 μm emission and gain coefficient of transparent glass ceramic containing BaF2: Ho3+,Tm3+ nanocrystals,” Opt. Express 17(23), 20952–20958 (2009). [CrossRef]   [PubMed]  

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    [CrossRef]
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    [CrossRef]
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    [CrossRef] [PubMed]
  48. V. K. Tikhomirov, D. Furniss, A. B. Seddon, I. M. Reaney, M. Beggiora, M. Ferrari, M. Montagna, and R. Rolli, “Fabrication and characterization of nanoscale, Er3+-doped, ultratransparent oxy-fluoride glass ceramics,” Appl. Phys. Lett.81(11), 1937–1939 (2002).
    [CrossRef]
  49. Q. Luo, X. S. Qiao, X. P. Fan, and X. H. Zhang, “Preparation and luminescence properties of Ce3+ and Tb3+ co-doped glasses and glass ceramics containing SrF2 nanocrystals,” J. Non-Cryst. Solids356(50–51), 2875–2879 (2010).
    [CrossRef]
  50. J. J. Pan, R. R. Xu, M. Wang, G. J. Gao, J. M. Chen, L. L. Hu, and J. J. Zhang, “Enhanced 2.0 μm emission in Tm3+/Ho3+ codoped transparent oxyfluoride glass ceramics containing β-PbF2 nano-crystals,” Solid State Commun.150(1–2), 78–80 (2010).
    [CrossRef]
  51. D. T. Tu, Y. S. Liu, H. M. Zhu, R. F. Li, L. Q. Liu, and X. Y. Chen, “Breakdown of crystallographic site symmetry in lanthanide-doped NaYF4 crystals,” Angew. Chem. Int. Ed. Engl.52(4), 1128–1133 (2013).
    [CrossRef] [PubMed]
  52. C. Bensalem, M. Mortier, D. Vivien, and M. Diaf, “Optical investigation of Eu3+:PbF2 ceramics and transparent glass-ceramics,” Opt. Mater.33(6), 791–798 (2011).
    [CrossRef]
  53. M. Gu, Q. C. Gao, S. M. Huang, X. L. Liu, B. Liu, and C. Ni, “Luminescence properties of Pr3+-doped transparent oxyfluoride glass-ceramics containing BaYF5 nanocrystals,” J. Lumin.132(10), 2531–2536 (2012).
    [CrossRef]
  54. B. C. Jamalaiah, M. V. Vijaya Kumar, and K. Rama Gopal, “Fluorescence properties and energy transfer mechanism of Sm3+ ion in lead telluroborate glasses,” Opt. Mater.33(11), 1643–1647 (2011).
    [CrossRef]
  55. P. Babu, K. H. Jang, E. S. Kim, L. Shi, R. Vijaya, V. Lavín, C. K. Jayasankar, and H. J. Seo, “Optical properties and energy transfer of Dy3+-doped transparent oxyfluoride glasses and glass-ceramics,” J. Non-Cryst. Solids356(4–5), 236–243 (2010).
    [CrossRef]
  56. Z. J. Hu, E. Ma, Y. S. Wang, and D. Q. Chen, “Fluorescence property investigations on Er3+-doped oxyfluoride glass ceramics containing LaF3 nanocrystals,” Mater. Chem. Phys.100(2–3), 308–312 (2006).
    [CrossRef]
  57. W. J. Zhang, Q. Y. Zhang, Q. J. Chen, Q. Qian, Z. M. Yang, J. R. Qiu, P. Huang, and Y. S. Wang, “Enhanced 2.0 μm emission and gain coefficient of transparent glass ceramic containing BaF2: Ho3+,Tm3+ nanocrystals,” Opt. Express17(23), 20952–20958 (2009).
    [CrossRef] [PubMed]

2013 (2)

C. Liu, X. J. Zhao, and J. Heo, “Direct imaging of inhomogeneous distribution of Er3 + ions in lead fluoride nanocrystals,” J. Non-Cryst. Solids365, 1–5 (2013).
[CrossRef]

D. T. Tu, Y. S. Liu, H. M. Zhu, R. F. Li, L. Q. Liu, and X. Y. Chen, “Breakdown of crystallographic site symmetry in lanthanide-doped NaYF4 crystals,” Angew. Chem. Int. Ed. Engl.52(4), 1128–1133 (2013).
[CrossRef] [PubMed]

2012 (5)

M. Gu, Q. C. Gao, S. M. Huang, X. L. Liu, B. Liu, and C. Ni, “Luminescence properties of Pr3+-doped transparent oxyfluoride glass-ceramics containing BaYF5 nanocrystals,” J. Lumin.132(10), 2531–2536 (2012).
[CrossRef]

S. W. Hao, L. Sun, G. Y. Chen, H. L. Qiu, C. Xu, T. N. Soitah, Y. Sun, and C. H. Yang, “Synthesis of monoclinic Na3ScF6:1 mol% Er3+/2 mol% Yb3+ microcrystals by a facile hydrothermal approach,” J. Alloy. Comp.522, 74–77 (2012).
[CrossRef]

A. Kar and A. Patra, “Impacts of core-shell structures on properties of lanthanide-based nanocrystals: crystal phase, lattice strain, downconversion, upconversion and energy transfer,” Nanoscale4(12), 3608–3619 (2012).
[CrossRef] [PubMed]

H. Yu, H. Guo, M. Zhang, Y. Liu, M. Liu, and L. J. Zhao, “Distribution of Nd3+ ions in oxyfluoride glass ceramics,” Nanoscale Res. Lett.7(1), 275 (2012).
[CrossRef] [PubMed]

C. Liu and J. Heo, “Electron energy loss spectroscopy analysis on the preferential incorporation of Er3+ ions into fluoride nanocrystals in oxyfluoride glass-ceramics,” J. Am. Ceram. Soc.95(7), 2100–2102 (2012).
[CrossRef]

2011 (7)

N. Hu, H. Yu, M. Zhang, P. Zhang, Y. Z. Wang, and L. J. Zhao, “The tetragonal structure of nanocrystals in rare-earth doped oxyfluoride glass ceramics,” Phys. Chem. Chem. Phys.13(4), 1499–1505 (2011).
[CrossRef] [PubMed]

Y. H. Wang, Y. S. Liu, Q. B. Xiao, H. M. Zhu, R. F. Li, and X. Y. Chen, “Eu3+ doped KYF4 nanocrystals: synthesis, electronic structure, and optical properties,” Nanoscale3(8), 3164–3169 (2011).
[CrossRef] [PubMed]

J. Chen, C. R. Guo, M. Wang, L. Huang, L. P. Wang, C. C. Mi, J. Li, X. X. Fang, C. B. Mao, and S. K. Xu, “Controllable synthesis of NaYF(4) : Yb,Er upconversion nanophosphors and their application to in vivo imaging of Caenorhabditis elegans,” J. Mater. Chem.21(8), 2632–2638 (2011).
[CrossRef] [PubMed]

J. W. Wang, J. H. Hao, and P. A. Tanner, “Upconversion luminescence of an insulator involving a band to band multiphoton excitation process,” Opt. Express19(12), 11753–11758 (2011).
[CrossRef] [PubMed]

M. Haase and H. Schäfer, “Nanopartikel für die Aufwärtskonversion,” Angew. Chem.123(26), 5928–5950 (2011).
[CrossRef]

B. C. Jamalaiah, M. V. Vijaya Kumar, and K. Rama Gopal, “Fluorescence properties and energy transfer mechanism of Sm3+ ion in lead telluroborate glasses,” Opt. Mater.33(11), 1643–1647 (2011).
[CrossRef]

C. Bensalem, M. Mortier, D. Vivien, and M. Diaf, “Optical investigation of Eu3+:PbF2 ceramics and transparent glass-ceramics,” Opt. Mater.33(6), 791–798 (2011).
[CrossRef]

2010 (8)

P. Babu, K. H. Jang, E. S. Kim, L. Shi, R. Vijaya, V. Lavín, C. K. Jayasankar, and H. J. Seo, “Optical properties and energy transfer of Dy3+-doped transparent oxyfluoride glasses and glass-ceramics,” J. Non-Cryst. Solids356(4–5), 236–243 (2010).
[CrossRef]

Q. Luo, X. S. Qiao, X. P. Fan, and X. H. Zhang, “Preparation and luminescence properties of Ce3+ and Tb3+ co-doped glasses and glass ceramics containing SrF2 nanocrystals,” J. Non-Cryst. Solids356(50–51), 2875–2879 (2010).
[CrossRef]

J. J. Pan, R. R. Xu, M. Wang, G. J. Gao, J. M. Chen, L. L. Hu, and J. J. Zhang, “Enhanced 2.0 μm emission in Tm3+/Ho3+ codoped transparent oxyfluoride glass ceramics containing β-PbF2 nano-crystals,” Solid State Commun.150(1–2), 78–80 (2010).
[CrossRef]

C. C. Lin, Z. R. Xiao, G. Y. Guo, T. S. Chan, and R. S. Liu, “Versatile phosphate phosphors ABPO(4) in white light-emitting diodes: collocated characteristic analysis and theoretical calculations,” J. Am. Chem. Soc.132(9), 3020–3028 (2010).
[CrossRef] [PubMed]

F. Wang, Y. Han, C. S. Lim, Y. H. Lu, J. Wang, J. Xu, H. Y. Chen, C. Zhang, M. H. Hong, and X. G. Liu, “Simultaneous phase and size control of upconversion nanocrystals through lanthanide doping,” Nature463(7284), 1061–1065 (2010).
[CrossRef] [PubMed]

D. K. Chatterjee, M. K. Gnanasammandhan, and Y. Zhang, “Small upconverting fluorescent nanoparticles for biomedical applications,” Small6(24), 2781–2795 (2010).
[CrossRef] [PubMed]

A. M. Cross, P. S. May, F. C. J. M. van Veggel, and M. T. Berry, “Dipicolinate sensitization of europium luminescence in dispersible 5%Eu:LaF3 nanoparticles,” J. Phys. Chem. C114(35), 14740–14747 (2010).
[CrossRef]

C. Bensalem, M. Mortier, D. Vivien, and M. Diaf, “Thermal and optical investigation of EuF3-doped lead fluorogermanate glasses,” J. Non-Cryst. Solids356(1), 56–64 (2010).
[CrossRef]

2009 (3)

B. R. Kumar, M. Nyk, T. Y. Ohulchanskyy, C. A. Flask, and P. N. Prasad, “Combined optical and MR bioimaging using rare earth ion doped NaYF4 nanocrystals,” Adv. Funct. Mater.19(6), 853–859 (2009).
[CrossRef]

Q. Ju, Y. S. Liu, R. F. Li, L. Q. Liu, W. Q. Luo, and X. Y. Chen, “Optical spectroscopy of Eu3+-doped BaFCl nanocrystals,” J. Phys. Chem. C113(6), 2309–2315 (2009).
[CrossRef]

W. J. Zhang, Q. Y. Zhang, Q. J. Chen, Q. Qian, Z. M. Yang, J. R. Qiu, P. Huang, and Y. S. Wang, “Enhanced 2.0 μm emission and gain coefficient of transparent glass ceramic containing BaF2: Ho3+,Tm3+ nanocrystals,” Opt. Express17(23), 20952–20958 (2009).
[CrossRef] [PubMed]

2008 (1)

H. Yu, N. Hu, Y. N. Wang, Z. L. Wang, Z. S. Gan, and L. J. Zhao, “The fabrication of nano-particles in aqueous solution from oxyfluoride glass ceramics by thermal induction and corrosion treatment,” Nanoscale Res. Lett.3(12), 516–520 (2008).
[CrossRef] [PubMed]

2007 (2)

K. Driesen, V. K. Tikhomirov, and C. Görller-Walrand, “Eu3+ as a probe for rare-earth dopant site structure in nano-glass-ceramics,” J. Appl. Phys.102(2), 024312 (2007).
[CrossRef]

V. K. Tikhomirov, K. Driesen, C. Görller-Walrand, and M. Mortier, “Broadband telecommunication wavelength emission in Yb(3+)-Er(3+)-Tm(3+) co-doped nano-glassceramics,” Opt. Express15(15), 9535–9540 (2007).
[CrossRef] [PubMed]

2006 (3)

K. Driesen, V. K. Tikhomirov, C. Görller-Walrand, V. D. Rodríguez, and A. B. Seddon, “Transparent Ho3+-doped nano-glass-ceramics for efficient infrared emission,” Appl. Phys. Lett.88(7), 073111 (2006).
[CrossRef]

H. X. Mai, Y. W. Zhang, R. Si, Z. G. Yan, L. D. Sun, L. P. You, and C. H. Yan, “High-quality sodium rare-earth fluoride nanocrystals: controlled synthesis and optical properties,” J. Am. Chem. Soc.128(19), 6426–6436 (2006).
[CrossRef] [PubMed]

Z. J. Hu, E. Ma, Y. S. Wang, and D. Q. Chen, “Fluorescence property investigations on Er3+-doped oxyfluoride glass ceramics containing LaF3 nanocrystals,” Mater. Chem. Phys.100(2–3), 308–312 (2006).
[CrossRef]

2005 (1)

G. Dantelle, M. Mortier, D. Vivien, and G. Patriarche, “Effect of CeF3 addition on the nucleation and up-conversion luminescence in transparent oxyfluoride glass-ceramics,” Chem. Mater.17(8), 2216–2222 (2005).
[CrossRef]

2004 (3)

F. Auzel, “Upconversion and anti-Stokes processes with f and d ions in solids,” Chem. Rev.104(1), 139–174 (2004).
[CrossRef] [PubMed]

O. Lehmann, K. Kömpe, and M. Haase, “Synthesis of Eu3+-doped core and core/shell nanoparticles and direct spectroscopic identification of dopant sites at the surface and in the interior of the particles,” J. Am. Chem. Soc.126(45), 14935–14942 (2004).
[CrossRef] [PubMed]

J. C. Boyer, F. Vetrone, J. A. Capobianco, A. Speghini, and M. Bettinelli, “Variation of fluorescence lifetimes and Judd-Ofelt parameters between Eu3+ doped bulk and nanocrystalline cubic Lu2O3,” J. Phys. Chem. B108(52), 20137–20143 (2004).
[CrossRef]

2003 (2)

M. Beggiora, I. M. Reaney, and M. S. Islam, “Structure of the nanocrystals in oxyfluoride glass ceramics,” Appl. Phys. Lett.83(3), 467–469 (2003).
[CrossRef]

J. Méndez-Ramos, V. Lavín, I. R. Martín, U. R. Rodríguez-Mendoza, V. D. Rodríguez, A. D. Lozano-Gorrín, and P. Núñez, “Site selective study of Eu3+-doped transparent oxyfluoride glass ceramics,” J. Appl. Phys.94(4), 2295–2301 (2003).
[CrossRef]

2002 (2)

M. H. V. Werts, R. T. F. Jukes, and J. W. Verhoeven, “The emission spectrum and the radiative lifetime of Eu3+ in luminescent Lanthanide complexes,” Phys. Chem. Chem. Phys.4(9), 1542–1548 (2002).
[CrossRef]

V. K. Tikhomirov, D. Furniss, A. B. Seddon, I. M. Reaney, M. Beggiora, M. Ferrari, M. Montagna, and R. Rolli, “Fabrication and characterization of nanoscale, Er3+-doped, ultratransparent oxy-fluoride glass ceramics,” Appl. Phys. Lett.81(11), 1937–1939 (2002).
[CrossRef]

2001 (1)

M. Mortier, P. Goldner, C. Chateau, and M. Genotelle, “Erbium doped glass-ceramics: concentration effect on crystal structure and energy transfer between active ions,” J. Alloys Compd. 323&324, 245–249 (2001).

2000 (2)

M. Bouffard, J. P. Jouart, and M. F. Joubert, “Red-to-blue up-conversion spectroscopy of Tm3+ in SrF2, CaF2, BaF2 and CdF2,” Opt. Mater.14(1), 73–79 (2000).
[CrossRef]

B. Delley, “From molecules to solids with the DMol3 approach,” J. Chem. Phys.113(18), 7756–7764 (2000).
[CrossRef]

1999 (1)

M. Mortier and F. Auzel, “Rare-earth doped transparent glass-ceramics with high cross-sections,” J. Non-Cryst. Solids 256&257, 361–365 (1999).

1995 (1)

P. A. Tick, N. F. Borrellia, L. K. Cornelius, and M. A. Newhouse, “Transparent glass ceramics for 1300 nm amplifier applications,” J. Appl. Phys.78(11), 6367–6374 (1995).
[CrossRef]

1993 (1)

Y. Wang and J. Ohwaki, “New transparent vitroceramics codoped with Er3+ and Yb3+ for efficient frequency upconversion,” Appl. Phys. Lett.63(24), 3268–3270 (1993).
[CrossRef]

1992 (1)

J. P. Perdew and Y. Wang, “Accurate and simple analytic representation of the electron-gas correlation energy,” Phys. Rev. B Condens. Matter45(23), 13244–13249 (1992).
[CrossRef] [PubMed]

1990 (2)

B. Delley, “An all-electron numerical method for solving the local density functional for polyatomic molecules,” J. Chem. Phys.92(1), 508–517 (1990).
[CrossRef]

K. Leśniak, “Crystal fields and dopant-ligand separations in cubic centres of rare-earth ions in fluorites,” J. Phys. Condens. Matter2(25), 5563–5574 (1990).
[CrossRef]

1986 (2)

K. Leśniak, “Model simulation of the tetragonal symmetry centre of a rare-earth ion in a fluorite lattice,” J. Phys. C Solid State Phys.19(15), 2721–2727 (1986).
[CrossRef]

F. J. Weesner, J. C. Wright, and J. J. Fontanella, “Laser spectroscopy of ion-size effects on point-defect equilibria in PbF2:Eu3+,” Phys. Rev. B Condens. Matter33(2), 1372–1380 (1986).
[CrossRef] [PubMed]

1982 (2)

R. J. Hamers, J. R. Wietfeld, and J. C. Wright, “Defect chemistry in CaF2:Eu3+,” J. Chem. Phys.77(2), 683–692 (1982).
[CrossRef]

S. Mho and J. C. Wright, “Site selective spectroscopy of defect chemistry in CdF2:Eu,” J. Chem. Phys.77(3), 1183–1192 (1982).
[CrossRef]

1966 (1)

C. W. Rector, B. C. Pandey, and H. W. Moos, “Electron paramagnetic resonance and optical Zeeman spectra of type II CaF2:Er3+,” J. Chem. Phys.45(1), 171–179 (1966).
[CrossRef]

1964 (1)

M. J. Weber and R. W. Bierig, “Paramagnetic resonance and relaxation of trivalent rare-earth ions in calcium fluoride. I. resonance spectra and crystal fields,” Phys. Rev.134(6A), A1492–A1503 (1964).
[CrossRef]

1959 (1)

J. M. Baker, W. Hayes, and D. A. Jones, “Paramagnetic resonance of impurities in CaF2,” Proc. Phys. Soc.73(6), 942–945 (1959).
[CrossRef]

Auzel, F.

F. Auzel, “Upconversion and anti-Stokes processes with f and d ions in solids,” Chem. Rev.104(1), 139–174 (2004).
[CrossRef] [PubMed]

M. Mortier and F. Auzel, “Rare-earth doped transparent glass-ceramics with high cross-sections,” J. Non-Cryst. Solids 256&257, 361–365 (1999).

Babu, P.

P. Babu, K. H. Jang, E. S. Kim, L. Shi, R. Vijaya, V. Lavín, C. K. Jayasankar, and H. J. Seo, “Optical properties and energy transfer of Dy3+-doped transparent oxyfluoride glasses and glass-ceramics,” J. Non-Cryst. Solids356(4–5), 236–243 (2010).
[CrossRef]

Baker, J. M.

J. M. Baker, W. Hayes, and D. A. Jones, “Paramagnetic resonance of impurities in CaF2,” Proc. Phys. Soc.73(6), 942–945 (1959).
[CrossRef]

Beggiora, M.

M. Beggiora, I. M. Reaney, and M. S. Islam, “Structure of the nanocrystals in oxyfluoride glass ceramics,” Appl. Phys. Lett.83(3), 467–469 (2003).
[CrossRef]

V. K. Tikhomirov, D. Furniss, A. B. Seddon, I. M. Reaney, M. Beggiora, M. Ferrari, M. Montagna, and R. Rolli, “Fabrication and characterization of nanoscale, Er3+-doped, ultratransparent oxy-fluoride glass ceramics,” Appl. Phys. Lett.81(11), 1937–1939 (2002).
[CrossRef]

Bensalem, C.

C. Bensalem, M. Mortier, D. Vivien, and M. Diaf, “Optical investigation of Eu3+:PbF2 ceramics and transparent glass-ceramics,” Opt. Mater.33(6), 791–798 (2011).
[CrossRef]

C. Bensalem, M. Mortier, D. Vivien, and M. Diaf, “Thermal and optical investigation of EuF3-doped lead fluorogermanate glasses,” J. Non-Cryst. Solids356(1), 56–64 (2010).
[CrossRef]

Berry, M. T.

A. M. Cross, P. S. May, F. C. J. M. van Veggel, and M. T. Berry, “Dipicolinate sensitization of europium luminescence in dispersible 5%Eu:LaF3 nanoparticles,” J. Phys. Chem. C114(35), 14740–14747 (2010).
[CrossRef]

Bettinelli, M.

J. C. Boyer, F. Vetrone, J. A. Capobianco, A. Speghini, and M. Bettinelli, “Variation of fluorescence lifetimes and Judd-Ofelt parameters between Eu3+ doped bulk and nanocrystalline cubic Lu2O3,” J. Phys. Chem. B108(52), 20137–20143 (2004).
[CrossRef]

Bierig, R. W.

M. J. Weber and R. W. Bierig, “Paramagnetic resonance and relaxation of trivalent rare-earth ions in calcium fluoride. I. resonance spectra and crystal fields,” Phys. Rev.134(6A), A1492–A1503 (1964).
[CrossRef]

Borrellia, N. F.

P. A. Tick, N. F. Borrellia, L. K. Cornelius, and M. A. Newhouse, “Transparent glass ceramics for 1300 nm amplifier applications,” J. Appl. Phys.78(11), 6367–6374 (1995).
[CrossRef]

Bouffard, M.

M. Bouffard, J. P. Jouart, and M. F. Joubert, “Red-to-blue up-conversion spectroscopy of Tm3+ in SrF2, CaF2, BaF2 and CdF2,” Opt. Mater.14(1), 73–79 (2000).
[CrossRef]

Boyer, J. C.

J. C. Boyer, F. Vetrone, J. A. Capobianco, A. Speghini, and M. Bettinelli, “Variation of fluorescence lifetimes and Judd-Ofelt parameters between Eu3+ doped bulk and nanocrystalline cubic Lu2O3,” J. Phys. Chem. B108(52), 20137–20143 (2004).
[CrossRef]

Capobianco, J. A.

J. C. Boyer, F. Vetrone, J. A. Capobianco, A. Speghini, and M. Bettinelli, “Variation of fluorescence lifetimes and Judd-Ofelt parameters between Eu3+ doped bulk and nanocrystalline cubic Lu2O3,” J. Phys. Chem. B108(52), 20137–20143 (2004).
[CrossRef]

Chan, T. S.

C. C. Lin, Z. R. Xiao, G. Y. Guo, T. S. Chan, and R. S. Liu, “Versatile phosphate phosphors ABPO(4) in white light-emitting diodes: collocated characteristic analysis and theoretical calculations,” J. Am. Chem. Soc.132(9), 3020–3028 (2010).
[CrossRef] [PubMed]

Chateau, C.

M. Mortier, P. Goldner, C. Chateau, and M. Genotelle, “Erbium doped glass-ceramics: concentration effect on crystal structure and energy transfer between active ions,” J. Alloys Compd. 323&324, 245–249 (2001).

Chatterjee, D. K.

D. K. Chatterjee, M. K. Gnanasammandhan, and Y. Zhang, “Small upconverting fluorescent nanoparticles for biomedical applications,” Small6(24), 2781–2795 (2010).
[CrossRef] [PubMed]

Chen, D. Q.

Z. J. Hu, E. Ma, Y. S. Wang, and D. Q. Chen, “Fluorescence property investigations on Er3+-doped oxyfluoride glass ceramics containing LaF3 nanocrystals,” Mater. Chem. Phys.100(2–3), 308–312 (2006).
[CrossRef]

Chen, G. Y.

S. W. Hao, L. Sun, G. Y. Chen, H. L. Qiu, C. Xu, T. N. Soitah, Y. Sun, and C. H. Yang, “Synthesis of monoclinic Na3ScF6:1 mol% Er3+/2 mol% Yb3+ microcrystals by a facile hydrothermal approach,” J. Alloy. Comp.522, 74–77 (2012).
[CrossRef]

Chen, H. Y.

F. Wang, Y. Han, C. S. Lim, Y. H. Lu, J. Wang, J. Xu, H. Y. Chen, C. Zhang, M. H. Hong, and X. G. Liu, “Simultaneous phase and size control of upconversion nanocrystals through lanthanide doping,” Nature463(7284), 1061–1065 (2010).
[CrossRef] [PubMed]

Chen, J.

J. Chen, C. R. Guo, M. Wang, L. Huang, L. P. Wang, C. C. Mi, J. Li, X. X. Fang, C. B. Mao, and S. K. Xu, “Controllable synthesis of NaYF(4) : Yb,Er upconversion nanophosphors and their application to in vivo imaging of Caenorhabditis elegans,” J. Mater. Chem.21(8), 2632–2638 (2011).
[CrossRef] [PubMed]

Chen, J. M.

J. J. Pan, R. R. Xu, M. Wang, G. J. Gao, J. M. Chen, L. L. Hu, and J. J. Zhang, “Enhanced 2.0 μm emission in Tm3+/Ho3+ codoped transparent oxyfluoride glass ceramics containing β-PbF2 nano-crystals,” Solid State Commun.150(1–2), 78–80 (2010).
[CrossRef]

Chen, Q. J.

Chen, X. Y.

D. T. Tu, Y. S. Liu, H. M. Zhu, R. F. Li, L. Q. Liu, and X. Y. Chen, “Breakdown of crystallographic site symmetry in lanthanide-doped NaYF4 crystals,” Angew. Chem. Int. Ed. Engl.52(4), 1128–1133 (2013).
[CrossRef] [PubMed]

Y. H. Wang, Y. S. Liu, Q. B. Xiao, H. M. Zhu, R. F. Li, and X. Y. Chen, “Eu3+ doped KYF4 nanocrystals: synthesis, electronic structure, and optical properties,” Nanoscale3(8), 3164–3169 (2011).
[CrossRef] [PubMed]

Q. Ju, Y. S. Liu, R. F. Li, L. Q. Liu, W. Q. Luo, and X. Y. Chen, “Optical spectroscopy of Eu3+-doped BaFCl nanocrystals,” J. Phys. Chem. C113(6), 2309–2315 (2009).
[CrossRef]

Cornelius, L. K.

P. A. Tick, N. F. Borrellia, L. K. Cornelius, and M. A. Newhouse, “Transparent glass ceramics for 1300 nm amplifier applications,” J. Appl. Phys.78(11), 6367–6374 (1995).
[CrossRef]

Cross, A. M.

A. M. Cross, P. S. May, F. C. J. M. van Veggel, and M. T. Berry, “Dipicolinate sensitization of europium luminescence in dispersible 5%Eu:LaF3 nanoparticles,” J. Phys. Chem. C114(35), 14740–14747 (2010).
[CrossRef]

Dantelle, G.

G. Dantelle, M. Mortier, D. Vivien, and G. Patriarche, “Effect of CeF3 addition on the nucleation and up-conversion luminescence in transparent oxyfluoride glass-ceramics,” Chem. Mater.17(8), 2216–2222 (2005).
[CrossRef]

Delley, B.

B. Delley, “From molecules to solids with the DMol3 approach,” J. Chem. Phys.113(18), 7756–7764 (2000).
[CrossRef]

B. Delley, “An all-electron numerical method for solving the local density functional for polyatomic molecules,” J. Chem. Phys.92(1), 508–517 (1990).
[CrossRef]

Diaf, M.

C. Bensalem, M. Mortier, D. Vivien, and M. Diaf, “Optical investigation of Eu3+:PbF2 ceramics and transparent glass-ceramics,” Opt. Mater.33(6), 791–798 (2011).
[CrossRef]

C. Bensalem, M. Mortier, D. Vivien, and M. Diaf, “Thermal and optical investigation of EuF3-doped lead fluorogermanate glasses,” J. Non-Cryst. Solids356(1), 56–64 (2010).
[CrossRef]

Driesen, K.

K. Driesen, V. K. Tikhomirov, and C. Görller-Walrand, “Eu3+ as a probe for rare-earth dopant site structure in nano-glass-ceramics,” J. Appl. Phys.102(2), 024312 (2007).
[CrossRef]

V. K. Tikhomirov, K. Driesen, C. Görller-Walrand, and M. Mortier, “Broadband telecommunication wavelength emission in Yb(3+)-Er(3+)-Tm(3+) co-doped nano-glassceramics,” Opt. Express15(15), 9535–9540 (2007).
[CrossRef] [PubMed]

K. Driesen, V. K. Tikhomirov, C. Görller-Walrand, V. D. Rodríguez, and A. B. Seddon, “Transparent Ho3+-doped nano-glass-ceramics for efficient infrared emission,” Appl. Phys. Lett.88(7), 073111 (2006).
[CrossRef]

Fan, X. P.

Q. Luo, X. S. Qiao, X. P. Fan, and X. H. Zhang, “Preparation and luminescence properties of Ce3+ and Tb3+ co-doped glasses and glass ceramics containing SrF2 nanocrystals,” J. Non-Cryst. Solids356(50–51), 2875–2879 (2010).
[CrossRef]

Fang, X. X.

J. Chen, C. R. Guo, M. Wang, L. Huang, L. P. Wang, C. C. Mi, J. Li, X. X. Fang, C. B. Mao, and S. K. Xu, “Controllable synthesis of NaYF(4) : Yb,Er upconversion nanophosphors and their application to in vivo imaging of Caenorhabditis elegans,” J. Mater. Chem.21(8), 2632–2638 (2011).
[CrossRef] [PubMed]

Ferrari, M.

V. K. Tikhomirov, D. Furniss, A. B. Seddon, I. M. Reaney, M. Beggiora, M. Ferrari, M. Montagna, and R. Rolli, “Fabrication and characterization of nanoscale, Er3+-doped, ultratransparent oxy-fluoride glass ceramics,” Appl. Phys. Lett.81(11), 1937–1939 (2002).
[CrossRef]

Flask, C. A.

B. R. Kumar, M. Nyk, T. Y. Ohulchanskyy, C. A. Flask, and P. N. Prasad, “Combined optical and MR bioimaging using rare earth ion doped NaYF4 nanocrystals,” Adv. Funct. Mater.19(6), 853–859 (2009).
[CrossRef]

Fontanella, J. J.

F. J. Weesner, J. C. Wright, and J. J. Fontanella, “Laser spectroscopy of ion-size effects on point-defect equilibria in PbF2:Eu3+,” Phys. Rev. B Condens. Matter33(2), 1372–1380 (1986).
[CrossRef] [PubMed]

Furniss, D.

V. K. Tikhomirov, D. Furniss, A. B. Seddon, I. M. Reaney, M. Beggiora, M. Ferrari, M. Montagna, and R. Rolli, “Fabrication and characterization of nanoscale, Er3+-doped, ultratransparent oxy-fluoride glass ceramics,” Appl. Phys. Lett.81(11), 1937–1939 (2002).
[CrossRef]

Gan, Z. S.

H. Yu, N. Hu, Y. N. Wang, Z. L. Wang, Z. S. Gan, and L. J. Zhao, “The fabrication of nano-particles in aqueous solution from oxyfluoride glass ceramics by thermal induction and corrosion treatment,” Nanoscale Res. Lett.3(12), 516–520 (2008).
[CrossRef] [PubMed]

Gao, G. J.

J. J. Pan, R. R. Xu, M. Wang, G. J. Gao, J. M. Chen, L. L. Hu, and J. J. Zhang, “Enhanced 2.0 μm emission in Tm3+/Ho3+ codoped transparent oxyfluoride glass ceramics containing β-PbF2 nano-crystals,” Solid State Commun.150(1–2), 78–80 (2010).
[CrossRef]

Gao, Q. C.

M. Gu, Q. C. Gao, S. M. Huang, X. L. Liu, B. Liu, and C. Ni, “Luminescence properties of Pr3+-doped transparent oxyfluoride glass-ceramics containing BaYF5 nanocrystals,” J. Lumin.132(10), 2531–2536 (2012).
[CrossRef]

Genotelle, M.

M. Mortier, P. Goldner, C. Chateau, and M. Genotelle, “Erbium doped glass-ceramics: concentration effect on crystal structure and energy transfer between active ions,” J. Alloys Compd. 323&324, 245–249 (2001).

Gnanasammandhan, M. K.

D. K. Chatterjee, M. K. Gnanasammandhan, and Y. Zhang, “Small upconverting fluorescent nanoparticles for biomedical applications,” Small6(24), 2781–2795 (2010).
[CrossRef] [PubMed]

Goldner, P.

M. Mortier, P. Goldner, C. Chateau, and M. Genotelle, “Erbium doped glass-ceramics: concentration effect on crystal structure and energy transfer between active ions,” J. Alloys Compd. 323&324, 245–249 (2001).

Görller-Walrand, C.

V. K. Tikhomirov, K. Driesen, C. Görller-Walrand, and M. Mortier, “Broadband telecommunication wavelength emission in Yb(3+)-Er(3+)-Tm(3+) co-doped nano-glassceramics,” Opt. Express15(15), 9535–9540 (2007).
[CrossRef] [PubMed]

K. Driesen, V. K. Tikhomirov, and C. Görller-Walrand, “Eu3+ as a probe for rare-earth dopant site structure in nano-glass-ceramics,” J. Appl. Phys.102(2), 024312 (2007).
[CrossRef]

K. Driesen, V. K. Tikhomirov, C. Görller-Walrand, V. D. Rodríguez, and A. B. Seddon, “Transparent Ho3+-doped nano-glass-ceramics for efficient infrared emission,” Appl. Phys. Lett.88(7), 073111 (2006).
[CrossRef]

Gu, M.

M. Gu, Q. C. Gao, S. M. Huang, X. L. Liu, B. Liu, and C. Ni, “Luminescence properties of Pr3+-doped transparent oxyfluoride glass-ceramics containing BaYF5 nanocrystals,” J. Lumin.132(10), 2531–2536 (2012).
[CrossRef]

Guo, C. R.

J. Chen, C. R. Guo, M. Wang, L. Huang, L. P. Wang, C. C. Mi, J. Li, X. X. Fang, C. B. Mao, and S. K. Xu, “Controllable synthesis of NaYF(4) : Yb,Er upconversion nanophosphors and their application to in vivo imaging of Caenorhabditis elegans,” J. Mater. Chem.21(8), 2632–2638 (2011).
[CrossRef] [PubMed]

Guo, G. Y.

C. C. Lin, Z. R. Xiao, G. Y. Guo, T. S. Chan, and R. S. Liu, “Versatile phosphate phosphors ABPO(4) in white light-emitting diodes: collocated characteristic analysis and theoretical calculations,” J. Am. Chem. Soc.132(9), 3020–3028 (2010).
[CrossRef] [PubMed]

Guo, H.

H. Yu, H. Guo, M. Zhang, Y. Liu, M. Liu, and L. J. Zhao, “Distribution of Nd3+ ions in oxyfluoride glass ceramics,” Nanoscale Res. Lett.7(1), 275 (2012).
[CrossRef] [PubMed]

Haase, M.

M. Haase and H. Schäfer, “Nanopartikel für die Aufwärtskonversion,” Angew. Chem.123(26), 5928–5950 (2011).
[CrossRef]

O. Lehmann, K. Kömpe, and M. Haase, “Synthesis of Eu3+-doped core and core/shell nanoparticles and direct spectroscopic identification of dopant sites at the surface and in the interior of the particles,” J. Am. Chem. Soc.126(45), 14935–14942 (2004).
[CrossRef] [PubMed]

Hamers, R. J.

R. J. Hamers, J. R. Wietfeld, and J. C. Wright, “Defect chemistry in CaF2:Eu3+,” J. Chem. Phys.77(2), 683–692 (1982).
[CrossRef]

Han, Y.

F. Wang, Y. Han, C. S. Lim, Y. H. Lu, J. Wang, J. Xu, H. Y. Chen, C. Zhang, M. H. Hong, and X. G. Liu, “Simultaneous phase and size control of upconversion nanocrystals through lanthanide doping,” Nature463(7284), 1061–1065 (2010).
[CrossRef] [PubMed]

Hao, J. H.

Hao, S. W.

S. W. Hao, L. Sun, G. Y. Chen, H. L. Qiu, C. Xu, T. N. Soitah, Y. Sun, and C. H. Yang, “Synthesis of monoclinic Na3ScF6:1 mol% Er3+/2 mol% Yb3+ microcrystals by a facile hydrothermal approach,” J. Alloy. Comp.522, 74–77 (2012).
[CrossRef]

Hayes, W.

J. M. Baker, W. Hayes, and D. A. Jones, “Paramagnetic resonance of impurities in CaF2,” Proc. Phys. Soc.73(6), 942–945 (1959).
[CrossRef]

Heo, J.

C. Liu, X. J. Zhao, and J. Heo, “Direct imaging of inhomogeneous distribution of Er3 + ions in lead fluoride nanocrystals,” J. Non-Cryst. Solids365, 1–5 (2013).
[CrossRef]

C. Liu and J. Heo, “Electron energy loss spectroscopy analysis on the preferential incorporation of Er3+ ions into fluoride nanocrystals in oxyfluoride glass-ceramics,” J. Am. Ceram. Soc.95(7), 2100–2102 (2012).
[CrossRef]

Hong, M. H.

F. Wang, Y. Han, C. S. Lim, Y. H. Lu, J. Wang, J. Xu, H. Y. Chen, C. Zhang, M. H. Hong, and X. G. Liu, “Simultaneous phase and size control of upconversion nanocrystals through lanthanide doping,” Nature463(7284), 1061–1065 (2010).
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Figures (6)

Fig. 1
Fig. 1

(a) XRD patterns of standard β-PbF2, lowly (0.05GC), moderately (0.5GC) and highly doped samples (1.5GC). The inset shows the enlarged three labeled diffraction peaks; HRTEM images of 0.05GC (b) and 1.5GC (c), and the corresponding fast-Fourier transform (FFT) of the diffraction patterns recorded from the area marked with the white circle.

Fig. 2
Fig. 2

A complete description of the phase transition: the phase transformation from cubic to tetragonal phase in lowly and highly doped nano-particles, respectively.

Fig. 3
Fig. 3

Emission spectra of the 5D07Fj (j = 0, 1, 2) transitions with the excitation at 393 nm. The inset shows the photoluminescence decays of the 5D07F1 transitions in 0.05GC, 0.5GC and 1.5GC.

Fig. 4
Fig. 4

Site-selective excitation spectra of the 7F05D1 transition monitoring at 586.5 nm, 589.0 nm and 591.7 nm.

Fig. 5
Fig. 5

XRD Rietveld refinements and the corresponding error curve of 0.05GC (a), 1.5GC (b) and 0.5GC (c). The positions of the Bragg reflections are represented by vertical bars (|).The simulated diffraction line spectra of the oxide phase PbSiO3 (d), the cubic phase Pb3EuF9 (e) and tetragonal phase PbEuF5 (f) are presented for comparisons.

Fig. 6
Fig. 6

XRD patterns of 0.5GC with thermal treatment temperature at 400 °C, 405 °C, and 410 °C. The left inset shows the enlarged diffraction peaks and the right inset presents the crystallized fraction of the cubic and tetragonal phase at different thermal treatment temperature.

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