Site-selective time-resolved spectroscopy of Eu3+ in KPb2Cl5 has been investigated by using fluorescence line narrowing technique. A crystal field analysis and simulation of the experimental results has been performed in order to parametrize the crystal field at the Eu3+ sites. Three symmetry independent crystal field sites for the rare-earth ion in this crystal were found. A plausible argument about the crystallographic nature of these sites is given.
© 2005 Optical Society of America
Over the past few years, the interest in the search of rare earth (RE) doped low-energy phonon host materials has increased, specially for solid state upconversion lasers and mid-infrared lasers [1–4]. Hosts with low phonon energy lead to low non-radiative transition rates due to multiphonon relaxation and high radiative transition rates, which increase the quantum efficiency from excited states of active ions. Sulfide-and chloride-based hosts have been studied as their phonon energies are lower than those in the most extensively studied fluoride compounds. However, these materials usually present poor mechanical properties, moisture sensitivity, and are difficult to synthesize. Recently, potassium lead chloride, KPb2Cl5, has been studied as a promising host for RE ions [5–13] because it is non-hygroscopic and readily incorporates RE ions. The crystal is biaxial, crystallizes in the monoclinic system  and it is transparent in the 0.3 to 20 µm spectral region. According to Raman-scattering measurements  the maximum phonon energy, measured at the highest energy peak of the spectrum, is 203 cm-1.
In RE doped KPb2Cl5 crystals the RE ions are supposed to substitute the lead (Pb2+) ions whereas potassium (K+) vacancies are assumed to provide charge compensation . However, it is well established that the presence of structural defects in the vicinity of the RE ions may modify the local crystal-field symmetry and strength and may lead to a variety of nonequivalent RE optical centers. Unfortunately at this moment no accordance exists with respect to either the number of independent positions for Pb atoms or their coordination [7,10,14]. If there are various nonequivalent crystallographic sites for Pb2+ ion in the material lattice one could expect a rather complex spectroscopic behavior for the RE active ions. However, a recent spectroscopic characterization of Er3+ in KPb2Cl5 performed by Jenkins el al.  concluded that the erbium ion replaced only one of the two available non-equivalent lead ion sites. In order to understand the underlying reasons for this behaviour and to clarify the nature of the RE environments in potassium lead chloride-type crystals we have undertaken the study of the site-resolved luminescence of Eu3+ in KPb2Cl5 account taken of the adequacy of the dopant ion as a structural probe. Since 5D0 state is nondegenerate under any symmetry, the structure of the 5D0→7FJ emission is only determined by the splitting of the terminal levels caused by the local crystal field. Moreover, as the 7F0 level is also nondegenerate, site-selective excitation within the inhomogeneous broadened 7F0→5D0 absorption band can be performed by using fluorescence line narrowing (FLN) technique to distinguish among different local environments around the rare-earth ions. On the ground of the experimental results a crystal-field analysis and simulation of the energy level schemes have also been performed in order to parametrize the crystal-field around the Eu3+ ions. As a conclusion, we found evidences about the existence of at least three symmetry independent crystal field sites for the RE ions in this crystal. A plausible argument about the crystallographic nature of these sites is finally given.
2. Experimental techniques
Single crystals of non-hygroscopic ternary potassium-lead chloride KPb2Cl5 doped with Eu3+ ions, typically 2 cm long and 1 cm in diameter, have been grown in our laboratory by the Bridgman technique, in a chlorine atmosphere, with a two-zone transparent furnace, a vertical temperature gradient of 18°C/cm, and a 1mm per hour growth rate. Quartz ampoules with a pointed end were used as seed selectors to promote single crystal growth. The pure crystals are transparent and colourless. The Eu3+ content was 0.5 mol % in the melt. The plates with approximate dimensions of 8×4×2 mm3 were cut from blocks and polished for spectroscopic measurements.
Resonant time-resolved FLN spectra were performed by exciting the sample with a pulsed frequency doubled Nd:YAG pumped tunable dye laser of 9 ns pulsed width and 0.08 cm-1 linewidth and detected by an EGG & PAR Optical Multichannel Analyzer. The measurements were carried out by keeping the sample temperature at 10 K in a closed cycle helium cryostat.
For lifetime measurements, the fluorescence was analyzed with a 0.25 m Jobin-Ybon monochromator and the signal detected by a Hamamatsu R636 photomultiplier. Data were processed by a Tektronix oscilloscope.
3. Experimental results
3.1 FLN spectra
Time-resolved line-narrowed fluorescence spectra of the 5D0→7F0-6 transitions of Eu3+ doped KPb2Cl5 crystal were obtained at 10 K by using different resonant excitation wavelengths into the 7F0→5D0 transition, and at different time delays after the laser pulse. Depending on the excitation wavelength the emission spectra present very diverse characteristics, mainly regarding the number of observed 5D0→7FJ transitions, their relative intensity and the magnitude of the observed crystal-field splitting for each 7FJ state. As an example Fig. 1 shows the spectra corresponding to the 5D0→7F0,1,2 transitions obtained with a time delay of 10 µs after the pump pulse at four different pumping wavelengths. At the lowest excitation wavelength, 578.5 nm, the 5D0→7F0 transition shows the presence of at least three resolved peaks which indicates the existence of different RE sites. Indeed, the 5D0→7F0,1,2 spectra obtained by exciting at 579.5, 580.1, and 581 nm respectively, selectively show the presence of the three isolated Eu3+ sites.
We shall hereafter refer to the optical features of these spectra as originating from sites I (λexc=579.5 nm), II (λexc=580.1 nm) and III (λexc=581 nm). On the other hand, while tuning the excitation pulse at other wavelengths the observed spectra consist of overlapped series of peaks corresponding to the three sites. The presence of the line for the 5D0→7F0 transition in each spectrum indicates a site of Cnv, Cn or Cs symmetry for Eu3+. These symmetries allow the transition as an electric dipole process, according to the group theory selection rules, with a linear term in the crystal-field expansion [16–18]. By making use of the selection rules for induced electric dipole (ED) and magnetic dipole (MD) transitions, that is, from the comparison between the derived number of possible and experimentally observed 5D0→ 7F0-6 transitions, it is in principle likely to discriminate between different symmetry point groups for these Eu3+ optical centers .
In this way, the spectra obtained with excitation wavelengths 579.5 and 580.1 nm display, in both cases, three Stark levels for the 5D0→7F1 transition and five levels in the hypersensitive 5D0→7F2 region, meaning that the degeneracy of these two states is completely lifted, that is, I and II Eu3+ optical centers are located in crystal sites with C2v or lower symmetry. These levels as well as those observed for remaining transitions from 5D0 to the ground 7FJ manifold are included in Table 1 (shown in the Appendix). However, the noticeably different patterns observed when comparing the fluorescence spectra from I and II sites indicate very distinct Eu3+ crystalline environments. As can be observed in Fig. 1, the intensity ratio of the 5D0→7F1 and 5D0→7F2 emissions is about one for site I, whereas the relationship is reduced to about one third for site II. Regarding the rest of the 5D0→7FJ transitions, shown in Fig. 2, it is also surprising to find the very low intensity of the 5D0→7F3 transition detected at excitation wavelength 580.1 nm. This transition, forbidden in first order by the electric/magnetic dipole selection rules, is observed only as a consequence of the J-mixing, which mixes the 7F3 wavefunctions with other 7FJ ones, through the second-and fourth-order crystal-field (CF) parameters. Finally, the quite large observed splitting of 7F1 for site I, 382 cm-1, clearly contrasts with the corresponding to site II, of about 87 cm-1.
On the other hand, the spectra obtained by exciting at 581 nm only show two CF levels for the 5D0→7F1 transition, and three levels (one of them with a very weak intensity) for the 5D0→7F2 emission. These results suggest that Eu3+ in site III is in the presence of a rather higher hexagonal, trigonal or tetragonal symmetry. A careful analysis of the whole set of levels from 5D0→7FJ transitions clearly allows to exclude higher hexagonal D6, C6v,C6, D3h, and C3h symmetries, as well as tetragonal D4, for which the number of energy levels to be likely observed would be lesser than in the identified set, whereas tetragonal C4v, D2d, S4 and C4, and trigonal C3v, D3 and C3 symmetries seem to be a better approach and deserve a more detailed evaluation. The results corresponding to attempts of parametrization of CF effects for Eu3+ located in site III developed on the basis of local C4v and C3v or D3 symmetries are presented in Table 2 (shown in the Appendix).
As could be expected, if there are different sites for the Eu3+ion, the lifetime of state 5D0 should depend on the excitation wavelengths. We have measured the lifetime of the 5D0 state at three different excitation wavelengths (579.5, 580.1, and 581 nm), which correspond to those at which the three sites are selectively resolved, and collecting the luminescence at the highest intensity Stark component of the 5D0→7F4 transition. The experimental decays are well described by a single exponential function to a good approximation. The values of the measured lifetime are 1.1 ms, 0.55 ms, and 0.14 ms for sites I, II, and III respectively.
4. Crystal-field analysis and simulation of the energy level schemes
It is well known that Eu3+ is the best choice for a ‘crystal-field probe’ in a given host. Their ground 7F0 as well as the fluorescent 5D0 states are non-degenerate, and have symmetry label Γ1, which largely simplifies the interpretation of the spectra. By making use of the selection rules for induced ED and MD transitions, it is possible to discriminate between different point symmetries for an observed optical center in a given host. Moreover, there is a straightforward relation between the CF splitting of 2S+1LJ levels with small J values, especially for J=1 and 2, and the CF parameters. In this case, CF parameters can be deduced directly from the experiment.
The phenomenological CF simulation of the Eu3+ energy level scheme can be accurately conducted on the strongly reduced basis of the 7FJM set alone, i.e., 49|SLJM J 〉 levels. The use of this truncation is enabled by two characteristics of the 4f6 configuration: firstly, the 7FJ (J=0-6) sextuplet is relatively well isolated from the rest of the configuration (the energy gap between 7F6 and 5D0 is ~12 000 cm-1), which renders the mixing of the wavefunctions negligible, and secondly, the CF operator only mixes levels with the same multiplicity. Evidently, even with the J-mixing included, not all the interactions are taken into account, as non-diagonal spin-orbit interactions that create small components of the 5DJ levels into the 7FJ wavefunctions. Therefore, some “intermediate parameters” have to be introduced, one for each 7FJ state, in order to overlap experimental and calculated barycenters.
The method used for calculating the energy levels of Eu3+ in a crystalline environment usually considers the single-particle CF theory. Following the formalism of Wybourne , the CF Hamiltonian is expressed as a sum of products of tensor operators ( ) i , with real and complex parameters as coefficients, these later appropriated to the Eu3+ site symmetry in the host
For I and II Eu3+sites in KPb2Cl5, which in accordance with the above fluorescence spectra present C2v or lower symmetries, we have carried out the parametrization by initially considering the nine real parameters of a C2v CF potential. Secondly, fourth and sixth-rank parameters were carefully determined from the adequate reproduction of 7F1, 7F2 and the remaining observed 7FJ splittings, respectively, before considering all observed energy levels and a free variation of all CF parameters. Thirty energy levels for site I and thirty three ones for site II, included in Table 1, were used to derive these parameters. The phenomenological sets of refined C2v parameters for sites I and II are presented in Table 2. After this first step, the preceding nine CF parameters can be considered as the starting ones for the simulation of the same energy level patterns in the lower Cs/C2 symmetry, which involves the corresponding non-zero complex parameters. The results are included in Table 2.
A scheme of 18 Stark levels was considered for the simulation of the sequence of Eu3+7FJ energy levels in site III, when the C4v symmetry is evaluated. Two more levels, which were seen from weak transitions, were added for the simulation with the C3v or D3 symmetry, Table 1. Results of refined CF parameters for both fittings C4v and C3v/D3 are included in Table 2. All the performed calculations were conducted with the aid of a matrix diagonalizing program  which took into account the J mixing between wavefunctions with different J and M values. The least squares refinement between the experimental and calculated energy levels was carried out by minimizing the rms function defined as σ=[∑(Eexp -Ecal )2/(Nlev -Npar )]1/2, where Eexp and Ecal correspond to the experimental and calculated energy level values, and Nlev and Npar the number of levels and parameters, respectively. Figure 3 shows the observed and calculated energy levels for the three sites.
The existence of diverse Eu3+ optical centers must be explained on the basis of the crystal structure of the KPb2Cl5 host in which they are embedded, that is, three different crystallographic sites for Eu3+ in europium-doped KPb2Cl5 crystal must be assumed.
According to X-ray analysis, biaxial crystals KPb2Cl5 present the monoclinic symmetry of the space-group P21/c, with lattice parameters (Å) α=8.831(2), b=7.886(2) and c=12.430(3), β=90.14 (2)°, V=865.6(4) and Z=4 . The β value near 90° indicates that the crystal is nearly orthorhombic. Although all studies on the crystal structure of KPb2Cl5 reveal that the [PbCl6]4-distorted octahedron is a principal element, no accordance exists with respect to either the number of independent positions for Pb atoms or their coordination. Thus, while some authors report the existence of four non-equivalent Pb sites, [7,22] others point to Pb occupying two symmetrically independent positions . In this last case, the Pb(1) coordination can be described as a distorted mono-capped octahedron (coordination number CN=7), with the seventh apex at distance >3.1 Å, and Pb(2) is located in the center of a tricapped trigonal prism of chlorine atoms (CN=9), where one of the capping ligands is at the large distance ~4.2 Å,  being thus more reasonable to regard this Pb(2) coordination polyhedron as a bi-capped trigonal prism (CN=8) of chlorine atoms. Anyway, if a smaller coordination sphere around Pb atoms is considered, Pb(1) will be in a purely octahedral environment, CN=6, with Pb-Cl distances <3Å, and Pb(2) possesses an umbrella-like environment with two other more remote Cl-, at around 3.2 Å, and then CN=7 .
The distribution of chlorine atoms around K corresponds to a tricapped trigonal prism TTP (CN=9), where one of the equatorial capping atoms is at distance ~3.8 Å. Figure 4 shows the coordination polyhedra around Pb(1) (CN=7), Pb(2) (CN=9) and K (CN=9) respectively, in KPb2Cl5.
Eu3+ ions are supposed to substitute Pb2+ in the above positions, and the charge compensation will be ensured by K+ vacancies. However taking into account that the K+ coordination polyhedron is similar to the one described for Pb(2), and that actually the planes occupied by Pb2+ and K+ ions alternate within the framework , the possibility of a further Eu3+ access to K+ sites should be also considered though it would suppose a hardly stable local distribution in the crystal host requiring a nearby Pb2+ vacancy for charge compensation.
Simulations of spectra for Eu3+ located in C2v I and II sites yielded energy levels schemes in very good agreement with the experimental data. They were, however, undoubtedly improved when the complex parameters of the symmetry C2/Cs were introduced, since σ deviations as well as residues decreased significantly. Anyway, complex CF parameters for site II are really weaker, with values of only a few cm-1, and this suggests that the deviation from the higher C2v is, in this case, negligible. By the contrary, results from the fit under C2/Cs symmetry for site I reveal rather important values for complex parameters, mainly for sixth-rank ones. Consequently, it seems that site I is far more distorted than site II from the initially considered C2v symmetry.
On the other hand, independently of the symmetry considered, the second-order CF parameters for site II are always small, and consequently the J-mixing through the 7F1 wavefunctions will be also small, which explains the above indicated low intensity for the 5D0→7F3 transition.
Finally, the important difference between absolute values of and parameters for each site reflects the considerably different splitting of their corresponding 7F1 levels.
From results of the CF analysis for Eu3+ in site III it is clear that it is consistent with a local environment related to a TTP coordination polyhedron CN=9, that is adopted by K+ cations in KPb2Cl5. However, the full corresponding D3h symmetry, described only by parameters , , and is not retained. If in TTP a twist for both top and base relative to each other over a distortion angle occurs, the symmetry is lowered first to D3h, and finally to D3 appearing two additional CF parameters, and , as it is currently derived from our CF analysis. On the other hand, the D3h symmetry is lowered to C3v if the three capping ligands are removed out of the equatorial plane, but all in the same direction and with the same distortion angle. The C3v potential is described by the same set of CF parameters than D3. It is worthy to consider that account taken of the crystallographic coordination polyhedron around K+ (CN=9) mentioned above (see Fig.4) a most realistic symmetry for site III could be C3.
Finally, some additional insight about the crystallographic nature of Eu3+ sites can be obtained from the measured lifetimes. As we have seen, the longest lifetime, 1.1 ms, corresponds to site I whose abundance, as estimated from the emission of the ED 5D0→7F2 transition, is only a 2 % of the one corresponding to site II. The less ED character of this transition if compared with the other RE sites suggests a less distorted ligand symmetry around the RE ion but, at the same time, the large Stark splitting of the 7F1 components (Fig. 1) indicates a stronger CF which could arise from shorter RE-Cl bond distances expected at this site  (see Fig. 4). Paying attention to these points, one could guess Pb(1) crystallographic site as a probable candidate for the Eu3+ (I) site. On the other hand, the lifetime of 5D0 state of Eu3+ in site II (the more abundant one), 0.55 ms, agrees well with the more distorted ligand symmetry related to Pb(2) crystallographic sites (see Fig.4) which is in accordance with a more ED character shown by the 5D0→7F2 transition. For both Pb sites, the charge compensation can be easily attained by K+ vacancies. Finally, the unexpected relatively high CF symmetry of Eu3+ in CF site III (abundance 1.2 % referred to site II) contrasts with the shortest lifetime of the 5D0 state in this matrix. Account taken of the crystallographic information shown in Fig. 4 one could suggest the crystallographic site of K+ ion as a rather possible place for Eu3+ in CF site III. Indeed, the ligand local environment is related to a TTP coordination polyhedron which has enough room for the RE and could display the required C3 symmetry. On the other hand, the short lifetime of the 5D0 state agrees well with the strong splitting of the 7F1 components and the pronounced ED character of the 5D0→7F2 transition which suggest a still remarkable CF distortion of the ligands promoted by the nearby Pb(2) vacancy needed for the charge compensation.
To further assess the likeliness of current phenomenological parametrizations of crystal field (CF) effects for the three Eu3+ crystal sites resolved in KPb2Cl5, we have also carried out semi-empirical simulations of the CF features observed. They have been conducted through the Simple Overlap Model (SOM) for the crystal field . Benefiting from the fact that the SOM model uses the crystallographic positions of the Eu(Pb1)Cl7, Eu(Pb2)Cl9 and Eu(K)Cl9 coordination polyhedra in the estimation of CF parameters, these calculations have supposed some little modifications (±10 %) in Eu (in Pb(1), Pb(2) and K sites) -Cl distances, with regards to those in Ref. 14. These atom displacements are needed to account for distortions induced in the actual KPb2Cl5 host, which arise from the existence of Pb vacancies necessary for charge compensation. Table 3 (shown in the Appendix) summarizes the main FLN experimental results together with the relationship between the second order calculated crystal field parameters and the ones simulated by using the crystallographic coordinates of the nearest ligands at the proposed lattice sites occupied by the Eu3+ ion.
In summary, if we take into account the proclivity of lanthanide ions for CN=9 within a more or less distorted tricapped trigonal prism  and that Pb vacancies are rather difficult to achieve in the KPb2Cl5 due to the framework, we can easily understand why Eu3+ in site Pb(2) is the more abundant one, nearly 97% in potassium lead chloride crystal.
By using fluorescence line narrowing technique we have demonstrated the existence of three different local environments around the RE ions in KPb2Cl5. On the ground of the experimental results, the crystal-field analysis and simulation of the energy level schemes allow to connect the predicted symmetry of the resolved sites with the crystal structure. In conclusion, the RE ions may occupy both the Pb and K sites but the luminescence results suggest that RE ions occupying the Pb(2) site is most likely to occur.
This work was supported by the Spanish Government MEC (MAT2004-03780 and MAT2004-02001) and Basque Country University (UPV13525/2001).
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