We report on a novel type of Bi-doped crystal that exhibits ultrabroadband photoluminescence in the near infrared (NIR). Emission centers can be generated and degenerated reversibly by annealing the material in CO atmosphere and air, respectively, indicating that emission is related to the presence of Bi-species in low valence states. Correlating static and dynamic excitation and emission data with the size and charge of available lattice sites suggests that two types of Bi0-species, each located on one of the two available Ba2+ lattice sites, are responsible for NIR photoemission. This is further confirmed by the absence of NIR emission in polycrystalline Ca2P2O7:Bi and Sr2P2O7:Bi. Excitation is assigned to transitions between the doubly degenerated ground state 4S3/2 and the degenerated excited levels 2D3/2, 2D5/2 and 2P1/2, respectively. NIR emission is attributed to 2D3/2→4S3/2. The NIR emission center can coexist with Bi2+ species. Then, also Bi2+ is accommodated on one of the two Ba2+-sites. Energy transfer between Bi2+ ions occurs within a critical distance of 25.9 Å.
©2010 Optical Society of America
Rapidly developing global demand for fast information transfer and ultra-high data capacity networks calls for novel types of superbroadband optical amplifiers. NIR emitting laser materials that can be processed into low-loss optical fiber are thought to offer a pathway towards this goal. In this context, glasses and glass ceramics doped with Ni2+, Cr4+ and Bismuth-ions of unknown valence, respectively, are presently receiving significant attention [1–6]. Over the last decade, especially Bi-doped glasses have seen the transition from original discovery  to the first demonstration of continuous wave lasing and efficient all-fiber optical amplifiers [7,8]. The optical properties of this class of materials offer the potential to replace classical fiber amplifiers: their emission spectrum covers the whole spectral range in which silica telecommunication fibers exhibit their lowest optical loss [5,6]. Only recently, a quantum efficiency of up to 1.0 ± 0.05 has been reported . However, while reports on Bi-activated NIR emission now cover almost all classes of inorganic glasses, only little is known on NIR-emitting Bi-doped crystalline materials [3–6,10,11]. So far, broadband emission has been observed from Bi-doped crystalline SrB4O7 , RbPb2Cl5 , 2MgO·2Al2O3·5SiO2, BaF2, α-BaB2O4, and zeolites [12–16]. While a crystalline environment may be favorable for the optoelectronic properties of the emission center (quantum yield, excitation cross section, quenching effects and lifetime), studying crystalline matrices may also allow new insights into the nature of the active center: as of today, the seemingly simple question which Bi species is responsible for NIR photoluminescence remains highly controversial. Single consensus is that NIR luminescence is totally different from what can be observed from Bi3+ and Bi2+-doped materials. Going further, arguments diverge between either Bismuth in lower valence states or Bi5+ [3–7,10–18]. Experimental evidence particular for or against Bi5+ often appears counterintuitive. For instance, Bi5+ was initially proposed as the most probable NIR emission center  and its existence in NIR emitting glasses was recently verified by X-ray photoelectron spectroscopy (XPS) and analyses of the extended X-ray absorption fine structure (EXAFS) [19,20]. Razdobreev et al. performed optically detected magnetic resonance spectroscopy (ODMR) on Bi-doped silica glass during excitation (808 nm, 1.8 K) and suggested the radiative e-h recombination between filled electronic configurations of Bi5+On2- molecules as the origin of NIR luminescence . At the same time, however, these experimental findings are contradicted by other observations [4,5,18]: (1) at elevated temperature, Bi2O3, conventionally used as source of the bismuth dopant, readily decomposes into BiO or even Bi atoms; (2) stabilization of Bi5+ appears to require elevated amounts of alkali or alkaline earth species (e.g. NaBiO3 or KBiO3), which are not present in most NIR-active Bi-doped glasses; (3) the known Bi5+-containing compounds become extremely unstable at temperature higher than 300 °C [22,23]; (4) Bi NIR emitting glasses can be prepared in reducing atmosphere  or vacuum ; (5) bismuth nanoparticles are present in NIR-emitting bismuthate glasses (although these nanoparticles are not the emission centers) ; (6) NIR-emission can be extinguished completely by adding oxidizing reagents (e.g. Sb2O5  or CeO2 ) to the glass; and (7) it was found in several compounds that NIR emission centers can coexist with Bi2+ [10,13,14]. In the framework of Duffy’s  concept of optical basicity, conditions that favour lower valence states of polyvalent constituents may be achieved in glass melts with low basicity. Vice versa, increasing basicity (obtained by, e.g., the addition of alkali or alkaline earth species to borate or silicate glasses) favours the formation of Bi5+. However, only glasses with low optical basicity seem to provide NIR luminescence [27–30].
These arguments suggest that NIR emission is related to the presence of low-valence bismuth species [4–8,10–18, 27-25]. Beyond that, however, the question as to which of the various possible low-valence Bi species is (or are) the actual emission center(s) remains intensively debated also on this side of the general discussion. While atomic spectral data suggest Bi0 as the active center , also Bi+ , Bi2 [17,31,32], Bi2- [12,17], Bi22-  and point defects  have been proposed, based on electron spin resonance spectroscopy (ESR), the quadratic dependence of center absorption on bismuth concentration and quantum chemical calculations, respectively. In the present study, a different strategy is followed to approach the nature of the emission center. Compared to corresponding glasses, crystals exhibit a much more defined lattice configuration and, usually, a much lower free volume. In glasses on the other hand, network connectivity and, hence, ligand field strength can easily and to a large extent be altered by introducing modifier ions. This property enables glass networks to accommodate relatively large clusters or even particles (e.g. Au, Ag or Bi [34,18,35]), what is difficult to achieve in crystalline matrices. In crystals, the size of a dopant should rather be comparable to the size of either a network constituent (or vacancy) or an interstitial network site. For example, in the case of SrB4O7:Bi, NIR-emission could be observed from the polycrystalline material, but not from single crystals [10,15], suggesting that emission centers precipitate at grain boundaries. Here, M2P2O7 (M = Ca, Sr, Ba) are chosen as the host crystal. It is demonstrated that Bi3+ and Bi2+ can be doped into all three lattices, but that NIR emission can be obtained only from Ba2P2O7. For this specific material, analyses provide evidence for Bi0 as NIR emission center.
Doped and un-doped samples of M2P2O7 (M = Ca2+, Sr2+ and Ba2+) were prepared via solid state reaction. For that, analytical grade reagents CaCO3, SrCO3, BaCO3, NH4H2PO4 and Bi2O3 were used as raw materials. Individual batches of 20 g were weighed according to nominal compositions M2(1-x)P2O7: 2xBi (x = 0, 0.001, 0.003, 0.005, 0.01, 0.02, 0.03, 0.05) and mixed thoroughly. NH4H2PO4 was added in excess of 3% to compensate for volatilization losses (in the following, all percentages are given in mol.%). To prevent batch foaming, heating and cooling rates were adjusted to between 80 K/h and 110 K/h. The complete reaction procedure comprised the following steps: (1) preheating of all batches at 500 °C for 6 h in air, using somewhat bigger alumina crucibles; (2) secondary grinding of the pre-reacted samples to improve homogeneity; (3) sintering at 1100°C, again in alumina crucibles. Step (3) was conducted in four different ways (A-D), respectively. (A) eight crucibles, containing samples of Ba2(1-x)P2O7: 2xBi, were put onto an alumina plate in a bigger alumina container. The container was covered with an alumina plate. A layer of ~2 cm of powdered graphite was put on the bottom of this container. At the reaction temperature of 1100 °C, this leads to an atmosphere where practically all oxygen and CO2 are converted to CO. Sintering was then conducted for 24 h. (B) another eight samples of Ba2(1-x)P2O7: 2xBi were first sintered for 24 h at 1100 °C in air to form solid cylinders. After this procedure, they were taken out of the crucible and put into an alumina container in a way similar to (A). Annealing in CO atmosphere was performed for 1 h at 1100 °C. (C) one sample of Ba1.994P2O7: 0.3%Bi was sintered in the same way as (B), but without the secondary treatment in CO atmosphere. (D) a final sample of Ba1.994P2O7: 0.3%Bi was sintered according to (B), but conducting a third annealing step (1h, 1100 °C), again in air. In parallel, Ca1.994P2O7: 0.3%Bi and Sr1.994P2O7: 0.3%Bi were prepared according to (A). Sample nomenclature is MXXY, whereby “M” represents the employed alkaline earth species (Ca, Sr, Ba), “XX” the concentration of Bi in mol.% (01, 03, 05, 10, 20, 30, 50) and “Y” the employed sintering procedure (A-D). Samples that were prepared via paths (A) and (B) are further referred to as A-type M2P2O7:Bi and B-type M2P2O7:Bi, respectively.
Crystal structures of obtained samples were examined by X-ray diffractometry (XRD, Siemens Kristalloflex D500, 30 kV/30 mA, Cu Kα, λ = 1.5405 Å, scan rate of 1 °/min). UV-Visible dynamic and static optical emission and excitation spectra, and fluorescent lifetimes of Bi3+ and Bi2+ were recorded with a high-resolution photoluminescence spectrometer (Horiba Jobin Yvon Fluorolog-3), using a static Xe lamp (450 W) and a Xe flashlamp (75 W) as excitation sources, respectively. NIR emission and uncorrected excitation spectra were obtained with the setup described in Ref. . For measuring the excitation spectra of NIR emissions at 1.1 and 1.15µm, an emission monochromator mounted with a grating of 300 g/mm blazing at 1250 nm and an excitation monochromator with a grating of 600 g/mm blazing at 750 nm were used. The grating of the emission monochromator was adjusted to exactly disperse emission at either 1.1 or 1.15µm, respectively. The grating of the excitation monochromator was then gradually turned to separate excitation light (500 nm-1050 nm) from the Xe light source. At the same time, variation of the emission intensity was recorded as a function of excitation wavelength. Corresponding lifetime of the NIR emission center was recorded with the help of an 80 Hz optical chopper and an oscilloscope with a bandwidth of 1.5 GHz at a sampling rate of 8 × 109s−1, and averaged over 1024 individual measurements. The experimental error of lifetime measurements is ± 1%.
3. Results and discussion
3.1 Crystal structure
XRD patterns of M2(1-x)P2O7: 2xBi are consistent with JCPDS cards 71-2123, 75-1490, 83-990 for M = Ca2+, Sr2+ and Ba2+, respectively. In a first consideration, this confirms that synthesized samples are of pure phase, tetragonal space group P41 for Ca2(1-x)P2O7: 2xBi, orthorhombic space group Pnma for Sr2(1-x)P2O7: 2xBi and hexagonal space group P2m for Ba2(1-x)P2O7: 2xBi, respectively. Figure 1 shows the XRD pattern of sample Ba03A and the result of Rietveld refinement (fullprof suite , ) to space group P2m. The calculation produces lattice parameters a = 9.423Å, c = 7.081Å and the unit cell volume V = 544Å3 with the accuracy RB = 0.086 and RF = 0.053. This agrees well with reported reference data, i.e. a = 9.415Å, c = 7.078Å and V = 543Å3 . Noteworthy for the following considerations, in Ba2P2O7, there are two types of barium sites, Ba(1) and Ba(2), two types of phosphorus sites, P(1) and P(2), and four types of oxygen sites, O(12), O(22), O(11) and O(21) (inset of Fig. 1). Ba(1) is surrounded by two O(12) and eight O(22). The bond length ranges from 2.545 to 2.977 Å (average: 2.767 Å). Surrounding Ba(2) are four O(12), four O(22) and four O(21) with bond lengths between 2.744 and 3.141 Å (average distance 2.910 Å). Ba(1) and Ba(2) sites have the symmetry of m2m (depicted in the insets A and B of Fig. 1). In one unit cell, layers Ba(1) and Ba(2) type are stacked along the c-axis in I-II-I order (inset C of Fig. 1).
3.2 Photoluminescence from A-type M2P2O7:Bi
Ba2P2O7:Bi samples that were prepared via path (A) (i.e., sintering for 24 h in CO atmosphere) exhibit strong NIR photoemission, centering at a wavelength of ~1.1 µm (for an excitation wavelength of 723 nm, Fig. 2A ). The full width at half maximum (FWHM) of this emission peak is 147 nm, narrower than for typical Bi-doped silicate and germanate glasses, but wider than for crystalline α-BaB2O4:Bi (108nm , ) and comparable to the emission bandwidth of Bi-doped zeolites (152 nm , ). The peak position corresponds to observations in other bismuth doped crystals [11,13,14] and remains unchanged with changing dopant concentration. On the other hand, emission intensity is strongly dependent on nominal bismuth concentration (inset of Fig. 2A). It exhibits a maximum at a dopant concentration of ~1 mol.%.
Lifetime of NIR emission was measured at room temperature, using a laser diode (980 nm, 200 mW) as excitation source. Temporal decay follows a single exponential decay equation with a lifetime of 634 μs. The uncorrected excitation spectrum (monitoring emission at 1.1 µm) is shown in Fig. 2B. It comprises at least eight individual peaks, at 586, 700, 723, 838, 900, 924, 955 and 997 nm (compared to only four structureless absorption bands that can be observed in typical Bi-doped glasses at ~500, 700, 800 and ~1000 nm ). While absorption at 723 nm is strongest, switching the excitation wavelength between either of the eight peaks does not affect the position of the emission peak (Fig. 2A). This finding is similar to observations of RbPb2Cl5:Bi , but differs from what was reported on BaF2:Bi. In the latter case, the ratio of the emission bands at 1070 and 1500 nm was reported to change when changing the excitation wavelength from 500 to 590 nm . Nevertheless, large similarity in emission, excitation and lifetime of Ba2P2O7:Bi as compared to other Bi-doped crystals allows to assume that in Ba2P2O7:Bi, the emission center is of the same nature as it is in other NIR-emitting Bi-doped crystals and glasses. In contrast to glassy hosts, the presence of much sharper excitation peaks is attributed to the fact that Bi-dopants are incorporated on strongly localized lattice sites. The appearance of additional absorption bands is related to the possible presence of Bi on more than one lattice site (see section 3.1 and the following paragraphs).
A more careful examination of the emission spectrum in Fig. 2A reveals an asymmetric shape with a tail extending towards the IR-side. The emission peak is best fit by two Gaussian functions, centering at 1.1 and 1.15 µm. FWHM of these individual emission peaks are 142 nm and 265 nm, respectively. The excitation spectrum that corresponds to the 1.15 µm emission is shown in Fig. 2B. While here, too, eight absorption peaks can be detected, a change occurs in the ratio between the excitation bands 924 nm and 723 nm: their intensities become comparable. Also the emission lifetime of the 1.15 µm band was found to differ from the 1.1 µm band, i.e. 656 μs as compared to 634 µs. These differences further indicate that two different centers contribute to NIR emission from Ba2P2O7:Bi. In sharp contrast, no NIR emission could be detected from Ca03A and Sr03A at excitation wavelength between 500 and 900 nm.
3.3 Photoluminescence from B-type M2P2O7:Bi
After sintering in air for 24 h (first step of synthesis path (B)), all Ba2P2O7:Bi samples exhibit typical Bi3+ UV photoluminescence. When subsequently annealing for another 1 h in CO atmosphere (second step of synthesis path (B)), intense red photoluminescence (peaking at 716 nm) can be detected together with emission in the NIR spectral range. Corresponding emission and excitation spectra are shown in Fig. 3A (sample Ba10B). Emission at 716 nm exhibits a FWHM of 67 nm. Monitoring this emission peak, the excitation spectrum consists of bands at ~286, 384 and 616 nm respectively. Shape and position of the emission peak are independent on excitation wavelength. The decay curve of the 716 nm emission can be fit to a single exponential equation (correlation coefficient of 99.06%), resulting in a lifetime of 14.18 μs (Ba10B, excitation at 286 nm). Both emission intensity and emission lifetime depend on bismuth concentration: maximum intensity was observed for a critical concentration of 1 mol.% (Fig. 3B). Differing from this observation, lifetime was found to decrease monotonically with increasing concentration (Fig. 3B).
Photoluminescence from Ba10B exhibits strong similarity to that of Bi2+-doped compounds [39,40]. Excitation bands at 286, 384, and 618 nm and emission at 716 nm can readily be assigned to the electronic transitions 2P1/2 → 2S1/2, 2P1/2 → 2P3/2 (2), 2P1/2 → 2P3/2 (1) and 2P3/2 (1) →2P1/2, respectively. Noteworthy, strong changes in the position of the emission peak in different host materials (588nm→660nm→716nm for Bi2+-doped SrB4O7, SrB6O10 and Ba2P2O7, respectively) imply a strong dependence of the optoelectronic properties of Bi2+ on ligand field strength [39,40].
In order to examine the distribution of Bi2+-species in the Ba2P2O7 host, dynamic emission spectra were recorded (excitation at 286 nm). Data are shown in Fig. 4 . For increasing temporal delay, a gradual change can be observed in the shape of the emission peak. Starting at 733 nm, the position of the emission peak starts to blue-shift after a delay of 3 μs. After 6 μs, the maximum of the peak has moved to 716 nm. For longer delay times, the position remains fixed. This observation shows the presence of at least two emission sites in Bi2+-doped Ba2P2O7 and agrees with crystallographic data: Bi2+-ions are assumed to occupy both of the two Ba2+-sites. A slight difference can be detected in the excitation spectra when monitoring emission at 716 nm and 733 nm, respectively (Fig. 3): for 733 nm emission, the two excitation peaks lie at 381 and 619 nm while for emission at 716 nm, they are shifted to 384 and 616 nm. This reflects differences in the Ba-O bond length for the two different Ba-sites (Ba(1)-O is somewhat shorter than Ba(2)-O, see 3.1). When Bi2+ is incorporated on a Ba(1) site, it experiences a slightly stronger crystal field as compared to the Ba(2) site. As a result, splitting of the two lowest excited states (2P3/2 (1) and 2P3/2 (2)) increases slightly for Bi2+ on Ba(1) (splitting energies of 10092 cm−1 and 9808 cm−1 are derived from the excitation spectra for Bi2+ on Ba(1) and on Ba(2), respectively). In the following, the two sites will be referred to as Bi2+ (1) and Bi2+ (2). We attribute emission at 716 nm and 733 nm to Bi2+ (2) and Bi2+ (1), respectively. Since the Ba(2) site offers a slightly larger free volume (inset of Fig. 1), Bi2+ may preferably occupy this site. This leads to dominance of the 716 nm emission in Fig. 3A.
Comparing static and dynamic emission curves (Figs. 3A and 4) reveals that at the initial stage, the peak 733 nm is dominant. This can be interpreted as a result of energy transfer from Bi2+ (2) to Bi2+ (1). Since the excitation spectra of 733nm and 1100nm emissions overlap partly with the emission at 716nm (see Figs. 5 and 6 ), it is assumed that emission at 716nm can partly be re-absorbed, contributing to the emission at 733 nm. This energy migration is fast and dominates particularly the early stage (< 6 μs, Fig. 4).
Normalizing the intensity of the excitation spectra of Ba01B to Ba50B to the height of the peak at 286 nm reveals a gradual change in the ratio of excitation intensities at 618 nm and 286 nm (R618nm/286nm). R618nm/286nm is 0.338 for a Bi-concentration of [Bi] = 0.1%, increases to 0.800 for [Bi] = 1.0%, and again decreases to 0.423 for [Bi] = 3.0%. The reason for this can presently not be explained.
The critical distance Rc, the mean separation between the nearest NIR emission centers at the critical concentration xc, can be estimated from Fig. 1C) is substituted by Bi2+ and dopant-induced network deformations are neglected, the longest distance between this site and another barium site is 24.7Ǻ (in two neighbouring unit cells). This means that energy transfer can occur from Bi2+ on this Ba-site to any other Bi2+-occupied Ba-site inside the same and neighbouring unit cells. Similar values of Rc were reported for Rb2ZnBr4:Eu2+ and Ba2B5O9Br:Eu2+, respectively . If the dopant concentration exceeds 1.0%, the probability of energy transfer increases and concentration quenching occurs (Fig. 3B).
If the transfer process occurs via an electric dipole-dipole interaction, Rc can also be estimated from excitation and emission spectra ,
Here, f is the oscillator strength of the 2P1/2 → 2P3/2 transition in Bi2+, ∫f(E)F(E)dE represents the overlap of normalized emission and excitation spectra and E is the energy of maximum spectral overlap. For E = 1.91 eV and ∫f(E)F(E)dE = 0.045eV−1, combining Eq. (1) and 2 produces a value of f = 0.03, comparable to the oscillator strength of the 4f7 → 4f65d transition in Eu2+ . Since in Bi-doped Ba2P2O7, there are coexisting Bi3+, Bi2+ and NIR emission centers, the actual value of xc is lower than the nominal critical concentration of 1.0% and, hence, Rc is somewhat longer than 25.9 Å. As a result, also f should be slightly larger than 0.03. This means that in the BaP2O7 matrix, the in principle parity forbidden transition 2P1/2 → 2P3/2 (Bi2+) is strongly allowed.
In a similar way, emission from Bi2+ can be observed in Ca2P2O7:Bi and Sr2P2O7:Bi (Fig. 5), peaking at 653 nm and 702 nm with FWHM of 68 nm and 80 nm, respectively. The absorption properties of all three components enable the use of blue LEDs (InGaN) for excitation, suggesting further examination with respect to potential use in white-light emitting devices (WLEDs) [39,40].
3.4 NIR-emission from Ba2P2O7:Bi
To assess the nature of NIR emission from Bi-doped Ba2P2O7, the various synthesis procedures A-D have to be considered. After a single-step sintering in air for 24h / 1100 °C (path C), only Bi3+-emission centers (active in the UV spectral range) can be observed (e.g. sample Ba03C). Neither red (typical for Bi2+) nor NIR photoluminescence occur. Emission from Bi3+, however, is strongly dependent on excitation wavelength: exciting at 290 nm produces a dominant peak at ~335 nm with a shoulder emission at ~356 nm. Switching the excitation wavelength to 310 nm results in dominance of the 356 nm emission peaks. Correspondingly, the excitation spectra peak at 290 and 310 nm when emissions at 335 and 356 nm, respectively, are monitored. This observation suggests that Bi3+ ions may occupy both of the two Ba-sites, Ba(1) and Ba(2) (Fig. 1). The presence of two kinds of Bi3+ emission centers is further confirmed by lifetime analyses: lifetime for the emission at 335 nm is 5.6 μs (290 nm excitation) while that of the 356 nm emission is 8.5 μs (310 nm excitation). These excitation and emission bands can be assigned to transitions between ground state 1S0 and excited state 3P1 in Bi3+ .
Bi2+-related emission from B-type Ba2P2O7:Bi (two-step sintering with second step 1 h in CO, 1100 °C) has been discussed in the previous chapter. In these samples, also broad NIR emission can be observed (sample Ba03B in Fig. 6). NIR emission strongly intensifies if the treatment time in CO is prolonged (sample Ba03A in Fig. 6; NIR-emission from A-type Ba2P2O7:Bi is discussed in paragraph 3.2). When sample Ba03B is annealed for a third time, but in air (path D), both Bi2+-related and NIR-photoluminescence can be completely erased (sample Ba03D in Fig. 6). These observations lead to the conclusion that Bi-based NIR emission centers can be created only in reducing atmosphere in Ba2P2O7. The related oxidation-reduction mechanism is reversible. The fact that Bi5+ cannot be stabilized at elevated temperature in CO atmosphere makes it rather unlikely that this species is the emission center. Annealing the sample in air appears to transform all Bismuth into its trivalent form. Further treating in CO results in partly and reversible reduction of Bi3+ to Bi2+ and Bi0: Bi3+ Bi2+, Bi0 Bi3+.
As discussed previously , atomic spectral data of Bi0 show best consistence with spectroscopic data of Bi-doped glasses and crystals.
The presence of NIR photoemission in Ba2P2O7:Bi, but not in Ca2P2O7:Bi and Sr2P2O7:Bi can be interpreted as follows: Conventionally, dopant ions are supposed to efficiently substitute a specific lattice atom of a crystalline compound only if the difference between ionic radii of the dopant and the lattice species should be as close as possible. This maximal allowed difference appears to be ~30% . Then, ionic radii of the potential Bismuth-species and their comparison to the available network sites must be considered. Relative ionic radius differences Dr between possible dopant species (Bi3+, Bi0, Bi dimer ions) and potential substituted ions (Ba2+, Sr2+, and Ca2+) in M2P2O7:Bi (M = Ca, Sr, Ba) can be calculated from44], ), bismuth dimer ions have been estimated close to ~3.2 Å. Furthermore, also data on Bi3+ with CN = 7, 9, 10, or 12 is not available. We have therefore chosen the value for CN = 8 as a reasonable approximation .
In Ca2P2O7, there are four types of calcium sites coordinated by 7, 7, 8, and 9 oxygen ions, respectively . In Sr2P2O7, there are two types of 9-coordinated strontium sites . The two Ba-sites in Ba2P2O7 (CN = 10 and 12, respectively) have already been discussed. Radius data of Ca2+, Sr2+ and Ba2+ with these coordination numbers is adopted from Ref. 45. Derived values of Dr are given in Table 1 . They allow the following conclusions: (1) Bi3+ can substitute alkaline earth ions in either of the three lattices (M2+ in M2P2O7). As a confirmation, when sintered in air, all three materials exhibit Bi3+-related photoluminescence. (2) According to the size mismatch, none of the three lattices can accommodate dimer ions. (3) Bi0 species can be accommodated in the Ba2P2O7 lattice, but not in Ca2P2O7. Accommodation in Sr2P2O7 appears borderline. Correspondingly, NIR emission is observed only from Ba2P2O7, but not from Sr2P2O7 or Ca2P2O7.
For judging whether a dopant may actually occupy a certain lattice site, besides radius matching, also the charge match (or mismatch, respectively) has to be taken into account. If Bi3+ substitutes M2+, the charge mismatch is + 1. If the charge can be compensated appropriately, ion size remains the dominant factor in deciding whether or not substitution is possible. However, for larger charge mismatches (e.g. + 2 as in the case of Bi0 ↔ M2+), if accompanied by a moderate radius mismatch, charge may become the dominant destabilizing factor. This is why we think that Sr2P2O7 does not accommodate Bi0 and, hence, does not exhibit NIR emission.
Commonly, Bi-doped glasses exhibit absorption bands at ~500, 700, 800 and 1000 nm. Emission is usually located within 1.1 - 1.65 μm [3–8,27–32,48]. However, additional absorption bands were recently reported by Hughes et al. (~1180 nm, Bi-doped chalcogenide ) and Dvoirin et al. (1.4μm of bismuth in silica fiber , ). For the case of chalcogenide glass, emission was found at 2.0 and 2.6 μm . Generally, spectroscopic data appear very complex. They may origin from different Bi-based centers, some of which can exist in an amorphous matrix but not in crystals. For example, Hughes et al. assigned absorption and emission to Bi22- dimers . On the other side, regarding this example, Bi22- cannot be stabilized on Ba-sites in Ba2P2O7 (see above discussion).
Thermal treatment of different Bi-doped silica glasses in hydrogen atmosphere [1,29,51] has been shown to promote precipitation of Bi nanoparticles, whereas in agreement with another study of Bi-particle containing glasses , no NIR emission could be observed in these studies. However, this observation does in fact not rule-out Bi0 or Bi-clusters as the active NIR emission centers.
Therefore, we attribute the absorption and NIR emission of polycrystalline Ba2P2O7: Bi to Bi0 species. Actually, laser oscillation of Bi0 atoms has been observed by Zhu and Liu  at 1171.1 nm as early as 1982. Chou and Cool reported pulsed laser action of Bi0 at 5.326μm . Laser operation was also realized in gaseous Bi2, but in the wavelength range of 590-790nm . In KCl:Bi0, Goovaerts et al. observed the Bi0 absorption at 629 and 725 nm. These bands were assigned to transitions between ground state 4S3/2 and excited states 2D3/2 and 2D5/2 .
Presently, for reasons of calibration, the absorption bands that were found here cannot be assigned to individual transitions. However, relying on Ref. 18, the groups of bands at 838, 900, 924, 955 and 997 nm peaks, 700 and 723 nm peaks, and the individual band at 586 nm are tentatively assigned to transitions in two types of Bi0 centers between a doubly degenerated ground state 4S3/2 and the degenerated excited levels 2D3/2, 2D5/2 and 2P1/2, respectively. NIR emission is then attributed to 2D3/2→4S3/2.
In summary, we have reported on a novel type of NIR-luminescent Bi-doped crystal, Ba2P2O7. The luminescence peaks around ~1.1 µm with a FWHM of ~140 nm and a lifetime of > 600 μs from two different lattice sites. The absorption peaks cover the spectral range from 550 nm to 1050 nm. NIR emission centers can be generated and removed reversibly by treating the polycrystalline material in CO atmosphere or air, respectively, at 1100 °C. This reveals directly lower valence bismuth as NIR emission centers. Based on previous study and analyses of available lattice sites, 1.1µm emission of Ba2P2O7:Bi is attributed to Bi0. For reasons of charge and radius mismatch, this species cannot replace Ca2+ and Sr2+ in Ca2P2O7 and Sr2P2O7, and accordingly, none of them exhibits NIR luminescence. In Ba2P2O7:Bi, two types of each Bi3+, Bi2+ and Bi0 may be accommodated on Ba(1) and Ba(2) sites, respectively. For the case of Bi2+, the critical distance for energy transfer is 25.9 Å. M2P2O7:Bi2+ (M = Ca, Sr and Ba) exhibit broad blue absorption and intense red emission.
Financial support by the Deutsche Forschungsgemeinschaft (DFG) under grant no. WO 1220/2-1 is gratefully acknowledged.
References and Links
1. S. Zhou, N. Jiang, B. Zhu, H. Yang, S. Ye, G. Lakshminarayana, J. Hao, and J. Qiu, “Multifunctional bismuth-doped nanoporous silica glass: from blue-green, orange, red, and white light sources to ultra-broadband infrared amplifiers,” Adv. Funct. Mater. 18(9), 1407–1413 (2008). [CrossRef]
2. S. Tanabe and X. Feng, “Temperature variation of near-infrared emission from Cr4+ in aluminate glass for broadband telecommunication,” Appl. Phys. Lett. 77(6), 818–820 (2000). [CrossRef]
3. Y. Fujimoto and M. Nakatsuka, “Infrared luminescence from bismuth-doped silica glass,” Jpn. J. Appl. Phys. 40(Part 2, No. 3B), L279–L281 (2001). [CrossRef]
4. M. Peng, J. Qiu, D. Chen, X. Meng, I. Yang, X. Jiang, and C. Zhu, “Bismuth- and aluminum-codoped germanium oxide glasses for super-broadband optical amplification,” Opt. Lett. 29(17), 1998–2000 (2004). [CrossRef] [PubMed]
6. Y. Arai, T. Suzuki, Y. Ohishi, S. Morimoto, and S. Khonthon, “Ultrabroadband near-infrared emission from a colorless bismuth-doped glass,” Appl. Phys. Lett. 90(26), 261110 (2007). [CrossRef]
7. E. Dianov, V. Dvoyrin, V. Mashinsky, A. Umnikov, M. Yashkov, and A. Gur’yanov, “CW bismuth fibre laser,” Quantum Electron. 35(12), 1083–1084 (2005). [CrossRef]
8. I. Razdobreev, L. Bigot, V. Pureur, A. Favre, G. Bouwmans, and M. Douay, “Efficient all-fiber bismuth-doped laser,” Appl. Phys. Lett. 90(3), 031103 (2007). [CrossRef]
10. M. Peng, D. Chen, J. Qiu, X. Jiang, and C. Zhu, “Bismuth-doped zinc aluminosilicate glasses and glass-ceramics with ultra-broadband infrared luminescence,” Opt. Mater. 29(5), 556–561 (2007). [CrossRef]
11. A. G. Okhrimchuk, L. N. Butvina, E. M. Dianov, N. V. Lichkova, V. N. Zagorodnev, and K. N. Boldyrev, “Near-infrared luminescence of RbPb2Cl5:Bi crystals,” Opt. Lett. 33(19), 2182–2184 (2008). [CrossRef] [PubMed]
14. L. Su, J. Yu, P. Zhou, H. Li, L. Zheng, Y. Yang, F. Wu, H. Xia, and J. Xu, “Broadband near-infrared luminescence in γ-irradiated Bi-doped α-BaB(2)O(4) single crystals,” Opt. Lett. 34(16), 2504–2506 (2009). [CrossRef] [PubMed]
15. L. Su, P. Zhou, J. Yu, H. Li, L. Zheng, F. Wu, Y. Yang, Q. Yang, and J. Xu, “Spectroscopic properties and near-infrared broadband luminescence of Bi-doped SrB4O7 glasses and crystalline materials,” Opt. Express 17(16), 13554–13560 (2009). [CrossRef] [PubMed]
16. H. T. Sun, Y. Miwa, F. Shimaoka, M. Fujii, A. Hosokawa, M. Mizuhata, S. Hayashi, and S. Deki, “Superbroadband near-IR nano-optical source based on bismuth-doped high-silica nanocrystalline zeolites,” Opt. Lett. 34(8), 1219–1221 (2009). [CrossRef] [PubMed]
17. S. Khonthon, S. Morimoto, Y. Arai, and Y. Ohishi, “Luminescence characteristics of Te- and Bi-doped glasses and glass-ceramics,” J. Ceram. Soc. Jpn. 115(1340), 259–263 (2007). [CrossRef]
18. M. Peng, C. Zollfrank, and L. Wondraczek, “Origin of broad NIR photoluminescence in bismuthate glass and Bi-doped glasses at room temperature,” J. Phys. Condens. Matter 21(28), 285106 (2009). [CrossRef] [PubMed]
19. T. Ohkura, Y. Fujimoto, M. Nakatsuka, and S. Young-Seok, “Local structures of bismuth ion in bismuth-doped silica glasses analyzed using Bi LIII X-Ray absorption fine structure,” J. Am. Ceram. Soc. 90(11), 3596–3600 (2007). [CrossRef]
20. Y. Fujimoto, “Local structure of the infrared bismuth luminescent center in bismuth-doped silica glass,” J. Am. Ceram. Soc. 93(2), 581–589 (2010). [CrossRef]
21. I. Razdobreev, V. Y. Ivanov, L. Bigot, M. Godlewski, and E. F. Kustov, “Optically detected magnetic resonance in bismuth-doped silica glass,” Opt. Lett. 34(17), 2691–2693 (2009). [CrossRef] [PubMed]
22. J. Ren, J. Qiu, D. Chen, X. Hu, X. Jiang, and C. Zhu, “Luminescence properties of bismuth-doped lime silicate glasses,” J. Alloy. Comp. 463(1-2), L5–L8 (2008). [CrossRef]
23. N. Kumada, N. Takahashi, N. Kinomura, and A. W. Sleight, “Preparation and crystal structure of a new lithium bismuth oxide: LiBiO3,” J. Solid State Chem. 126(1), 121–126 (1996). [CrossRef]
24. G. Dong, X. Xiao, J. Ren, J. Ruan, X. Liu, J. Qiu, C. Lin, H. Tao, and X. Zhao, “Broadband infrared luminescence from bismuth-doped GeS2-Ga2S3 chalcogenide glasses,” Chin. Phys. Lett. 25(5), 1891–1894 (2008). [CrossRef]
25. B. Denker, B. Galagan, V. Osiko, S. Sverchkov, and E. Dianov, “Luminescent properties of Bi-doped boro-alumino-phosphate glasses,” Appl. Phys. B 87(1), 135–137 (2007). [CrossRef]
26. J. Duffy, “Redox equilibria in glass,” J. Non-Cryst. Solids 196, 45–50 (1996). [CrossRef]
27. X. G. Meng, J. R. Qiu, M. Y. Peng, D. P. Chen, Q. Z. Zhao, X. W. Jiang, and C. S. Zhu, “Infrared broadband emission of bismuth-doped barium-aluminum-borate glasses,” Opt. Express 13(5), 1635–1642 (2005). [CrossRef] [PubMed]
28. J. Ren, J. Qiu, D. Chen, C. Wang, X. Jiang, and C. Zhu, “Infrared luminescence properties of bismuth-doped barium silicate glasses,” J. Mater. Res. 22(7), 1954–1958 (2007). [CrossRef]
29. S. Zhou, W. Lei, N. Jiang, J. Hao, E. Wu, H. Zeng, and J. Qiu, “Space-selective control of luminescence inside the Bi-doped mesoporous silica glass by a femtosecond laser,” J. Mater. Chem. 19(26), 4603–4608 (2009). [CrossRef]
30. M. Peng, B. Wu, N. Da, C. Wang, D. Chen, C. Zhu, and J. Qiu, “Bismuth-activated luminescent materials for broadband optical amplifier in WDM system,” J. Non-Cryst. Solids 354(12-13), 1221–1225 (2008). [CrossRef]
31. B. Denker, B. Galagan, V. Osiko, I. Shulman, S. Sverchkov, and E. Dianov, “Absorption and emission properties of Bi-doped Mg-Al-Si oxide glass system,” Appl. Phys. B 95(4), 801–805 (2009). [CrossRef]
32. M. Hughes, T. Suzuki, and Y. Ohishi, “Compositional optimization of bismuth-doped yttria–alumina–silica glass,” Opt. Mater. 32(2), 368–373 (2009). [CrossRef]
33. M. Y. Sharonov, A. B. Bykov, V. Petricevic, and R. R. Alfano, “Spectroscopic study of optical centers formed in Bi-, Pb-, Sb-, Sn-, Te-, and In-doped germanate glasses,” Opt. Lett. 33(18), 2131–2133 (2008). [CrossRef] [PubMed]
34. J. Qiu, X. Jiang, C. Zhu, M. Shirai, J. Si, N. Jiang, and K. Hirao, “Manipulation of gold nanoparticles inside transparent materials,” Angew. Chem. Int. Ed. 43(17), 2230–2234 (2004). [CrossRef]
35. M. Peng, Q. Zhao, J. Qiu, and L. Wondraczek, “Generation of emission centers for broadband NIR luminescence in bismuthate glass by femtosecond laser irradiation,” J. Am. Ceram. Soc. 92(2), 542–544 (2009). [CrossRef]
36. M. Peng and L. Wondraczek, “Bismuth-doped oxide glasses as potential solar spectral converters and concentrators,” J. Mater. Chem. 19(5), 627–630 (2009). [CrossRef]
37. http://www.ill.eu/sites/fullprof/ (2009).
38. A. ElBelghitti, A. Elmarzouki, A. Boukhari, and E. M. Holt, “σ-dibarium pyrophosphate,” Acta Crystallogr. C 51(8), 1478–1480 (1995). [CrossRef]
40. M. Peng, N. Da, S. Krolikowski, A. Stiegelschmitt, and L. Wondraczek, “Luminescence from Bi2+-activated alkali earth borophosphates for white LEDs,” Opt. Express 17(23), 21169–21178 (2009). [CrossRef] [PubMed]
41. G. Blasse, “Energy transfer in oxidic phosphors,” Philips Res. Rep. 24, 131–144 (1969).
42. G. Blasse, “Energy transfer between inequivalent Eu2+ ions,” J. Solid State Chem. 62(2), 207–211 (1986). [CrossRef]
43. M. Peng, Z. Pei, G. Hong, and Q. Su, “The reduction of Eu3+ to Eu2+ in BaMgSiO4:Eu prepared in air and the luminescence of BaMgSiO4: Eu2+ phosphor,” J. Mater. Chem. 13(5), 1202–1205 (2003). [CrossRef]
44. J. Slater, Quantum theory of molecules and solids, Symmetry and energy bands in crystals, (McGraw-Hill Inc. 1965) Vol. 2 pg. 55.
45. R. Shannon, “Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides,” Acta Crystallogr. A 32(5), 751–767 (1976). [CrossRef]
46. S. Boudin, A. Grandin, M. Borel, A. Leclaire, and B. Raveau, “Redetermination of the β-Ca2P207 structure,” Acta Crystallogr. C 49(12), 2062 (1993). [CrossRef]
47. J. Barbier and J. Echard, “A new refinement of α-Sr2P2O7,” Acta Crystallogr. C 54(12), IUC9800070 (1998). [CrossRef]
48. Y. Qiu and Y. Shen, “Investigation on the spectral characteristics of bismuth doped silica fibers,” Opt. Mater. 31(2), 223–228 (2008). [CrossRef]
50. V. Dvoirin, V. Mashinsky, O. Medvedkov, A. Umnikov, A. Gur’yanov, and E. Dianov, “Bismuth-doped telecommunication fibres for lasers and amplifiers in the 1400-1500-nm region,” Quantum Electron. 39(6), 583–584 (2009). [CrossRef]
51. V. Truong, L. Bigot, A. Lerouge, M. Douay, and I. Razdobreev, “Study of thermal stability and luminescence quenching properties of bismuth doped silicate glasses for fiber laser applications,” Appl. Phys. Lett. 92(4), 041908 (2008). [CrossRef]
52. Z. Xu-hui and L. Jian-bang, “A hollow cathode bismuth ion laser,” Appl. Phys. B 29(4), 291–292 (1982). [CrossRef]
53. M. Chou and T. Cool, “Laser operation by dissociation of metal complexes: New transitions in arsenic, bismuth, gallium, germanium, mercury, indium, lead, antimony, and thallium,” J. Appl. Phys. 47, 1055–1061 (1976). [CrossRef]
54. S. Drosch and G. Gerber, “Optically pumped cw molecular bismuth laser,” J. Chem. Phys. 77(1), 123–130 (1982). [CrossRef]
55. E. Goovaerts, S. Nistor, and D. Schoemaker, “Electron-spin-resonance and optical study of the Bi0(6p3) center in KCl,” Phys. Rev. B 42(7), 3810–3817 (1990). [CrossRef]