Luminescence and Valence of Tb Ions in Alkaline Earth Stannates and

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Luminescence and Valence of Tb Ions in Alkaline Earth Stannates and Zirconates Examined by X‑ray Absorption Fine Structures Kazushige Ueda,*,† Yuhei Shimizu,† Kouta Nagamizu,† Masashi Matsuo,† and Tetsuo Honma‡ †

Department of Materials Science, Graduate School of Engineering, Kyushu Institute of Technology, 1-1 Sensui, Tobata, Kitakyushu 804-8550, Japan ‡ Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto, Sayou-cho, Sayou-gun, Hyogo 679-5198, Japan

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S Supporting Information *

ABSTRACT: The difference in Tb3+ green luminescence intensities in doped perovskite(ABO3)-type alkaline earth stannates, AeSnO3 (Ae = Ca, Sr, Ba), and the Mg codoping effect on the luminescence intensities in doped CaMO3 (M = Sn, Zr) were investigated utilizing the X-ray absorption fine structures (XAFS) of the Tb LIII absorption edge. It is considered that the local symmetry at A sites is responsible for the different Tb3+ luminescence intensities in AeSnO3 (Ae = Ca, Sr, Ba) doped with Tb ions at A sites. However, it was found from the XAFS spectra that some Tb ions are unintentionally stabilized at B sites as Tb4+, especially in BaSnO3. Not only the central symmetry for Tb3+ at A sites but also the presence of Tb4+ at B sites were considered to bring about the absence of Tb3+ luminescence in doped cubic BaSnO3. No obvious changes in the Tb3+ local structure at A sites were detected between Tb single doped and Tb−Mg codoped CaMO3 (M = Sn, Zr) from the extended XAFS oscillation, but the trace of Tb4+ at B sites in the Tb single doped sample was observed in the X-ray absorption near edge structures. It is, therefore, considered that the Tb3+ luminescence enhancement by Mg codoping is primarily attributed to the charge compensation rather than the changes in the local structure around Tb3+ at A sites. initial and final states in the electronic dipole transitions. However, the control of the orbital hybridization and its experimental evaluation is usually difficult. Doping Tb3+ ions at crystallographic sites without a center of symmetry is an effective way to achieve the efficient Tb3+ luminescence. In this case, the environments at Tb sites can be examined experimentally by some analyses. X-ray or neutron diffraction (XRD) analysis provides overall crystal structures and averaged local structures, whereas X-ray absorption fine structure (XAFS) spectroscopy gives the local structures specific to observed ions along with their valence states.11,12 The results of these measurements are, therefore, very useful for the feedback related to the development of new phosphors. Several perovskite-type oxides (ABO3) are known to show not only photoluminescence (PL) by doping lanthanide ions,13−22 but also cathodoluminescence and electroluminescence.23−29 Because the perovskite-type crystal structure is not complicated and the combinations of various cations are available in the structure, it is important to understand the luminescence properties of doped perovskite-type oxides in both scientific and industrial viewpoints.30 Tb doped perovskite-type CaSnO3 (CaSnO3:Tb) and CaZrO3 (CaZrO3:Tb) have been reported to show green PL,31−35 which contrasts

1. INTRODUCTION Tb ions are frequently used as emission centers in green luminescent phosphors such as LaPO 4 :Ce 3+ ,Tb 3+ and Y2SiO5:Ce3+,Tb3+.1,2 They are trivalent in the host crystals, and the green luminescence is mainly derived from 4f−4f transitions from the 5D4 to 7F5 states in the Tb3+ ions.3,4 In contrast, tetravalent Tb ions are also known to be stable in crystals due to the half-filled 4f shells. They are occasionally used as absorption centers in yellow pigments such as Y2O3:Tb4+,5−7 where the absorption by the Tb4+ ions is primarily attributed to the electronic transitions from O 2p to Tb 5d orbitals. Tb4+ ions do not only show any luminescence but also reduce the Tb3+ green luminescence when they coexist in crystals.8−10 This is probably because excitation energy transfers from host lattices or Tb3+ ions to Tb4+ ions owing to the lower lying Tb 5d orbitals, and then results in a nonradiative transition without exciting the 4f7 semiclosed shell in Tb4+. It is, therefore, important to know the valence of Tb ions in detail to develop efficient green luminescent phosphors. Local structures of Tb3+ ions, as well as the valence, influence the intensity of the green luminescence because the 4f−4f transitions in lanthanide ions are basically forbidden at a crystallographic site with a center of symmetry.1,2 To obtain an efficient Tb3+ luminescence, hybridizing of Tb 4f orbitals with other orbitals is necessary which modifies wave functions of © 2017 American Chemical Society

Received: August 22, 2017 Published: October 3, 2017 12625

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Inorganic Chemistry with the absence of green PL in Tb doped CaTiO3.36,37 In addition, the enhancement of the PL intensity in CaSnO3:Tb, CaZrO3:Tb, and CaZrO3:Eu was observed by Mg codoping,38−40 which markedly contrasts with the decrease of the PL intensity in Y3Al5O12:Tb by Mg codoping.41 However, the occupying sites of Tb ions and their valence states are not fully understood, and the origin of the Mg codoping effect is still unknown. In this study, the XAFS measurements of the Tb LIII edge in Tb doped perovskite-type stannates and zirconates were carried out to understand the detailed states of doped Tb ions and their luminescence properties.

structural parameters of the host stannates are summarized in Table 1. The CaSnO3:Tb and SrSnO3:Tb samples exhibited Table 1. Crystal Structural Parameters of AeSnO3 (Ae = Ca, Sr, Ba) space group crystal system tolerance factora site symmetry (A) distortion indexb (A)

2. EXPERIMENTS

a

CaSnO3

SrSnO3

BaSnO3

Pbnm (No. 62) orthorhombic 0.927 m 0.141

Pbnm (No. 62) orthorhombic 0.961 m 0.084

Pm3m ̅ (No. 221) cubic 1.018 m3̅m 0.000

Tolerance factor: t =

The samples were prepared by a conventional solid state reaction method using starting materials of AeCO3 (Ae = Ca, Sr, Ba) and MO2 (M = Sn, Zr) for host materials and Tb4O7 and MgCO3 for dopant. Both Tb and Mg ions were substituted for Ca ions at A sites in nominal composition; the Tb concentration was 0.5 at. % at A sites, and Mg ions were substituted for Ca ions at 3.0 at. %. Although the Tb concentration is rather small and disadvantageous to the XAFS measurements, the optimal concentration for Tb3+ luminescence in the stannate samples was adopted because of its sharp concentration quenching. The starting materials were mixed thoroughly with ethanol, and the dried mixed powders were heated at 1400 °C for 6 h in air to obtain the samples. The XRD patterns of the obtained samples were measured using an XRD diffractometer (Rigaku, RINT-2500), and crystalline phases in the samples were identified. As a result, it was confirmed that each sample was composed of a pure perovskite-type phase. The PL spectra of the samples were measured using a conventional spectrofluorometer (Jasco, FP-6500). The XAFS spectra of the Tb LIII edge were measured at the BL14B2 beamline of the SPring-8 synchrotron radiation facility. Perovskite-type TbAlO3 and AeTbO3 (Ae = Sr, Ba) with the space group Pbnm (No. 62) were used as reference samples to examine the occupying sites and the valence of Tb ions. The reference samples were also prepared by a solid state reaction with the same heating conditions. The fluorescence XAFS measurements using a 19element Ge solid state detector were carried out on the samples doped with dilute Tb ions, while the standard XAFS measurements using the gas-flow-type ionization chambers were made on the reference samples. The crystal structures of samples were visualized using the VESTA code,42 and the obtained XAFS spectra were analyzed using the IFEFFIT code.43

rA + rO .r ,r , 2 (rB + rO) A B

and rO are ionic radii of A and

B site cations and oxide ion, respectively. 1 n |l − l | Distortion index: Dl = n ∑i = 1 i l av . n, lav, and li are the number

b

av

of neighboring oxide ions and averaged bond length between central cation and coordinated oxide ions and the ith individual bond length, respectively.

green luminescence due to the 4f−4f electronic transition from the 5D4 to 7FJ states in Tb3+ ions. The luminescence intensity in the CaSnO3:Tb was higher than that in the SrSnO3:Tb. In contrast, the green luminescence was not observed in the BaSnO3:Tb. Because the 4f−4f transition is originally forbidden at a crystal site with an inversion center, Tb3+ luminescence will be influenced by the local structure of the A site in the host perovskite structure. In the Ca and Sr stannate samples,44 the site symmetry at the A site is m without an inversion center, and the local structure is distorted as shown in the inset in Figure 1. The degree of distortion from an ideal cubic perovskite structure is frequently evaluated by the tolerance factor.45−47 The tolerance factors of the Ca and Sr samples with orthorhombic distortion were 0.927 and 0.961, respectively, and they largely differed from the ideal value of 1.0 for a cubic perovskite structure. The distortion index is also useful to estimate the degree of local distortion around a cation.48,49 To compare the local distortion at A sites straightforwardly in AeSnO3 (Ae = Ca, Sr, Ba), 12 Ae−O bond lengths at A sites were used for the distortion index evaluation. The distortion indexes at A site in the Ca and Sr samples were 0.141 and 0.084, respectively, which largely deviated from zero. The values of the tolerance factors and distortion indexes indicated that the influence of the distortion on Tb ions doped in the Ca sample is much larger than that in the Sr sample. This difference probably caused the higher PL intensity in the Ca sample. In the Ba sample, the crystal system is cubic, and the site symmetry at the A site is m3̅m with an inversion center. In addition, the tolerance factor of 1.018 is very close to 1.0, and the distortion index is zero. These parameters clearly verified that the Ba sample is an ideal cubic perovskite structure. It is, therefore, reasonable to consider first that the 4f−4f transitions in Tb ions were strongly forbidden in the Ba sample, which resulted in the absence of the green luminescence. To examine the local structures of Tb ions directly, XAFS spectra of the Tb LIII edge were measured in AeSnO3:Tb (Ae = Ca, Sr, Ba). The X-ray absorption near edge structures (XANES) of the Tb LIII edge of the samples are shown in Figure 2 with those of the reference samples. The reference samples of TbAlO3 and AeTbO3 (Ae = Sr, Ba), which are orthorhombic perovskite-type structures, are isostructural with each other and also with the Ca and Sr samples. Although the

3. RESULTS AND DISCUSSION 3.1. Tb Occupying Sites and Tb Valence. Figure 1 shows PL spectra of Tb doped alkaline earth stannates, AeSnO3:Tb (Ae = Ca, Sr, Ba), and local crystal structures at A sites. Crystal

Figure 1. PL spectra and photographs of Tb doped alkaline earth stannates, AeSnO3:Tb [Ae = Ca (), Sr (···), Ba (---)], and local crystal structures of A sites, in which each Ae ion is located in the center and blue and white balls are Sn and O ions, respectively. The samples were excited by UV irradiation at 254 nm. 12626

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Ba) owing to the large difference in electronegativity between Ba and O. To evaluate the concentration ratio of the Tb3+ at an A site to Tb4+ at a B site, the XANES spectrum of the Ba sample was fitted by the linear combination of the spectra of the Ca sample as a Tb3+ component and BaTbO3 as a Tb4+ component. Although the spectrum of TbAlO3 should be used for the linear combination fitting as a reference of Tb3+ at the A site, the peak at 7517 eV for TbAlO3 was not as sharp as those for the Ca and Sr stannate samples, which gave worse and improper fitting results inevitably. The best fitting result was obtained by the ratio Tb3+:Tb4+ = 0.61:0.39, revealing that approximately 40% of 0.5 at. % doped Tb ions occupy B sites with tetravalent states in the Ba sample. The presence of Tb4+ ions, as well as inversion symmetry, was responsible for the absence of green luminescence in the Ba sample. The difference of Tb occupying sites in the Ba samples was also observed in the extended XAFS (EXAFS). Figure 3 shows

Figure 2. XANES spectra of Tb LIII absorption edge in AeSnO3:Tb [Ae = Ca (), Sr (···), Ba (---)] (top) along with those in reference samples, TbAlO3 (), SrTbO3 (···), and BaTbO3 (---) (bottom). Thin lines in the top are the component spectra of the Tb3+ at A site (Ca sample) (···) and Tb4+ at B site (BaTbO3) (---) for linear combination fitting and the obtained spectrum ().

crystal structures are the same between TbAlO3 and AeTbO3 (Ae = Sr, Ba), Tb3+ and Tb4+ occupy two distinguishable atomic sites. Tb3+ with a larger ionic radius of rTb3+(XII) = 1.41 Å, which is roughly estimated from extrapolation, occupies A sites in TbAlO3, and Tb4+ with a smaller ionic radius of rTb4+(VI) = 0.90 Å occupies B sites in AeTbO3 (Ae = Sr, Ba), respectively.50 The XANES spectra of the reference samples exhibited a clear difference between Tb3+ at the A site and Tb4+ at the B site. Tb3+ at the A site in TbAlO3 gave an intensely sharp absorption peak, viz., a white line, at 7517 eV, whereas Tb4+ at the B site in AeTbO3 (Ae = Sr, Ba) provided a broad peak at 7525 eV with a shoulder at 7529 eV. The XANES spectra of SrTbO3 and BaTbO3 are almost the same, and the Tb4+ absorption edges showed the chemical shift to the higher energy side because of the higher valence state. The XANES spectra of the Ca and Sr samples are nearly the same as each other although the peak intensity of the Ca sample is slightly higher than that of the Sr sample. The shapes of their XANES spectra were very similar to that of TbAlO3, which indicates that Tb ions doped into the Ca and Sr samples are trivalent states at A sites. On the other hand, the XANES spectrum of the Ba sample showed low peak intensity at 7517 eV and large shoulder peaks at 7525 and 7530 eV. Because the shape of the shoulder peaks is analogous to the XANES spectra of AeTbO3 (Ae = Sr, Ba), Tb ions doped into the Ba samples are considered to be not only trivalent states at A sites but also tetravalent states at B sites. The ionic radii of Ae ions at A sites increase from Ca (rCa(XII) = 1.48 Å) to Ba (rBa(XII) = 1.75 Å), increasing the difference from the ionic radius of Tb3+ (rTb3+(XII) = 1.41 Å). The difference becomes over 15% only in the case of Ba ions (19%), indicating limited substitution may occur in the Ba samples. On the other hand, the ionic radius difference at B sites between Sn (rSn (VI) = 0.83 Å) and Tb ions is small for Tb4+ (rTb4+(VI) = 0.90 Å) but becomes as large as 28% for Tb3+ (rTb3+(VI) = 1.06 Å). Therefore, it is reasonable from the viewpoint of ionic sizes that some Tb ions occupied B sites in tetravalent states in the Ba samples. In addition to the ionic sizes, the absence of a compound of CaTbO3 and the presence of BaTbO3 also gained the stabilization of Tb4+ at the B site in the Ba samples. Namely, it is considered that some Tb4+ ions at B sites were stabilized in the Ba samples because the BaSnO3 host lattice is the most oxidizing among AeSnO3 (Ae = Ca, Sr,

Figure 3. Tb LIII edge k2χ(k) EXAFS oscillation spectra of AeSnO3:Tb [Ae = Ca (), Sr (···), Ba (---)] (top) and reference samples, TbAlO3 (), SrTbO3 (···), and BaTbO3 (---) (bottom).

k2χ(k) EXAFS oscillations of the AeSnO3:Tb (Ae = Ca, Sr, Ba) samples along with the TbAlO3 and AeTbO3 (Ae = Sr, Ba) references. Although the EXAFS oscillations of the samples were not strong enough to conduct Fourier transformation analysis because of a small Tb concentration, the features of the oscillations showed the difference between Tb occupying sites in the Ca and Sr samples and those in the Ba sample. In the reference samples, the phase of the EXAFS oscillation clearly differs between TbAlO3 and AeTbO3 (Ae = Sr, Ba). Tb ions doped in the Ca and Sr samples showed the EXAFS oscillations similar to that in TbAlO3, indicating that the Tb ions were located at A sites. A slight disagreement between the Ca and Sr samples demonstrates the difference in the local structure of Tb ions at A sites between them. In contrast, the EXAFS oscillation of Tb ions in the Ba sample was similar to those in AeTbO3 (Ae = Sr, Ba) rather than that in TbAlO3, suggesting that some amount of Tb ions were located at B sites. 3.2. Effects of Mg Codoping. Figure 4 shows PL spectra of Tb single doped and Tb−Mg codoped CaMO3 (M = Sn, Zr) at room temperature. The nominal chemical compositions of the samples are (Ca0.995Tb0.005)MO3+δ for CaMO3:Tb, and {(Ca0.97Mg0.03)0.995Tb0.005}MO3+δ for CaMO3:Tb−Mg (M = Sn, Zr). The enhancement of the Tb3+ green luminescence by Mg codoping was observed in both the stannate and zirconate samples, even though no obvious differences in the diffuse reflectance spectra were observed in the visible range between CaMO3:Tb and CaMO3:Tb−Mg. The external quantum 12627

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codoping. It is also implied that the mechanisms of the luminescence enhancement by Mg codoping might be different between Tb3+ and Eu3+ activators in terms of the occupying sites and the local structures,39,40,51 even though the sizes of lanthanide ions are not much different from each other. Figure 6 shows the XANES spectra of the Tb single doped and Tb−Mg codped CaSnO3 and CaZrO3. The XANES

Figure 4. PL spectra of Tb single doped (---) and Tb−Mg codoped () CaSnO3 (left) and CaZrO3 (right) under UV excitation at 254 nm.

efficiency evaluated using an integrating sphere was approximately 35% for CaSnO3:Tb−Mg and 20% for CaZrO3:Tb− Mg. As reported previously,38 two possible reasons are considered to explain the origin of the enhancement. One is the local lattice distortion around Tb ions induced by small Mg ions at Ca sites. The local distortion might promote the hybridization of 4f orbitals with other orbitals and increases a radiative transition probability as observed above between the Ca and Sr samples. The other is a charge compensation caused by Mg ions at Sn sites. The valence of Mg ions is the same with that of Ca ions at A sites, whereas the size of Mg ions is much closer to that of the B site ions: rMg(VI) = 0.86 Å, rSn(VI) = 0.83 Å, rZr(VI) = 0.86 Å.50 To examine the possibility of the local lattice distortion induced by Mg ions, k2χ(k) EXAFS oscillations of the CaSnO3:Tb and CaSnO3:Tb−Mg samples are compared in Figure 5. Both of the samples showed EXAFS oscillations

Figure 6. XANES spectra of Tb LIII absorption edge in Tb single doped (---) and Tb−Mg codoped () CaSnO3 (top) and CaZrO3 (bottom). Thin lines in the bottom are the component spectra of the Tb3+ at the A site (CaZrO3:Tb−Mg) (···) and Tb4+ at the B site (BaTbO3) (---) for linear combination fitting and the obtained spectrum ().

spectrum of the CaSnO3:Tb is almost the same as that of the CaSnO3:Tb−Mg, clarifying that most Tb ions are located at A sites in both the stannate samples, irrespective of Mg codoping. However, the intensity of the white line at 7517 eV for the CaSnO3:Tb is slightly smaller than that for the CaSnO3:Tb− Mg. A similar phenomenon was observed in the zirconate samples. The XANES spectra of the CaZrO3:Tb and CaZrO3:Tb−Mg exhibited that most Tb ions are located at A sites in trivalent states in both the zirconate samples. Nevertheless, the XANES spectra of the CaZrO3:Tb showed not only the lower intensity of the white line at 7517 eV but also a shoulder peak at 7525 eV, implying the presence of a very small amount of Tb4+ at B sites. The linear combination fitting of the CaZrO3:Tb spectrum by the spectra of CaZrO3:Tb−Mg (Tb3+ component) and BaTbO3 (Tb4+ component) gave the best fitting result at the ratio of Tb3+:Tb4+ = 0.83:0.17. Accordingly, the XANES spectra of the Tb single doped samples, CaSnO3:Tb and CaZrO3:Tb, demonstrated that these would include a small amount of Tb4+ at B sites in contrast to the Tb−Mg codoped samples. It is, therefore, considered that Mg codoping prevented the generation of the Tb4+ ions and the enhancement of Tb3+ luminescence is primarily attributed to a charge compensation caused by the Mg codoping. Na or K codoping at A sites was attempted in the CaSnO3 samples to compare the influence on the Tb3+ luminescence by codoping. Figure 7 shows PL spectra of Tb single doped and codoped CaSnO3 samples. The Tb3+ luminescence was enhanced by Na or K codoping, as well as Mg codoping, implying that the charge compensation occurred in these codoped samples. The most probable defect models for Na or K codoping are expressed in the following with Kröger−Vink notation.

Figure 5. Tb LIII edge k2χ(k) EXAFS oscillation spectra of Tb single doped (---) and Tb−Mg codoped () CaSnO3 (top) and reference samples, TbAlO3 (), SrTbO3 (···), and BaTbO3 (---) (bottom).

similar to that of TbAlO3 indicating that the doped Tb ions were located at A sites in these samples. In addition, the EXAFS oscillation of the CaSnO3:Tb−Mg was found to be almost the same as that of the CaSnO3:Tb, compared with the EXAFS oscillations of the Ca and Sr samples in Figure 3. This result suggests that the local lattice distortion around Tb ions induced by Mg ions is extremely small or negligible to influence the Tb3+ luminescence. Consequently, it is considered that the enhancement of the Tb3+ luminescence is not mainly caused by the increase in the local distortion around Tb ions by Mg

Tb•Ca + Na′Ca 12628

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codoping effect was attributed to the charge compensation by a few Mg ions probably occupying B sites rather than the increase in the local distortion at A sites induced by a small Mg ion.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02165. Summary of Tb3+:Tb4+ and additional ionic radius information (PDF)

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Figure 7. PL spectra of Tb−Na codoped (···) and Tb−K codoped () CaSnO3 compared with those of Tb single doped (thin ---) and Tb−Mg codoped (thin ) CaSnO3 under UV excitation at 254 nm.

*E-mail: [email protected]. Phone: +81 93 884 3339. ORCID

Both Tb3+ and Na+ ions occupy Ca2+ sites without generating extra charges or Tb4+ ions. In contrast, the possible model of the charge compensation for Mg codoping is the following. 2Tb•Ca

AUTHOR INFORMATION

Corresponding Author

Kazushige Ueda: 0000-0002-4527-799X Notes

The authors declare no competing financial interest.

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+ Mg″Sn

ACKNOWLEDGMENTS This work was partly supported by JSPS KAKENHI Grant JP16K06724. The synchrotron radiation experiments were performed at the BL14B2 of Spring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposals 2009B1804 and 2016B1567).

A few Mg2+ ions unintentionally occupy Sn4+ sites to compensate for the positive charge of Tb3+ ions at Ca2+ sites without generating Tb4+ ions at Sn4+ sites, even though Mg ions were nominally doped at Ca2+ sites. Mg ions are reported to occupy B sites preferentially in BaTiO3, and the change of Dy3+ occupying sites in BaTiO3:Dy3+ by Mg codoping was observed theoretically and experimentally.51−53 Because the Tb concentration is small and the powder samples have various defects including the surface, it is considerably difficult to clarify the details of the defects and the charge balance quantitatively. However, these models are able to explain the Tb 3+ luminescence enhancement reasonably by the charge compensation due to the Mg codoping.

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REFERENCES

(1) Phosphor Handbook; Yen, W. M.; Shionoya, S.; Yamamoto, H., Eds.; CRC Press: Boca Raton, FL, 2006. (2) Blasse, G.; Grabmaier, B. C. Luminescent Materials; SpringerVerlag: Berlin, 1994. (3) Hoshina, T. Radiative Transition probabilities in Tb3+ and fluorescence colors producible by Tb3+-activated phosphors. Jpn. J. Appl. Phys. 1967, 6, 1203−1211. (4) Cavalli, E.; Boutinaud, P.; Mahiou, R.; Bettinelli, M.; Dorenbos, P. Luminescence dynamics in Tb3+ -doped CaWO4 and CaMoO4 crystals. Inorg. Chem. 2010, 49, 4916−4921. (5) De la Luz, V.; Prades, M.; Beltran, H.; Cordoncillo, E. Environmental-friendly yellow pigment based on Tb and M (M = Ca or Ba) co-doped Y2O3. J. Eur. Ceram. Soc. 2013, 33, 3359−3368. (6) Luñaḱ ová, P.; Trojan, M.; Luxová, J.; Trojan, J. BaSn1‑xTbxO3: a new yellow pigment based on a perovskite structure. Dyes Pigm. 2013, 96, 264−268. (7) Raj, A. K. V.; Rao, P. P.; Divya, S.; Ajuthara, T. R. Terbium doped Sr2MO4 [M = Sn and Zr] yellow pigments with high infrared reflectance for energy saving applications. Powder Technol. 2017, 311, 52−58. (8) Flores-Gonzalez, M. A.; Ledoux, G.; Roux, S.; Lebbou, K.; Perriat, P.; Tillement, O. Preparing nanometer scaled Tb-doped Y2O3 luminescent powders by the polyol method. J. Solid State Chem. 2005, 178, 989−997. (9) Zych, E.; Deren, P. J.; Strek, W.; Meijerink, A.; Mielcarek, W.; Domagala, K. Preparation, X-ray analysis and spectroscopic investigation of nanostructured Lu2O3:Tb. J. Alloys Compd. 2001, 323−324, 8−12. (10) Verma, R. K.; Kumar, K.; Rai, S. B. Inter-conversion of Tb3+ and Tb4+ states and its fluorescence properties in MO-Al2O3: Tb (M = Mg, Ca, Sr, Ba) phosphor materials. Solid State Sci. 2010, 12, 1146−1151. (11) Bunker, G. Introduction to XAFS; Cambridge University Press: Cambridge, 2010. (12) Agarwal, B. K. X-Ray Spectroscopy; Springer-Verlag: Berlin, 1991. (13) Makishima, S.; Yamamoto, H.; Tomotsu, T.; Shionoya, S. Luminescence spectra of Sm3+ in BaTiO3 host lattice. J. Phys. Soc. Jpn. 1965, 20, 2147−2151.

4. CONCLUSIONS The PL properties of Tb doped alkaline earth stannates and zirconates with the perovskite structure were examined from the viewpoints of the surroundings and valence states of Tb ions measuring the XAFS spectra of Tb LIII absorption edge. CaSnO3:Tb showed the most intense Tb3+ PL among AeSnO3:Tb (Ae = Ca, Sr, Ba) because of the lack of the center symmetry and the large distortion index at A sites. In contrast, Tb3+ luminescence was not observed in BaSnO3:Tb because Tb ions not only occupy A sites with an inversion center but also partially occupy B sites in tetravalent states. The presence of Tb4+ ions at B sites in the BaSnO3:Tb was revealed obviously by the XANES and EXAFS spectra. It is, consequently, understood that Tb3+ luminescence in the BaSnO3:Tb was not simply forbidden by the Laporté rule but also inhibited by some Tb4+ ions. Tb−Mg codoping in CaSnO3 or CaZrO3 enhanced the Tb3+ luminescence, and the origin of the enhancement was analyzed by the XAFS measurements. The EXAFS oscillation spectra did not exhibit obvious changes in the local structure around Tb3+ ions by Mg codoping. On the other hand, the XANES spectra of Tb−Mg codoped stannate and zirconate samples showed the higher peak intensity of the white lines and no shoulder peaks at the higher energy side, indicating that doped Tb ions become almost purely Tb3+ at A sites by Mg codoping. The absence of Tb4+ ions in the Tb−Mg codoped samples is considered to increase Tb3+ luminescence as observed in Na or K doped samples. Consequently, the Mg 12629

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Article

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.7b02165 Inorg. Chem. 2017, 56, 12625−12630