Structural Redetermination and Photoluminescence Properties of the

Article pubs.acs.org/IC

Structural Redetermination and Photoluminescence Properties of the Niobium Oxyphosphate (NbO)2P4O13 Daniel Schildhammer,† Gerda Fuhrmann,† Lucas L. Petschnig,† Klaus Wurst,† Daniela Vitzthum,† Markus Seibald,‡ Herwig Schottenberger,† and Hubert Huppertz*,† †

Faculty of Chemistry and Pharmacy, Institute of General, Inorganic and Theoretical Chemistry, Leopold-Franzens-University Innsbruck, Innrain 80-82, 6020 Innsbruck, Austria ‡ OSRAM GmbH, Corporate Innovation, Mittelstetter Weg 2, 86830 Schwabmuenchen, Germany S Supporting Information *

ABSTRACT: The structure of (NbO)2P4O13 was solved and refined based on new single-crystal diffraction data revealing considerably more complexity than previously described. (NbO)2P4O13 crystallizes in the triclinic space group P1̅ with Z = 6. The lattice parameters determined at room temperature are a = 1066.42(4) pm, b = 1083.09(4) pm, c = 1560.46(5) pm, α = 98.55(1)°, β = 95.57(1)°, γ = 102.92(1)°, and V = 1.7213(2) nm3. The superstructure contains 64 unique atoms including two disordered semioccupied oxygen positions. An unusual 180° bond angle between two [P4O13]6− groups was refined to form half-occupied, split positions in agreement with previous reports. The IR and Raman spectra reflect the appearance of overlapping bands assignable to specific group vibrations as well as P−O−P linkages present in the [P4O13]6− entities. Investigation of the powdered product concerning its photoluminescence properties revealed an excitability in the UV at 270 nm assigned to O2p−Nb4d charge transfer transitions. A resulting broad-band emission with the maximum in the visible region at 455 nm was determined.

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luminescence of these materials was observed.12 Condensed materials containing [NbO6]7− octahedra often exhibit very efficient luminescence. The reason is supposed to be the presence of edge- and face-sharing octahedra. Thus, cornersharing [NbO6]7−octahedra are predicted to reduce the luminescence efficiency.13 In supplementary studies, the presence of NbO (niobyl) bonds was identified to be responsible for the luminescence behavior.14−16 Recently, studies postulated a triple bond in the oxo metal complexes (VO)2+, (CrO)3+, (MoO)3+, and (WO)3+.17 However, the niobium complex was not mentioned in this study, and therefore, the widely used niobium oxide double bond is assumed for the here presented compound (NbO)2P4O13. In our study, we discovered a superstructure of (NbO)2P4O13, a compound previously published by Nikolaev et al., containing NbO6 octahedra corner shared with [P4O13]6− groups.18 Spectroscopic investigations of the compound (NbO)2P4O13 revealed UV absorption accompanied by a broad-band emission in the visible region.

INTRODUCTION Framework structured inorganic materials are of common interest due to their utilization as host systems in lithium electrochemical devices.1−3 They offer high architectural diversity and also enable applications as intercalation hosts in the area of radioisotope sequestration and waste remediation materials.4 Recently, a topological analysis of vacancies in phosphate frameworks pointed out the possibility to store environmentally and technically relevant guest species, e.g., CO2, H2O, CH4, and H2.5 In a structural context of the title compound, niobium phosphates with enhanced proton conductivity and selective catalytic properties already exist.6−8 Previous reports of a cubic zirconium phosphate ZrP2O7 with a structure similar to NbP2O7.5 showed a weak luminescence at a temperature of 4.2 K with an emission maximum at 460 nm.9,10 The structure of NbP2O7.5 represents a framework structure containing niobium octahedra linked to phosphate tetrahedra. In general, the luminescence properties of compounds are influenced by their particular atomic environment. Interestingly, no luminescence was observed in niobium phosphates structured in analogy to Sc2(WO4)3, which also possesses a framework built up of corner-shared NbO6 octahedra together with PO4 tetrahedra.9,11 In other studies, the absorption properties of Cs2GeP4O13 containing [P4O13]6− anionic groups were assigned to charge transfer transitions from the O 2p orbitals to the corresponding cations Cs+ or Ge4+. However, no © XXXX American Chemical Society

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EXPERIMENTAL METHODS

Synthesis. Single-crystals of (NbO)2P4O13 were prepared by a classical solid state reaction. Crystal growth was achieved by a Received: December 2, 2016

A

DOI: 10.1021/acs.inorgchem.6b02891 Inorg. Chem. XXXX, XXX, XXX−XXX

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658.6(1) pm, b = 840.0(1) pm, c = 1084.2(1) pm, α = 73.63(1)°, β = 89.65(1)°, γ = 89.97(1)°, and V = 0.5754(1) nm3.18 Interestingly, there exists a second entry in the database originating from the same reference (Nikolaev et al.) revealing a centrosymmetric setting in the space group P1̅ (no. 2) (ICSD 56793). No further information is given. In striking contrast, the present study revealed a three times larger superstructure of (NbO)2P4O13 with lattice parameters of a = 1066.42(4) pm, b = 1083.09(4) pm, c = 1560.46(5) pm, α = 98.55(1)°, β = 95.57(1)°, γ = 102.92(1)°, and a volume of 1.7213(2) nm3 (Table 1). The structure solution and refinement converged best in the centrosymmetric triclinic space group P1̅ (no. 2).

nonstoichiometric mixture of Nb:P in a ratio of 1:4. The educts consisting of 201.3 mg of Nb2O5 (Chempur, 99.9%) and 799.0 mg of (NH4)2HPO4 (Merck, >99.0%) were homogenized with a planetary mill (FRITSCH, Pulverisette 7) 4 × 20 min at 400 rpm. Subsequently, the homogeneous mixture was transferred into a platinum crucible and placed in a silica tube positioned in a tube furnace (Tmax = 1200 °C). The temperature was raised with 4 °C min−1 to 400 °C and held for 2 h. Afterward, the temperature was further increased to 750 °C with a heating rate of 4 °C min−1. The temperature program was finalized after 5 h at 750 °C by slowly cooling down to 400 °C with a rate of 0.1 °C min−1. The synthesis was conducted under ambient atmosphere. The resulting colorless single-crystals were found embedded in a white amorphous phase. Single-Crystal Structure Analysis. The single-crystal data collection was carried out under ambient atmosphere by using a Bruker D8 Quest Kappa diffractometer with Mo Kα radiation (λ = 71.073 pm) with multiscan absorption correction. The candidate space groups P1 and P1̅ were identified due to the absence of systematic extinctions. The structure solution and parameter refinement (fullmatrix least-squares on F2) in the centrosymmetric space group P1̅ were performed by using SHELXL-2013.19,20 Anisotropic refinement of all positions was performed. A final difference Fourier analysis revealed no significant peaks. The X-ray powder diffraction data of (NbO)2P4O13 were obtained by measuring in transmission geometry using a STOE Stadi P powder diffractometer with Ge(111)monochromatized Mo Kα1 (λ = 70.930 pm) radiation. As detector, the silicon microstrip solid state detector Mythen 1K was used. The data were obtained by measuring in 2θ steps of 0.005° from 2° to 70°. Rietveld refinement was done by using the program Topas 4.2. The peak shape was adjusted by means of LaB6 standard measurements. Further details of the crystal structure investigation may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: crysdata@fizkarlsruhe.de) on quoting deposition number CSD-432261. Luminescence. Luminescence investigations of the powder material were carried out using a HAMAMATSU QUANTAURUSQY spectrometer equipped with a full integrating sphere (diameter approximately 8.4 cm) and a 150 W xenon excitation light source. In order to suppress fluorescence, the sample was measured inside a closed silica-glass cell (outer diameter 17 mm) positioned on a Tefloncovered sample holder. An excitation wavelength of 270 nm was chosen with a maximum spectral full width at half-maximum (fwhm) of 10 nm. A spectrum was measured in the wavelength range between 250 and 960 nm with a 0.77 nm step size. Vibrational Spectroscopy. The transmission FT-IR spectrum of a single-crystal was recorded in the range between 600 and 1400 cm−1 with a Vertex 70 FT-IR spectrometer (wavelength resolution 4 cm−1). The spectrometer was equipped with a KBr beam splitter, an LNMCT (Mercury Cadmium Telluride) detector and a Hyperion 3000 microscope (Bruker, Vienna, Austria). Three hundred twenty scans of the sample were conducted using a Globar (silicon carbide) rod as mid-IR source and a 15× IR objective as focus. The sample was positioned on a BaF2 window during the measurement. The atmospheric influences were corrected using the software OPUS 6.5. The single-crystal Raman spectrum was recorded using the Horiba Jobin Yvon LabRam-HR 800 Raman microspectrometer in the range between 600 and 1400 cm−1. The crystal was excited with the emission line of 532 nm through an Olympus objective (magnification 100×) with a frequency-doubled 100 mW Nd:YAG laser. An optical grating (1800 lines mm−1) served for the dispersion of the scattered light, which was collected subsequently by an open-electrode CCD detector (1024 × 256). The spectrum was recorded under ambient conditions with background correction.

Table 1. Single-Crystal Data and Structure Refinement of (NbO)2P4O13a empirical formula molar mass, g·mol−1 cryst syst space group single-crystal diffractometer radiation a, pm b, pm c, pm α, deg β, deg γ, deg V, nm3 formula units per cell, Z calcd density, g·cm−3 cryst size, mm3 temperature, K abs coeff, mm−1 F(000) θ range, deg range in hkl total no. of reflns no. of independent reflns Rint no. of reflns with I ≥ 2σ(I) Rσ data/ref parameters abs corr goodness-of-fit on Fi2 final R1/wR2 indices [I ≥ 2σ(I)] R1/wR2 indices (all data) largest diff. peak and hole, e·Å−3 a

(NbO)2P4O13 221.10 triclinic P1̅ (no. 2) Bruker D8 Quest Mo Kα (λ = 71.073 pm) 1066.42(4) 1083.09(4) 1560.46(5) 98.55(1) 95.57(1) 102.92(1) 1.7213(2) 6 3.182 0.06 × 0.05 × 0.025 300(2) 2.654 1572 2.53−35.05 ±16, ±16, ±23 79 602 12 461 0.0770 8558 0.0542 12461/577 multiscan (SADABS) 1.041 0.0447/0.0933 0.0815/0.1034 2.388/−2.015

Space group P1̅ (no. 2); standard deviations in parentheses.

The enlarged superstructure was identified through weak reflections in the X-ray diffraction pattern, since a twin could be excluded due to nonperiodical extinctions. The smaller cell reported by Nikolaev could be successfully indexed by neglecting the weak reflections as well. However, in contrast to the previously assigned space group P1, in our study, both cases converged in the space group P1̅ (no. 2). Interestingly, a similar amendment of the structure was made in the case of the structurally related molybdenum phosphate Mo2P4O15 (≙ (MoO)2P4O13). First results revealed a monoclinic structure crystallizing in the space group P21 (no. 14).21,22 By a later report, the smaller cell was corrected to a supercell structure

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RESULTS AND DISCUSSION Description of the Structure. In the above-mentioned previous work by Nikolaev et al., single-crystal analysis revealed (NbO)2P4O13 crystallizing in the noncentrosymmetric space group P1 (no. 1) (ICSD 62471) with lattice parameters a = B

DOI: 10.1021/acs.inorgchem.6b02891 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry crystallizing in the space group Pn (no. 7) containing 441 crystallographically unique atoms in its asymmetric unit.23,24 As reported earlier, the Mo2P4O15 substructure can be found in the Mo2P4O15 superstructure in the identical viewing direction (Figure 1, right, orange unit cell). In contrast, the

equations

asuper =

2 2 asub + bsub

and

csuper

=

2 (2 × asub)2 + bsub . The newly explored superstructure of (NbO) 2 P 4 O 13 represents a complex supercell containing 64 distinct atoms in the asymmetric unit composed of four [P4O13]6− entities and six NbO6 octahedra. The perpendicularly orientated [P4O13]6− anionic groups consist of four corner-sharing PO43− tetrahedra. Figure 2 shows the two separate [P4O13]6− groups orientated along the c axis (blue) and the other two [P4O13]6− groups orientated along the a axis (green). In Figure 3, the 4 distinct [P4O13]6− groups coordinated via corner sharing to 10 NbO6 octahedra are highlighted separately.

Figure 1. Relationship of the predescribed subcells of (NbO)2P4O13 (left, top) and Mo2P4O15 (left, bottom) in comparison with the title supercell (NbO)2P4O13 (center) and the predescribed Mo2P4O15 (right).21−24

substructure of (NbO)2P4O13 can be found in the corresponding superstructure by rotational change of the viewing direction. Therefore, the [01̅0] direction of the subcell of Mo2P4O15 fits in the [1̅01] direction of the supercell of (NbO)2P4O13. However, both subcells can be found in the supercell of Mo2P4O15 and the title superstructure of (NbO)2P4O13, clearly demonstrating the structural relationship of these compounds. Figure 2 illustrates the relation between the subcell and the supercell of the compound (NbO)2P4O13. Whereas the vector

Figure 3. Coordination of the four distinct [P4O13]6− groups corner shared to the NbO6 octahedra. (a and b) Centrosymmetricinterrelated [P4O13]6− groups with their split position of oxygen atoms, and (c and d) two other [P4O13]6− groups.

While two perpendicularly orientated [P4O13]6− groups are related by the inversion center (P8−P4−P4−P8 and P5−P7− P7−P5) (Figure 3a and 3b), the other two [P4O13]6− groups are evolved from four unique phosphorus atoms (P1−P9−P6− P3 and P12−P11−P10−P2) (Figure 3c and 3d). The two symmetry-originated [P4O13]6− groups exhibit disordered, half-occupied oxygen positions around the inversion center. These split semipopulated oxygen positions were refined based on an unusual 180° bond angle on two distinct P−O−P linkages in P4−O45−P4 and P7−O44−P7 (Figure 4) where O44 and O45 resulted in primarily cigarshaped displacement ellipsoids. Such an unusual 180° P−O−P bond angle was reported previously for [P2O7]4− polyanions in cubic/orthorhombically structured ZrP2O7 and for the monoclinic phosphate system in MnP2O7.25,26 However, further examination revealed a disordered bridging oxygen position instead of the 180° P− O−P bridge, which is in accordance to our studies, where the 180° angled bridging oxygen atoms (O44 and O55) were populationally split in half-occupied positions.27 Accordingly, the new reasonable assigned bond angles P4−O45−P4 and P7−O44−P7 have values of 157.5° and 165.2°, respectively. These angles correlate with the expected P−O−P bond angles in the [P4O13]6− groups revealing usual values in the range between 130° and 160°.24−27 The P−O distances within the [P4O13]6− groups vary between 145.7(3) and 162.0(3) pm, exhibiting a mean value of 152.3 pm in the PO4 linkages. The local environments of the six NbO6 octahedra include one terminal niobyl oxygen in addition to the five shared

Figure 2. View of the superstructure of (NbO)2P4O13 in direction (a) [10̅ 0], (b) [001]̅ , and (c) [010̅ ] in comparison to their substructure in direction (d) [11̅0], (e) [2̅1̅0], and (f) [001̅].

b⃗super = [01̅0] is identical with cs⃗ ub = [001̅] (Figure 2c and 2f), the vectors for as⃗ uper and cs⃗ uper were received by rotation of the subcell around the c axis as depicted in Figure 2d and 2e. ⃗ to obtain as⃗ uper Therefore, the vector as⃗ ub was subtracted by bsub = [11̅0] and the double value of the vector −asub was subtracted by b⃗sub to get cs⃗ uper = [2̅1̅0]. The values of the supercell lattice parameters can then be calculated from the trigonometric C

DOI: 10.1021/acs.inorgchem.6b02891 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. P−O−P linkages in the two symmetry-generated [P4O13]6− groups emphasized with the 180° angle and the oxygen split position, respectively: P7−O44−P7 (left) and P4−O45−P4 (right).

oxygen positions of the PO4 tetrahedra (Figure 5). While the mean value of the bond distance of Nb to the terminally bound

Figure 5. Local environment of a representatively selected NbO6 octahedron occurring in (NbO)2P4O13, showing five corner-sharing PO4 tetrahedra and one terminal niobyl oxygen.

Figure 6. Experimental (black cross marks), calculated (red line), and difference powder diffraction pattern (blue line) of niobium oxyphosphate (NbO)2P4O13 combined with NbP1.8O7. Vertical ticks (top/pink, (NbO)2P4O13; bottom/green, NbP1.8O7) indicate the position of the reflections.

oxygen atom exhibits a short distance of 168.1 pm, the opposite oxygen atom reveals a long distance of 227.2 pm, indicating an off-centered niobium cation. The other bond distances to the shared oxygen positions have a mean value of 202 pm. A similar situation was described recently for the coordination mode of MoO6 octahedra in the superstructure of Mo2P4O15.24 Rietveld refinement of the powder data revealed a good conformity to the single-crystal structure analysis of (NbO)2P4O13. As secondary phase in the powder pattern, the cubically structured compound NbP1.8O710 (Figure 6) could be identified. The weight ratios of (NbO)2P4O13 and NbP1.8O7 were determined to be 82% and 18%, respectively. The final refinement value was 4.84% for Rexp and 5.06% for Rwp. The difference curve revealed a good fit for both the triclinic phase (NbO)2P4O13 (Table 2) and the cubic structured compound NbP1.8O7. Figure 7 shows the powdered white product (NbO)2P4O13 in an agate mortar. For the first examination of the luminescence behavior, an excitation wavelength of 254 nm was used revealing a strong white to light blue emission for the compound (Figure 7b).

Further investigations of the photoluminescence properties of the powdered sample showed a broad-band emission in the range of 350−600 nm when excited with a wavelength of 270 nm. The maximum was localized at ∼455 nm (Figure 8). As generally agreed, such broad-band emissions excited by UV arise from the charge transfer transition of O2p anions to the cationic counterions.14,28 According to the literature, the charge transfer of the short NbO niobyl double bonds in the NbO6 octahedra are attributable to the luminescence properties.15 Usually most photoluminescence materials are composed of nonluminescent host constituents containing rare earth dopants that offer narrow-banded emission spectra due to their d−f electronic transitions. However, the naturally occurring luminescent mineral CaWO4 displays a broad-band emission similar to (NbO)2P4O13, and when activating CaWO4 with rare earth cations, the [WO4]2− anionic group can also act as sensitizers to activators.28,29 D

DOI: 10.1021/acs.inorgchem.6b02891 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 2. Crystallographic Data of (NbO)2P4O13 from the Rietveld Refinement of the XRPD Data formula space group a, pm b, pm c, pm α, deg β, deg γ, deg V, nm3 formula units per cell Z temperature, K no. of reflns radiation wavelength, pm Rexp Rwp Rp χ2 ρcalc, g cm−3 starting angle, deg final angle, deg step width, deg

(NbO)2P4O13 P1̅ 1066.88(2) 1083.03(1) 1560.75(1) 98.40(1) 95.56(1) 103.02(1) 1.7224(1) 6 300(2) 15 788 Mo Kα1 70.93 0.0484 0.0506 0.0390 1.04 3.18(1) 2.0 70.0 0.005

Figure 9. FT-IR (bottom) and Raman (top) spectra of a (NbO)2P4O13 single crystal.

Raman spectrum revealed a very intense absorption band at about 980 cm−1 assignable to the formal NbO double bonds. The intensity of the other bands is almost inconsiderable; however, the absorption bands in both IR and Raman are in accordance with expected vibrations.

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SUMMARY AND CONCLUSIONS Crystalline (NbO)2P4O13 represents a complex superstructure consisting of NbO6 octahedra, including the terminal niobyl oxygen and four distinct [P4O13]6− groups. The more precisely characterized compound crystallizes in the triclinic space group P1̅ and forms a supercell with the volume exceeding by three times the subcell reported previously by others. An unusual 180° P−O−P bond angle in the middle of two [P4O13]6− groups was refined to split oxygen positions. The photoluminescence of the new compound is attributed to transitions within the NbO6 octahedra. In particular, the short NbO niobyl bonds are assumed to be responsible for the luminescence phenomena. Inorganic photoluminescent materials containing no rare earth cations are very uncommon. Consequently, the unique compound (NbO)2P4O13 offers an inviting starting base for the development of innovative photonic materials with lightfastness only attainable by inorganic phosphors.

Figure 7. Powdered product of (NbO)2P4O13 (a) irradiated by visible light and (b) excited by UV radiation (254 nm).

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ASSOCIATED CONTENT

S Supporting Information *

Figure 8. Photoluminescence emission (PL) spectra of the powdered product.

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02891. Detailed information on the single-crystal solution, e.g., bond length, displacement parameters and atomic coordinates. (CIF)

Complementary spectroscopic investigations were conducted by IR and Raman measurements (Figure 9). The asymmetric and symmetric bands of the [P4O13]6− groups as well as the niobyl (NbO) stretching modes were identified in the literature.8 The stretching modes of the [P4O13]6− group can also be interpreted as two (PO3)2− terminal groups and two (PO2)− inner linkages, with P−O−P bridges connecting the single linking moieties.30 In this formalism, the charge of the bridging oxygen atoms has to be counted at half value. Strong overlapping of the bands between the wavenumbers 1320 and 650 cm−1 reflecting the presence of four different [P4O13]6− species can be observed. In particular, the niobyl (NbO) stretching modes are located at wavenumbers around 900 and 1000 cm−1, leading to additional band overlaps.8 The

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hubert Huppertz: 0000-0002-2098-6087 Notes

The authors declare no competing financial interest. E

DOI: 10.1021/acs.inorgchem.6b02891 Inorg. Chem. XXXX, XXX, XXX−XXX

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(18) Nikolaev, V. P.; Sadikov, G. G.; Lavrov, A. V.; Poraj-Koshits, M. A. Crystal structure of niobyl tetraphosphate (NbO)2P4O13. Izv. Akad. Nauk SSSR, Neorg. Mater. 1986, 22, 1364−1368. (19) Sheldrick, G. A short history of SHELX. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122. (20) Gruene, T.; Hahn, H. W.; Luebben, A. V.; Meilleur, F.; Sheldrick, G. M. Refinement of macromolecular structures against neutron data with SHELXL2013. J. Appl. Crystallogr. 2014, 47, 462− 466. (21) Costentin, G.; Leclaire, A.; Borel, M. M.; Grandin, A.; Raveau, B. Determination of the crystal structure of Mo2P4O15. Z. Kristallogr. 1992, 201, 53−58. (22) Costentin, G.; Leclaire, A.; Borel, M. M.; Grandin, A.; Raveau, B. Molybdenum (V) Phosphates: Structural Relationships and classification. Rev. Inorg. Chem. 1993, 13, 77−101. (23) Lister, S. E.; Radosavljevic Evans, I.; Howard, J. A. K.; Coelho, A.; Evans, J. S. O. Mo2P4O15 - the most complex oxide structure solved by single crystal methods? Chem. Commun. 2004, 2540−2541. (24) Lister, S. E.; Evans, I. R.; Evans, J. S. O. Complex Superstructures of Mo2P4O15. Inorg. Chem. 2009, 48, 9271−9281. (25) Khosrovani, N.; Korthuis, V.; Sleight, A. W.; Vogt, T. Unusual 180° P−O−P Bond Angles in ZrP2O7. Inorg. Chem. 1996, 35, 485− 489. (26) Stefanidis, T.; Nord, A. G. Structure studies of thortveitite-like dimanganese diphosphate, Mn2P2O7. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1984, 40, 1995−1999. (27) Birkedal, H.; Krogh Andersen, A. M.; Arakcheeva, A.; Chapuis, G.; Norby, P.; Pattison, P. The Room-Temperature Superstructure of ZrP2O7 Is Orthorhombic: There Are No Unusual 180° P−O−P Bond Angles. Inorg. Chem. 2006, 45, 4346−4351. (28) Thongtem, T.; Kungwankunakorn, S.; Kuntalue, B.; Phuruangrat, A.; Thongtem, S. Luminescence and absorbance of highly crystalline CaMoO4, SrMoO4, CaWO4 and SrWO4 nanoparticles synthesized by co-precipitation method at room temperature. J. Alloys Compd. 2010, 506, 475−481. (29) Xie, W.; Liu, G.; Dong, X.; Wang, J.; Yu, W. Doping Eu3+/Sm3+ into CaWO4:Tm3+, Dy3+ phosphors and their luminescence properties, tunable color and energy transfer. RSC Adv. 2016, 6, 26239−26246. (30) Efimov, A. M. IR fundamental spectra and structure of pyrophosphate glasses along the 2ZnO·P2O5−2Me2O·P2O5 join (Me being Na and Li). J. Non-Cryst. Solids 1997, 209, 209−226.

ACKNOWLEDGMENTS We gratefully acknowledge Dr. Joachim Bastian for the Raman measurements, Stephanie Dirksmeyer for the luminescence measurement, Prof. Dr. Roland Stalder for giving us access to the single-crystal IR spectrometer, the Austrian Research Promotion Agency (FFG), and Durst Phototechnik Digital Technology GmbH, Lienz, Austria, by funding this research.

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REFERENCES

(1) Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. D. Phospho-olivines as positive-electrode materials for rechargeable lithium batteries. J. Electrochem. Soc. 1997, 144, 1188−1194. (2) Tarascon, J.-M.; Armand, M. Issues and challenges facing rechargeable lithium batteries. Nature 2001, 414, 359−367. (3) Armand, M.; Tarascon, J.-M. Building better batteries. Nature 2008, 451, 652−657. (4) Scheetz, B. E.; Agrawal, D. K.; Breval, E.; Roy, R. Sodium zirconium phosphate (NZP) as a host structure for nuclear waste immobilization: A review. Waste Manage. 1994, 14, 489−505. (5) Cramer, A. J.; Cole, J. M. Topological Analysis of Void Space in Phosphate Frameworks: Assessing Storage Properties for the Environmentally Important Guest Molecules and Ions: CO2, H2O, UO2, PuO2, U, Pu, Sr2+, Cs+, CH4, and H2. ACS Sustainable Chem. Eng. 2016, 4, 4094−4112. (6) Huang, Y.; Li, Q.; Anfimova, T. V.; Christensen, E.; Yin, M.; Jensen, J. O.; Bjerrum, N. J.; Xing, W. Indium doped niobium phosphates as intermediate temperature proton conductors. Int. J. Hydrogen Energy 2013, 38, 2464−2470. (7) Huang, Y.; Yu, L.; Li, H.; Wang, C.; Li, Q.; Shuai, Q. Enhanced proton conductivity of niobium phosphates by interfacing crystal grains with an amorphous functional phase. Solid State Ionics 2016, 294, 54−58. (8) Weng, W.; Davies, M.; Whiting, G.; Solsona, B.; Kiely, C. J.; Carley, A. F.; Taylor, S. H. Niobium phosphates as new highly selective catalysts for the oxidative dehydrogenation of ethane. Phys. Chem. Chem. Phys. 2011, 13, 17395−17404. (9) Hazenkamp, M. F.; Blasse, G.; Zah-Letho, J. J.; Varbaere, A. Luminescence of some phosphatoniobates. Eur. J. Solid State Inorg. Chem. 1992, 29, 1263−1268. (10) Oyetola, S.; Verbaere, A.; Guyomard, D.; Crosnier, M.; Piffard, Y.; Tournoux, M., New ZrP2O7-Like Diphosphates of Either Mixed (M (III)1/2 M′(V)1/2) Cations (M: Sb, Bi, Nd, Eu; M′: Sb, Nb, Ta) or M′(V) Cations (M′: Ta, Nb): Synthesis and Structure. ChemInform 1991, 22.10.1002/chin.199117006 (11) Blasse, G.; Piffard, Y.; Struye, L. The luminescence of βZr2(PO4)2SO4 in comparison to that of isomorphous SC2(WO4)3. Chem. Phys. Lett. 1988, 147, 514−516. (12) Zhao, D.; Xie, Z.; Hu, J.-M.; Zhang, H.; Zhang, W.-l.; Yang, S.L.; Cheng, W.-D. Structure determination, electronic and optical properties of NaGe2P3O12 and Cs2GeP4O13. J. Mol. Struct. 2009, 922, 127−134. (13) Wiegel, M.; Blasse, G.; Ouwerkerk, M. Luminescence of potassium lithium niobate compositions. Mater. Res. Bull. 1992, 27, 617−621. (14) Blasse, G.; Bril, A. The Influence of Crystal Structure on the Fluorescence of Oxidic Niobates and Related Compounds. Z. Phys. Chem. 1968, 57, 187−202. (15) Kudo, A.; Sakata, T. Effect of Ion Exchange on Photoluminescence of Layered Niobates K4Nb6O17 and KNb3O8. J. Phys. Chem. 1996, 100, 17323−17326. (16) Blasse, G. The luminescence of closed-shell transition-metal complexes. New developments. In Luminescence and Energy Transfer; Springer Berlin Heidelberg: Berlin, Heidelberg, 1980; pp 1−41. (17) Winkler, J. R.; Gray, H. B. Electronic Structures of Oxo-Metal Ions. In Molecular Electronic Structures of Transition Metal Complexes I; Mingos, D. M. P., Day, P., Dahl, J. P., Eds.; Springer Berlin Heidelberg: Berlin, Heidelberg, 2012; pp 17−28. F

DOI: 10.1021/acs.inorgchem.6b02891 Inorg. Chem. XXXX, XXX, XXX−XXX