A3Sr2P7O21 (A = Rb, Cs): Two Polyphosphates Based on Different

Mar 15, 2017 - Interestingly, Rb3Sr2P7O21 is the first example of two kinds of [PO3]∞ linear chains coexisting in one phosphate structure. However, ...
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A3Sr2P7O21 (A = Rb, Cs): Two Polyphosphates Based on Different Types of P−O Chains and Ring Structures Maierhaba Abudoureheman,†,‡ Shujuan Han,*,† Ying Wang,† Bing-Hua Lei,†,‡ Zhihua Yang,† and Shilie Pan*,† †

Key Laboratory of Functional Materials and Devices for Special Environments, Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Sciences, and Xinjiang Key Laboratory of Electronic Information Materials and Devices, 40-1 South Beijing Road, Urumqi 830011, China ‡ University of the Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: Two polyphosphates containing two types of polymerization of the [PO 4 ] groups, Rb 3 Sr 2 P 7 O 21 and Cs3Sr2P7O21, were grown through a spontaneous nucleation method. Single-crystal X-ray diffraction data were collected in order to determine their structures. Interestingly, Rb3Sr2P7O21 is the first example of two kinds of [PO3]∞ linear chains coexisting in one phosphate structure. However, in the structure of Cs3Sr2P7O21, the isolated [P4O12] ring and the 1D [PO3]∞ chain can be observed, which is also rare in phosphates. After careful structural analysis, the alkali-metal cations have an effect on the polymerization of the [PO4] groups and make Rb3Sr2P7O21 and Cs3Sr2P7O21 crystallize in different space groups. What is more, IR spectra, UV−vis−NIR diffuse reflectance spectroscopy data, and first-principles theoretical calculations were adopted to determine the optical properties and the structure−properties relationship of the compounds.

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Na4Ni5(PO4)2(P2O7)2,31 and Na7M4(PO4)(P2O7)4 (M = Al, Fe, Cr),32 which have excellent sodium ion transportability, contain isolated [PO4] tetrahedra and [P2O7] dimers. In addition, the Pb12[Li2(P2O7)2(P4O13)2](P4O13) compound,33 whose structure contains three types of discrete P−O groups, has been reported by our group, recently. To date, phosphates containing two kinds of infinite [PO3]∞ chains have not been reported. Moreover, the isolated [P4O12] rings and [PO3]∞ chains coexisting in one phosphate structure is also rare. Herein, we report two phosphates A3Sr2P7O21 (A = Rb, Cs), which crystallize in different space groups although they have identical stoichiometries. Rb3Sr2P7O21 is the first example that two kinds of [PO3]∞ linear chains coexist in one phosphate. In the structure of Cs3Sr2P7O21, the isolated [P4O12] ring and the 1D [PO3]∞ chain can be observed, which is also rare in phosphates. Meanwhile, we describe how the “A” cation size affects the framework structure in A3Sr2P7O21 (A = Rb, Cs). In addition, a structural comparation between A3Sr2P7O21 (A = Rb, Cs) and the phosphates containing two types of anionic groups has been discussed. We summarize the effect of the polymerization of P−O units on the framework structures. Further, the reason for the coexistence of two kinds of [PO3]∞ linear chains in Rb3Sr2P7O21 is discussed. Moreover, we also

INTRODUCTION Inorganic phosphates have attracted considerable interest for their potential applications in the area of catalysis, ion-exchange materials, ionic conductors, solid electrolyte batteries, and nonlinear-optical materials.1−12 For example, KH2PO413 and KTiOPO414 are mature nonlinear-optical materials and are capable of laser-frequency conversion; LiCs2PO4,15 Ba5P6O20, etc.16−21 are deep-UV nonlinear optical crystals, which possess a short absorption edge and moderate second-harmonic generating response. The olivine-type LiFePO422 and NASICON-type Li3V2(PO4)323 are excellent cathode materials for rechargeable lithium batteries. Owing to the various anionic groups, as well as polymerization of the [PO4] groups, the phosphates have different performances. According to the amount of interconnected [PO4] groups, anionic groups in phosphates can be divided into orthophosphates (isolated [PO4]), pyrophosphates [P2O7], and polyphosphates (isolated [PnO3n+1], cyclic framework [PnO3n], or linear chain [PO3]∞ for n ≥ 3) groups.24−26 Usually, phosphates include one type of anionic group mentioned above. As a special kind of structure, two or more than two types of isolated P−O groups can be observed in one phosphate. For example, isolated [P2O7] and [P3O10] are included in the structures of KMg6(P2O7)2P3O1027 and (NH4)Cd6(P2O7)2(P3O10).28 The structures of Na 4 M 3 (PO 4 ) 2 (P 2 O 7 ) (M = Mn, Co, Ni, Mg), 29, 30 © 2017 American Chemical Society

Received: December 12, 2016 Published: March 15, 2017 3939

DOI: 10.1021/acs.inorgchem.6b03032 Inorg. Chem. 2017, 56, 3939−3945

Article

Inorganic Chemistry discuss the optical properties and first-principles theoretical calculation results of the compounds.

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Table 1. Crystal Data and Structural Refinement for Rb3Sr2P7O21 and Cs3Sr2P7O21 empirical formula fw cryst syst space group, Z unit cell dimens

EXPERIMENTAL SECTION

Reagents. Rb2CO3, Cs2CO3, Sr(NO3)2, and NH4H2PO4 were used as reagents. All of them are analytical grade and purchased without any treatment. Crystal Growth. Single crystals of Rb3Sr2P7O21 and Cs3Sr2P7O21 were grown through a spontaneous nucleation method. The stoichiometric mixtures of Rb 2 CO 3 /Cs 2 CO 3 , Sr(NO 3 ) 2 , and NH4H2PO4 were put into a Pt crucible, respectively. We heated the mixtures to 850 °C and held them for 2 h at this temperature to homogenize the melt. Then the melt was cooled slowly to 750 °C (1 °C/h), cooled to 650 °C (2 °C/h), and finally cooled to 25 °C (10 °C/h). Solid-State Synthesis. Polycrystalline states of the two crystals were synthesized via conventional high-temperature solid-state reactions with stoichiometric amounts of Rb2CO3/Cs2CO3, Sr(NO3)2, and NH4H2PO4. The mixtures were presintered at 300 °C for 24 h in a muffle furnace. The mixtures were sintered at 700 °C for 72 h with intermittent grindings. Powder X-ray diffraction (XRD) was used to verify phase purity of the two compounds. The XRD data were taken on an automated Bruker D2 X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å) at room temperature. The results indicate that the experimental values are in good agreement with the calculated ones (Figures S1(a) and (b) in the Supporting Information (SI)). Single-Crystal X-ray Diffraction. The crystal structure determination of A3Sr2P7O21 (A = Rb, Cs) was done by single-crystal XRD on an APEX II CCD diffractometer using monochromatic Mo Kα radiation with λ = 0.710 73 Å and integrated with the SAINT program.34 All calculations were performed with programs from the SHELXTL crystallographic program package.35 The structure determination and refinements of Rb3Sr2P7O21 were straightforward (R1 = 0.037, wR2 = 0.070), whereas Cs3Sr2P7O21 shows a little bit higher wR2 value possibly due to a twinning structure as shown in Figure S2. It is known that the R1 and wR2 values can be improved if the twin structure is properly handled. The second domain (twinning) can be indexed as a monoclinic unit cell (almost the same as the original unit cell) by using the CELL_NOW program. The twin operator is a 180° rotation about the b-axis. In order to give a better refinement result, we have tried to grow Cs3Sr2P7O21 crystals and collected single-crystal data several times. Unfortunately, we still cannot obtain a crystal with high quality and detwined structure. No higher symmetries checked by the PLATON program were found.36 Table 1 lists crystal data, details of data collections, and structure refinement information. Table S1 in the SI summarizes equivalent isotropic displacement parameters and atomic coordinates. Table S2 in the SI presents selected bond distances and angles. Optical Properties. The optical diffuse reflectance data of the A3Sr2P7O21 (A = Rb, Cs) powder samples were collected with a Shimadzu SolidSpec-3700 UV−vis−NIR spectrophotometer at room temperature. The IR spectra were collected on a Shimadzu IR Affinity1 Fourier transform IR spectrometer in the range 400−4000 cm−1 at room temperature. The sample was mixed thoroughly with dried KBr. Calculation Details. A plane-wave pseudopotential package based on the density functional theory, CASTEP,37 was employed to execute the ab initio calculations of Rb3Sr2P7O21. During the calculation, geometry optimization was performed using the BFGS minimization technique, and the criterion is that the residual forces on the atoms were less than 0.01 eV/Å, the displacements of atoms were less than 5 × 10−4 Å, and the energy change was less than 5.0 × 10−6 eV/atom. The generalized gradient approximation (GGA) with the Perdew− Burke−Ernzerhof (PBE) functional and the norm-conserving pseudopotential (NCP) were chosen as the exchange−correlation functional and pseudopotential. Only Rb-4s24p65s1 Sr-4s24p65s2, O2s22p4, and P-3s23p3 were considered as the valence electrons. Firstprinciples calculation results were convergent under the conditions of plane wave’s basis whose energy cutoff was 830 eV as well as 4 × 2 × 1 Monkhorst−Pack k-point sampling with a separation of 0.035 Å−1 in

volume (Å3) density (calcd) (mg/m3) θ range for data collection limiting indices reflns collected/ unique completeness to θ (%) goodness of fit on F2 final R indices [Fo2 > 2σ(Fo2)]a R indices (all data)a extinction coeff largest diff peak and hole (e/Å3)

Rb3Sr2P7O21 984.44 monoclinic P21/n, 4 a = 7.6148(7) Å b = 12.4146(11) Å c = 21.862(2) Å β = 90.497(5) 2066.7(3) 3.164

Cs3Sr2P7O21 1126.76 monoclinic C2/c, 4 a = 6.9636(3) Å b = 13.9950(6) Å c = 22.1957(11) Å β = 98.114(3) 2141.44(17) 3.495

1.86−27.49°

1.85−27.46°

−9 ≤ h ≤ 9, − 16 ≤ k ≤ 16, − 28 ≤ l ≤ 20 18 255/4707 [R(int) = 0.0400] 99.4

−9 ≤ h ≤ 8, − 18 ≤ k ≤ 18, − 28 ≤ l ≤ 25 8397/2405 [R(int) = 0.0387] 98.0

1.081 R1 = 0.0370, wR2 = 0.0695 R1 = 0.0554, wR2 = 0.0740 0.00098(8) 1.254 and −1.061

1.048 R1 = 0.0473, wR2 = 0.0977 R1 = 0.0657, wR2 = 0.1070 0.0077(11) 3.219 and −2.166

R1 = ∑||Fo| − |Fc||/∑|Fo|, and wR2 = [∑w(Fo2 − Fc2)2/∑wFo4]1/2 for Fo2 > 2σ(Fo2). a

the Brillouin zone. We kept the default values of the CASTEP code for the other calculation parameters and convergent criteria.

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RESULTS AND DISCUSSION Crystal Structure. Rb3Sr2P7O21 crystallizes in space group P21/n of the monoclinic crystal system. It exhibits a 3D framework that is composed of the [RbO10], [SrO7], and [PO4] polyhedra. The main outstanding feature of Rb3Sr2P7O21 is the coexistence of two kinds of 1D [PO3]∞ chains with different arrangements in the structure, which has not been found in other phosphates. The two 1D [PO3]∞ chains are linked by [SrO7] via sharing O atoms to construct the 3D network, while Rb+ ions are located in the cavities (Figure 1a). The asymmetrical unit includes three Rb, two Sr, seven P, and 21 O ions (Table S1 in the SI). The [PO4] tetrahedron is connected via corner-sharing to form two types of infinite 1D A chain and B chain. The A chain is built up of the three [PO4] tetrahedra that are linked to form a repeat unit running wavily up and down along the b-axis (Figure 1b), while the B chain consists of four [PO4] tetrahedra via sharing a vertex O atom, which runs along the a-axis (Figure 1c). Ten-coordinated Rb atoms form the [RbO10] polyhedra with Rb−O distances varying from 2.835(4) to 3.497(4) Å. The [Rb(1)O10] and [Rb(2)O10] polyhedra are interconnected by edge-sharing or corner-sharing to build a 2D layer extending in the (110) plane (Figure S3 in the SI). Interestingly, the neighboring [Rb(3)O10] polyhedra are cosharing four O atoms to build a 0D [Rb2O16] unit (Figure S4 in the SI). Both Sr(1) and Sr(2) atoms are seven-coordinated with the Sr−O distances 2.432(4)−2.653(4) Å. Four oxygen atoms are from the [PO4] tetrahedra in the B chain, while other three oxygen atoms are from the A chain. The Sr(1), Sr(2), and 11 [PO4] 3940

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Figure 3. (a) Anionic groups arrangement of Cs3Sr2P7O21. (b) Structure of [PO3]∞ chains. (c) [P4O12] rings.

Figure 1. (a) 3D structure of Rb3Sr2P7O21. The structure of 1D [PO3]∞ chains: (b) A chains; (c) B chains.

tetrahedra were bonded via O atoms to give a [Sr2P11O14] cluster (Figure 2).

Figure 4. 3D structure of Cs3Sr2P7O21 and the [SrP7O26] clusters.

in the range of 3.079(1)−3.630(8) Å, respectively. The [Cs(1)O10] and [Cs(2)O10] polyhedra are connected via three O atoms (face sharing) to give a [Cs2O17] dimer, which are interconnected by sharing one vertex to give a 1D chain (Figure S5 in the SI). The Sr(1) atom is seven-coordinated with the Sr−O bond lengths ranging from 2.402(7) to 2.722(8) Å, in which three of the O atoms are from the [PO4] tetrahedra in chains, and other four O atoms are from the [P4O12] rings. The [SrO7] polyhedra and seven [PO4] tetrahedra are bonded to form the [SrP7O26] cluster (Figure 4). Structure Comparison among the Phosphates with Two Types of Polymerization of the [PO4] Groups. As shown in Figure 5, different types of P−O groups, rings, or chains can be found based on the publications and Inorganic Crystal Structure Database (ICSD). Phosphates including two kinds of isolated P−O groups are shown in Table S3 in the SI. Based on their containing isolated anionic groups, the listed phosphates can be summarized as five classes: (1) compounds with the [PO4] tetrahedra and the [P2O7] dimers,29−32,38−53 (2) compounds with the [P2O7] dimers and the [P3O10] trimers,27,28,54 (3) compounds with the [PO4] tetrahedra and the [P3O10] trimers,55,56 (4) compounds with the [P2O7] dimers and the [PO3]∞ chains,57,58 (5) compounds with the [P4O12] rings and the [PO3]∞ chains.59 It is worth noting that the Rb3Sr2P7O21 compound can be attributed to another class, which contains two different kinds of [PO3]∞ chains. To further understand the stability of those compounds, the electron localization function map of Rb3Sr2P7O21 has been

Figure 2. Structure of the [Sr2P11O14] cluster and the 3D network of Sr−P−O.

Differently, Cs3Sr2P7O21 belongs to the monoclinic system with space group C2/c (no. 15). It has a 3D network that is constructed by [CsO10], [SrO7], and [PO4] polyhedra. Interestingly, it contains a linear infinite 1D [PO3]∞ chain and a crystallographically independent [P4O12] ring, which is rare in phosphates (Figure 3a). In the asymmetric unit of Cs3Sr2P7O21, there are two crystallographically independent Cs, one Sr, four P, and nine O. The O(5) and O(10) atoms are disordered and split into O(5A), O(5B), O(10A), and O(10B) positions. The rotations of the [PO4] tetrahedra result in the split of the O atoms. The 1D chain is formed by the [P(1)O4] and [P(2)O4] tetrahedra (Figure 3b), while two [P(3)O4] tetrahedra and two [P(4)O4] tetrahedra are corner-connected by vertex oxygen atoms to construct independent [P4O12] rings (Figure 3c). The 1D chain and isolated [P4O12] rings are bridged by the [SrO7] polyhedra constructing the 3D network, and the Cs+ ions are located in cavities (Figure 4). The P−O distances range from 1.441(7) to 1.626(2) (1.695(2)) Å, while the 10-coordinated Cs−O distances are 3941

DOI: 10.1021/acs.inorgchem.6b03032 Inorg. Chem. 2017, 56, 3939−3945

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anions can result in less resistance of interconnections among the groups in the 3D framework.61 Further, the arrangement of different groups decreases the conflict with Pauling’s fifth rule.62 Those features may be the reason for the two kinds of P−O groups coexisting in the structure. Effect of Cation Sizes. Interestingly, although the two stoichiometrically equivalent metal phosphates, Rb3Sr2P7O21 and Cs3Sr2P7O21, consist of similar structural feature groups, such as [SrO7] and [PO4], they exhibit different structures. As mentioned above, in the structure of Rb3Sr2P7O21, the [PO4] tetrahedra are linked via sharing O to form two kinds of [PO3]∞ chains, which are different from that observed in Cs3Sr2P7O21 (Figures 1 and 3). In the structure of Cs3Sr2P7O21, the anionic groups contain an isolated [P4O12] ring and a 1D [PO3]∞ chain. The different sizes of the alkali-metal cations lead to the dissimilar arrangement of the anionic groups. The bond distances of Rb−O vary from 2.835(4) to 3.497(4) Å for Rb3Sr2P7O21, while Cs−O distances are 3.079(1)−3.630(8) Å for Cs3Sr2P7O21. On comparison, the larger Cs+ cations are unable to lie in the spaces filled by Rb+ ions in the structure of Rb3Sr2P7O21. In other words, when the larger Cs+ ions are placed in the 3D framework of Rb3Sr2P7O21, a structural strain may be produced in the structure. In order to reduce this strain, one kind of 1D [PO3]∞ chain in Rb3Sr2P7O21 is changed to [P4O12] rings to accommodate the larger Cs+ cations. In addition, the cation sizes of Rb and Cs also affect the P−P−P angles between neighboring [PO4] tetrahedra. As shown in Figures 1 and 3, the P−P−P angles vary from 105.1° to 151.4° for the A chain and from 104.5° to 122.4° for the B chain in Rb3Sr2P7O21, while from 86.9° to 133.2° for the chains and from 83.5° to 96.5° for rings in Cs3Sr2P7O21. In addition, Cs3Sr2P7O21 and Cs3Pb2P7O2159 are also stoichiometrically equivalent phosphates, and both of them contain [PO3]∞ chains and [P4O12] rings in the structure. However, Cs3Sr2P7O21 and Cs3Pb2P7O21 belong to different space groups: Cs3Sr2P7O21 belongs to the space group C2/c (no. 15), while Cs3Pb2P7O21 crystallizes in P1̅ (no. 2). Interestingly, the asymmetric unit of Cs3Pb2P7O21 consists of three Cs, two Pb, seven P, and 21 O (ICSD no. 60503). Cs3Pb2P7O21 shows the 3D framework, which includes the [CsO10], [PbO7], and [PO4] polyhedra (Figure S6 in the SI). In Cs3Pb2P7O21, the Cs atoms are 10-coordinated, while the Pb atoms are seven-coordinated and the [PO4] tetrahedra form 1D [PO3]∞ chains and [P4O12] rings. Four unique [PO4] tetrahedra are interconnected by edge-sharing to form two different rings (Figure S6 in the SI), which is different from the ring structure in Cs3Sr2P7O21. Further, the P−P−P angles are in the range of 80.8−99.2° for the rings and 83.1−139.2° for the chains in the structure of Cs3Pb2P7O21. The different number of atoms in the asymmetric units and the different symmetry actions in the structures between Cs3Sr2P7O21 and Cs3Pb2P7O21 may be the reasons for unsimilar ring structures as well as the values of bond angles. Moreover, the substitution of Pb2+ by Sr2+ results in the difference of bond distances and angles of the [PO4] tetrahedra. Optical Properties. Figure S7 in the SI shows the UV− vis−NIR diffuse reflectance spectra of the two compounds. It illustrates that the UV cutoff edges of A3Sr2P7O21 (A = Rb, Cs) are below 260 nm. Figure S8 and Table S4 in the SI show the IR spectra and assignment of the absorption peaks observed in the IR spectra for Rb3Sr2P7O21 and Cs3Sr2P7O21. The peaks obtained at 1322, 1299 and 1272 cm−1 for Rb3Sr2P7O21 and 1312, 1286, and 1266 cm−1 for Cs3Sr2P7O21 belong to the

Figure 5. Isolated P−O structures including groups, rings, and [PO3]∞ chains.

given as a representative. From the illustration in Figure 6, it is clear that the charges are spherically distributed at the Sr atoms,

Figure 6. Electron localization function map of Rb3Sr2P7O21.

which is characteristic for systems having ionic interactions. Those Sr−O bonds act as a bridge to connect the A and B [PO3]∞ chains. Further, we calculated the electrostatic force between the Sr−O bonds based on Coulomb’s law (in the SI).60 On the other hand, in the structure of Rb3Sr2P7O21, the only difference between two types of chains is the P−P−P angles and number of cycling [PO4] units. Similar polyphosphate 3942

DOI: 10.1021/acs.inorgchem.6b03032 Inorg. Chem. 2017, 56, 3939−3945

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stretching vibrations of νas(O−P−O).63 The peaks observed around 1175, 1146, 1120, 1098, 1041, and 1010 cm−1 for Rb3Sr2P7O21 and 1162, 1125, 1096, and 1046 cm−1 for Cs3Sr2P7O21 are deformation vibrations of the νs(O−P−O).19 The peaks around 982, 919, and 864 cm−1 for Rb3Sr2P7O21 and 984 and 853 cm−1 for Cs3Sr2P7O21 can be assigned to νas(P− O−P) and νs(P−O−P).64 The peaks obtained at 768, 741, and 706 cm−1 for Rb3Sr2P7O21 and 752, 728, and 703 cm−1 for Cs3Sr2P7O21 are attributed to νs(P−O−P).65 Several peaks at about 605, 576, and 550 cm−1 for Rb3Sr2P7O21 and 579, 538, and 515 cm−1 for Cs3Sr2P7O21 belong to δas(O−P−O), while the peaks at 502, 458, and 437 cm−1 for Rb3Sr2P7O21 and 464 cm−1 for Cs3Sr2P7O21 can be assigned to δs(O−P−O).21,64 Band Structures and Density of States. The band structure of Rb3Sr2P7O21 is shown in Figure 7, which indicates

CONCLUSIONS Two polyphosphates, A3Sr2P7O21 (A = Rb, Cs), have been successfully obtained. Rb3Sr2P7O21 shows an unusual structure that is composed of two kinds of 1D [PO3]∞ chains, which is the first report of such kind of structure. For Cs3Sr2P7O21, its structure contains a linear infinite 1D [PO3]∞ chain and a crystallographically independent [P4O12] ring, which is also rare in phosphates. According to the structural analysis, the cations in A3Sr2P7O21 (A = Rb, Cs) make a vital contribution to the overall structure, as they determine the polymerization of the [PO4] groups. UV−vis−NIR diffuse reflectance spectra results show that their UV cutoff edges are below 260 nm. The theoretical calculation suggests that Rb3Sr2P7O21 has a direct band gap of 4.97 eV, which is very close to the corresponding experimental value.

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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.6b03032. Atomic coordinates and equivalent isotropic displacement parameters; selected bond lengths and angles; Xray powder diffraction patterns; the connection of the Rb(1)O10 and Rb(2)O10 polyhedra, the [Rb2O16] unit in Rb3Sr2P7O21, and the [Cs2O17] dimer in Cs3Sr2P7O21; the 3D structure of Cs3Pb2P7O21; UV−vis−NIR diffuse reflectance spectra; IR spectra (PDF) Crystallographic data for Rb3Sr2P7O21 (CCDC no. 1502573) (CIF) Crystallographic data for Cs3Sr2P7O21 (CCDC no. 1502572) (CIF)

Figure 7. Band structure of Rb3Sr2P7O21.

that Rb3Sr2P7O21 has a direct band gap of 4.97 eV and the valence-band maximum (VBM) and conduction-band minimum (CBM) are located the same k point (G). A little underestimate of the band gap compared with the experimental result is because of the exchange−correlation energy used in the calculation.66,67 The partial density of states is shown in Figure 8. At the top of the VBM (−6 to 0 eV), the main

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.-J. Han). *E-mail: [email protected] (S.-L. Pan). ORCID

Ying Wang: 0000-0001-6642-543X Shilie Pan: 0000-0003-4521-4507 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Urumqi Science and Technology Plan (Grant No. P151010004), the National Key Research Project (Grant No. SQ2016YFGX070132), and the Science and Technology Project of Urumqi (Grant No. P141010002).

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REFERENCES

(1) Halasyamani, P. S.; Poeppelmeier, K. R. Noncentrosymmetric Oxides. Chem. Mater. 1998, 10, 2753−2769. (2) Bang, S.; Lee, D. W.; Ok, K. M. Variable Framework Structures and Centricities in Alkali Metal Yttrium Selenites, AY(SeO3)2 (A = Na, K, Rb, and Cs). Inorg. Chem. 2014, 53, 4756−4762. (3) Hubert, H.; Stefanie, A. H.; Carmen, E. Z.; Stefan, L.; Oliver, O.; Ralf, R.; Isabel, K. High-Pressure Synthesis, Electron Energy-Loss Spectroscopy Investigations, and Single Crystal Structure Determination of a Spinel-Type Gallium Oxonitride Ga2.79□0.21(O3.05N0.76□0.19). Chem. Mater. 2009, 21, 2101−2107.

Figure 8. Partial density of states of Rb3Sr2P7O21.

occupants are O-p, P-p, and few P-s orbitals, which come from the sp3 hybridization of the [PO4] tetrahedra and the nonbonding orbital of oxygen, while the bottom of the CBM is mainly occupied by the Rb-s and Rb-p orbitals. Therefore, the interactions of the [PO4] tetrahedra as well as rubidium determine the band gap of Rb3Sr2P7O21. 3943

DOI: 10.1021/acs.inorgchem.6b03032 Inorg. Chem. 2017, 56, 3939−3945

Article

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.6b03032 Inorg. Chem. 2017, 56, 3939−3945

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DOI: 10.1021/acs.inorgchem.6b03032 Inorg. Chem. 2017, 56, 3939−3945