552 Russian Chemical Bulletin, International Edition, Vol. 57, No. 3, pp. 552—556, March, 2008 Synthesis and crystal structure of new titanyl phosphate Sr2TiO(PO4)2 R. V. Shpanchenko,a A. A. Tsirlin,a J. Hadermann,b and E. V. Antipova aDepartment of Chemistry, M. V. Lomonosov Moscow State University, 1 Leninskie Gory, 119992 Moscow, Russian Federation. Fax: +7 (495) 939 4788. Email: shpanchenko@icr.chem.msu.ru; bElectron Microscopy for Materials Science (EMAT), University of Antwerp, 171 Groenenborgerlaan, 2020 Antwerp, Belgium New strontium titanyl phosphate Sr2TiO(PO4)2 (1) was synthesized and characterized by Xray powder diffraction, electron diffraction, highresolution electron microscopy, and band structure calculations. Titanyl phosphate 1 is isostructural with vanadyl phosphate Sr2VO(PO4)2 and has a layered structure. The titanium atoms are shifted from the centers of the TiO6 octahedra and form short (1.74 Å) titanyl bonds. The structure of 1 is an unusual example of the disordered orientation of the chains formed by TiO6 octahedra in complex titanium phosphates. Key words: strontium titanyl phosphate, complex transition metal phosphates, Xray diffraction study, electron diffraction, highresolution electron microscopy. Titanium phosphates have attracted attention as prom ising ferroelectric materials, ionic conductors,1—4 non linear optical materials,5,6 catalysts,7 materials with low thermal expansion,8 and materials for lithium batteries.9 Most of compounds known to date belong to sim ple phosphates or alkali metalcontaining complex phosphates. Data on complex titanium phosphates with divalent cations are scarce. For example, only com pounds having the NASICON structure M0.5Ti2(PO4)3 (M = Mg, Ca, of Sr) were characterized in detail in the MII—TiIV—P–O systems.10,11 One of the possible approaches to a search for new titanium phosphates is based on the structural similarity of titanium and vanadium compounds. Both tetravalent vanadium and tetravalent titanium are characterized by the distorted octahedral environment with a short me tal—oxygen bond. Many complex TiIV and VIV phosphates are isostructural. For instance, LiMPO5 (see Refs 12 and 13) and Na4MO(PO4)2 (M = Ti or V)14,15 may be men tioned as examples. However, in many compounds VIV has coordination number 5,16 which is very rarely ob served in titanium oxides. Therefore, certain TiIV com pounds differ substantially from their vanadium ana logs. For example, MgTi2O5 (see Ref. 17) and MgV2O5 (see Ref. 18) have different structures. The compositions of many vanadium phosphates can be described by the general formula A2VO(PO4)2 (A is a divalent cation). Such compounds, though having dif ferent structures, are typical of several A cations. Va nadyl phosphates Zn 2VO(PO4) 2,19 Sr 2VO(PO 4) 2,20 Pb2VO(PO4)2,21 and Ba2VO(PO4)2 (Ref. 22) can be cited as examples. Tetrahedral phosphate groups can be re placed by vanadate or arsenate groups, these replace ments being generally accompanied by a considerable transformation of the crystal structure.23,24 Strontium is the only А cation that allows the retention of the structure upon changes in the anion sublattice. Thus, all three compounds Sr2VO(XO4)2 (X = P, As, or VV) have simi lar layered structures (Fig. 1).20,22,25 Therefore, only the Sr2VO(XO4)2 structural type is resistant to changes in the anion sublattice. This structure would also be expect ed to remain unchanged upon changes in the cation sub lattice (replacement of vanadium by titanium). In the present study, we synthesized and structu rally characterized new strontium titanyl phosphate Sr2TiO(PO4)2. a b c a Fig. 1. Crystal structure of Sr2TiO(PO4)2 projected along the b and c axes. Strontium atoms are shown by circles. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 540—544, March, 2008. 10665285/08/5703552 © 2008 Springer Science+Business Media, Inc. New titanyl phosphate Sr2TiO(PO4)2 Russ.Chem.Bull., Int.Ed., Vol. 57, No. 3, March, 2008 Experimental A singlephase polycrystalline sample of Sr2TiO(PO4)2 was prepared by annealing a stoichiometric mixture of Sr2P2O7 and TiO2 in a corundum crucible in air at 900 °C for three days with intermediate regrindings. Strontium pyrophosphate Sr2P2O7 was prepared by annealing a stoichiometric mixture of SrCO3 and NH4H2PO4 in air at 800 °C for 48 h. All reagents were of at least analytical grade ("pure for analysis"). Xray diffraction (XRD) data were collected on a Stoe STADI/P diffractometer (CuKα1 radiation, germanium monochromator, lin ear positionsensitive detector, transmission mode). The XRD pat terns were processed and indexed using the WinXPow program package. The fullprofile refinement of the crystal structure was carried out with the use of the GSAS software.26 Principal crystallo graphic parameters and the Xray data collection and refinement statistics for Sr2TiO(PO4)2 are given in Table 1. The atomic coordi nates were deposited in the Inorganic Crystal Structure Database (ICSD, code 419295). Electron diffraction (ED) and highresolution electron micros copy (HREM) studies were performed on a JEOL 4000EX trans mission electron microscope. The HREM image simulation was carried out using the MacTempas software.27 The electronic band structure calculations were performed with the use of the FPLO program2 implementing the DFT method for crystalline solids with the use of the localorbital basis. The PW92 exchangecorrelation potential29 and the partition of the first Bril louin zone into 768 k points (432 points in the symmetryirreduc ible part) were used in the calculations. The convergence of the total energy with respect to the number of k points was checked. Results and Discussion The XRD pattern of Sr2TiO(PO4)2 was indexed in the monoclinic system with the unit cell parameters Table 1. Principal crystallographic parameters and the Xray diffrac tion data collection and refinement statistics for Sr2TiO(PO4)2 Parameter Characteristics Molecular formula Space group a/Å b/Å c/Å β/deg Z V/Å3 ρcalc/g cm–3 Diffractometer Т/°C Radiation λ/Å 2θ range/deg Scan step/deg wRp, Rp, RF2 Sr2TiO(PO4)2 I2/a (No. 15) 6.75893(6) 15.8589(1) 7.03130(6) 115.5369(3) 4 680.05(2) 4.191 STOE STADI/P 20 CuKα1 1.5406 9 ≤ 2θ ≤ 105 0.01 0.040, 0.030, 0.023 1.48 1.22 63 χ2 GOOF Number of refinement parameters 553 a = 6.75893(6) Å, b = 15.8589(1) Å, c = 7.03130(6) Å, β = 15.5369(3)°. The XRD pattern and the fact that the unit cell parameters of Sr2VO(PO4)2 (a = 6.744(4) Å, b = 15.866(4) Å, c = 7.032(2) Å, β = 115.41(2)°, I2/a)20 are similar to those of Sr2TiO(PO4)2 indicate that stronti um titanyl phosphate is isostructural with strontium vana dyl phosphate. The systematic absences hkl, h + k + l = 2n, and h0l, h = 2n, were confirmed by the electron diffrac tion patterns (see below) corresponding to the space groups Ia or I2/a. Hence, the structural model of Sr2TiO(PO4)2 was built based on the atomic coordinates for the Sr2VO(PO4)2 structure. The Sr2VO(PO4)2 structure is characterized by the presence of vanadium atoms in the general position (8f) with an occupancy of 0.5, thus implying the statisti cal distribution of vanadium atoms on either side of the equatorial plane of the octahedron. This arrangement is possible due to the tendency of vanadium atoms to form short vanadyl bonds with oxygen. Titanium is also characterized by the formation of a short (titanyl) bond with oxygen. However, in some compounds this bond is not formed. For example, the TiO6 octahedra in Na4TiO(PO4)2 are almost regular;14 whereas, by contrast, the distorted VO6 octahedra with the vanadyl bond were found in isostructural Na4VO(PO4)2.15 These facts sug gest two modes of arrangement of the titanium atoms in the Sr2TiO(PO4)2 structure. First, the titanium atom can occupy the special position 4d in the center of the octahedron. Second, the titanium atom can be shift ed from the center of the octahedron. In the later case, the titanium atoms can be either statistically disordered on either side of the equatorial plane of the octahedron (general position 8f with an occupancy of 0.5; space group I2/a) or arranged in an ordered fashion (space group Ia). The refinement in the space group Ia resulted in a strong distortion of the PO4 tetrahedra (d(P—O) = 1.4—1.7 Å). The refinement in the space group I2/a gave rise to the almost regular tetrahedra (d(P—O) = 1.53—1.58 Å), which is more typical of titanium phos phates.12 In addition, the reliability factor was somewhat lower in the space group I2/a (RF2 = 0.023 for I2/a and RF2 = 0.027 for Ia). Therefore, we conclusively decided that the latter space group is more realistic. The crystal structure refinement of Sr2TiO(PO4)2 in the space group I2/a was performed with titanium atoms placed either in the general position 8f with an occupancy of 0.5 or in the special position 4d with an occupancy of 1. Both refinements converged to the virtually equal R fac tor (RF2 = 0.023 and RF2 = 0.024, respectively). Hence, we failed to unambiguously locate the titanium atoms based on the Xray powder diffraction data. Nevertheless, a comparison of the total energies for the corresponding structural models (see below) indicates that the titanium atom is shifted from the center of the octahedron. There 554 Russ.Chem.Bull., Int.Ed., Vol. 57, No. 3, March, 2008 Shpanchenko et al. Table 2. Selected interatomic distances (d) in the Sr2TiO(PO4)2 structure 1.74Å Distance d/Å Ti—O(1) Ti—O(1) Ti—O(3) Ti—O(3) Ti—O(5) Ti—O(5) Ti—Ti P(1)—O(2) P(1)—O(5) P(2)—O(3) 1.747(4) 2.042(5) 2.022(6) 2.000(5) 2.010(6) 2.032(6) 0.295(9) 1.534(3) 1.563(3) 1.583(2) Distance d/Å P(2)—O(4) Sr(1)—O(2) Sr(1)—O(3) Sr(1)—O(4) Sr(1)—O(5) Sr(2)—O(1) Sr(2)—O(2) Sr(2)—O(2) Sr(2)—O(3) Sr(2)—O(4) 1.532(3) 2.539(2) 2.483(3) 2.664(3) 2.724(3) 2.577(4) 2.653(3) 2.680(3) 2.859(2) 2.630(3) fore, the final structure refinement was carried out with titanium atoms located in the general position. All atomic displacement parameters were refined isotropically. For the oxygen atoms, the common displacement parameter was used. Selected interatomic distances in the Sr2TiO(PO4)2 structure are given in Table 2. The experi mental, calculated, and difference XRD patterns are shown in Fig. 2. Strontium titanyl phosphate Sr2TiO(PO4)2 is isos tructural with strontium vanadyl phosphate Sr2VO(PO4)2. The coordination polyhedra of the titanium atoms can be described as distorted octahedra with short titanyl bonds. The TiO 6 octahedra share vertices to form chains, the titanyl bonds being aligned along the chain axes. The chains are linked to each other by the P(1)O4 tetrahe dra. The P(2)O4 tetrahedra additionally link the octahe dra within the chains. The strontium cations are located between the layers (see Fig. 1). The TiO6 octahedra are characterized by one short (1.747(4) Å) and five longer (2.000(5)—2.032(6) Å) Ti—O distances (Fig. 3). The titanyl bond is 0.1 Å longer I 10–3 (rel. units) 20 10 0 20 40 60 80 2θ/deg Fig. 2. Experimental, calculated, and difference XRD patterns of Sr2TiO(PO4)2. 2.04Å Ti Sr(1) Sr(2) Fig. 3. Coordination polyhedra of the Ti, Sr(1), and Sr(2) atoms in the Sr2TiO(PO4)2 structure. than the vanadyl bond in Sr2VO(PO4)2, which is also typical of other titanium compounds.12 The Ti—O bond in the trans position (Ti—O(1)) of the octahedron is virtually equal in length to the equatorial bonds, whereas the distorted V+4O6 octahedra always contain the long bond in the trans position (2.1—2.3 Å).16 The calculated bond valence sum30 for the titanium atom is 4.07, which is close to the expected value (+4). The PO 4 tetrahedra in the Sr 2TiO(PO 4) 2 struc ture are virtually regular. The P—O distances are 1.532(3)—1.583(2) Å. The bond valence sum calcula tions gave 4.92 and 4.72 for the P(1) and P(2) atoms, respectively. The strontium atoms in the Sr(1) and Sr(2) sites are coordinated by eight and nine oxygen atoms, respectively, which form complexshaped polyhedra (see Fig. 3). The bond valence sums are 2.20 and 1.97 for Sr(1) and Sr(2), respectively. The electron diffraction patterns for Sr2TiO(PO4)2 (Fig. 4) were indexed based on the unit cell parameters determined from the XRD data. The systematic absences hkl, h + k + l = 2n, unambiguously determined a body centered unit cell. The absences h0l, h = 2n, are also consistent with the XRD data and confirm the space group I2/a. The presence of reflections h0l with odd h indices in the [101] zone can be attributed to double diffraction, because these reflections disappear upon slight rotation of the crystallite. The HREM image is presented in Fig. 5. A good agreement between the simulated and experimen tal HREM images confirms the correctness of the struc ture refinement of Sr2TiO(PO4)2. The electronic structure of Sr2TiO(PO4)2 was cal culated for two ordered structural models. In one model, the titanium atom is located in the center of the octa hedron, whereas this atom is shifted from the equatori al plane in another model (positions 4d and 8f, respec tively, in the space group I2/a). The calculations for both models gave similar results. However, the second model corresponds to the lower total energy of the system (ener gy difference E I — EII = 0.059 eV, i.e., ~700 K). In addition, the calculation for the second model gave a larger energy gap (Eg = 2.68 eV as opposed to Eg = 2.32 eV for the first model), which is in better agreement with the observed yellow color of Sr2TiO(PO4)2. Therefore, New titanyl phosphate Sr2TiO(PO4)2 Russ.Chem.Bull., Int.Ed., Vol. 57, No. 3, March, 2008 [100]* 020 002 [001]* 020 555 [101]* 020 200 202 Fig. 4. Electron diffraction patterns of Sr2TiO(PO4)2. a b [001]* Fig. 5. HREM image of Sr2TiO(PO4)2. The black rectangle repre sents the simulated image; the small white rectangle, the unit cell. the electronic structure calculations confirmed the shift of the titanium atom from the center of the TiO6 octahe dron and the formation of the titanyl bond. The disorder of titanium atoms is an unusual feature of the crystal structures of strontium titanyl phosphates. This can be interpreted as disordered orientations of the titanyl bonds. However, all titanyl bonds within a single chain should have the same direction; otherwise, some oxygen atoms would be involved in two short Ti—O bonds (1.747(4) Å). At the same time, the correlation between the direction of the titanyl bonds in the adjacent chains is rather weak, resulting in the experimentally observed disorder of the titanium atoms in the TiO6 octahedra. These qualitative considerations can be supported by quantitative estimates* and are indicative of the disordered orientation of the chains formed by the TiO6 octahedra (rather than the disordered orienations of the titanyl bonds) in Sr2TiO(PO4)2. * R. V. Shpanchenko, A. A. Tsirlin, E. V. Antipov, J. Hadermann, Order and Disorder in Sr2VO(XO4)2 (X = V, P) phases, J. Solid State Chemistry, 2008, in press. In several known structures of titanium phosphates (for example, in LiTiPO512), the distorted TiO6 octahedra share vertices to form chains, all chains being in the same orientation. The Sr2TiO(PO4)2 structure is an unusual example of the disordered orientation of the chains formed by octahedra in complex titanium phosphates. It should be noted that the analogous situation is often observed in vanadium compounds (for example, in (Sr2VO(XO 4)2,20,22,25 Pb2VOP 2O 7,31 and Cs2V3P4O17 (see Ref. 32)). The V—V distances between two positions in the octahedra are in the range of 0.45—0.50 Å. The titanyl bonds are longer than the vanadyl bonds. Hence, the Ti—Ti distance in Sr2TiO(PO4)2 is as short as 0.295(9) Å. In turn, the shift of the titanium atom from the center of the octahedron results in the disordered orientation of the chains formed by octahedra, which is characteristic of the Sr2VO(PO4)2 structural type. It should be noted that in complex titanium phos phates, the disordered orientation of the chains formed by TiO6 octahedra was observed for compounds with the lomonosovite structure;33 the Ti—Ti distance is 0.4 Å. In these compounds, the TiO6 octahedra are similar to the VO6 octahedra in Sr2VO(XO4)2, i.e., they contain one short (1.74 Å), four equatorial (2.00—2.05 Å), and one long (2.12 Å) Ti—O bonds. In Sr2TiO(PO4)2, the TiO6 octahedra are less distorted (one short and five longer Ti—O bonds), but the tendency toward disordering persists. 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