Syntheses and characterization of noncentrosymmetric layered phosphates, Sr3Nb2P2O13 and Sr3Ta2P2O13

Two new isostructural quaternary phosphates, Sr3Nb2P2O13 and Sr3Ta2P2O13, were synthesized by solid-state reactions and characterized by X-ray diffraction, thermal and spectroscopic techniques. Sr3Nb2P2O13 compound, as determined by single crystal X-ray diffraction method, crystallizes in noncentrosymmetric P212121 space group with orthorhombic unit cell parameter values of a = 5.2713 (1) angstrom, b = 10.6570 (2) and c = 19.7155 (4) angstrom and has a unique layered structure consisting of Sr2+ and discrete PO43- ions between anionic [Nb2PO9]3framework layers. The noncentrosymmetric structure is attributed to acentric coordination polyhedra, namely, distorted NbO6 octahedron and PO43- tetrahedron. The isostructural nature of Sr3Ta2P2O13 compound was deduced from powder X-ray diffraction data. Sr3Nb2P2O13 and Sr3Ta2P2O13 compounds, respectively, melt and decompose at -1200 degrees C and have band gap values of 3.86 and 3.64 eV.