Sr9In(VO4)7 as a model ferroelectric in the structural family of β-Ca3(PO4)2-type phosphates and vanadates

Sr9In(VO4)(7) was prepared by a solid-state method at 1270 K in air. This vanadate has the beta-Ca-3(PO4)(2)-type structure and crystallizes in polar space group R3c. The structural parameters of Sr9In(VO4)(7) were refined by the Rietveld method from laboratory powder X-ray diffraction data (XRD): the lattice parameters are a = 11.18016(9) angstrom and c = 39.6170(3) angstrom with Z = 6. In3+ cations occupy the octahedral M5 site, Sr2+ cations occupy the M1, M2, and M3 sites of the beta-Ca-3(PO4)(2)-type structure, and the M4 site remains vacant. Sr9In(VO4)(7) was characterized by differential thermal analysis (DTA), optical second-harmonic generation (SHG), high-temperature XRD, and dielectric measurements. All these methods prove the existence of a ferroelectric-paraelectric phase transition at T-c = 974 K. This transition is compared with a similar transition in Ca9In(PO4)(7) with lower T-c = 902 K. The polar-to-centrosymmetric phase transition in such compounds has a quite unique mechanism of the order-disorder type. The structural transition involves slight shifts of the M1, M2, M3 cations and the E2O(4), E3O(4) tetrahedra, while half of the E1O(4) tetrahedra (E = P or V) statistically reverse their orientation along the three-fold axis, so that the centre of symmetry appears in the structure as a whole. To invert the E1O(4) tetrahedron, one oxygen anion should pass a large neighbouring cation (Sr2+ or Ca2+) that is only possible when intense rotational vibrations of the tetrahedra are excited at high temperatures. The lower Curie temperature in Ca9In(PO4)(7) corresponds to the smaller rotational vibration amplitude of the P1O(4) tetrahedron required to reverse this tetrahedra at T-c in comparison with V1O(4) in Sr9In(VO4)(7).