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

Abstract

Sr₉In(VO₄)₇ was prepared by a solid-state method at 1270 K in air. This vanadate has the β-Ca₃(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) Å and c = 39.6170(3) Å with Z = 6. In³⁺ cations occupy the octahedral M5 site, Sr²⁺ cations occupy the M1, M2, and M3 sites of the β-Ca₃(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 E2O₄, E3O₄ tetrahedra, while half of the E1O₄ 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 E1O₄ 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 P1O₄ tetrahedron required to reverse this tetrahedra at T_c in comparison with V1O₄ in Sr₉In(VO₄)₇.