Sr 9 In(VO 4 ) 7 as a model ferroelectric in the structural family of β-Ca 3 (PO 4 ) 2 -type phosphates and vanadates.

Sr 9 In(VO 4 ) 7 was prepared by a solid-state method at 1270 K in air. This vanadate has the β-Ca 3 (PO 4 ) 2 -type structure and crystallizes in polar space group R 3 c . The structural parameters of Sr 9 In(VO 4 ) 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. In 3+ cations occupy the octahedral M5 site, Sr 2+ cations occupy the M1, M2, and M3 sites of the β-Ca 3 (PO 4 ) 2 -type structure, and the M4 site remains vacant. Sr 9 In(VO 4 ) 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 Ca 9 In(PO 4 ) 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 (Sr 2+ or Ca 2+ ) that is only possible when intense rotational vibrations of the tetrahedra are excited at high temperatures. The lower Curie temperature in Ca 9 In(PO 4 ) 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 Sr 9 In(VO 4 ) 7 .