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 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.

Pure, lithium- or magnesium-doped ferroelectric single crystals of Ca9Y(VO4)7: cation arrangements and phase transitions

Single crystals of Ca9Y(VO4)7 (1), Ca9Y(VO4)7:Li+ (2) and Ca9Y(VO4)7:Mg2+ (3) were grown by the Czochralski method. Their chemical composition was analyzed by ICP spectroscopy and their crystal structure was examined by single crystal X-ray analysis. The crystals are characterized by trigonal symmetry, space group R3c. Hexagonal unit-cell parameters are as follows: a=10.8552(1) Å, c=38.0373(2) Å, V=3881.65(1) Å3 for 1; a=10.8570(1) Å, c=38.0161(3) Å, V=3880.77(4) Å3 for 2; a=10.8465(1) Å, c=38.0366(2) Å, V=3875.36(3) Å3 for 3. All crystals are characterized by β-Ca3(PO4)2-type structure with statistical distribution of Ca2+ and Y3+ over M1, M2 and M5 sites in different ratios and with completely empty M4-cationsite. The impurity of Mg2+cations in structure 2 has been detected in octahedral M5 site. Ferroelectric phase transitions are evidenced by DSC and SHG. At about 1220 and 1300 K, they demonstrate phase transitions. Upon heating the symmetry of the crystal structure changes according to the scheme R3c→R3̅c→R3̅m and is restored during consequent cooling. The first of them is of ferroelectric and the second of non-ferroelectric nature. Even a small amount of impurities in Ca9Y(VO4)7 structure is accompanied by a noticeable decrease in the temperature of the ferroelectric-paraelectric phase transition.

Influence of lithium and magnesium on the real structure and dielectric properties of Ca9Y(VO4)7 single crystals

The real crystal structure was oxyaminated by complex methods; the influence of impurity and point defects is shown.