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) ų for 1; a=10.8570(1) Å, c=38.0161(3) Å, V=3880.77(4) ų for 2; a=10.8465(1) Å, c=38.0366(2) Å, V=3875.36(3) ų 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→R3c→R3m 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.