Ferroelectric crystal Ca9Yb(VO4)7 in the series of Ca9R(VO4)7 non-linear optical materials (R = REE, Bi, Y)

Abstract

The crystal structure, thermal, dielectric and second harmonic generation (SHG), and nonlinear optical activity data for whitlockite-type Ca₉Yb(VO₄)₇ single crystals were obtained on one and the same sample produced by means of the Czochralski method. The crystal structure refinement has revealed that Yb³⁺ cations substitute for Ca²⁺ ions only in the M1, M2 and M5 positions of the whitlockite-type structure. Dielectric, differential thermal analysis and SHG data have shown that Ca₉Yb(VO₄)₇ belongs to the family of high-temperature Ca₃(VO₄)₂ ferroelectrics with Curie temperature T_c = 1221 K, where the symmetry changes from R3c to Rc. At higher temperatures a previously unknown complementary phase transition is discovered at T₂ = 1276 K and is associated with the symmetry change during heating from Rc to Rm. Unlike other whitlockites, two phase transitions in Ca₉Yb(VO₄)₇ are separated by a broad interval (ΔT = 55 K) which allows one to register two phase transitions by DSC and dielectric measurements. According to the thermal type both transitions are classified as first-order transformations and their structural mechanisms are considered. Inhomogeneity in the cation distribution is argued to have a crucial influence on the optical quality and ferroelectric domain structures of Ca₉Yb(VO₄)₇ and other whitlockite-type laser crystals.