Optical non-linearity tuning in Ca_8-xPb_xMBi(VO₄)₇ whitlockite-type systems

Ca_8-xPb_xMBi(VO₄)₇, M²⁺ = Mg²⁺, Zn²⁺ (0 ≤ x ≤ 1.5) and M²⁺ = Ca²⁺, Cd²⁺ (0 ≤ x ≤ 2) solid solutions with whitlockite-type structure have been prepared by solid state reactions in the form of powders and ceramics. Ferroelectric phase transitions (FPT) are revealed in the cause of dielectric, calorimetric and temperature second harmonic generation (SHG) investigations. It is found that ferroelectric Curie points in solid solutions decrease with x from 1000–1070 K to 730–800 K. The intensity of SHG firstly increases with x and then decreases, the highest SHG signal belongs to Ca₇PbCdBi(VO₄)₇ and Ca_6.5Pb_1.5CdBi(VO₄)₇. Variation of Pb²⁺ concentration is shown to have a minor influence on the coherence length, therefore the SHG augmentation in solid solutions is attributed to optical nonlinear coefficient . The structure of Ca_6.5Pb_1.5MgBi(VO₄)₇ is resolved and crystal structure changes in solid solutions are analyzed. The results are interpreted in view of site occupancy by two- and trivalent cations and their influence on formation of spare space essential for Pb²⁺ and Bi³⁺ non-bonded electrons stereo-activity in whitlockite-type crystal structures. For the first time contribution to optical nonlinearity from VO₄³⁻ anionic groups in different positions is also discussed. - Highlights: • The Curie temperatures of ferroelectric Ca_8-xPb_xMBi(VO₄)₇ solid solutions are chemically controlled from 1070 down to 730–800 K. • Optical SHG non-linear coefficient in Ca_8-xPb_xMBi(VO₄) strongly increase up to 3.3 pm/V at an optimized Pb and Bi concentration. • Structure-property correlations in the whitlockites includes crystal site positions occupancy by two- and trivalent cations.