Ferroelectricity, ionic conductivity and structural paths for large cation migration in Ca10.5−xPbx(VO4)7 single crystals, x = 1.9, 3.5, 4.9

Abstract

Crystal structure, and thermal, dielectric and optical second-harmonic activities were investigated for whitlockite-type Ca_8.6Pb_1.9(VO₄)₇ (1), Ca₇Pb_3.5(VO₄)₇ (2) and Ca_5.6Pb_4.9(VO₄)₇ (3) single crystals. The refinement of the structure revealed splitting of the M3 and M4 sites into two and three sub-positions, respectively. Splitting of the M3 and M4 sites strongly enhances nonlinear optical activity, measured on powders, with the effect vanishing at ferroelectric phase transitions at elevated temperatures. Single crystals (1) and (2) undergo two structural transformations: ferroelectric one between 750–1150 K and a subsequent phase transition at higher temperatures. The structural mechanism of the ferroelectric phase transition is analyzed with respect to the rearrangement of Ca²⁺ and Pb²⁺ over their positions in the ferroelectric and paraelectric phases. Large Pb²⁺ ion substitution for calcium results in unit cell expansion with consequent extending of ion migration channels, thus leading to enhanced ionic conductivity in the Pb-rich materials. It is concluded that in spite of steric hindrances for lead cations, they effectively move between positions M2 → M4 → M1 or M1 → M4 → M2 together with calcium cations across the crystal. The cation conduction pathways as well as the migration energies of Ca²⁺ and Pb²⁺ cations were calculated by the bond valence energy landscape (BVEL) method. Among the obtained single crystals, (3) demonstrated the highest ionic conductivity, maximum second harmonic generation (SHS) signal and the lowest temperature of ferroelectric phase transition.