Enhanced nonlinear optical activity and Ca2+-conductivity in Са10.5-xPbx(VO4)7 ferroelectrics

Са3(VO4)2-based compositions are considered as promising multifunctional materials combining ferroelectric, optical nonlinear, and Ca2+-ion conductive properties. Their ferroelectric Curie points stretch from minimal for β-Ca3(PO4)2-type compounds temperatures of about 800 K to very high. Investigated in this paper lead substitution for calcium is as a factor controlling ferroelectricity, ionic-conductivity and non-linear optical activity in Ca3(VO4)2-based materials. Polar phase containing powders and ceramics in Ca10.5-xPbx(VO4)7 system are synthesized for 0 ≤ x ≤ 9.5 by the solid state method, and structurally characterized with X-ray powder diffraction and transmission electron microscopy. Dielectric properties, differential thermal analysis and second harmonic generation (SHG) evidence that Ca10.5-xPbx(VO4)7 solid solution (0 ≤ x ≤ 4.5) belongs to whitlockite-type ferroelectrics. SHG activity strongly increases with x up to its maximum at x = 4.5, where it has a record value among all studied before Ca3(XO4)2-related compounds (X = P,V). Ferroelectric Curie temperatures of Ca3(VO4)2 drops from its known value Tc = 1368 K (x = 0) to 770 K (x = 4.5). Crystal symmetry at Tc changes from R3c to R3¯c. After this, one more phase transition to the symmetry R3¯m takes place, its temperature bringing down from 1387 K (x = 0) to 804 K (x = 4.5). Ferroelectric and non-ferroelectric phase transitions in the Са10.5-xPbx(VO4)7 are separated by a broad interval ΔT = 20–50 K and both classified as first-order transformations going in the sequence: R3c↔R3¯c↔R3¯m. Structures of Са10.5-xPbx(VO4)7 compositions with x = 0.5–4 were refined by the Rietveld method and peculiarities of the Pb2+cations distribution in the M1 - M3 and M4 sites of β-Ca3(PO4)2-type structure are discussed regarding the optical nonlinear, ferroelectric and ion-conductive properties. Manifold increased Ca2+- ion conductivity of Са10.5-xPbx(VO4)7 in vicinity of 1000 K at x = 4–4.5 in combination with their ferroelectric and optical nonlinear properties extends applicability of ion-exchange technologies to new promising materials. © 2017 Elsevier B.V.