Ca10.5 − x Pb x (PO4)7 and Ca9.5 − x Pb x M(PO4)7 ferroelectrics with the whitlockite structure

Abstract

We have prepared Ca9.5 − xPbxM(PO4)7 (M = Mg, Zn, Cd) and Ca10.5 − xPbx(PO4)7 solid solutions. A polar whitlockite-like (sp. gr. R3c) crystal structure exists in the range 0 ≤ x ≤ 1.5 for all of the M cations in Ca9.5 − xPbxM(PO4)7 and in the range 0 ≤ x ≤ 2.5 for Ca10.5 − xPbx(PO4)7. X-ray powder diffraction profile analysis results for Ca8.5PbCd(PO4)7 powder demonstrate that the small divalent M cations reside predominantly on the octahedral site M5 of the whitlockite structure, the calcium cation occupy the M1–M3 sites, and the lead cations are located primarily on the M4 site. Differential scanning calorimetry, second-harmonic generation, and dielectric permittivity data indicate that all of the synthesized phosphates are high-temperature ferroelectrics. The highest Curie temperatures are offered by the x = 0.5 materials, in which most of the lead resides in the spacious oxygen polyhedra M4 and only a small amount of lead is incorporated into the smaller polyhedra around M1–M3. The nonlinear optical activity has a maximum in the middle of the solid-solution series and is an order of magnitude higher than that of the parent, lead-free phases.