Ca10.5-xPbx(PO4)7 and Ca9.5 - xPbxM(PO4)7 Ferroelectrics with the Whitlockite Structure

We have prepared Ca-9.5 (-) xPbxM(PO4)(7) (M = Mg, Zn, Cd) and Ca-10.5 Pb-- x(x)(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.