Structure and Properties of Ca_(9-x)Pb_xR(PO_4)_7 (R = Sc, Cr, Fe, Ga, In) Whitlockite-Like Solid Solutions

摘要

We have prepared solid solutions based on whitlockite-struclure ferroelectrics, Ca_(9-x)Pb_xR(PO_4)_7 (R = Sc, Cr, Fe, Ga, In), through Pb substitution for Ca. Single-phase Ca_(9-x)Pb_xR(PO_4)_7 materials with a polar whitlockite-like structure (sp.gr. R3c) exist in the range 0≤x≤ 1.5 for all of the R metals studied. X-ray powder diffraction profile analysis results for R = In (x = 0.5,0.8, and 1.0) demonstrate that the trivalent cations reside on the octahedral site M5 of the whitlockite structure, the calcium cation occupy the Ml — M3 sites, and the lead cations are located predominantly on the M3 site. Differential scanning calorimetry, dielectric permittivity, and second-harmonic generation data attest to a ferroelectric phase transition, whose temperature is 580—610℃ in Ca_9R(PO_4)_7 and decreases monotonically to 480—520℃ asx increases to 1.5.