β-Ca3(PO4)2-related structure and dielectric properties of Ca8CdLa(PO4)7

Abstract

A Ca₈CdLa(PO₄)₇ phase was prepared by a high-temperature solid-state reaction in air. The bulk and local cation composition was determined by inductively coupled plasma atomic emission spectroscopy and by energy-dispersive X-ray spectrometry. Second-harmonic generation, differential scanning calorimetry and dielectric measurements revealed the presence of a reversible ferroelectric first-order phase transition from a ferroelectric β-phase (space group (SG) R3c) to a paraelectric β′-phase (SG Rc). The phase transition temperature, T_c, is 865 ± 10 K. The β-Ca₃(PO₄)₂ (β-TCP) related structure was refined by the Rietveld method in the SG R3c using powder synchrotron X-ray diffraction data. The distribution of Ca²⁺, Cd²⁺ and La³⁺ cations among the sites of the β-TCP-type structure was found. According to crystal structure refinement, M5O₆ octahedra are occupied by Ca²⁺ and Cd²⁺ (M5 = 0.307(6)Ca²⁺ + 0.693(6)Cd²⁺) while Ca²⁺ and La³⁺ cations occupy M1 and M3 sites of the β-TCP-type structure. Analysis of the difference electron density map [ρ_dif: (x; y; z)] revealed the presence of small residual electron density at the M4 site. The M4 position is partially occupied by Ca²⁺ cations (M4 = 0.131(6)Ca²⁺). The elemental composition after the Rietveld refinement is determined as Ca_8.61Cd_0.70La_0.82(PO₄)₇. Conductivity measurements revealed that the calculated σ_bulk (σ_bulk = 4.01 × 10⁻⁶ S cm⁻¹ at 900 K and σ_bulk = 1.49 × 10⁻⁴ S cm⁻¹ at 1270 K) for Ca_8.61Cd_0.70La_0.82(PO₄)₇ was lower than one for other β-TCP-type compounds.