Polar and centrosymmetric phases in solid solutions Ca3-xSrx(PO4)2 (0≤x≤16/7)

Solid solutions Ca3-xSrx(PO4)(2) (0 less than or equal to x less than or equal to 16/7) were studied by X-ray powder diffraction, infrared spectroscopy, differential scanning calorimetry, electrical-conductivity measurements, and second-harmonic generation. Phosphates with 0 less than or equal to x less than or equal to 12/7 (beta-phase) crystallize in space group R3c, Z = 21, a approximate to 10 Angstrom, and c approximate to 38 Angstrom. Sr-rich phosphates with 13/7 less than or equal to x less than or equal to 16/7 V-phase) have a somewhat different crystal structure: space group R (3) over barm, Z = 10.5, a approximate to 10 Angstrom, and c approximate to 19 Angstrom. The solid solutions with 0 less than or equal to x less than or equal to 12/7 showed a reversible high-temperature phase transition beta reversible arrow beta', where the beta and beta' phases have noncentrosymmetric and centosymmetric structures, respectively. The temperature of the phase transition decreased with increasing Sr content. In Ca-3(PO4)(2), a new phase transition, beta reversible arrow beta', was observed at 920 degreesC in the electrical-conductivity measurements. The structure parameters of polar beta-Ca2Sr(PO4)(2) (x = 1) and centrosymmetric beta'-Ca5/7Sr16/7(PO4)(2) (x = 16/7) at room temperature were refined by Rietveld refinements of the X-ray diffraction data: space group R3c, Z = 21, a = 10.5612(2) Angstrom, and c = 38.0588(5) Angstrom for beta-Ca2Sr(PO4)(2) and space group R (3) over barm, Z = 10.5, a = 10.7015(2) Angstrom, and c = 19.5787(2) Angstrom for beta'-Ca5/7Sr16/7(PO4)(2).beta-Ca2Sr(PO4)(2) is isotypic with beta-Ca-3(PO4)(2). In beta-Ca2Sr(PO4)(2), the M4 site is 50% occupied by Sr2+ ions, and the M6 site is vacant. In both structures, Ca2+ and Sr2+ ions enter the M1-M3 sites whereas Ca2+ ions selectively occupy the M5 site. beta'-Ca5/7Sr16/7(PO4)2 contains some disordered atoms: (1) cations at the M3 site are statistically distributed among several positions near the center of symmetry, (2) P1O(4) tetrahedra are orientationally disordered, and (3) the M4 site (M6 site in beta-Ca2Sr(PO4)(2)) is occupied by 0.165Sr(2+) + 0.085Ca(2+) + 0.75rectangle. The mechanism of the beta reversible arrow beta' phase transition in Ca3-xSrx(PO4)(2) (0 less than or equal to x less than or equal to 12/7) is discussed on the basis of the crystal data.