Polar and Centrosymmetric Phases in Solid Solutions Ca_3-xSr_x(PO₄)₂ (0 ⩽ x ⩽ 16/7)

Solid solutions Ca 3 - x Sr x (PO 4 ) 2 (0 ≤ x ≤ 16/7) were studied by X-ray powder diffraction, infrared spectroscopy, differential scanning calorimetry, electrical-conductivity measurements, and second-harmonic generation. Phosphates with 0 ≤ x ≤ 12/7 (β-phase) crystallize in space group R 3 c, Z = 21, a ≈ 10 Å, and c ≈ 38 Å. Sr-rich phosphates with 13/7 ≤ x ≤ 16/7 (β‘-phase) have a somewhat different crystal structure: space group R 3̄ m, Z = 10.5, a ≈ 10 Å, and c ≈ 19 Å. The solid solutions with 0 ≤ x ≤ 12/7 showed a reversible high-temperature phase transition β ⇄ β‘, where the β and β‘ phases have noncentrosymmetric and centosymmetric structures, respectively. The temperature of the phase transition decreased with increasing Sr content. In Ca 3 (PO 4 ) 2, a new phase transition, β ⇄ β‘, was observed at 920 °C in the electrical-conductivity measurements. The structure parameters of polar β-Ca 2 Sr(PO 4 ) 2 ( x = 1) and centrosymmetric β‘-Ca 5/7 Sr 16/7 (PO 4 ) 2 ( x = 16/7) at room temperature were refined by Rietveld refinements of the X-ray diffraction data: space group R 3 c, Z = 21, a = 10.5612(2) Å, and c = 38.0588(5) Å for β-Ca 2 Sr(PO 4 ) 2 and space group R 3̄ m, Z = 10.5, a = 10.7015(2) Å, and c = 19.5787(2) Å for β‘-Ca 5/7 Sr 16/7 (PO 4 ) 2 . β-Ca 2 Sr(PO 4 ) 2 is isotypic with β-Ca 3 (PO 4 ) 2 . In β-Ca 2 Sr(PO 4 ) 2, the M4 site is 50% occupied by Sr 2+ ions, and the M6 site is vacant. In both structures, Ca 2+ and Sr 2+ ions enter the M1−M3 sites whereas Ca 2+ ions selectively occupy the M5 site. β‘-Ca 5/7 Sr 16/7 (PO 4 ) 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 β-Ca 2 Sr(PO 4 ) 2 ) is occupied by 0.165Sr 2+ + 0.085Ca 2+ + 0.75□. The mechanism of the β ⇄ β‘ phase transition in Ca 3 - x Sr x (PO 4 ) 2 (0 ≤ x ≤ 12/7) is discussed on the basis of the crystal data.