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Solid solutions Ca 3-x Sr x (PO 4 ) 2 (0 e x e 16/7) were studied by X-ray powder diffraction, infrared spectroscopy, differential scanning calorimetry, electrical-conductivity measurements, and second-harmonic generation. Phosphates with 0 e x e 12/7 ( -phase) crystallize in space group R3c, Z ) 21, a ≈ 10 Å, and c ≈ 38 Å. Sr-rich phosphates with 13/7 e x e 16/7 ( ′-phase) have a somewhat different crystal structure: space group R3 hm, Z ) 10.5, a ≈ 10 Å, and c ≈ 19 Å. The solid solutions with 0 e x e 12/7 showed a reversible high-temperature phase transition a ′, where the and ′ 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, a ′, was observed at 920°C in the electrical-conductivity measurements. The structure parameters of polar -Ca 2 Sr(PO 4 ) 2 (x ) 1) and centrosymmetric ′-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 R3c, Z ) 21, a ) 10.5612(2) Å, and c ) 38.0588 (5) Å for -Ca 2 Sr(PO 4 ) 2 and space group R3 hm, Z ) 10.5, a ) 10.7015(2) Å, and c ) 19.5787(2) Å for ′-Ca 5/7 Sr 16/7 (PO 4 ) 2 . -Ca 2 Sr(PO 4 ) 2 is isotypic with -Ca 3 (PO 4 ) 2 . In -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. ′-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 -Ca 2 Sr(PO 4 ) 2 ) is occupied by 0.165Sr 2+ + 0.085Ca 2+ + 0.750. The mechanism of the a ′ phase transition in Ca 3-x Sr x (PO 4 ) 2 (0 e x e 12/7) is discussed on the basis of the crystal data.

Structure of the Paraelectric Phase at 923 K. -The ferroelectric phase transition of the title compound occurring at 890 K is characterized by high-temperature powder XRD, electron diffraction, HREM, second-harmonic generation, DSC, and dielectric and electrical conductivity measurements. The phase transition from a polar ferroelectric form (space group R3c) to a centrosymmetric paraelectric form (space group R3c) is reversible and of first order. The structural parameters of the paraelectric phase are refined by the Rietveld method from XRD data at 923 K in the space group R3c with Z = 6. -(LAZORYAK, B. I.; MOROZOV, V. A.; BELIK, A. A.; STEFANOVICH, S. Y.; GREBENEV, V. V.; LEONIDOV, I. A.; MITBERG, E. B.; DAVYDOV, S. A.; LEBEDEV*, O. I.; VAN TENDELOO, G.; Solid State Sci. 6 (2004) 2, 185-195; EMAT, Univ. Antwerp, B-2020 Antwerpen, Belg.; Eng.) -W. Pewestorf 43-005