Structural changes and phase transitions in whitlockite-like phosphates

New compounds with a beta-Ca-3(PO4)(2) structure type were found in three systems: Sr9+xM1.5-x(PO4)(7) (M = Mn, Fe, Co, Ni, Cu, and Cd; space group Rim; Z = 3), Sr9R(PO4)(7) (R = Al, Sc, Cr, Fe, Ga, In, and Gd-Lu; space group P2/c, Z = 4), and Sr(9+2x)M(1+x)A(1-6x)(PO4)(7) (M = Mn, Ni, Cd; space group R3c and Z = 6 for A = Na, K; space group P2/m and Z = 4 for A = Li). Crystal structures of these compounds were determined by time-of-flight neutron, synchrotron X-ray, and laboratory X-ray powder diffraction. Reversible polar-to-centrosymmetric phase transitions (R3creversible arrowR (3) over barm) were observed at high temperatures in Ca3-xSrx(PO4)(2) (0 less than or equal to x less than or equal to 12/7), Ca10.5-1.5xFex(PO4)(7) (0 less than or equal to x less than or equal to 1), and Ca9R(PO4)(7). Solid solutions Ca3-xSrx(PO4)(2) (13/7 less than or equal to x less than or equal to 16/7) are centosymmetric with space group R (3) over barm at room temperature. These phase transitions were studied by high-temperature X-ray diffraction, second-harmonic generation, DSC, electric-conductivity and dielectric measurements.