Polar-to-centrosymmetric phase transition in Ca1.5Sr1.5(VO4)(2) and the polar phase structure

Ca1.5Sr1.5(VO4)(2) was synthesized by the solid-state method at 1150 degree sC followed by quenching in air. Crystal structure of Ca1.5Sr1.5(VO4)(2) wa s refined by Rietveld method: space group R3c, a = 11.0152(1) Angstrom, c = 38.8937(2) Angstrom, Z = 21. The structure of Ca1.5Sr1.5(VO4)(2) is isotyp ic with Ca-3(VO4)(2). Here the MS site is fully occupied by calcium canons and the M4 site is half-occupied by strontium canons. The M1-M3 sites are o ccupied by strontium and calcium canons. The amount of strontium canons in the M1-M3 sites correlates with the size of these positions in the host str ucture of Ca-3(VO4)(2). The second harmonic generation in Ca1.5Sr1.5(VO4)(2 ) (I-2 omega/I-2 omega(SiO2) approximate to 8) is close to Ca-3(VO4)(2) (I- 2 omega/I-2 omega(SiO2) approximate to 10) but substantially less than in C a9R(VO4)(7) (for R = Dy I-2 omega/ I-2 omega(SiO2) approximate to 30). (C) 2001 Elsevier Science Ltd. All rights reserved.