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°C followed by quenching in air. Crystal structure of Ca1.5Sr1.5(VO4)2 was refined by Rietveld method: space group R3c, a = 11.0152(1) Å, c = 38.8937(2) Å, Z = 21. The structure of Ca1.5Sr1.5(VO4)2 is isotypic with Ca3(VO4)2. Here the M5 site is fully occupied by calcium cations and the M4 site is half-occupied by strontium cations. The M1–M3 sites are occupied by strontium and calcium cations. The amount of strontium cations in the M1–M3 sites correlates with the size of these positions in the host structure of Ca3(VO4)2. The second harmonic generation in Ca1.5Sr1.5(VO4)2 (I2ω/I2ω(SiO2) ≈ 8) is close to Ca3(VO4)2 (I2ω/I2ω(SiO2) ≈ 10) but substantially less than in Ca9R(VO4)7 (for R = Dy I2ω/I2ω(SiO2) ≈ 30).