Phase transitions in Sr-containing phosphates and vanadates with β-Ca3(PO4)2-related structures

Phase transitions (PTs) in Sr9+1.5xM1−x(PO4)7 (M=Fe (1), Tb (2), and Gd (3)), Sr9NiLi(PO4)7 (4), Sr9.03In0.98(VO4)7 (5), and Sr9M(VO4)7 (M=Tm (6), Yb (7), and Lu (8)) were studied by laboratory and synchrotron X-ray powder diffraction (XRD) and thermal analysis. High-temperature modifications of these compounds crystallize in space group (SG) R3̄m with a≈11 Å and c≈20 Å (prototype phase). At room temperature, they exhibit different types of distortions of the prototype phase: 1 and 2 belong to SG I2/a, 4 crystallizes in SG I2/m, and 5–8 are isotypic with β-Ca3(PO4)2 and belong to SG R3c. 3 does not change the symmetry (R3̄m) down to 10 K. 5–8 are ferroelectric materials and they show two PTs of the first order near 960 K with very close transition temperatures. 4 exhibits one PT of the first order near 890 K while PT seems to be the second order in 1 and 2. XRD data showed that the PT temperature is about 700 K in 1, and 660 K in 2.