Phase transitions (PTs) in Sr9+1.5xM1-x(PO4)(7) (M=Fe (1), Tb (2), and Gd-(3)), Sr9NiLi(PO4)(7) (4), Sr(9.03)ln(0.98)(VOP4)(7) (5), and Sr9M(VO4)(7) (M = Tin (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) R (3) over barm with a = 11 Angstrom and c = 20 Angstrom (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 beta-Ca-3(PO4)(2) and belong to SG Me. 3 does not change the symmetry (R (3) over barm) 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. (C) 2004 Elsevier B.V. All rights reserved.