Phase transitions (PTs) in Sr 9 + 1 . 5 x M 1 - x (PO 4 ) 7 (M=Fe (1), Tb (2), and Gd (3)), Sr 9 NiLi(PO 4 ) 7 (4), Sr 9 . 0 3 In 0 . 9 8 (VO 4 ) 7 (5), and Sr 9 M(VO 4 ) 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) R3m with a~11 A and c~20 A (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-Ca 3 (PO 4 ) 2 and belong to SG R3c. 3 does not change the symmetry (R3m) 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.