Phase transitions (PTs) in Sr 9+1.5 x M 1− x (PO 4) 7 (M=Fe ( 1), Tb ( 2), and Gd ( 3)), Sr 9NiLi(PO 4) 7 ( 4), Sr 9.03In 0.98(VO 4) 7 ( 5), and Sr 9M(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) R 3 ̄ m 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-Ca 3(PO 4) 2 and belong to SG R3 c. 3 does not change the symmetry ( R 3 ̄ m ) 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.