Thermal expansion of solid solutions in apatite binary systems

High-temperature in situ X-ray diffraction was used to investigate isomorphism and the thermal expansion of apatite-structured compounds in three binary systems in the entire temperature range of the existence of its hexagonal modifications. Most of the studied compounds are highly expandable (αl > 8 × 106 (K−1)). In Pb5(PO4)3F–Pb5(PO4)3Cl system, volume thermal expansion coefficient is independence from the composition at 573 K. In Pb5(PO4)3Cl–Pb5(VO4)3Cl, the compound with equimolar ratio of substituted atoms has constant volume thermal expansion coefficient in temperature range 298–973 K. Ca5(PO4)3Cl–Pb5(PO4)3Cl system is characterized by the most thermal sensitive composition, in which there is an equal ratio of isomorphic substituted atoms.