Search for new apatite-like phases for lead utilization based on crystal structure and thermal expansion

Apatites, being one of the most numerous mineral-like families of compounds, are considered as a matrix for binding lead ions, which is dangerous for the biosphere. The crystal-chemical (composition, structure) and thermophysical aspects (thermal expansion) are considered as the basis for analysing the properties of this kind of material. It is suggested that substances of the composition Pb5(AIVO4)2(BVIO4), Pb5(AIVO4)(CVO4)2 can be a perspective form of lead binding materials based on compounds with the structure of apatite (AIV = Si, Ge; BVI = S, Cr; CV = P). Such compounds, as it was shown by DTA and HTXRD experiments, are distinguished by the absence of polymerphism and the abnormal ordering of structure. Also, they have relatively low values of thermophysical indicators (the rate of change of linear thermal expansion coefficients is 0.02–0.03×106 K-1; values of the volume thermal expansion coefficients are 40–70×106 K-1). Compounds Pb5(SiO4)(PO4)2 (a = 9.78782(16) Å, c = 7.31084(16) Å, V = 606.555(23) Å3, R-bragg = 4.694 %) and Pb5(GeO4)(PO4)2 (a = 9.87697(12) Å, c = 7.33136(11) Å, V = 619.388(17) Å3, R-bragg = 1.730 %) were obtained, identified and crystallographically characterised for the first time.