Effect of strontium loading on the structural and thermodynamic properties of Ca10-xSrx(PO4)6F2(x= 2, 4, 6, 8) solid solutions for radioactive waste immobilization

Abstract

Fluorapatite is being contemplated as a potential host for the incorporation of fluoride bearing radioactive nuclear waste. Ca10(PO4)6F2 and Sr10(PO4)6F2 are two fluorapatites which are studied extensively for the purpose of radioactive waste immobilization. Here, in this work a series of solid solutions having compositions Ca10-xSrx(PO4)6F2(x = 2, 4, 6 and 8) were synthesized by solid state chemistry route. The detailed structural properties of these solid solutions were analysed by powder XRD, FTIR and Raman spectroscopy. Rietveld refinement was used to determine the structural changes as well as micro-structural strain in these compositions. At lower loading of Sr2+, there is a preference on site occupation of Sr2+ between two crystallographic Ca2+ sites as evidenced by Rietveld refinement. The calculated average strain values in the calcium rich compositions (x = 2 and 4) are higher than the strontium rich compositions (x = 6 and 8). Thermo-physical properties such as heat capacity and thermal expansion of Ca10-xSrx(PO4)6F2 (x = 2, 4, 6, 8) solid solutions were experimentally measured for the first time.Heat capacity values follow the usual solid solution behaviour. Electron beam irradiation studies show that these substituted solutions are resistant to phase degradation up to 1000 kGy as revealed by powder XRD. Moreover, combined FTIR, Raman and EPR studies confirmed that higher strontium content is detrimental to the fluorapatite through surface modification. This study further gives insight into design of better matrix in terms of 90Sr loading capacity and stability.