Structural and thermophysical properties of Sr7U(PO4)6 and Ba7U(PO4)6

Compounds Sr7U(PO4)6(1) and Ba7U(PO4)6 (2) were prepared by the solid state reaction route, by mixing appropriate molar proportions of alkaline earth metal carbonates (Sr and Ba), UO2 and NH4H2PO4 (ADHP) and heating in high purity Ar atmosphere at 1373 K for 10 h with intermittent grinding. Powder X-ray Diffraction (XRD) was done to ascertain the phase purity of the compounds. Structural characterization was carried out by Rietveld refinement method which shows the octahedral coordination for U4+ in both the compounds. Thermo-gravimetric (TG) measurement revealed that the compounds are stable up to 1000 K and, then, they oxidize in the air. High Temperature X-ray Diffraction (HT-XRD) study of both the compounds in vacuum showed positive thermal expansion over the temperature range from 298 to 1273 K. Heat capacity (Cp) of the compounds was also measured by heat flow Differential Scanning Calorimeter (DSC) under argon atmosphere from 303 to 858 K.