Structural and thermal study of Sr(Th1-xUx)(PO4)2 compounds

Abstract

A series of compositions covering the complete range in SrTh(PO4)2–SrU(PO4)2 system were synthesized by conventional solid state (dry) route at 1473 ​K in argon atmosphere. The products were examined by powder X-ray diffraction (XRD) technique. X-ray diffraction (XRD) patterns of thorium rich Sr(Th1-xUx)(PO4)2 with compositions x ​≤ ​0.15 showed cheralite type structures similar to SrTh(PO4)2 having (Sr/Th/U)O9 polyhedra (Th, U and Sr occupying same atomic position) and PO4 tetrahedra. Uranium rich compounds with composition x ​≥ ​0.6 were isomorphs with SrU(PO4)2 having ordered cheralite (orthorhombic) structures with separate specific sites for SrO10 and (Th, U)O8 polyhedra. Compounds with 0.15 ​< ​x ​< ​0.6 were composed of both orthorhombic and monoclinic phases confirming formation of partial solid solution in SrTh(PO4)2 and SrU(PO4)2 systems. Oxidation study of single phase compounds was carried out up to 1673 ​K using thermogravimetric (TG) method. Study reveals that addition of thorium in SrU(PO4)2 hinders the oxidation of uranium and stabilizes it in 4+ oxidation state. Thermal expansion behaviour of single phase compounds was studied from ambient to 1273 ​K in vacuum using high temperature X–ray diffractometer. All the compounds show regular positive thermal expansion up to 1273 ​K and the values of volume thermal expansion coefficient range between 20–30 ​× ​10−6 ​K−1. Heat capacities of all the compounds were measured using Differential Scanning Calorimeter in the temperature range of 300–873 ​K.