Structural and thermal investigations of CaU(PO4)2; phase diagram study of MO−UO2–P2O5 systems (M = Ca, Sr, Ba)

Abstract

Structure of monoclinic CaU(PO4)2, synthesized in argon atmosphere at 1473 K, was derived from powder X−ray diffraction (XRD) data using Rietveld refinement method. Thermogravimetric (TG) Curve recorded up to 1573 K in argon atmosphere showed thermal as well as structural stability of CaU(PO4)2. When heated in air, TG curve of CaU(PO4)2 showed weight gain between 1100–1673 K due to oxidation of U(IV) to U(VI). Molar heat capacity of CaU(PO4)2 as well as some ternary alkaline earth phosphates were calculated using differential scanning calorimeter technique. Phase diagrams of pseudo ternary alkaline earth metal−uranium (IV)−phosphates were derived and phase boundaries were established. All the ternary and quaternary compounds used for deriving the phase diagrams were synthesized in an argon atmosphere at 1473 K. X-ray diffraction (XRD) was used as a prime analytical tool to investigate the phase mixtures and draw phase boundaries. Phase diagram of SrO–UO2–P2O5 and BaO–UO2–P2O5 systems confirm the existence two quaternary compounds, (Sr/Ba)U(PO4)2 and (Sr/Ba)7U(PO4)6 in the systems whereas in CaO–UO2–P2O5 system only one quaternary compound, CaU(PO4)2, was identified.