Phase study in Sr–Th–P–O system: Structural and thermal investigations of quaternary compounds

The sub-solidus phase relations in Sr–Th–P–O quaternary system were determined at 1223 K in air. To confirm the formation and stability of reported phases, ternary and quaternary compounds in Sr–Th–O, Sr–P–O, Th–P–O and Sr–Th–P–O systems were synthesized by solid state reactions of SrCO3, ThO2 and NH4H2PO4 in desired molar proportions at 1223 K. A pseudo-ternary phase diagram of SrO–ThO2–P2O5 system was drawn on the basis of the phase analysis of various phase mixtures and phase fields were established by powder X-ray diffraction. In the phase diagram, three quaternary compounds SrTh(PO4)2, SrTh4(PO4)6 and Sr7Th(PO4)6 were identified. When heated in air at 1673 K, these compounds decompose to ThO2. Structures of SrTh(PO4)2, SrTh4(PO4)6 and Sr7Th(PO4)6 were derived from X-ray powder data using the Rietveld refinement method. Thermal expansion behaviors of SrTh(PO4)2, SrTh4(PO4)6 and Sr7Th(PO4)6 were investigated using high-temperature X-ray diffraction in the temperature range of 298–1273 K.