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

Abstract

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 SrCO 3, ThO 2 and NH 4H 2PO 4 in desired molar proportions at 1223 K. A pseudo-ternary phase diagram of SrO–ThO 2–P 2O 5 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(PO 4) 2, SrTh 4(PO 4) 6 and Sr 7Th(PO 4) 6 were identified. When heated in air at 1673 K, these compounds decompose to ThO 2. Structures of SrTh(PO 4) 2, SrTh 4(PO 4) 6 and Sr 7Th(PO 4) 6 were derived from X-ray powder data using the Rietveld refinement method. Thermal expansion behaviors of SrTh(PO 4) 2, SrTh 4(PO 4) 6 and Sr 7Th(PO 4) 6 were investigated using high-temperature X-ray diffraction in the temperature range of 298–1273 K.