Investigation of phase relations in SrO–ZrO2–P2O5 system at 1573 K; phase analysis and thermal study of phosphate compounds

A pseudo ternary phase diagram of the SrO–ZrO₂–P₂O₅ system was constructed at 1573 K and phase relations were established. A detailed investigation into the synthesis of reported ternary and quaternary compounds was carried out. These compounds as well as phase mixtures, used to draw the phase diagram, were prepared via solid state method at 1573 K. Powder X-ray diffraction (XRD) was employed to identify all phases present in the products. At 1573 K, no ternary compound could be established in ZrO₂–P₂O₅ system whereas only one compound SrZrO₃ was confirmed in SrO–ZrO₂ system. In SrO–ZrO₂–P₂O₅ system, SrZr₄(PO₄)₆ [SZ4P6] and Sr₇Zr(PO₄)₆ [S7ZP6] were synthesized in pure form whereas SrZr(PO₄)₂ [SZP2] could not be obtained in pure phase. A detailed structural study of cubic S7ZP6 was performed by refining its XRD data. Thermal expansion coefficients of SZ4P6 and S7ZP6 were calculated from 298 to 1273 K using high temperature X-ray diffraction data. SZ4P6 exhibited ultra low volume expansion while S7ZP6 showed positive expansion over the experimental range. Molar heat capacities of these compounds were measured from 308 to 863 K using Differential Scanning Calorimeter (DSC).