Investigation of phase equilibria in SrO–La2O3–P2O5 system and thermo physical properties of established compounds

Abstract

Sub solidus phase relations in pseudo ternary SrO–La2O3–P2O5 system were determined experimentally at 1523 ​K in air. To confirm the formation of phases at this temperature, ternary and quaternary compounds in SrO–P2O5, SrO–La2O3, La2O3–P2O5 and SrO–La2O3–P2O5 systems were synthesized by solid state route. Powder X-ray diffraction (XRD) technique was used to characterize product phases. A phase diagram of SrO–La2O3–P2O5 system has been drawn and phase boundaries are established on the basis of phase analysis of synthetic phase mixtures and compounds, identified at 1523 ​K. No ternary compound was identified in SrO–La2O3 system. Formation of three quaternary compounds, Sr4La(PO4)3O, Sr3La(PO4)3 and Sr9La(PO4)7 was confirmed in the phase diagram. Thermal expansion behavior of these compounds along with some ternary lanthanum and strontium phosphates was investigated from ambient to 1273 ​K using High Temperature X-ray diffraction (HT–XRD) technique. All the compounds showed positive thermal expansion over the range of temperature studied. Average volume expansion coefficients for quaternary phosphate compounds are in the range of 33–36 × 10−6 ​K−1. Heat capacities of the ternary and quaternary phosphate compounds were measured between 300 and 863 ​K using Differential Scanning Calorimeter.