Phase diagram study of Na2O–ThO2–P2O5 system and investigation of thermo physical properties of quaternary compounds

Abstract

The subsolidus phase relations in Na2O–ThO2–P2O5 pseudo ternary system were determined at 1223 K. Synthesis of all the phases, used for drawing phase diagram, was carried out by solid state route. Formation of ternary and quaternary compounds in Na2O–ThO2, Na2O–P2O5, ThO2–P2O5 and Na2O–ThO2–P2O5 systems was confirmed at 1223 K. With the help of these compounds and phase analysis of number of synthetic mixtures, prepared by reacting different mol fractions of Na2O, ThO2 and P2O5 at 1223 K, phase diagram was drawn and phase boundaries were established. No ternary compound was identified in Na2O–ThO2 system. Formation of three quaternary phosphates, Na2Th(PO4)2, NaTh2(PO4)3 and Na5Th(PO4)3 was confirmed at 1223 K. XRD pattern of monoclinic Na5Th(PO4)3, synthesized for the first time and found iso structural with Na5Th(AsO4)3 and its lattice parameters were obtained by Le Bail method. Thermogravimetric studies of quaternary phosphates were carried out in air to investigate their thermal stability. All the three compounds decompose to ThO2 at 1673 K. Two step weight loss in TG curve of Na2Th(PO4)2 confirmed its decomposition to ThO2 via NaTh2(PO4)3 as an intermediate. To calculate thermal expansion coefficients, high temperature XRD (HT-XRD) data of Na2Th(PO4)2, NaTh2(PO4)3 and Na5Th(PO4)3 was collected in air between the temperature range of 298–1173 K. All these compounds exhibit positive thermal expansion with the average volume expansion coefficient values ranging between 30 and 60 × 10−6 K−1. Heat capacities of these compounds were measured in the temperature range of 308–873 K using Differential Scanning Calorimetry (DSC) technique.