Solid Solubility and Thermal Expansion in a LaPO ₄ –YPO ₄ System

Abstract

The composition and lattice parameters of co‐precipitated (La 0.3 Y 0.7 ) orthophosphate were studied using X‐ray diffraction (XRD), transmission electron microscopy (TEM), and energy dispersive X‐ray spectroscopy (EDX). The results indicate that the as‐precipitated powder consists of single‐phase (La 0.3 Y 0.7 PO 4 ·H 2 O) rhabdophane nanoparticles. Heat treatment at 950°C caused the decomposition of rhabdophane into a (La 1− x Y x )PO 4 monazite solid solution and YPO 4 xenotime. The solid solubility of Y in LaPO 4 monazite from 1000° to 1600°C was studied using XRD, TEM, and EDX. The implications of the findings for controlling the coefficient of thermal expansion of the prospective two‐phase monazite–xenotime fiber coatings for ceramic composites applications are discussed.