Solid Solubility and Thermal Expansion in a LaPO4–YPO4 System

The composition and lattice parameters of co-precipitated (La_0.3Y_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_0.3Y_0.7PO₄·H₂O) rhabdophane nanoparticles. Heat treatment at 950°C caused the decomposition of rhabdophane into a (La_{1? x} Y _x )PO₄ monazite solid solution and YPO₄ xenotime. The solid solubility of Y in LaPO₄ 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.