Structural and thermodynamic mixing properties of La1−xNdxPO4 monazite-type solid solutions

Abstract La 1 − x Nd x PO 4 monazite-type ceramics were synthesized by precipitation and sintering. The obtained crystalline powders and pellets were structurally characterized by X-ray diffraction, electron microscopy, and infrared and Raman spectroscopy. The determined unit cell constants agree with published values for the pure end-members ranging from LaPO 4 to DyPO 4 , as well as with values for known mixed monazite-type lanthanide orthophosphates, indicating a homogeneous solid solution. However, Raman spectroscopy shows a change in short-range order and the results of high temperature oxide melt solution calorimetry suggest a subregular solid solution, which requires two Margules parameters to calculate the observed enthalpy of mixing Δ H mix for La 1 − x Nd x PO 4 , which is positive and asymmetric and shows a maximum at x = 0.299 ( Δ H mix = 7.53 ± 0.01 kJ mol − 1 ) at 298 K.