Structural and thermodynamic mixing properties of La 1 − x Nd x PO 4 monazite-type solid solutions

${\mathrm{La}}_{1-x}{\mathrm{Nd}}_x{\mathrm{PO}}_4$ 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 ${\mathrm{LaPO}}_4$ to ${\mathrm{DyPO}}_4$, 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 $\Delta H_{\mathrm{mix}}$ for ${\mathrm{La}}_{1-x}{\mathrm{Nd}}_x{\mathrm{PO}}_4$, which is positive and asymmetric and shows a maximum at x = 0.299 ($\Delta H_{\mathrm{mix}}=7.53\pm 0.01\mathrm{kJ}{\mathrm{mol}}^{-1}$) at 298 K.