Monazite solid solutions. Characterisation of structural, thermal and physical properties

The aim of this extensive characterisation at ambient and non-ambient conditions was to obtain insight into the structural, chemical and physical behaviour of monazite and to complement the knowledge of monazite solid solutions. Monazite (LnPO${}_4$) has been proposed as a promising host matrix for the long-term storage of long-lived radionuclides. Six monazite solid solutions (La${}_{1-x}$Pr${}_x$PO${}_4$, Pr${}_{1-x}$Nd${}_x$PO${}_4$, Pr${}_{1-x}$Sm${}_x$PO${}_4$, Nd${}_{1-x}$Eu${}_x$PO${}_4$, Nd${}_{1-x}$Sm${}_x$PO${}_4$, and Sm${}_{1-x}$Gd${}_x$PO${}_4$) were synthesised as powder samples, single crystals and ceramics. To model trivalent actinides in the monazite structure, lanthanides (Ln) were used as surrogates. Powder samples were prepared using a solid state reaction with an excess of NH${}_4$H${}_2$PO${}_4$. Single crystals were grown from these powders using the high-temperature solution (flux) method. Crystals were obtained by evaporation of the Li${}_2$MoO${}_4$ flux material and/or temperature reduction. Ceramics were produced after cold isostatic pressing by a two-step sintering procedure. Structural investigations using X-ray diffraction showed a linear increase of the lattice parameters $\overrightarrow{a}$, $\overrightarrow{b}$, and $\overrightarrow{c}$, the unit cell volume, and the Ln-O bond lengths with increasing Ln radius, while the monoclinic angle $\beta$ decreased. The PO${}_4$ tetrahedron was found to be a 'rigid body' because of constant P-O bond lengths and O-P-O angles. This 'rigidity' led to geometric constraints within the chain structure of monazite yielding an increasing distortion of the LnO${}_9$ polyhedron with increasing Ln radii. The wave numbers of the P-O modes, observed by Raman spectroscopy, decreased with increasing Ln radius which was attributed to the increase in the Ln-O bond lengths. Single crystals were found to be inclusion-free and showed no sign of zoning in electron micro-probe analyses. At high pressure, the coordination of the Ln changed from nine to twelve oxygens which was accompanied by an increase of the Ln-O bond lengths and a rapid decrease in the unit cell volume. The structure type found using high-pressure single-crystal diffraction is the post-barite type in space group P2${}_1$2${}_1$2${}_1$. Ceramics were homogeneous and highly dense (up to 95% of the theoretical densities). The thermal expansion coefficients measured on the powders showed an increase with increasing Ln radii. Thermal expansion coefficients of the ceramics obtained from dilatometry were strongly influenced by the microstructure of the samples. The elastic moduli obtained from ultrasound spectroscopy on the ceramics decreased with increasing Ln radius. Both, the thermal expansion coefficients and the elastic moduli depended on the density of the ceramics and, hence, on the degree of sintering. The enthalpy of solution, measured with high-temperature drop solution calorimetry, and the deduced standard enthalpies of formation increased with increasing Ln radius. The molar heat capacity, the standard molar entropy and the molar entropy change decreased with increasing Ln radius as seen in low-temperature microcalorimetry. An increasing Schottky contribution to the molar heat with increasing Pr content was attributed to the excitation of f-orbital electrons of the Pr${}^{3+}$ ion. These extensive investigations show that monazite can form thermodynamically stable, ideal solid solutions from La to Gd without a miscibility gap. This is essential when considering monazite as a potential ceramic matrix material for nuclear waste.