Exploring the influence of lanthanides on the structure and thermodynamic stability of SrLnTh(PO4)3 (Ln = La, Nd, Sm, Eu, and Gd)

The disposal of high-level nuclear waste (HLW) presents major challenges due to the presence of a wide range of long-lived radionuclides, necessitating the development of an effective and safe disposal strategy. SrLnTh(PO4)3 (Ln = La–Gd) system was examined to understand the impact of lanthanides on the structure and thermodynamic stability as a host matrix for HLW immobilisation. Solid-state method was used for the synthesis and, powder X-ray diffraction with infrared spectroscopy techniques for their characterization. All compounds were found to crystallize in monazite-type phase with cations Mn+ (Sr2+, Ln3+ and Th4+) randomly occupying the same lattice site, forming MO9 polyhedron. Heat capacities were measured using differential scanning calorimeter. Dissolution enthalpies were measured using solution calorimeter in lead-borate solvent kept at 1067 K and used for determination of enthalpy of formation of the compounds from their constituent oxides (ΔHf-oxo\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Delta H^{\text o}_{\text{f-ox}} $$\end{document}) and elements (ΔHf-eleo\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Delta H^{\text o}_{\text{f-ele}} $$\end{document}) at 298 K. The results indicate that the enthalpy of formation progressively becomes less exothermic from La3+ to Gd3+ ions, which is also supported by lattice energy calculation using Born–Haber cycle.