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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 ( Δ H f-ox o ) and elements ( Δ H f-ele o ) 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. [ABSTRACT FROM AUTHOR]

• Monazite solid-solution were synthesized by solid state method. • Thermodynamic stability of charge-coupled substituted monazite solid-solution have been investigated using high temperature oxide melt calorimetry. • Thermodynamic parameters for the solid solution such as standard enthalpy of formation and heat capacity have been determined. • Leaching behaviour of solid solution were investigated under off-normal conditions. Monazite (LnPO 4 , Ln = light rare-earth elements; La-Gd) and its solid-solutions are potential candidates for the immobilization of long-lived minor actinides. Thermodynamic stability and chemical durability of monazite-based solid solutions are important to evaluate the matrix for waste disposal. In the present work, thermodynamic stability of Ba and Th charge-coupled substituted monazite solid-solution, (La 1- x Ba x /2 Th x /2)PO 4 (x = 0.1, 0.2 and 0.3) have been investigated for the immobilization of radioactive waste. Thermodynamic parameters for the solid solution such as standard enthalpy of formation (Δ H f o) , heat capacity C p o (T) have been determined and Gibbs energy of formation (Δ G f o) have been derived as a function of temperature. Leaching behaviour of (La 1- x M x /2 Th x /2)PO 4 (M = Sr and Ba) solid solution revealed that the release of Sr ions is lower than that of Ba ions and in accordance with their corresponding thermodynamic stability. Thermodynamic and leaching studies results established long-term stability of charge-couple substituted monazite solid solution. [Display omitted] [ABSTRACT FROM AUTHOR]

Highlights: [•] Heat capacity of Cr2TeO6 (s), Fe2TeO6 (s) and Ni3TeO6 (s) were measured. [•] Thermal expansion and Gibbs energy of formation of Fe2TeO6 (s) and Ni3TeO6 (s) were measured. [•] Enthalpy of formation of A n TeO6 (s) (where A=metal atom, n =2, 3 or 6) was estimated. [•] Heat capacity of TeO3 (s) was estimated. [•] Chemical potential diagram was generated for A–Te–O system (A=Fe,Cr,Ni). [ABSTRACT FROM AUTHOR]