Phosphate-based monazite ceramic is considered worldwide as a potential crystalline host matrix for immobilization of long-lived tri- and tetra-valent actinides present in high-level nuclear waste. Monazite is chemically stable with respect to the leaching processes and has high radiation stability. The present paper describes the influence of charged coupled (Ca²⁺, Th⁴⁺) substitution in place of La³⁺ on thermodynamic stability of synthetic monazite ceramics. XRD-analysis of Ca, Th substituted LaPO₄ viz.,      La_1-xCa_x/2Th_x/₂PO₄ (0 ≤ x ≤ 1) points to the formation of ideal solid-solution in the entire range of composition. However, thermodynamic analysis indicates deviation from ideal solid-solution with a minima at x = 0.25. The substituted La_1xCa_x/₂Th_x/₂PO₄ system is found to be iso-entropic and stabilized mainly by enthalpy. Enthalpies of formation as a function of Ca²⁺, Th⁴⁺ substitution were analysed to provide insights into the development of thermodynamically stable nuclear waste matrix.