In the present study, structural and thermodynamic investigations are performed on the ternary Na2MnF5(s) compound, which can be formed due to interaction of molten fluoride salts with structural materials of the molten salt reactor (MSR). The standard molar enthalpy of formation ( \(\Delta {\text{H}}_{\text{f}, 298.15\text{K}}^{\text{o}}\) ) at 298.15 K of this compound was determined by room temperature solution calorimetry and the corresponding value is found to be − 2318.84 ± 15.72 kJ mol−1. Critical insights into the thermal stability of this compound were obtained by performing TG–DTA–MS analysis supported by parallel XRD measurements. At temperatures between 573 and 623 K, the compound was found to undergo thermal decomposition. A thorough decomposition mechanism for this compound is proposed based on the inference. Utilizing a combined approach of relaxation calorimetry and differential scanning calorimetry, molar heat capacity of this compound from 2 to 573 K is obtained. Using enthalpy of formation value at 298.15 K and molar heat capacity of this compound other thermodynamic functions are derived in the current investigation. The temperature dependence of the Gibbs energy, which reflects the thermodynamic stability, was determined using the thermodynamic parameters of Na₂MnF₅(s).
