Low temperature heat capacity of α-Na2NpO4

The heat capacity of α-Na2NpO4 has been measured for the first time in the temperature range 2.1-301 K using a Quantum Design PPMS (Physical Property Measurement System) calorimeter. The heat capacity and entropy values at 298.15 K have been derived as Cp,mo(α-Na2NpO4, cr, 298.15 K) = (151.9 ± 4.6) J K−1 mol−1 and Smo(α-Na2NpO4, cr, 298.15 K) = (178.3 ± 4.3) J K−1 mol−1, respectively. The standard entropy of α-Na2NpO4 follows the trend shown by the isostructural compound α-Na2UO4 and the alkaline earth uranates MgUO4, SrUO4, BaUO4, and Cs2UO4. When combined with the entropies of the constituent elements, these data yield a standard entropy of formation at 298.15 K as ΔfSmo(α-Na2NpO4, cr, 298.15 K) = − (385.1 ± 4.3) J K−1 mol−1. Using the enthalpy of formation tabulated in the literature, the standard Gibbs energy of formation was calculated at 298.15 K as ΔfGmo(α-Na2NpO4, cr, 298.15 K) =(−1649.0  ± 5.8) kJ mol−1.