Low temperature heat capacity of Na4UO5 and Na4NpO5

The low temperature heat capacities of Na4UO5 and Na4NpO5 have been measured for the first time in the temperature range (1.9 to 292) K using a Quantum Design PPMS (Physical Property Measurement System) calorimeter. The experimental data have been fitted to theoretical functions below 20 K, and to a combination of Debye and Einstein functions above this temperature. The heat capacity and entropy values at T = 298.15 K have been derived as Cp,mo(Na4UO5,cr,298.15K)=(220.6±6.7) J · K−1 · mol−1, Smo(Na4UO5,cr,298.15K)=(247.5±6.2) J · K−1 · mol−1, Cp,mo(Na4NpO5,cr,298.15K)=(219.0±6.6) J · K−1 · mol−1, and Smo(Na4NpO5,cr,298.15K)=(247.5±6.2) J · K−1 · mol−1. When combined with the enthalpies of formation reported in the literature, these data yield standard entropies and Gibbs energies of formation as ΔfSmo(Na4UO5,cr,298.15K)=-(520.8±6.3) J · K−1 · mol−1, ΔfSmo(Na4NpO5,cr,298.15K)=-(521.0±6.3) J · K−1 · mol−1, ΔfGmo(Na4UO5,cr,298.15K)=-(2301.7±2.9) kJ · mol−1 and ΔfGmo(Na4NpO5,cr,298.15K)=-(2159.7±6.0) kJ · mol−1.