Low temperature heat capacity of α-Na2NpO4

Abstract

a b s t r a c tThe heat capacity of˛-Na 2 NpO 4 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 C o p,m (˛-Na 2 NpO 4 , cr, 298.15 K) = (151.9 ± 4.6) J K −1 mol −1 and S o m (˛-Na 2 NpO 4 , cr, 298.15 K) = (178.3 ± 4.3) J K −1 mol −1 , respectively. The standard entropy of˛-Na 2 NpO 4 follows the trend shown by the isostructural compound -Na 2 UO 4 and the alkaline earth uranates MgUO 4 , SrUO 4 , BaUO 4 , and Cs 2 UO 4 . When combined with the entropies of the constituent elements, these data yield a standard entropy of formation at 298.15 K as f S o m (˛-Na 2 NpO 4 , 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 f G o m (˛-Na 2 NpO 4 , cr, 298.15 K) =(−1649.0 ± 5.8) kJ mol −1 .

A New Look at the Structural and Magnetic Properties of Potassium Neptunate K2NpO4 Combining XRD, XANES Spectroscopy, and Low-Temperature Heat Capacity

“…Hence, the physical origin of this feature remains unclear. The appearance of a nuclear Schottky effect arising from the magnetic hyperfine splitting interaction between the unpaired 5f electron and the magnetic moment at the Np nucleus ( I = 5/2) was suggested for Na 2 NpO 4 , as the corresponding data showed a reincrease below 3.7 K. 37 K 2 NpO 4 might show similar behavior (Figure 7), but we cannot conclude in the absence of data below 2.0 K, which would require complementary measurements using a 3 He refrigerator.…”

A Combined Experimental and First-Principles Based Assessment of Finite-Temperature Thermodynamic Properties of Intermetallic Al3Sc

“…In many cases [ 80 , 81 , 82 , 83 , 84 , 85 ] the predictive power of the NKR turned out to be surprisingly high, but there have also been several reports about significant deviations of calorimetric data over extended temperature ranges and even at low temperatures[ 82 , 86 , 87 ]. Grimvall [ 73 ] stated certain conditions for the inapplicability of NKR, which included the presence of anharmonic effects and significant non-vibrational contributions in either one or both of the elemental constituents.…”

X-ray Diffraction, Mössbauer Spectroscopy, Magnetic Susceptibility, and Specific Heat Investigations of Na4NpO5 and Na5NpO6

“…The former, used for Mossbauer spectroscopy and magnetic susceptibility measurements, contained 1.8 wt % of Na 5 NpO 6 impurity, while the latter, used for specific heat measurements at low temperatures, showed 0.5 wt % of α-Na 2 NpO 4 impurity. The characterization of the sample's purity for the specific heat measurement was reported in detail in another publication, 10 and the collected data were corrected for the α-Na 2 NpO 4 contribution. As for the Na 5 NpO 6 material, no secondary phases were detected by Xray diffraction.…”

“…The lattice contribution to the heat capacity of Na 4 NpO 5 was approximated with that of the isostructural compound Na 4 UO 5 , with the electronic configuration [Rn]5f 0 , reported in another publication. 10 The excess electronic heat capacity obtained from the difference between these two data is shown in Figure 10. Above 35 K, this electronic heat capacity becomes slightly negative, but this is insignificant considering the accuracy of the measurement.…”

“…Figure 10. Electronic contribution to the heat capacity in Na 4 NpO 5 obtained by subtracting the data of Na 4 UO 510 (black •), and comparison with the simulation for energy levels at 0, 14, 5910 cm −1 (blue •) using a homemade Fortran code.…”