Mossbauer spectroscopy, magnetization, magnetic susceptibility, and low temperature heat capacity of a Na2NpO4.
The physical and chemical properties at low temperatures of hexavalent disodium neptunate a Na2NpO4 are investigated for the first time in this work using Mossbauer spectroscopy, magnetization, magnetic susceptibility, and heat capacity measurements. The Np(VI) valence state is confirmed by the isomer shift value of the Mossbauer spectra, and the local structural environment around the neptunium cation is related to the fitted quadrupole coupling constant and asymmetry parameters. Moreover, magnetic hyperfine splitting is reported below 12.5 K, which could indicate magnetic ordering at this temperature. This interpretation is further substantiated by the existence of a l peak at 12.5 K in the heat capacity curve, which is shifted to lower temperatures with the application of a magnetic field, suggesting antiferromagnetic ordering. However, the absence of any anomaly in the magnetization and magnetic susceptibility data shows that the observed transition is more intricate. In addition, the heat capacity measurements suggest the existence of a Schottky type anomaly above 15 K associated with a low lying electronic doublet found about 60 cm( 1) above the ground state doublet. The possibility of a quadrupolar transition associated with a ground state pseudoquartet is thereafter discussed. The present results finally bring new insights into the complex magnetic and electronic peculiarities of a Na2NpO4.
Journal of physics. Condensed matter an Institute of Physics journal