title: Mössbauer spectroscopy, magnetization, magnetic susceptibility, and low temperature heat capacity of α-Na₂NpO₄.
abstract: The physical and chemical properties at low temperatures of hexavalent disodium neptunate α-Na2NpO4 are investigated for the first time in this work using Mössbauer spectroscopy, magnetization, magnetic susceptibility, and heat capacity measurements. The Np(VI) valence state is confirmed by the isomer shift value of the Mössbauer 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 λ-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 α-Na2NpO4.
authors.affiliation: European Commission, Joint Research Centre (JRC), Institute for Transuranium Elements (ITU), PO Box 2340, D-76125 Karlsruhe, Germany. Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, UK.
authors.forename: Anna L
authors.initials: AL
authors.lastname: Smith
authors.forename: Amir
authors.initials: A
authors.lastname: Hen
authors.forename: Nicola
authors.initials: N
authors.lastname: Magnani
authors.forename: Jean-Pierre
authors.initials: JP
authors.lastname: Sanchez
authors.forename: Eric
authors.initials: E
authors.lastname: Colineau
authors.forename: Jean-Christophe
authors.initials: JC
authors.lastname: Griveau
authors.forename: Philippe E
authors.initials: PE
authors.lastname: Raison
authors.forename: Roberto
authors.initials: R
authors.lastname: Caciuffo
authors.forename: Rudy J M
authors.initials: RJ
authors.lastname: Konings
authors.forename: Anthony K
authors.initials: AK
authors.lastname: Cheetham