Mössbauer spectroscopy, magnetization, magnetic susceptibility, and low temperature heat capacity of α-Na&lt;sub&gt;2&lt;/sub&gt;NpO&lt;sub&gt;4&lt;/sub&gt;

Smith, A.L.; Hen, Amir; Magnani, Nicola; Sanchez, Jean Pierre; Colineau, Eric; Griveau, Jean Christophe; Raison, Philippe E.; Caciuffo, Roberto; Konings, R.; Cheetham, Anthony K.

doi:10.1088/0953-8984/28/8/086002

Mössbauer spectroscopy, magnetization, magnetic susceptibility, and low temperature heat capacity of α-Na₂NpO₄

Title

Mössbauer spectroscopy, magnetization, magnetic susceptibility, and low temperature heat capacity of α-Na₂NpO₄

Author

Smith, A.L. (TU Delft RST/Reactor Physics and Nuclear Materials; European Commission; University of Cambridge)
Hen, Amir (European Commission)
Magnani, Nicola (European Commission)
Sanchez, Jean Pierre (Université Grenoble Alpes)
Colineau, Eric (European Commission)
Griveau, Jean Christophe (European Commission)
Raison, Philippe E. (European Commission)
Caciuffo, Roberto (European Commission)
Konings, R. (European Commission)
Cheetham, Anthony K. (University of Cambridge)

Date

2016

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.

Subject

heat capacity
magnetic susceptibility
magnetization
Mössbauer spectroscopy
sodium neptunate

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DOI

https://doi.org/10.1088/0953-8984/28/8/086002

ISSN

0953-8984

Source

Journal of Physics: Condensed Matter, 28 (8)

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