A New Look at the Structural and Magnetic Properties of Potassium Neptunate K 2 NpO 4 Combining XRD, XANES Spectroscopy, and Low-Temperature Heat Capacity.

The physicochemical properties of the potassium neptunate K ₂ NpO ₄ have been investigated in this work using X-ray diffraction, X-ray absorption near edge structure (XANES) spectroscopy at the Np-L ₃ edge, and low-temperature heat capacity measurements. A Rietveld refinement of the crystal structure is reported for the first time. The Np(VI) valence state has been confirmed by the XANES data, and the absorption edge threshold of the XANES spectrum has been correlated to the Mössbauer isomer shift value reported in the literature. The standard entropy and heat capacity of K ₂ NpO ₄ have been derived at 298.15 K from the low-temperature heat capacity data. The latter suggest the existence of a magnetic ordering transition around 25.9 K, most probably of the ferromagnetic type.