Electrical resistance and magnetic properties of the neptunium monopnictides NpAs, NpSb, and NpBi at high pressures

Abstract

We report on high-pressure studies performed on the neptunium pnictides NpAs and NpBi via electrical resistance up to $\ensuremath{\sim}25\mathrm{GPa}$ between 1.3 K and room temperature, and on a high-pressure investigation up to 9 GPa and at 4.2 K on NpSb using ${}^{237}\mathrm{Np}$ M\"ossbauer spectroscopy. This work extends previous high-pressure studies carried out on NpAs via M\"ossbauer spectroscopy, on NpSb via resistance, and on all pnictides via x-ray study. In $\mathrm{Np}X$ $(X=\mathrm{A}\mathrm{s},\mathrm{S}\mathrm{b},\mathrm{B}\mathrm{i})$ crystallizing in the cubic-NaCl phase the ground state is antiferromagnetic and displays a noncollinear 3k spin structure. The strong increase of the resistivity with decreasing temperature observed in the temperature range of the 3k order at ambient pressure collapses at 0.23 (NpAs), 2.7 (NpSb), and 3.9 GPa (NpBi). No significant change of the hyperfine interactions is found in NpAs or NpSb at the pressure where the resistance collapse is observed. The Kondo anomaly of the resistivity observed at ambient pressure disappears above 25 GPa (NpAs), 2.7 GPa (NpSb), and 3 GPa (NpBi). The N\'eel temperature ${T}_{N}$ of all compounds and the ordered moment of NpAs and NpSb decrease with reduced volume. For NpAs and NpBi the resistance indicates the presence of magnetic order at least up to 16 GPa. The compounds undergo a pressure-induced structural transition with a volume reduction by $\ensuremath{\sim}10%.$ Although in the resistance of NpSb the signature of magnetic order is lost already at 8 GPa in the high-pressure phase, a magnetic hyperfine field is present, which is reduced by $\ensuremath{\sim}30%$ relative to the NaCl phase. It is suggested that the resistance collapse is caused by a change of the magnetic structure, that the decrease of ${T}_{N}$ is due to a modification of the Fermi surface besides a small $5f$ delocalization, and that in NpSb the volume reduction in the structural high-pressure phase leads to an enhanced $5f$ delocalization.