V. Ichas et al., ELECTRICAL-RESISTANCE AND MAGNETIC-PROPERTIES OF THE NEPTUNIUM MONOPNICTIDES NPAS, NPSB, AND NPBI AT HIGH-PRESSURES, Physical review. B, Condensed matter, 56(22), 1997, pp. 14481-14492
We report on high-pressure studies performed on the neptunium pnictide
s NpAs and NpBi via electrical resistance up to similar to 25 GPa betw
een 1.3 K and room temperature, and on a high-pressure investigation u
p to 9 GPa and at 4.2 K on NpSb using Np-237 Mossbauer spectroscopy. T
his work extends previous high-pressure studies carried out on NpAs vi
a Mossbauer spectroscopy, on NpSb via resistance, and on all pnictides
via x-ray study. In NpX (X=As,Sb,Bi) crystallizing in the cubic-NaCl
phase the ground state is antiferromagnetic and displays a noncollinea
r 3k spin structure. The strong increase of the resistivity with decre
asing 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 (N
pBi). 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 di
sappears above 25 GPa (NpAs), 2.7 GPa (NpSb), and 3 GPa (NpBi). The Ne
el temperature TN Of all compounds and the ordered moment of NpAs and
NpSb decrease with reduced volume. For NpAs and NpBi the resistance in
dicates the presence of magnetic order at least up to 16 GPa. The comp
ounds undergo a pressure-induced structural transition with a volume r
eduction by similar to 10%. Although in the resistance of NpSb the sig
nature of magnetic order is lost already at 8 Ga in the high-pressure
phase, a magnetic hyperfine field is present, which is reduced by simi
lar to 30% relative to the NaCl phase. It is suggested that the resist
ance 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 t
he structural high-pressure phase leads to an enhanced 5f delocalizati
on.