Valence instability across magnetostructural transition in USb₂

Pressure dependent transport measurements on USb₂ indicate an abrupt antiferromagnetic–ferromagnetic (AFM–FM) transition near P = 8 GPa, accompanied by a reduction in ordering temperature by more than 100 K. The AFM–FM transition has been the subject of several recent theoretical calculations, though only few experimental probes have been reported. To elucidate the behavior near this magnetic transition, we performed pressure dependent X-ray diffraction and resonant X-ray emission spectroscopy experiments on USb₂. The magnetic transition coincides with a tetragonal to orthorhombic structural transition resulting in a 17% volume collapse as well as a transient f-occupation enhancement. The AFM–FM transition width determined from structural and spectroscopic measurements agrees well with previously published transport measurements and suggests a sluggish transition from about 8 to 11 GPa. Except for the enhancement across the transition region, the f-occupation decreases steadily with pressure, demonstrating the multiconfigurational nature of actinide materials.