valence transition at low temperatures and high magnetic fields

CeOs₄Sb₁₂ exhibits a very unusual field- and temperature-driven valence transition that is accompanied by a drastic change in Fermi-surface topology. It is also a candidate heavy-fermion topological semimetal and sports a field-induced quantum-critical point at about 20 T. Single crystal x-ray diffraction in pulsed magnetic fields of up to 30 T using a unique instrumental combination at the Advanced Photon Source permitted a study of the change in unit-cell volume as the field swept through the valence transition. In addition, neutron-diffraction measurements probed the analogous structural changes as a function of temperature in zero magnetic field. Both measurements shed light on the unique “wedge-shaped” phase boundary that marks the valence transition in CeOs₄Sb₁₂.