"Compressibility Collapse" Transition in ReO3

We have used Fermi-surface pressure-derivative measurements at 2 K and singlecrystal and powder x-ray studies as a function of pressure at room temperature to demonstrate that recently published anomalous Fermi-surface and lattice properties of Re${\mathrm{O}}_3$ are related to a novel second-order phase transition. This transition probably involves a tetragonal distortion of the lattice accompanying a displacement of the O atoms from the linear O-Re-O chains of cubic Re${\mathrm{O}}_3$ to a "hinged" arrangement at the oxygens and is characterized by an enormous increase in the compressibility of the high-pressure phase.