Neutron scattering study of the pressure induced phase transformation in Re03.

We have studied the pressure induced phase transformation in a large crystal of Re03 at 300K and 0-6 kbar hydrostatic pressure. Inelastic measurements at 1 bar show a pronounced minimum in the T1 mode near the BZ boundary (M-point). Above P(c) = 5.2 kbar superlattice reflections with intensities consistent with M3-like rotations of Re06 octahedra appear. The transformation transformation seems to be continuous (second order), but an anomaly in the superlattice intensity at p = 5.4 kbar suggests a further transformation. The M3-phonon softens dramatically as p -> p(c) from below, but no substantial changes occur in the long wavelength acoustic velocities between 0 and 6 kbar.