Neutron scattering study of the pressure-induced phase transformation inReO3

We have studied the pressure-induced phase transformation in a large single crystal of ${\mathrm{ReO}}_3$ at 300 K and (0–6)-kbar hydrostatic pressure. Inelastic measurements at 1 bar show a pronounced minimum in the ${\mathrm{T}}_1$ mode near the Brillouin-zone boundary (M point). Above $P_c$=5.2 kbar superlattice reflections with intensities consistent with $M_3$-like rotations of ${\mathrm{ReO}}_6$ octahedra appear. The transformation seems to be continuous (second order), but an anomaly in the superlattice intensity at P=5.4 kbar suggests a further transformation. The $M_3$ 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.