Order parameter and critical exponent for the pressure-induced phase transitions inReO3

Neutron powder diffraction data for ${\mathrm{ReO}}_3$ show a sequence of phase transitions starting at 5 kbar consistent with the condensation of $M_3$ phonons. The structure is cubic Pm3m at ambient pressure, tetragonal P4/mbm at 5.2 kbar, and cubic Im3 at 7.3 kbar and higher pressures. A single order parameter, the rotation angle of the ${\mathrm{ReO}}_6$ octahedra, describes the distortion in the high-pressure phases. The rotation angle varies as (P-$P_c$ ${)}^{\beta }$ with β=0.322(5) over the entire pressure range of the measurements 5.0 to 27.4 kbar. This value of β indicates an unusually large critical region extending to rotation angles of more than 14°.