Order parameter and critical exponent for the pressure-induced phase transitions in Re0(3).

Neutron powder diffraction data for Re0(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 Re0(6) octahedra, describes the distortion in the high pressure phases. The rotation angle varies as (P-Pc)(beta) with beta = 0.322(5) over the entire pressure range of the measurements, 5.0 to 27.4 kbar. This value of beta indicates an unusually large critical region extending to rotation angles of more than 14 degrees.