Ferrorotative transition in the antifluorite crystalK2OsCl6

The structural phase transition in potassium hexachloro-osmate (${\mathrm{K}}_2$Os${\mathrm{Cl}}_6$) has been investigated at the NRU reactor, Chalk River, by elastic neutron-scattering methods. The observed change in the crystal structure in passing through $T_c$ indicates that the Os${\mathrm{Cl}}_6$ octahedra undergo a collective ferrorotation. The data are consistent with both a displacive and an order-disorder model. The transition occurs at $T_c=44.5\mathrm{}\pm 0.4$ K and is continuous, with a critical exponent $\beta =0.35\mathrm{}\pm 0.06$ characteristic of three-dimensional phase transitions.