  High temperature powder neutron diffraction experiments have been conducted
around the reported beta-gamma, insulator-metal phase transition (~ 930 C) in
BiFeO3. The results demonstrate that while a small volume contraction is
observed at the transition temperature, consistent with an insulator-metal
transition, both the beta and gamma phase of BiFeO3 exhibit orthorhombic
symmetry i.e. no further increase of symmetry occurs under the present
experimental conditions, contrary to previous suggestions. Furthermore we
observe the gamma orthorhombic phase to persist up to a temperature of
approximately 950 C before complete decomposition into Bi2Fe4O9 (and liquid
Bi2O3), which subsequently begins to decompose at approximately 960 C.
