Electron diffraction study of the space group of Bi₅Ti₃FeO₁₅ multiferroic ceramic

Bi₅Ti₃FeO₁₅ (pentabismuth trititanium iron pentadecaoxide), which is a multiferroic four-layer Aurivillius phase com­pound, has received much attention in recent years. However, three mutually inconsistent ortho­rhom­bic space groups, i.e. A2₁am, Fmm2 and Pnn2, have been reported for the room-temperature phase of Bi₅Ti₃FeO₁₅ by X-ray and neutron diffraction investigations. Here, electron diffraction results are presented and discussed for the first time to unambiguously clarify the room-temperature space group of ceramic Bi₅Ti₃FeO₁₅. It has been found that all the observed reflections from the ceramic agree with those expected in A2₁am, while the observed reflections 011, 013 and 015 should be forbidden in the case of Fmm2, and no 107 and 109 reflections were observed although allowed for Pnn2. The present study has demonstrated that the space group of Bi₅Ti₃FeO₁₅ ceramic is A2₁am rather than Fmm2 or Pnn2, an identification that proved to be a challenge for X-ray diffraction. On the basis of the space group A2₁am, the lattice parameters of the Bi₅Ti₃FeO₁₅ ceramic were calculated from its X-ray diffraction data.