Triclinic crystal structure distortion of multiferroic BiMn₇O₁₂

The quadruple perovskite BiMn₇O₁₂ obtained via high-pressure synthesis was investigated by high-resolution synchrotron X-ray powder diffraction over a temperature range of 10 to 295 K. Careful Rietveld analysis reveals triclinic lattice distortion of BiMn₇O₁₂ at 295 K, which increases upon cooling to 10 K. Also hkl-dependent anisotropic Bragg reflection shape was introduced to give a precise description of the diffracted intensities. Importantly BiMn₇O₁₂ crystal structure was described in the non-centrosymmetric I1 triclinic space group. We also demonstrate the use of irreducible representations analysis (ISODISTORT program) for crystal structure distortion from Im to I1 space group. The irreducible representation which describes crystal structure distortion points towards possible ferroelectricity. Finally anisotropic thermal lattice expansion was observed.