Symmetry of the Low-Temperature Phase of BaTiO3

X-ray Weissenberg patterns of a barium titanate crystal in the low-temperature phase (below -90°C) show that the spacings along the perovskite cubic axes are equal and that the cube faces are sheared through a small angle, i.e., the unit cell has rhombohedral character. Optical examination of these crystals in electric fields, together with dielectric constant measurements, suggest that the cell is only pseudorhombohedral. The true symmetry appears to be lower and may possible be monoclinic (point group 2 or $m$). The contradictions in the early dielectric measurements are explained by this assumption.