Observation of a low-symmetry phase in Na_0.5Bi_0.5TiO₃ crystals by optical birefringence microscopy

Single crystals of sodium bismuth titanate (Na_0.5Bi_0.5TiO₃, NBT) (pure and Mn-doped) have been investigated by means of optical birefringence microscopy. For both crystals, it was found that, above ∼573 K, the optical axis is perfectly aligned with one of the pseudocubic 〈001〉_PC directions, while the domain walls appearing in the images are parallel to the pseudocubic 〈011〉_PC directions. These observations are consistent with a tetragonal symmetry. Below ∼573 K, the observed optical orientation of both crystals shows a broad angular distribution, with no clearly visible crystallographically oriented domain walls. These results are consistent with monoclinic symmetry. Most importantly, the direct observation of the temperature-driven rotation of the optical axis within the monoclinic plane is reported for the first time in a perovskite oxide material, in the present case for Mn-doped NBT. This has consequences for the understanding of phase transitions in technologically relevant compounds, such as PbZr_1−xTi_xO₃, which are considered to have monoclinic structure in some parts of the phase diagram.