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 001PC directions, while the domain walls appearing in the images are parallel to the pseudocubic 011PC 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-x Ti_x O₃, which are considered to have monoclinic structure in some parts of the phase diagram.