X-ray and optical studies on phase transition of PbTiO3at low temperatures

Accurate measurements of the lattice parameters and the anisotropy of the optical susceptibilities, i.e., optical birefringence and activity of PbTi${\mathrm{O}}_3$ were made in the low-temperature region by using a special x-ray strainmeter and a newly developed polarimeter. It was found that PbTi${\mathrm{O}}_3$ undergoes a phase transition at -90°C between $C_{4v}$ and $C_{2v}$ point groups. This transition is different from the corresponding transition of BaTi${\mathrm{O}}_3$ in that the transition is of second order and the direction of polarization is unchanged. Lattice and optical anisotropies featuring the $C_{2v}$ phase are smaller than those ever found in other perovskite-type crystals.