Structure of the defect perovskite [Pb_0.85La_0.10] TiO₃ between 10 and 1023 K

The structure of the defect perovskite [Pb_0.85D'Alembertian sign_0.05La_0.10]TiO₃ was investigated by powder neutron and x-ray diffraction and by specific heat measurements made at various temperatures. Refinement of the structure by the Rietveld technique at 10, 200, and 300 K from time-of-flight powder neutron diffraction data revealed no low temperature transitions from the ferroelectric tetragonal (4mm) phase. On average, the oxygen octahedral framework of the parent compound was not distorted by the incorporation of lanthanum and vacancies, and the ratio of cation displacements along the polar axis was comparable to that observed for pure PbTiO₃. However, the material showed a large nonuniform strain that increased with decreasing temperature. As a result, the effective coupling between the uniform tetragonal deformation and cation displacement was reduced. X-ray diffraction and specific heat measurements at 298-1023 K indicated that the ferroelectric m3m→4mm transition was nearly tricritical and took place at 565 K. The diffuse nature of the transition was reflected in a temperature-dependent x-ray profile broadening and anomalous excess specific heat that persisted to ≊775 K, close to the phase transition temperature of pure lead titanate.