First-principles study of tetragonal PbTiO3: Phonon and thermal expansion

• The NTE behavior from RT to Tc of tetragonal PbTiO3 is concerned.
• The contribution of overall phonons to thermal expansion of PbTiO3 is positive.
• Different phonon modes have different effects to thermal expansion.
• Acoustic phonons promote larger volume expansion, while optical phonons cause volume contraction.
• The normal thermal expansion below RT can be explained.

Tetragonal PbTiO3 is a ferroelectric material and is reported to exhibit an unusual negative thermal expansion (NTE) behavior from room temperature to Curie temperature in experimental measurements. Since thermal expansion is closely associated with the anharmonicity of the lattice vibrations, the role of phonon in NTE is necessary to be given a detailed investigation. The vibrational properties indicate that: despite the NTE behavior in experiments, the contribution from overall phonons to thermal expansion is positive. Moreover, different phonon modes have different effects. Acoustic phonons promote larger volume expansion. In contrast, optical phonons lead to volume contraction. Due to the distinction of occupied phonon modes in different temperatures, this can be used to explain the normal thermal expansion below room temperature based on the related experiments.

Tetragonal PbTiO3 is a ferroelectric material and reported to exhibit an unusual NTE behavior from RT to Tc. Our phonon calculation indicates that: despite the NTE behavior in experiments, the contribution from overall phonons is positive to thermal expansion. Moreover, different phonon modes have different effects. Acoustic phonons can promote larger volume expansion, while optical phonons can lead to volumetric shrinkage. Acoustic phonons, which are most likely excited at low temperatures, play a major role in low-temperature thermal behavior. Combined with related experiments, this can be used to explain the normal thermal expansion below RT and provide complement for better understanding the origin of NTE.Figure optionsDownload as PowerPoint slide