Role of ferrons in the heat capacity and thermal transport of displacive ferroelectrics

The collective amplitude mode of the order parameter in displacive ferroelectrics, termed the ferron, represents the amplitude fluctuations of long-range ordered polarization. At temperatures well below phase-transition temperature Tc, the energy of ferron excitation is significantly gapped in the long-wavelength limit. As Tc is approached, this gap softens dramatically to minimal or gapless values, and thereby should lead to a substantial contribution to thermal properties. In this context, we explore the role of ferrons in heat capacity and thermal transport by incorporating a microscopic self-consistent phase-transition theory for displacive ferroelectricity, in contrast to the conventional treatment of attributing thermal properties solely to acoustic phonons. Using ferroelectric PbTiO3 as a case study, we show that the softening of ferrons near the phase transition is essential to accurately capturing the experimental temperature and electric-field dependencies of thermal properties.