Critical behaviour study of ferroelectric semiconductors (PbxSn1−x)2P2S6 from thermal diffusivity measurements

An ac photopyroelectric calorimeter has been used to study the thermal diffusivity of the ferroelectric semiconductors family (PbxSn1−x)2P2S6 (x = 0.1 to 1) from low temperature to room temperature. A typical behaviour of poor thermal conduction has been observed, ascertaining the role of phonons as heat carriers. The substitution of Sn by Pb leads to an increase in thermal conduction due to the different ionic size. Thermal anisotropy has been found in this monoclinic structure, with heat diffusion being easier in the (1 0 0) direction. The second order character of the paraelectric to ferroelectric transition has been checked and a crossover in the evolution of its critical behaviour has been found, from a clear non-mean field model at x = 0.1 (where a model including both first-order fluctuations as well as the presence of defects needs to be considered) to a mean field one at x = 0.3. Besides, phenomenological parameters in the Landau expansion applied to the ferroelectric phase have given values in agreement with literature.