Low-Temperature Thermal and Dielectric Properties of Na0.5Bi0.5TiO3

Thermal conductivity (at 5 to 300 K), dielectric (at 5 to 250 K) and sound velocity (at 5 to 150 K) measurements have been performed for the first time on ceramic samples of Na0.5Bi0.5TiO3. The data revealed a nearly temperature-independent plateau in thermal conductivity λ(T) at about 7 to 30 K and ≈︂0.2 W/mK. This plateau corresponds to a broad maximum in the heat capacity CexptT—3 plot for this material. Strong temperature and frequency dependences of electrical permittivity were observed. The results of the measurements presented here indicate a glass-like behaviour of Na0.5Bi0.5TiO3 at low temperatures. Strongly frequency-dependent phonon scattering and/or interactions of phonons and domain walls with excitations, which arise from fluctuations between the rhombohedral matrix and regions of another phase, are responsible for this behaviour. Additional measurements performed for Na0.5Bi0.5TiO3 single crystals (at 80 to 440 K) reveal the existence of very weak paramagnetic properties in this temperature range. The sound velocity, elastic stiffness moduli, phonon mean free path and dielectric losses have a small anomaly near 55 K indicating the presence of a new phase transition inferred in the present paper. The deduced elastic Debye temperature at 5 K is ΘelastD = 190 K (180 and 185 K was recently obtained from calorimetric and pyroelectric measurements, respectively).