LOW-TEMPERATURE THERMAL AND DIELECTRIC-PROPERTIES OF NA0.5BI0.5TIO3

Thermal conductivity (at 5 to 300 K), dielectric (at 5 to 250 K) and s ound velocity (at 5 to 150 K) measurements have been performed for the first time on ceramic samples of Na0.5Bi0.5Ti03 The data revealed a n early temperature-independent plateau in thermal conductivity lambda ( T) at about 7 to 30 K and approximate to 0.2 W/mK. This plateau corres ponds to a broad maximum in the heat capacity CexptT-3 plot for this m aterial. Strong temperature and frequency dependences of electrical pe rmittivity were observed. The results of the measurements presented he re indicate a glass-like behaviour of Na0.5Bi0.5TiO3 at low temperatur es. Strongly frequency-dependent phonon scattering and/or interactions of phonons and domain walls with excitations, which arise from fluctu ations between the rhombohedral matrix and regions of another phase, a re responsible for this behaviour. Additional measurements performed f or Na0.5Bi0.5TiO3 single crystals (at 80 to 440 K) reveal the existenc e of very weak paramagnetic properties in this temperature range. The sound velocity, elastic stiffness moduli, phonon mean free path and di electric 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 Theta(D)(elast) = 190 K (180 and 1 85 K was recently obtained from calorimetric and pyroelectric measurem ents, respectively).