Thermal and Dielectric Behaviour of Pure and Doped Na0.5Bi0.5TiO3 at Low Temperatures

Low-temperature (4.2 to 300 K) thermal conductivity measurements of undoped NBT crystals and of Ba, Mn and Nb doped NBT are presented for the first time. First low-temperature dielectric measurements of NBT single crystals are also presented. It is shown that a nearly temperature independent regime (plateau) of thermal conductivities λ appears, as in glasses. This plateau correlates with an anomaly in the CexptT—3 curve. These non-Debye low-temperature features are observed in polycrystalline as well as crystalline samples, so they cannot be related to grain boundary mechanisms. Introducing substitutional impurities into crystalline or polycrystalline NBT has only a small influence on the temperature dependence of the thermal conductivity at low temperatures (the plateau region exists in all samples). This fact indicates that this anomaly is related rather to intrinsic mechanisms and not to impurity doping. Strong temperature and frequency dependences of the dielectric permittivity were observed. A low-temperature loss anomaly was reported. This anomaly arises from dielectric relaxation which can be connected with polarons localised on defects (oxygen vacancies) and/or on twinning walls between rhombohedral domains and on rhombohedral–another phase (probably tetragonal) interphases.