Phase Transitions and Phonon Mode Dynamics of Ba(Cu1/3Nb2/3)O3 and Sr(Cu1/3Nb2/3)O3 for Understanding Thermoelectric Response

We report electrical and thermal properties of perovskite-type Ba-(Cu1/3Nb2/3)O-3 (BCN) and Sr(Cu1/3Nb2/3)O-3 (SCN). The BCN and SCN ceramics were synthesized by using the conventional solid state (CS) reaction method. The transmission electron microscopy analysis exhibited needle- and comb-type domain structures in BCN. Interestingly, SCN did not exhibit domain structure; however, it exhibited superlattice reflections due to ordering that were quite prominent in the selected area electron diffraction patterns. The temperature dependence of the dielectric response for BCN and SCN systems exhibits peaks due to structural phase transitions. The change in the Raman modes with increasing temperature also indicated the presence of phase transition in the temperature range 300-400 degrees C. BCN exhibited a lower value of the thermal conductivity (1.6 W/m.K at 600 degrees C) as compared to that of SCN (2.1 W/m.K at 600 degrees C) because of multiple phonon modes as identified through terahertz frequency domain spectroscopy.