Microstructural and high-temperature impedance spectroscopy study of Ba₆MNb₉O₃₀ (M=Ga, Sc, In) relaxor dielectric ceramics with tetragonal tungsten bronze structure

This work reports on the microstructural and high-temperature impedance spectroscopy study of a family of dielectric ceramics Ba₆MNb₉O₃₀ (M=Ga, Sc, In) of tetragonal tungsten bronze (TTB) structure with relaxor properties. For Ba₆GaNb₉O₃₀ and Ba₆InNb₉O₃₀ pellets, the SEM images have revealed good, dense internal microstructures, with well-bonded grains and only discrete porosity; in contrast Ba₆ScNb₉O₃₀ pellets had a poorer microstructure, with many small and poorly-bonded grains gathered in agglomerates, resulting in significant continuous porosity and poorly defined grain boundary regions. The electroactive regions were characterised by the bulk and grain boundaries capacitances and resistances, while their contribution to the electrical conduction process was estimated by determining activation energies from the temperature (Arrhenius) dependence of both electric conductivities and time constants. For Ga and In analogues the electronic conductivity are dominated by the bulk response, while for Sc analogue, the poorly defined grain boundaries give a bulk-like response, mixing with the main bulk contribution.