The electronic properties of complex oxides of bismuth with the mullite structure

Bismuth forms double oxides with the oxides of aluminium, gallium, iron and manganese, all of which have the mullite structure. Since other bismuth-based complex oxides show useful functional properties including high-temperature pyroelectricity and piezoelectricity, the electronic and magnetic properties of the mullite-structured bismuth compounds are of potential interest. In the present study, all the above compounds were synthesised in pure form by solid-state reaction, and their thermal behaviour and structures characterised by thermal analysis, powder X-ray diffraction, solid-state MAS NMR and Mőssbauer spectroscopy as appropriate. The thermal analysis information allowed highly sintered polycrystalline samples to be produced. After the application of silver electrodes, the electronic properties of the pellet samples were determined as a function of frequency and temperature up to 900 °C. Frequency-dependent inflexions in the relative permittivity and loss angle curves of the ferrate and gallate may be due to losses due to dielectric relaxation, but the most notable and consistent electronic phenomenon in all samples is a steep rise in the dc conductivity at higher temperatures. The P – E hysteresis loops for the ferrate and manganate indicate that these are very poor ferroelectric materials, consistent with the centrosymmetric crystal structure of all these compounds.