Available online at www.sciencedirect.com Journal of the European Ceramic Society 28 (2008) 499–504 The electronic properties of complex oxides of bismuth with the mullite structure Kenneth J.D. MacKenzie ∗ , Troy Dougherty, Jeremie Barrel MacDiarmid Institute for Advanced Materials and Nanotechnology, School of Chemical and Physical Sciences, Victoria University of Wellington, New Zealand Available online 6 April 2007 Abstract 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 M˝ ossbauer 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. © 2007 Elsevier Ltd. All rights reserved. Keywords: Bismuth compounds; Powders-solid state reaction; Spectroscopy; X-ray methods; Electrical properties 1. Introduction In the last decade, ceramic materials previously considered interesting for their mechanical or engineering properties have increasingly been reinvestigated for other potentially useful functional properties, especially their electronic properties. The aluminosilicate mullite is an example of a material with excel- lent thermal and mechanical properties at elevated temperatures, making is an important engineering material. 1 Pure mullite is an insulator, allowing it to be used as a substrate for electronic devices, 1 but doping with transition metals is reported 2 to lower the resistivity by two orders of magnitude, to about 10 11  cm at room temperature. The dielectric properties of aluminosilicate mullite, which are also important in determining its usefulness as an electronic substrate material, have also been determined. 3,4 The spread in the reported dielectric constants of 6.7–7.5 at 1 MHz has been ∗ Corresponding author. Fax: +64 4463 5237. E-mail address: kenneth.mackenzie@vuw.ac.nz (K.J.D. MacKenzie). ascribed to differences in chemical composition and microstruc- ture of the ceramic samples, and in the proportion of glassy intergranular phases. 3 The dielectric constants remain more or less constant over a wide frequency range, up to 14 GHz. 4 These data, and the low dielectric loss of pure mullite confirm its useful high-frequency insulating properties. These conclusions are not unexpected, but indicate that materials with the mullite structure which may also display useful electronic functions should be sought in systems con- taining other elements. One such group of mullite-structured compounds based on bismuth oxide (bismuth aluminate, fer- rate, gallate and manganate) was first reported by Levin and Roth 5 without supporting X-ray structural data. The mullite- related structures of the aluminate and ferrate compounds were determined by single-crystal X-ray studies in the same year by Koizumi and Ikeda, 6 with X-ray determinations of the manganate 7 and gallate 8 following in 1967 and 1971, respec- tively. Subsequent refinements of these crystal structures 9–12 indicate that they all have orthorhombic unit cells in the space group Pbam with z = 2. The unit cell parameters are shown in Table 1, together with mullite for comparison. 0955-2219/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2007.03.012