Alkali Metal Aurivillius Phases Structure and Electrical Properties

Abstract: The quest for new materials is crucial for the development of smaller and more efficient electronic components. Aurivillius phases have recently received considerable attention due to their wide-ranging properties and are considered promising candidates in e.g. memory devices, microwave filters and fuel cells.

In this thesis, alkali metal Aurivillius phase compounds within the system Bi_2.5Me_m-1.5B_mO_3m+3 (Me = Li, Na, K; B = Nb, Ta, and m = 2-4) have been studied. Polycrystalline samples were successfully prepared by a high-temperature solid-state synthesis of metal oxides and carbonates. The samples were initially characterized by X-ray powder diffraction and the crystal structures were refined by using the Rietveld method on neutron powder diffraction data. High-resolution transmission microscopy was used to examine the intergrowth and complex impedance measurements were performed to elucidate the electrical properties.

Results show that the compounds with m = 2 are isostructural and the structural distortion decreases with increasing size of the Me cation. The tantalate compound is less distorted than the corresponding niobate, and bond valence sum calculations indicate that the Ta-O bonds are somewhat stronger than the Nb-O bonds. The a and b unit cell parameters increase with increasing m and approach the value of a pure perovskite, causing greater structural strain. Stacking faults, i.e. extended planar defects of thinner and thicker perovskite slabs, also increase.

High-temperature studies of Bi_2.5K_0.5Nb₂O₉ demonstrate that the ferroelectric-paraelectric transition occurs at the Curie temperature (T_C) without passing through an intermediate phase. The T_C can be correlated to the structural distortion of the investigated compounds and it rises with increasing structural distortion. Bi_2.5Li_0.5Nb₂O₉ behaves differently from the other m = 2 compounds on heating and shows a much more complex permittivity spectrum.