The Effect of Thermal History on the Structural and Dielectric Properties of Tetragonal Tungsten Bronzes

Abstract

Tetragonal tungsten bronzes (TTB) are remarkable materials for their potential for chemical substitution and disorder. The niobates of this kind are usually ferroelectric, and they can comprise both ordered and disordered cation configurations, depending on the thermal history. The structural and ferroelectric properties are linked to the degree of disorder.

Phase pure and dense samples of the unfilled tungsten bronze strontium barium niobate (SBN) were synthesized by the solid-state route. The compositions SBN20, SBN25, SBN35, and SBN45 were quenched from the high sintering temperature, and differentiated by thermal history by heat-treating some of them at a lower temperature. The structural and dielectric properties were studied as a function of thermal history, using powder X-ray diffraction and dielectric spectroscopy. Phase pure samples of the filled tungsten bronzes potassium bismuth niobate (KBiN) and potassium neodymium niobate (KNdN) were also synthesized and their structural properties investigated by powder X-ray diffraction.

The as sintered samples of SBN were found to be more disordered than the heat-treated samples. This affects the critical temperature of the ferroelectric phase transformation, where Tc was increased after heat treatment by 32.2 degrees celsius for SBN20 and by 46.5 degrees celsius for SBN25. The room temperature unit cell parameters were found to change after heat treatments, in particular the c axes were expanded, allowing for a larger and more stable ferroelectric distortion.

The effect of thermal history on the phase transformation in SBN comes from the change in cation distribution, which in turn affects the lattice parameters and ferroelectric properties. The cations in SBN are mobile at a much lower temperature and shorter relaxation time than what was expected. This eclipses some of the quantitative results from this thesis, but is promising regarding how amenable tungsten bronzes are by thermal history.