Abstract
The versatile crystal structure of tetragonal tungsten bronzes (A12A24C4B10O30) can accommodate complex stoichiometries including cations in different valence states and vacant cation sites. Here, we report on the effect of thermally induced cation-vacancy disorder in the tetragonal tungsten bronze SrxBa1−xNb2O6 (SBNX). SBNX (x = 0.25, 0.33, 0.50, 0.61) ceramics, prepared by conventional solid-state synthesis, were annealed at varying temperatures and subsequently quenched to room temperature. The Curie temperature of all the SBNX materials increased with higher quenching temperatures, accompanied with ferroelectric hardening. The variation in thermal history also caused structural changes, specifically a contraction of the a lattice parameter and a minor elongation of the c parameter. These effects are discussed in relation to recent first principles calculations of the energy landscape of the cation-vacancy configurations and experimental evidence of thermally induced cation-vacancy disordering.
Highlights
The tetragonal tungsten bronze (TTB) structure has the general formula A12 A24 C4 B10 O30, where the two A1 and A2 cation sublattices have sites of relatively similar size
Oxygen vacancies and reduction of Nb were expected to result in chemical expansion of the crystal lattice in tungsten bronzes, as they do in perovskites [20], but our observations show a contraction of the total volume of the unit cell with increasing quenching temperature
We have performed a systematic study of the effect of thermal history on the structural and electrical properties of four compositions in the SBN system
Summary
The tetragonal tungsten bronze (TTB) structure has the general formula A12 A24 C4 B10 O30 , where the two A1 and A2 cation sublattices have sites of relatively similar size. The Curie temperature of the paraelectric to ferroelectric phase transition is known to decrease as the transition gradually becomes more relaxor-like with increasing strontium content, from a true ferroelectric for low strontium content and typical relaxor behavior for high strontium content [3]. The mechanism for the relaxor behavior in SBN is debated; it has recently been shown that incommensurate modulations accompany relaxor behavior in many tungsten bronzes, but in the case of SBN, the incommensurate modulation is present regardless of the barium content and√. The lowering of the Curie temperature is accompanied by a decrease of the a/c 10 ratio, which is analogous to tetragonality in perovskite structures. The sintering behavior of SBN is reported to be composition dependent due to the convex shape of the solidus line in the phase diagram [4], and it is challenging to control the microstructure due to abnormal grain growth [5]
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