Sammendrag
Tetragonal Tungsten Bronze (TTB) systems with morphotropic phase boundaries (MPBs) allow for broader compositional engineering compared to the more extensively studied perovskites with MPBs, and thus provide unexplored opportunities for functional properties. In this study, we investigate the solid solution system Ba₄Na₂Nb₁₀O₃₀ (BNN)-K₄Bi₂Nb₁₀O₃₀ (KBiN). The crystal structures of the ceramics were characterised by powder X-ray diffraction, which indicated the system exhibits a transition from orthorhombic (Ama2) to tetragonal symmetry (P4/mbm) between 30 and 40 % KBiN content, accompanied by converging of the a- and b- lattice parameters. In general, an expansion of the in-plane (a, b) and a contraction of the out-of-plane (c) parameters are observed as the KBiN content is increased. The maxima in permittivity observed for compositions containing 20-35 % KBiN, suggest that ferroelectric phase transitions occur as a function of temperature, while more diffuse dielectric behaviour is exhibited for compositions with 40 % KBiN and higher. Non-linear P-E loops are evident up to 35 % KBiN, and vaguely present for 40 %. The most prominent ferroelectric character, assigned by large remanent polarisation and opening of the strain hysteresis response is exhibited at 35 % KBiN. The structural, electrical and electromechanical data suggests that a ferroelastic-paraelastic phase transition occurs between 30-35 % KBiN and a ferroelectric-paraelectric phase transition between 35 and 50 % KBiN. Thus, we suggest that two distinct phase transitions occur between 30 and 50 % KBiN content, mimicking the temperature dependent structural evolution observed in BNN. This work thus provides new insight into the solid solution of BNN and KBiN and showcases the properties manifested by ferroelastic-ferroelectric and ferroelectric-paraelectric phase transition in close proximity in a TTB system.
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