Electrical and transparent properties induced by structural modulation in (Sr0.925Ca0.075)2.5-0.5xNa xNb5O15 ceramics

The effects of Na+ concentration on the microstructural, dielectric, ferroelectric and transparent properties of (Sr0.925Ca0.075)2.5-0.5xNa xNb5O15 (SCNN; 0.4 ≤ x ≤ 1.0) ceramics prepared using a conventional solid-state reaction method were investigated. X-ray diffraction results showed that the phase structure transitioned from tetragonal to orthorhombic tungsten bronze along with the structural type changing from 'unfilled' to 'filled' state. The NbO6 octahedron became more distorted and had higher interaction strength when introducing Na+ in A-sites to decrease the structural vacancies, leading to the improvement of dielectric and ferroelectric properties. The dielectric spectra of all specimens showed two broad dielectric anomalies: a high-temperature anomaly (the ferroelectric phase transition) and a low-temperature anomaly caused by the subtle structural effects in 'unfilled' SCNN ceramics, which evolved into the ferroelastic phase transition in 'filled' SCNN ceramics.