Structural origins of dielectric anomalies in the filled tetragonal tungsten bronze, Sr2NaNb5O15

Abstract

The tetragonal tungsten bronze, Sr₂NaNb₅O₁₅, shows promise for application in high-temperature high-efficiency capacitors vital for the sustainable energy revolution. Previously, the structural complexity of this and related materials has obscured the mechanisms underpinning two large anomalies in relative permittivity (ε_r) which give rise to their exceptionally broad dielectric response. We comprehensively investigate the structural evolution from − 173 to 627°C, combining electron, X-ray and neutron diffraction, electron microscopy, and first principles electronic structure calculations to unambiguously identify the structural origins of both anomalies. The peak in ε_r at 305°C is associated with a polar-nonpolar phase transition, wherein cations displace along the c-axis. Guided by DFT, we identify a further transition upon cooling, associated with the second peak at − 14°C, linked to the softening of an in-plane polar distortion with a correlation length limited by ferroelastic nano-domains arising from rigid-unit-like tilting of NbO₆ octahedra at high temperature, imparting relaxor-like behaviour. Thus, the two dielectric anomalies in Sr₂NaNb₅O₁₅ are associated with two distinct crystallographic phase transitions and their interplay with a microstructure that arises from a third, non-polar structural distortion. Chemical control of these will enable development of tuneable materials with dielectric properties suitable for high-temperature energy storage applications.