Origin and stability of the dipolar response in a family of tetragonal tungsten bronze relaxors

TL;DR

This study investigates the origin and stability of dipolar responses in a new family of tetragonal tungsten bronze relaxors, revealing that dipole stability correlates with cation size and tetragonality, and suggesting uniaxial relaxor behavior.

Contribution

The paper provides new insights into the relationship between crystal structure, cation size, and relaxor dielectric behavior in tetragonal tungsten bronze materials.

Findings

Dipole stability increases with larger M3+ cation size.

Dipolar response correlates with crystallographic strain and tetragonality.

Materials are uniaxial relaxors with dipoles mainly at B1 sites.

Abstract

A new family of relaxor dielectrics with the tetragonal tungsten bronze structure (nominal composition Ba6M3+Nb9O30, M3+ = Ga, Sc or In) were studied using dielectric spectroscopy to probe the dynamic dipole response and correlate this with the crystal structure as determined from powder neutron diffraction. Independent analyses of real and imaginary parts of the complex dielectric function were used to determine characteristic temperature parameters, TVF, and TUDR, respectively. In each composition both these temperatures correlated with the temperature of maximum crystallographic strain, Tc/a determined from diffraction data. The overall behaviour is consistent with dipole freezing and the data indicate that the dipole stability increases with increasing M3+ cation size as a result of increased tetragonality of the unit cell. Crystallographic data suggests that these materials are uniaxial relaxors with the dipole moment predominantly restricted to the B1 cation site in the structure. Possible origins of the relaxor behaviour are discussed.