Active and passive defects in tetragonal tungsten bronze relaxor ferroelectrics

Area of Science:

• Materials Science
• Solid State Physics
• Crystallography

Background:

• Tetragonal tungsten bronze (TTB) oxides are dielectric materials known for complex distortions and modulated superstructures.
• Local structural deviations in TTB materials cause diffuse scattering in diffraction experiments.
• These materials' structure and charge dynamics are sensitive to defects like cation or oxygen vacancies.

Purpose of the Study:

• To investigate the impact of cation and oxygen vacancies on the structural and charge dynamical properties of TTB materials.
• To understand how these defects influence diffuse scattering patterns.
• To compare defect responses in 'filled' and 'unfilled' TTB structures.

Main Methods:

• Performed dielectric susceptibility measurements.
• Conducted single crystal diffraction experiments.
• Investigated TTB materials with both filled (Ba2NdFeNb4O15, Ba2PrFeNb4O15) and unfilled (Sr0.5Ba0.5Nb2O6) cation sublattices.
• Carried out measurements before and after oxygen annealing to modify oxygen vacancy concentrations.

Main Results:

• Diffuse scattering features observed in unfilled TTB were also present in filled TTB.
• This suggests that random fields and disorder in unfilled TTB are not the sole cause of many observed local structural features.
• Oxygen annealing altered color and dielectric properties, indicating reduced oxygen vacancies, but had minimal impact on diffuse scattering patterns.

Conclusions:

• The presence of similar diffuse scattering in both filled and unfilled TTB structures indicates complex defect-induced phenomena beyond simple random field disorder.
• Oxygen vacancies significantly influence dielectric properties and color but not the primary diffuse scattering characteristics.
• Further research is needed to fully elucidate the origins of local structural deviations and diffuse scattering in TTB materials.