Peculiar Bi-ion dynamics in Na_1/2Bi_1/2TiO₃ from terahertz and microwave dielectric spectroscopy

Abstract

Dynamics of the main dielectric anomaly in Na_1/2Bi_1/2TiO₃ (NBT) were studied by time-domain THz and microwave spectroscopy, using also previously published data and their new overall fits. Above the dielectric maximum temperature T_m ≈ 600 K, the response consists of coupled sub-THz oscillator and a relaxation mode, assigned to strongly anharmonic Bi-ion vibrations and hopping, whose slowing down explains the paraelectric-like permittivity increase to T_m. Below T_m, the main relaxation continues slowing down and additional relaxation, assigned to quasi-Debye losses, appears in the 10¹¹ Hz range. The oscillator hardens on cooling and takes over the whole oscillator strength. The permittivity decrease below T_m is caused by the reduced strength of the relaxations due to dominance of the rhombohedral phase within the coexistence region with the tetragonal phase. The anharmonic dynamics of Bi are supported by previous structural studies. NBT represents a hybrid between standard and relaxor ferroelectric behaviour.

Keywords:

• sodium bismuth titanate
• terahertz spectroscopy
• broadband dielectric spectroscopy
• phase-transition dynamics
• Bi-ion dynamics

Acknowledgements

The authors thank M. Savinov for the discussion on Arrhenius behaviour. The research was supported by the Czech Science Foundation (project 13-15110S). R. Dittmer acknowledges support from the German Research Foundation DFG within the collaborative research centre SFB595.