Peculiar Bi-ion dynamics in Na1/2Bi1/2TiO3 from terahertz and microwave dielectric spectroscopy

TL;DR

This study investigates the complex dielectric dynamics of Na1/2Bi1/2TiO3 using terahertz and microwave spectroscopy, revealing anharmonic Bi-ion vibrations and relaxation processes that underpin its hybrid ferroelectric behavior.

Contribution

It provides a detailed analysis of Bi-ion dynamics and relaxation modes in NBT, combining new spectroscopic data with previous findings to elucidate its dielectric anomaly.

Findings

Coupled sub-THz oscillator and relaxation mode above Tm

Slowing down of relaxations explains permittivity increase to Tm

Dominance of rhombohedral phase reduces relaxation strength below Tm

Abstract

Dynamics of the main dielectric anomaly in Na1/2Bi1/2TiO3 (NBT) was studied by time-domain THz and microwave spectroscopy, using also previously published data and their new overall fits. Above the dielectric maximum temperature Tm ~ 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 Tm. Below Tm, the main relaxation continues slowing down and additional relaxation, assigned to quasi-Debye losses, appears in the 10^11 Hz range. The oscillator hardens on cooling and takes over the whole oscillator strength. The permittivity decrease below Tm 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 is supported by previous structural studies. NBT represents a hybrid between standard and relaxor ferroelectric behaviour.