Relaxor behaviour of BaBi4Ti4O15 ceramics

Abstract

Family of bismuth oxides was discovered more than 50 years ago by Aurivillius [1]. The lattice structure of the Aurivillius family of compounds is composed of n number of like perovskite (An-1BnO3n+3)2- unit cells sandwiched between (Bi2O2)2+ slabs along pseudo tetragonal c-axis. Majority of Aurivillius oxides are normal ferroelectrics with fairly high Curie temperature, while only a few of them such as BaBi4Ti4O15, BaBi2Nb2O9, BaBi2Ta2O9, etc. exhibit relaxor behaviour. Relaxor materials are characterized by frequency dispersion having broad dielectric anomaly in their dielectric behavior near the dielectric maximum. The latter properties are very useful for wide range of applications owing to their excellent high dielectric and piezoelectric responses over a wide range of temperatures [2].

In present work, BaBi4Ti4O15 - BBiT was prepared by noncoventional solid state reaction from mixture of oxide: BaO, TiO2 and Bi2O3 which was milled for 6 h. The powders were heated at 750oC for 4 h. Ceramic samples were prepared by conventional sintering of isostatic pressed (298 MPa) pellets at 1130 oC for 1 h. Dielectric properties were investigated in a wide range of temperatures (20-800°C) and frequencies (1 kHz-1 MHz) Figure 1a. displays temperature dependence of dielectric constant (ε’) of BBiT ceramics determined during cooling in BBiT ceramics. The results give an evidence of a diffuse phase transition accompanied by a relaxation of the permittivity. The temperature Tm of the maximum permittivity (εm) is shifted from 402°C to 417°C as the frequency increases from 1 kHz to 1 MHz. The temperature corresponding to also increases with the increase of frequency (Fig. 1b). The associated imaginary permittivity (ε’’) exibits a maximum at temperature close to the low temperature inflexion point of ε’ and this maximum is also shiftes towards the high temperature side as the frequency increases This behavior is typical for relaxor ferroelectric. The broad dielectric constant peaks at temperature Tm was frequently dependent. A modified Curie-Weiss relationship is used to study the diffuseness behavior of a ferroelectric phase transition where value of γ indicates the degree of diffuseness of BBiT material. The value of γ calculated from the slot of the curve is found to be 1.88 which reveals the near-relaxor nature of BBiT ceramics.

The variation of temperature of the dielectric maximum Tm as a function of ln(ν) is presented on Fig. 2. The dielectric relaxation rate follows the Vogel-Fulcher relation with Ea = 0.013 eV, ν0 = 2.09 × 108 Hz, and Tf = 651 K. All these parameters indicate that BaBi4Ti4O15 is a relaxor ferroelectric