ELECTRICAL PROPERTIES OF BARIUM BISMUTH TITANATE

Abstract

Family of bismuth oxides was discovered more than 50 years ago by Aurivillius [1]. Recently, there has been renewed interest in the properties of the Aurivillius phases as temperature-stable ferro-piezoelectrics [2]. Several bismuth-layered crystal structures and their properties have been investigated in detail. However, a lot of aspects of the preparation and properties of barium bismuth titanate, BaBi4Ti4O15 [BBiT] remain unexplored, whereas being promising candidate for high-temperature piezoelectric applications, memory application and ferroelectric nonvolatile memories (Fe-RAM). 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. The 12 coordinate perovskite-like A-site is typically occupied by a large cation such as Na+, K+, Ca2+, Sr2+, Ba2+, Pb2+, Bi3+ or Ln3+ and the 6-coordinate perovskite-like B-site by smaller cations such as Fe3+, Cr3+, Ti4+, Nb5+ or W6+ [2]. BBiT, as the n=4 member of the Aurivillius family has Ba and Bi ions at the A sites and Ti ions at the B sites of the perovskite block [(Bi2O2)2+· ((BaBi2)Ti4O13)2-] [3].

In present work, BBiT was prepared by solid state reaction from different starting oxides. The first rout (BBiT-1) is from stoichiometric quantities of BaTi3O12 and Bi4Ti3O12 obtained via mechanically assisted synthesis in planetary ball mill. BaTi3O12 has been synthesised from mixture of BaO and TiO2 and Bi4Ti3O12 was prepared starting from Bi2O3 and TiO2, commercially available. The second rout (BBiT-2) is from mixture of oxide: BaO, TiO2 and Bi2O3 which was milled for 6 h. The both powders were heated at 750 oC for 4 h. Ceramic samples were prepared by conventional sintering of isostatic pressed (298 MPa) pellets at 1130 oC for 1 h.

Figure 1. displays the real part of dielectric permittivity as a function of temperature determined during cooling in BBiT-1 and BBiT-2 ceramics measured in frequency interval from 1 kHz to 1 MHz. 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 for BBiT-1 and from 385 °C to 406 °C for BBiT-2 as the frequency increases from 1 kHz to 1 MHz. This behavior is typical for relaxor ferroelectric. Such a phenomenon has already been observed in several Ba-bearing Aurivillius compounds [4].

The value of dielectric constant for BBiT-1 ceramic in frequency interval from 1 kHz to 1 MHz is apprximately 1300 and for BBiT-2 approximately 2300.