Structure, dielectric and ferroelectric anisotropy of Sr_2−xCa_xBi₄Ti₅O₁₈ ceramics

Abstract

Sr_2−xCa_xBi₄Ti₅O₁₈(x = 0, 0.05) powders synthesized by solid state route were uniaxially pressed and sintered at 1225 °C for 2 h. The obtained dense ceramics exhibited crystallographic anisotropy with a dominant c axis parallel to the uniaxial pressing direction which was quantified in terms of the Lotgering factor. Microstructure anisotropy of both compositions (x = 0, 0.05) consisted of plate like grains exhibiting their larger surfaces mostly perpendicular to the uniaxial pressing direction. Dielectric and ferroelectric properties of Sr_2−xCa_xBi₄Ti₅O₁₈ ceramics were measured under an electric field (E) applied parallel and perpendicularly to uniaxial pressing direction. The obtained dielectric ɛ_R(T) and polarization (P–E) curves depended strongly on E direction thus denoting a significant effect from microstructure and crystallographic texture. Sr_2−xCa_xBi₄Ti₅O₁₈ properties were also significantly affected by Ca content (x): Curie temperature increased from 280 °C (x = 0) to 310 °C (x = 0.05) while ɛ_R and remnant polarization decreased for x = 0.05. The present results are discussed within the framework of the processing and crystal structure–properties relationships of Aurivillius oxides ceramics.

Introduction

The wide family of Aurivillius oxides (AO) is described by the general formula (Bi₂O₂)²⁺(A_n−1B_nO_3n+1)²⁻, where (A_n−1B_nO_3n+1)²⁻ stands for pseudo-perovskite blocks interleaved with bismuth oxide layers (Bi₂O₂)²⁺ along the c axis. A stands for a mono-, di- or trivalent ions or a combination of them, B represents a small ion with high valency as Ti⁴⁺, Nb⁵⁺, Ta⁵⁺, etc, or a combination of them, and n denotes the number of these oxides frequently referred as bismuth layer-structured ferroelectrics (BLSF). These materials were repeatedly investigated during the last two decades for modern microelectronic applications, in particular for ferroelectric random access memories (FeRAM) and dynamic random access memories (DRAM) [1], [2], [3], [4]. It is the case of SrBi₂M₂O₉ (M = Ta, Nb) compounds that became renowned due to their excellent fatigue endurance to the repetitive switching of polarization thus explaining their competitive position regarding the until then hegemony of lead-based PZT compositions [5]. Considering that toxicity issues associated to BLSF are of lower concern as compared to PZT [6], the interest for replacing PZT in its innumerous applications either as thin films or bulk ceramics by BLSF offering equivalent or better properties is naturally addressed. BLSF are strongly anisotropic with a spontaneous electrical polarization vector lying almost perpendicular to c axis thus explaining the strong dependence on crystallographic orientation of their dielectric and ferroelectric properties such as dielectric constant, remnant polarization and coercive field [7], [8], [9], [10], [11]. As examples SrBi₂Ta₂O₉ (n = 2), Bi₄Ti₃O₁₂ (n = 3) and SrBi₄Ti₄O₁₅ (n = 4) ceramics and/or single crystal were effectively reported with different dielectric and ferroelectric properties on a, b plane and along c axis [7], [12], [13]. These and other studies on BLSF have focused the structure, phase transition and ferroelectric properties of low n oxides (n = 2, 3, 4) [14], [15], [16], [17], [18], being n ≥ 5 a much less studied case [19], [20], [21], [22], [23]. Sr₂Bi₄Ti₅O₁₈ is a BLSF having n = 5 where Bi₂O₂ layers alternate with (Sr₂Bi₂Ti₅O₁₆) perovskite blocks built by five TiO₆ octahedral layers and hosting Sr and Bi at the A site. Being a lead free compound with a relatively high Curie temperature it has been pointed out as a useful ceramic candidate for several piezoelectric applications including piezoelectric resonators, filters and high temperature sensors [21]. For some of these applications Sr₂Bi₄Ti₅O₁₈ ceramics oriented by hot-forging as compared to nonoriented ones showed superior piezoelectric properties futhermore enhanced by Ca doping [21], [24]. Although dielectric and piezoelectric properties of Sr₂Bi₄Ti₅O₁₈ were already measured on ceramics textured by hot-forging [21], ferroelectric properties anisotropy of Sr₂Bi₄Ti₅O₁₈ ceramics, either with or without Ca addition, is still an uncovered issue.

The present study aims at contributing for the search of lead free ferroelectric materials offering competitive properties for electronic functional devices applications. The target compositions include Sr₂Bi₄Ti₅O₁₈ and a Ca doped composition, hereafter referred as Sr_2−xCa_xBi₄Ti₅O₁₈, which are synthesized by a solid state route and submitted to a conventional ceramic processing including uniaxial pressing and ordinary firing steps. As previous reports on Ca doped Sr₂Bi₄Ti₅O₁₈ have corroborated the detrimental effects of high Ca contents on the ferroelectric properties of Sr_2−xCa_xBi₄Ti₅O₁₈ [24], [25] a low Ca amount was selected for the present study. Moreover, in spite of the variability of the reported effects for small Ca contents on the polarization of the doped ceramics, x = 0.05 was envisaged as a convenient value since it is one of the low Ca dopant levels within the x range (0 < x < 0.20) where a consistent increasing tendency of remanent polarization was found by Quiang et al. [26]. The obtained ceramics were then fully characterized in terms of crystal structure, microstructure, dielectric and ferroelectric properties along two particular directions: (i) a direction parallel to the axis of the uniaxial pressure used for powder consolidation and (ii) a direction perpendicular to that same axis. The correlations between crystallographic orientation, microstructure and ferroelectric properties as well as their variation upon Ca addition are here presented and discussed.