Elsevier

Applied Surface Science

Volume 253, Issue 1, 31 October 2006, Pages 354-357
Applied Surface Science

BST solid solutions, temperature evolution of the ferroelectric transitions

https://doi.org/10.1016/j.apsusc.2006.06.011Get rights and content

Abstract

Solid solutions Ba1  xSrxTiO3 (BST) are of high technological importance, particularly in microwave domain. Barium titanate has “naturally” three transitions, between four stable ferroelectric phases: (C) cubic, (T) tetragonal, (O) orthorhombic, (R) rhombohedral. Jaffe et al. [B. Jaffe, W.R. Cook, H. Jaffe, Piezoelectric Ceramics, Academic Press, 1971] has given the dependence of the transition temperatures up to 30% of Sr content. We have extrapolated these temperatures and we have found that some phases might disappear at higher Sr concentrations. A family of solid solutions with x = 25, 50, 75, 90% was prepared by standard solid-state reaction and sintered at 1230 and 1260 °C, respectively. The permitivities and the dielectric losses were measured with a self-acting bridge (1 kHz), on a large temperature range (±200 °C). The composition x = 25% shows three peak values of permittivity as expected, while the composition x = 50%, only two peak values, corresponding to phase transitions cubic–tetragonal–rhombohedral, phase orthorhombic being excluded. Compositions with x  63%, Sr shows only one peak value corresponding to a genuine transition cubic–rhombohedral. The cubic transition to several lower phases shows almost a linear decrease of the Curie point with the increase of Sr fraction. For Sr concentration x  80%, the Curie point appears to fall more rapidly than linear. To our best knowledge, there is for the first time, this effect is reported.

Introduction

Ferroelectric materials have a large range of applications, as dielectric, piezoelectric, pyroelectric, electro-optical material. The high dielectric permittivity of perovskite materials has stimulated their applications in ferroelectric non-volatile memories (FeRAM), dynamic random access memories (DRAM) and even more important in the microwave domain.

Barium titanate is the earliest known perovskite-type ferroelectric materials used in electronics. Thin films of high crystallinity are required for the new generation of devices. Crystal structure, ferroelectric properties of nanocrystalline thin layers perovskite materials, integrated on Si substrate, deposited by several methods are studied [1]. Integrated heterostructures on silicon are tested in order to replace SiO2 in Si CMOS devices. High-k materials are important candidates for gate dielectrics. An equivalent oxide of ∼20 Å, of high dielectric constant, can eliminate the leakage current, experienced with equivalent thick SiO2 layer. Low densities of traps, at the oxide silicon interface (due to the decrease of the number of dandling bonds), increase the carrier mobility [2].

Thickness dependence of ferroelectric domains, in thin crystalline layers, has been noticed [3]. In very thin samples of a few hundred angstroms, the BaTiO3 ferroelectric domains may disappear at 70 °C instead of 120–130 °C in bulk ceramics. The thickness of the surface relaxation layer, which is totally non-ferroelectric, was estimated to be of the order of 10 nm [3].

More recently, it was found that the dielectric properties of sintered barium titanate ceramic depend on the grain boundary width. A generalized diagram of size induced phase transition in BaTiO3 ceramic was proposed [4]. From Fig. 9, in ref. [4], it is clearly seen that at the grain dimensions of 20–40 nm, undergo “genuine” ferroelectric transitions between cubic and several other crystallographic states. This fact suggests not only a technological limitation for miniaturization of multilayer ceramic capacitors, but also a fundamental scientific problem encountered.

Pure BaTiO3 ceramic undergoes a para-ferro phase transition at 130 °C from cubic (C) to tetragonal (T) phase, as a typical first order phase transition of displacement type [5]. Other ferroelectric transitions around 5 °C (or ∼0 °C) from tetragonal (T) to orthorhombic (O) and the other, around −80 °C (up to −100 °C) between orthorhombic (O) and rhombohedral (R) appear also as first order transitions [6]. Barium substitution by lead or strontium are used to rise or to lower the Curie point in several applications.

Niobium oxide in a few percent, in the system Ba(Ti, Nb)O3 severely decreases the C–T transition temperature and rises the other T–O and O–R transitions [7]. At some precise compositions, the transitions C–O and C–R become possible (see Fig. 4 in ref. [7]). The permittivity in transitions shows large maxima not typical for the first order transition. A theoretical analysis of a typical transition between several polymorphic phases was presented in ref. [8].

Section snippets

Experimental

Barium strontium titanate ceramic (BST for short), with the formula Ba1  xSrxTiO3 was prepared as BST 25, 50, 75, 90, for x = 25, 50, 75, 90, respectively, by standard solid-state reaction technology. Details are given in refs. [9], [10]. Two sets of samples sintered at (I) 1230 and (II) 1260 °C, for 2 h shall be discussed. Dopants MgO and MnO2 (1 wt% each) were added to the set (II) in order to control the porosity of the structure [10]. Morphological and structural analyses were performed on the

Polymorphic diagram

Strontium substitution in BaTiO3 lattice lowers the Curie temperature of the cubic–tetragonal transition. We have used Fig. 5.28 from the book of Jaffe et al. [5], and we have extrapolated the lines representing the border of several crystallographic states of the Sr alloys by doted lines in Fig. 2. Surprisingly, we have learned that orthorhombic (O) and even tetragonal (T) phases are limited on the whole range of Sr concentrations.

Three distinct regions can be discerned in Fig. 2. In the

First transition analysis

Curie points of the first transition cubic–tetragonal/rhombohedral for both set of samples sintered at (I) 1230 and (II) 1260 °C are presented in Fig. 4. Similar data for BST samples sintered at 1320 °C were drawn from ref. [12]. The first transition temperatures and Curie constants are presented in Table 2. Curie points of this samples (empty stars in Fig. 4), fit very well the same curve as our samples (II). It is interesting to notice that our set of data (I), triangle in Fig. 4, fit quite

Discussions and conclusions

There is a good correspondence between ferroelectric transitions predicted by the Jaffe extrapolated lines, representing the border between several polymorphic states and our experimental data (Fig. 3). Lower fired (sintered) samples in the middle of the phase diagram fit much better the Jaffe line of the first transition. Our samples, from the set (II), sintered at higher temperature (1260 °C) and the samples fired at 1320 °C [12] (shown by short arrows in Fig. 4), have the transition

Acknowledgement

Authors acknowledge the financial support through the grants CERES 4-99/2004.

References (12)

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