Consequences of Ca multisite occupation for the conducting properties of BaTiO3

doi:10.1016/j.jssc.2016.08.013

Journal of Solid State Chemistry

Volume 243, November 2016, Pages 77-82

https://doi.org/10.1016/j.jssc.2016.08.013 Get rights and content

Abstract

In combination with the dielectric modulus formalism and theoretical calculations, a newly developed defect incorporation mode, which is a combination of the standard A- and B-site doping mechanisms, is used to explain the conducting properties in 5 mol% Ca-doped BaTiO₃. Simulation results for Ca solution energies in the BaTiO₃ lattice show that the new oxygen vacancy inducing mixed mode exhibits low defect energies. A reduction in dc conductivity compared with undoped BaTiO₃ is witnessed for the incorporation of Ca. The conducting properties of 5 mol% Ca-doped BaTiO₃ are analyzed using molecular dynamics and impedance spectroscopy. The ionic conductivity activation energies for each incorporation mode are calculated and good agreement with experimental data for oxygen migration is observed. The likely existence of the proposed defect configuration is also analyzed on the basis of these methods.

Graphical abstract

Oxygen vacancy formation as a result of self-compensation in Ca-doped BaTiO₃.

Introduction

Barium titanate (BaTiO₃ or BT) has many industrial and technological applications as a result of its powerful electrical properties. It is well known for its ferroelectric properties and high dielectric constant [1], [2], as well as its use in lead-free piezoelectric devices [1], [2], [3]. In addition, its positive temperature coefficient of resistivity [2], [3] underlines its importance in temperature sensors. Other important applications of BT include random access memories (RAMs), infrared sensors and optic fibers [1], [2], [3], [4], [5].

The natural ability of the perovskite structure (given in Fig. 1) to accept dopants with different ionic radii is essential for modifying the desired properties and applications of BT-based materials. Which cation site becomes occupied by the dopant is dependent on the ionic radii of the dopant [6], [7], [8], [9], [10]. The ionic radii of Ba²⁺ and Ti⁴⁺, in six-fold coordination, are 1.35 and 0.68 Å [11], respectively. Ca-doped BT (BCT) is an interesting case as the Ca²⁺ (1.00 Å (six-fold coordination) [11]) dopant can occupy either the octahedral B-site as an acceptor dopant or undergo isovalent substitution at the A-site, depending on the BT stoichiometry and formation conditions [12], [13], [14], [15]. This interesting phenomenon of Ca²⁺ multisite occupancy in BT has been previously experimentally explored [12], [13], [14], [15]. These previous studies revealed that extrinsic oxygen vacancies, as a result of B-site doping, can contribute to the total conductivity, relaxor behavior and other properties of BT-based materials [12], [13], [14], [15], [16].

In a previous study, we obtained the defect energetics for several dopant incorporation modes and the dielectric-electric properties in 1 mol% Er-doped BT [17]. Two incorporation mechanisms, involving oxygen vacancy formation, were proposed and their existence was demonstrated using both experimental and molecular dynamics (MD) results. A somewhat similar, but alternative mechanism for introducing oxygen vacancies into La-doped BT has also been computationally proposed [18].

Oxygen vacancies are responsible for the conductive behavior observed in BaTiO₃ up to the Curie temperature and are used to explain the origin of the dielectric anomaly [19]. Traditional incorporation mechanisms that involve the substitution of Ca²⁺ at Ba sites (in stoichiometric Ba_1-xCa_xTiO₃) do not require/involve the creation of oxygen vacancies in the lattice structure [10], [12]. Therefore, such mechanisms cannot be used to explain the conductive behavior [20].

Here, joint computational-experimental studies were carried out to explore the defect chemistry and conducting properties of 5 mol% Ca-doped BT. The standard incorporation modes of Ca²⁺ into BT are analyzed, together with an additional oxygen vacancy inducing incorporation mechanism that combines both of the standard mechanisms. The feasibility of our model is supported by comparison with the solution energies for each substitution mode, the conduction behavior and by the experimental measurements from impedance spectroscopy. The results are divided into three sections. The first section concerns the analysis of the defect energetics of the Ca-doped structure and the second and third sections deal with the conducting properties of BCT at high temperatures obtained from MD and impedance spectroscopy measurements.

Section snippets

Computational details

Lattice statics simulations were employed to calculate the defect energies. Ions are modeled as core-shell structures and the interactions between them are taken into account by a force field or potential model (also known as empirical potentials). All calculations were completed using the cubic phase of BT with a lattice parameter of 4.01 Å. This phase is the most stable at high processing temperatures. Generally, incorporation of Ca²⁺ ions in the BT structure is usually accounted for by two

Defect chemistry

On the basis of the defect mechanisms in Eqs. (1), (2), (3), the solution energies related to the three aforementioned incorporation mechanisms are defined as: $E_{s} = E_{subs, Ba}^{{Ca}^{2 +}} + E_{L}^{BaO} - E_{L}^{CaO}$ $E_{s} = E_{subs, Ti}^{{Ca}^{2 +}} + E_{vac}^{O^{2 -}} + E_{L}^{Ti O_{2}} - E_{L}^{CaO}$ $E_{s} = \frac{1}{2} (E_{subs, Ti}^{{Ca}^{2 +}} + E_{subs, Ba}^{{Ca}^{2 +}} + E_{vac}^{O^{2 -}} + E_{L}^{BaO} + E_{L}^{Ti O_{2}} - E_{L}^{CaO})$ for the A-site, B-site and mixed incorporation mechanisms, respectively. The values of the individual terms in these mechanisms, such as substitution $(E_{subs, Ti}^{{Ca}^{2 +}} and E_{subs, Ba}^{{Ca}^{2 +}})$ , lattice ( $E_{L}^{CaO}, E_{L}^{BaO} and E_{L}^{Ti O_{2}}$ ) and

Conclusions

We have used a wide-range of computational and experimental techniques to analyze the defect energetics and conducting properties of 5 mol% Ca-doped BT. The defect energy calculations show that out of the various dopant incorporation modes used, the A-site substitution mode is the preferred mechanism. However, the mixed mechanism (where Ca²⁺ occupy both Ba²⁺ and Ti⁴⁺ sites, yielding oxygen vacancies) is lower in energy than the B-site mode and provides us with an alternative way of looking at

Acknowledgments

This research has been supported by the Brazilian Agencies, CNPq, CAPES and FAPEAM, and the Belgian Development Cooperation through the VLIR-UOS (www.vliruos.be). VLIR-UOS supports partnerships between universities and colleges in Flanders (Belgium) and the South looking for innovative responses to global and local challenges.

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Consequences of Ca multisite occupation for the conducting properties of BaTiO3

Abstract

Graphical abstract

Introduction

Section snippets

Computational details

Defect chemistry

Conclusions

Acknowledgments

J. Mater. Sci. Technol.

J. Eur. Ceram. Soc.

Mater. Sci. Eng. B.

Physica B

Mat. Sci. Eng. B.

J. Solid. State Chem.

J. Phys. Chem. Solids

J. Comp. Phys.

Mat. Sci. Eng. B.

Solid State Commun.

Adv. Mater.

Jpn. J. Appl. Phys.

Mater. Res. Lett.

J. Ceram. Soc. Jpn.

J. Am. Ceram. Soc.

Electroceramics: Materials, Properties, Applications

J. Am. Ceram. Soc.

Inorg. Chem.

Appl. Phys. Lett.

Appl. Phys. Lett.

J. Sol-Gel Sci. Technol.

Consequences of Ca multisite occupation for the conducting properties of BaTiO₃