Yoed Tsur

Crystal and Defect Chemistry of Rare Earth Cations in BaTiO3

This study revisits the issue of rare earth cation substitutions into barium titanate. Analysis based upon crystal chemistry, defect chemistry and metastable states is presented to aid interpretation of experimental data. Recent detailed and highly precise X-ray powder diffraction and Electron Paramagnetic Resonance experiments performed on samples produced with different A/B ratios and fired under different oxygen partial pressure conditions give rise to new insights into the material. Specifically, the site occupancy and the valence states for the rare-earth dopants in barium titanate are considered. Earlier work is also reviewed and compared to the studies performed here. Collectively a classification of the various types of behavior observed for the rare-earth series in barium titanate is presented.

Site Occupancy of Rare-Earth Cations in BaTiO3

A systematic study on the site occupancy of rare-earth cations in Ba-rich and Ti-rich BaTiO3 fired in reducing atmosphere is performed. The corresponding lattice relaxation is used as an indicator of site occupancy and, to a lesser extent, defect structures. Accurate lattice parameters are obtained from X-ray diffraction data that is analyzed with the maximum likelihood method to account for correlated errors in the c and a lattice parameters. Comparisons of lattice volume as a function of ionic radius of the dopant reveals three regimes, with the intermediate sized cations (0.087 nm \leqslantr \leqslant0.094 nm where r is the ionic radius in six-fold coordination) demonstrating amphoteric behavior (occupying A- or B-sites). Defect chemistry analysis of site occupancy links the importance of metal vacancy ratios and oxygen vacancy concentrations with site occupancy. The trends predicted from the defect chemistry analysis are consistent with the observed lattice relaxations.

Analysis of impedance spectroscopy data: Finding the best system function

Impedance spectroscopy gains much attention as a non-destructive analysis technique in many areas of materials science and device manufacturing. While it is relatively easy to collect data, the correct analysis or the data interpretation is not a straightforward task. In this paper, a novel analysis technique that provides a simple mean to identify the best system function is shown.A new taxonomy of all the possible circuit models that are based on RC lumped elements is given. The taxonomy divides the various circuit models into groups of increasing complexity. Its order and family, where for RC elements there are four different families, identify each group. A “black box”, rather than a pre-assumed circuit model, represents the sample under test (SUT). The simplest group (order and family) that describes the SUT accurately within the experimental limitations can be found in a single experiment. In some cases, the best circuit model within the group can also be found by investigating the behavior of the SUT under various changes (i.e., temperature, radiation, other environmental conditions, sample construction, etc.).The technique is demonstrated on various circuits with lumped capacitors and resistors. This is done both on actual systems and on synthetic data with artificial noise. A comparison of this method with a standard Cole-Cole identification demonstrates the power of the new approach.

Identification of the Early Stage of Sintering of Nano- BaTiO3 A Comparative Study

The very early stage of nano-BaTiO 3 sintering was monitored using a number of experimental methods. It was found that in situ impedance spectroscopy provides the most sensitive probe to determine the beginning of the sintering process. Dramatic changes in the impedance can be detected in situ on time scales of minutes at temperature as low as 500°C. The mechanisms behind these changes hardly influence the surface area and do not result in shrinkage. A microscopic model of the process that complies with the experimental observations is suggested and discussed. This includes oxygen vacancy formation in the very early low temperature stage, and particle rearrangement at the beginning of the shrinkage stage.

How trivalent amphoteric dopants in BaTiO3 ceramics improve reliability of capacitors

A new hypothesis regarding the role of rare earth dopants in improving the lifetime of multilayer capacitors with base metal electrodes is presented. The hypothesis is based on the experimental findings, which have shown that the site occupancy of the important dopants is a function of the overall A/B ratio in the perovskite dielectric material. The main assumption is that deviation from A/B stoichiometry significantly controls the mobility of oxygen vacancies along the grain boundaries.

Doping of ionic compounds : solubility limit and self-compensation

Abstract An analysis is presented that allows calculating properties of primary solid solutions in ionic crystals. These properties are expressed in terms of properties of the undoped host material and the impurity. One can calculate the solubilities of the impurities with a knowledge of the concentration of the ‘corresponding native defects’, the standard chemical potential of the materials involved and the shear modulus of the host material. The changes in concentrations of ionic point defects as well as electron/hole concentrations due to doping are also calculated. In order to do so one has to know the concentrations of the native point defects in the undoped host material and the concentration of the dopant. It is shown that in a native p-type semiconductor (e.g. Cu 2 O) donors are mainly self-compensated by ionic defects, while acceptors are compensated significantly by holes.

Electrochemical impedance spectroscopy of supercapacitors: A novel analysis approach using evolutionary programming

In this contribution we present a novel approach to analyze impedance spectroscopy measurements of supercapacitors. Transforming the impedance data into frequency-dependent capacitance allows us to use Impedance Spectroscopy Genetic Programming (ISGP) in order to find the distribution function of relaxation times (DFRT) of the processes taking place in the tested device. Synthetic data was generated in order to demonstrate this technique and a model for supercapacitor ageing process has been obtained.

Kinetic Considerations in the Formation of Electrical Active Grain Boundaries in Barium Titanate and Similar Perovskites

Grain boundaries in ceramic barium titanate and related materials can be engineered in order to obtain desired transport behavior. Our ability to do so is closely related to kinetic limitations during the preparation. The close-packed structure of perovskites excludes native or foreign interstitials in the bulk. (Interstitial protons are regarded as OHO•, using the Kröger-Vink notation). Antisites are also unlikely due to size, charge and coordination number mismatch. The possible point defects are, therefore, substitutionals and vacancies. The kinetic limitations of these species, and the results in terms of grain boundary engineering, are considered in this contribution.A clear distinction between three different conditions is made. At very high temperatures, it is assumed that all the relevant defects are mobile and can equilibrate, at least locally. Hence, their concentrations are all functions of the degrees of freedom of the system. At lower temperatures, the cation sublattice is frozen. Therefore, the concentrations of metal vacancies and substitutional cations are constants and, from local electrical neutrality point of view, a new parameter becomes important: the concentration of frozen charge. The concentrations of electronic defects and oxygen vacancies in this metastable state are functions of temperature, oxygen partial pressure and frozen charge. The normalized concentration of frozen metal vacancies is calculated as a function of the doping factor, f (defined as the ratio between the electron concentration at a given state and at a reference state), and a nonstoichiometry parameter. Around room temperature, the anion sublattice is also frozen, and only electrons and holes exhibit significant transport properties.

Effect of Y-Doping on the Dielectric Properties of BatiO 3 Films Deposited in Reducing Atmospheres Using Pulsed Laser Deposition

BaTiO 3 -based multilayer capacitors with Ni electrodes are fired at low Po 2 to prevent the oxidation of Ni. Amphoteric dopants such as Y, Ho and Dy are used to prevent electrical degradation due to oxygen vacancy migration. This paper describes use of this approach in preparing thin film BaTiO 3 capacitors. Tolerance of reducing atmospheres is particularly important in films which are co-processed with resistors such as TaN for integrated resistor-capacitor networks. To assess this approach, films with and without Y-doping were grown in pure N 2 atmospheres by pulsed laser deposition from undoped and 0.2wt% Y 2 O 3 doped BaTiO 3 targets, respectively. Films were deposited onto Pt/Ti/SiO 2 /Si substrates using a 5∼10Hz laser repetition rate, an energy density of 4.2J/cm 2 and a working pressure of 100mTorr. It was found that polycrystalline perovskite films could be grown at 700°C and that Y-doped films had better crystallinity than the undoped ones. The dielectric constants of films without and with Y-doping are ∼1100 and ∼1300 at lkHz, respectively. The dielectric losses of Y-doped films were lower than for undoped films, possibly because the concentration of mobile charges including oxygen vacancies was decreased. Y-doped films also have a smaller coercive field (25kV/cm) and higher maximum polarization (∼20μC/cm 2 ) than undoped films. The defect chemistry of the system is also discussed.

The correlation between non-stoichiometry and charge compensation in perovskites

Non-stoichiometry in ternary oxide perovskites is analyzed to examine its donor/acceptor-like behavior using the doping factor concept. The general form, A+ αB+ βO3, represents the three ternary oxide perovskite systems: III-III, II-IV and I-V. The five most important native point defects in those systems are dealt with, namely, A- and B- metal vacancies, oxygen vacancies, electrons and holes. The influence of non-stoichiometry on the electron and hole concentrations compared to a reference state can be described by expansion of the doping factor concept from binary to ternary systems. The doping factor, f, is the parameter that quantifies the concentration change of quasi-free electrons in a solid upon changes in composition. In particular, f  > 1 when the solid is doped with donors and f < 1 when it is doped with acceptors. In the ternary system, changes in the A/B ratio in the undoped material are accompanied by small deviations of the metal vacancy concentrations, and therefore result in deviation of the doping factor f from unity. This deviation is expressed using an additional doping factor, which results from the added degree of freedom in the ternary case as compared to the binary system. It is shown that despite the well-known fact that frozen-in metal vacancies behave as acceptors, in the perovskite systems II-IV and I-V, there is a finite range in the A/B ratio where an increase in metal vacancy concentration results in donor-like behavior. In addition to its theoretical aspects, the importance of this analysis stems from the fact that, in practice, small deviations from stoichiometry in the cation sub-lattice are inevitable. Understanding the way in which these materials respond to non-stoichiometry is crucial for comprehending the observed electrical phenomena.