Redox‐Based Resistive Switching Memories – Nanoionic Mechanisms, Prospects, and Challenges

II. Structure of Perovskites 1982 A. Crystal Structure 1982 B. Nonstoichiometry in Perovskites 1983 1. Oxygen Nonstoichiometry 1983 2. Cation Nonstoichiometry 1984 C. Physical Properties 1985 D. Adsorption Properties 1986 1. CO and NO Adsorption 1986 2. Oxygen Adsorption 1987 E. Specific Surface and Porosity 1987 F. Thermal Stability in a Reducing Atmosphere 1989 III. Acid−Base and Redox Properties 1990 A. Acidity and Basicity 1990 B. Redox Processes 1991 1. Kinetics and Mechanisms 1992 2. Reduction−Oxidation Cycles 1993 C. Ion Mobility 1993 1. Oxygen Transport 1993 2. Cation Transport 1994 IV. Heterogeneous Catalysis 1995 A. Oxidation Reactions 1995 1. CO Oxidation 1995 2. Oxidation of Hydrocarbons 1996 B. Pollution Abatement 2001 1. NOx Decomposition 2001 2. Exhaust Treatment 2002 3. Stability 2004 C. Hydrogenation and Hydrogenolysis Reactions 2004 1. Hydrogenation of Carbon Oxides 2004 2. Hydrogenation and Hydrogenolysis Reactions 2006

Chemical structures and performance of perovskite oxides.

This review covers important advances in recent years in the physics of thin-film ferroelectric oxides, the strongest emphasis being on those aspects particular to ferroelectrics in thin-film form. The authors introduce the current state of development in the application of ferroelectric thin films for electronic devices and discuss the physics relevant for the performance and failure of these devices. Following this the review covers the enormous progress that has been made in the first-principles computational approach to understanding ferroelectrics. The authors then discuss in detail the important role that strain plays in determining the properties of epitaxial thin ferroelectric films. Finally, this review ends with a look at the emerging possibilities for nanoscale ferroelectrics, with particular emphasis on ferroelectrics in nonconventional nanoscale geometries.

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Physics of thin-film ferroelectric oxides

After twenty years of partly quiet and ten years of partly enthusiastic research into lead-free piezoceramics there are now clear prospects for transfer into applications in some areas. This mimics prior research into eliminating lead from other technologies that resulted in restricted lead use in batteries and dwindling use in other applications. A figure of merit analysis for key devices is presented and used to contrast lead-containing and lead-free piezoceramics. A number of existing applications emerge, where the usage of lead-free piezoceramics may be envisaged in the near future. A sufficient transition period to ensure reliability, however, is required. The use of lead-free piezoceramics for demanding applications with high reliability, displacements and frequency as well as a wide temperature range appears to remain in the distant future. New devices are outlined, where the figure of merit suggests skipping lead-containing piezoceramics altogether. Suggestions for the next pertinent research requirements are provided.

Transferring lead-free piezoelectric ceramics into application

We present a critical review that encompasses the fundamentals and state-of-the-art knowledge of barium titanate-based piezoelectrics. First, the essential crystallography, thermodynamic relations, and concepts necessary to understand piezoelectricity and ferroelectricity in barium titanate are discussed. Strategies to optimize piezoelectric properties through microstructure control and chemical modification are also introduced. Thereafter, we systematically review the synthesis, microstructure, and phase diagrams of barium titanate-based piezoelectrics and provide a detailed compilation of their functional and mechanical properties. The most salient materials treated include the (Ba,Ca)(Zr,Ti)O3, (Ba,Ca)(Sn,Ti)O3, and (Ba,Ca)(Hf,Ti)O3 solid solution systems. The technological relevance of barium titanate-based piezoelectrics is also discussed and some potential market indicators are outlined. Finally, perspectives on productive lines of future research and promising areas for the applications of these ma...

BaTiO3-based piezoelectrics: Fundamentals, current status, and perspectives

The defect chemistry of polycrystalline has been studied by means of the equilibrium electrical conductivity as a function of temperature, oxygen activity, Sr/Ti ratio, and impurity additions. Reduction, excess , and acceptor impurities all contribute to the oxygen vacancy content and their effects are therefore highly interdependent. The effect of added donor‐impurities,e.g., 500 ppm Nb, is highly dependent on the presence and amount of excess .

https://iopscience.iop.org/article/10.1149/1.2127727/pdf

Nonstoichiometry in SrTiO3

Equilibrium electrical conductivity data for large-grained, poly crystalline, undoped BaTiO3, as a function of temperature, 750° to 1000°C, and oxygen partial pressure, 10−20<PO2<10−1 MPa, were quantitatively fit to a defect model involving only doubly ionized oxygen vacancies, electrons, holes, and accidental acceptor impurities. The latter are invariably present in sufficient excess to control the defect concentrations through the compensating oxygen vacancies, except under the most severely reducing conditions. Singly ionized oxygen vacancies play no discernible role in the defect chemistry of BaTiO3 within this experimental range. The derived accidental acceptor content has a slight temperature dependence which may reflect some small amount of defect association. Deviation of the conductivity minima from the ideal shape yields a small PO2-independent conductivity contribution, which is tentatively identified as oxygen vacancy conduction.

Nonstoichiometry in Undoped BaTiO3

The effect of added Al, as an acceptor impurity, on the equilibrium electrical conductivity of large-grained, polycrystalline BaTiO3, is consistent with a previously proposed defect model which involves only doubly ionized oxygen vacancies, electrons, holes, and acceptor impurities. The behavior is an extension of that of undoped BaTiO3, in which an accidental net acceptor excess already plays an important role. Comparison of the derived active acceptor content with the amount of added Al indicates that Al is <50% effective in creating acceptor levels. The magnitude of a small Po2-independent conductivity component, necessary to fit the observed conductivity minima, increases with added Al content. This is consistent with a contribution from extrinsic oxygen vacancy conduction.

Nonstoichiometry in Acceptor‐Doped BaTiO3

Donor additions to BaTiO3 up to a few tenths atom percent are compensated by electrons and the resulting electrical conductivity is independent of temperature and Po2 at 700° to 1000°C. The conductivities are impurity-insensitive at very low Poz and high temperature where reduction is the major source of defects. Variation in the site occupation ratio (A/B in ABO3 has a small effect on the conductivities for donor additions in the 100 ppm range. Nb is more effective as a donor than is Al as an acceptor, and Nb can compensate approximately 2½ times as much Al on an atomic basis.

Defect Chemistry of Donor‐Doped BaTiO3

Scanning electron microscopy and electron probe micro-analysis were used to investigate the microstructure of both slow-cooled and quenched polycrystalline BaTiO3 specimens with a small excess of TiO2 (Ba/Ti=0.995 to 0.999) or of BaO (Ba/Ti=1.002 and 1.005). The electron micrographs of polished and etched TiO2-excess BaTiOs samples, and of fracture surfaces of quenched samples, showed a second phase in the grain boundaries and triple-point regions, whereas no second phase was observed in samples having Ba/Ti=1.000. Microprobe analysis of the second phase gave compositions near that of the reported adjacent phase of higher TiO2 content, Ba6Ti17O40. The results indicate that the solubility of TiO2 in BaTiO3 is <0.1 mol%.

N.-H. Chan

Papers

Nonstoichiometry in SrTiO3

Nonstoichiometry in Undoped BaTiO3

Nonstoichiometry in Acceptor‐Doped BaTiO3

Defect Chemistry of Donor‐Doped BaTiO3

Solubility of TiO2 in BaTiO3