The properties of TiO 2 ͑110͒ single crystals are greatly changed after hydrogen is doped into them through electrochemical hydrogen charging, in which the crystals have been placed in 0.01M NaOH solution to deposit hydrogen on their electrodes through the electrolysis of water. The changes in properties, including great increases in leakage current, strong dielectric dispersions over 10 2 -10 6 Hz, and a strong aging phenomenon suggest that hydrogen is a donor in TiO 2 while ionized hydrogen is not bound to an oxygen site by a strong O-H bond. This behavior of hydrogen, which can be characterized as a metastable donor, is different from the usual donor behavior of hydrogen in oxides and should be interesting for a better understanding of the behavior of hydrogen in oxides. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2900957͔The behavior of hydrogen in oxides has been a topic of extensive investigations. On one hand, hydrogen is a ubiquitous impurity in various oxides. It can be introduced into oxides from precursors, from annealing gases, 1 or even from aqueous vapor in ambient atmosphere.2 On the other hand, hydrogen behaves quite differently from one oxide to another. In some oxides, hydrogen forms deep gap states in all of its three charge states, H + , H 0 , and H − ; while in other oxides, hydrogen gives rise to a shallow level at the conduction band edge and acts as a donor. [3][4][5] As an example, hydrogen has been predicted theoretically 6 and proven experimentally 7 as a shallow donor in ZnO. It must possess a high stability in ZnO to survive high-temperature sintering or crystal growth. As a matter of fact, it is generally believed that as a donor, hydrogen ionizes to H + with the electron being delocalized in the oxide conduction band and then H + forms a single strong O-H bond to an O site; in contrast, when hydrogen is a deep impurity, it will only form some longer, weaker bonds to more than one neighboring oxygen and exist as a metastable interstitial.5 Metastable interstitial hydrogen has been observed in proton-exchanged LiNbO 3 . A sharp peak at around 3500 cm −1 can always be observed in the infrared absorption spectra of proton exchange processtreated LiNbO 3 , which is the characteristic absorption peak for the stretch mode of the O-H bond, as hydrogen has replaced some lithium ions in LiNbO 3 . When the acidity of the melt in the proton exchange process is high enough, another broad peak at 3280 cm −1 can also be observed, which is only metastable and will disappear through an annealing process. Obviously, these two peaks should correspond to two different states of hydrogen with quite different stabilities. The broad peak should indicate the existence in LiNbO 3 of hydrogen with a much lower stability, which is possibly a deep impurity forming only long and weak bonds to neighboring oxygen sites. In a previous investigation, hydrogen has been doped into TiO 2 single crystals through an electrochemical method.9 Presently, a study on the behavior of hydrogen in TiO 2 single crysta...

Degradation of TiO 2 -based ceramic capacitors was observed after hydrogen incorporation on the termination electrodes of the capacitors via electrolysis of water. Fourier-transform infrared ͑FTIR͒ absorption spectra analysis of polished rutile single crystals clearly showed that hydrogen was incorporated into the TiO 2 lattice through the treatment. Hydrogen reduces Ti 4ϩ to Ti 3ϩ and increases the concentration of charge carriers. The degradation was found to exhibit a strong dependence on time at room temperature. The degraded properties were spontaneously recovered through an aging process, showing a spontaneous recovery unique to TiO 2 -based ceramic capacitors. It is proposed that hydrogen is metastable in TiO 2 and that hydrogen-induced degradation has different stabilities among various oxide-based components and devices. © 2004 American Institute of Physics. ͓DOI: 10.1063/1.1637942͔The effects of hydrogen incorporation in materials have been extensively studied, particularly on metals ͑hydrogen embrittlement͒ and silicon-based devices ͑passivation of dangling bonds͒. In recent years, however, more and more attention has been paid to the reactions between hydrogen and metal oxide materials. [1][2][3] In our previous studies, we found that hydrogen reacts with many metal oxides at an ambient temperature in some electrochemical processes, including electroplating 4 and the electrolysis of water. 5,6 These ambient temperature reactions usually induce obvious degradations in the properties of these oxide-based components and devices. A systematic understanding of the reactions is of great importance for improving the reliability of metal oxide-based electronic components and devices. 7For many oxide-based components and devices, the degradation induced by the reaction of hydrogen is permanent and stable at room temperature. Sometimes, an annealing in an oxidizing atmosphere can recover the degraded properties. 4,8 Obviously, the hydrogen-induced defects in those oxides responsible for the degradation can only be eliminated by high-temperature oxidizing reactions. When we studied hydrogen-induced degradation in TiO 2 -based ceramic capacitors, however, we obtained a quite unexpected result. Although hydrogen induces an obvious degradation in the capacitors, the degraded properties exhibit a strong dependence on time, and the degradation is gradually recovered through an aging process without any thermal treatment. This indicates that hydrogen-induced defects are only metastable in TiO 2 -based ceramic capacitors at room temperature, in contrast to hydrogen-induced defects in many other oxides.The composition and sintering of the TiO 2 -based temperature-compensating ceramic capacitors used in the present investigation were reported in another paper. 9 The capacitors were 0.70 mm thick and 7.5 mm in diameter, with a 4.5 mm diameter silver electrode on one surface and a 5.5 mm diameter silver electrode on the other surface. Some capacitors were placed in a 0.01 M NaOH solution and dc voltages were imposed betwe...

Barium titanate single crystals had been placed in 0.01M NaOH solution to deposit hydrogen on their electrodes through the electrolysis of water. Two kinds of time-dependent changes in dielectric properties of the single crystals were observed: One occurred in the course of hydrogen deposition and the other lasted for a long period of time after the deposition. The diffusion of hydrogen and in and out of the single crystals may be the cause for the changes. This hydrogen-related dynamic dielectric behavior clearly shows that hydrogen is an important mobile ion other than oxygen vacancy in perovskite-type lattice. Extra attention should be paid to the role of hydrogen in time-dependent property changes, including resistance degradation and ferroelectric aging, of perovskite-type ferroelectric titanates. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2206686͔A wide variety of point defects can be formed in perovskite-type titanate lattices and they play a vital role in tailoring perovskite-type titanates to many important and diverse applications-from high dielectric constant ceramic capacitors and piezoelectric ceramic devices of high resistivity, 1 to semiconducting thermistors with positive temperature coefficient of resistivity ͑PTCR͒, and to oxide superconductors. Point defects are normally characterized by concentration, and the relationship between the physical properties and the concentration of various point defects has been extensively investigated for perovskite-type titanates. On the other hand, the mobility or the movement of point defects in perovskite lattices has also attracted more and more attention. Waser et al. demonstrated that oxygen vacancies in perovskite-type titanates would electromigrate when a dc field is applied and a concentration polarization of oxygen vacancies between the anode and the cathode would be formed, which is believed to be responsible for the well known resistance degradation in perovskite-type titanates and the lifetime of components and devices based on them is greatly reduced. 2 Recently, Ren observed a huge recoverable electrostrain in aged barium titanate single crystals, and he ascribed this amazing phenomenon to the so-called symmetry-conforming property of point defects in perovskite-type titanate lattices. 3 In the course of aging, oxygen vacancies migrate to a distribution conforming to the ferroelectric domain structure formed after the cubic to tetragonal transition and this distribution provides a restoring force for a reverse domain switching when electric field is removed. This discovery may lead to some applications in ultralarge stroke and nonlinear actuators for barium titanate single crystals. The migration of oxygen vacancies in aging was also found to result in a stabilization effect in ferroelectric materials. 4 Obviously, it is meaningful to establish a relatively full understanding of the behavior of various point defects in perovskite-type titanate lattices. In this letter, we report an interesting dynamic dielectric behavior of barium tit...