D. Samuelis

A chemically driven insulator-metal transition in non-stoichiometric and amorphous gallium oxide.

Insulator-metal transitions are well known in transition-metal oxides, but inducing an insulator-metal transition in the oxide of a main group element is a major challenge. Here, we report the observation of an insulator-metal transition, with a conductivity jump of seven orders of magnitude, in highly non-stoichiometric, amorphous gallium oxide of approximate composition GaO(1.2) at a temperature around 670 K. We demonstrate through experimental studies and density-functional-theory calculations that the conductivity jump takes place at a critical gallium concentration and is induced by crystallization of stoichiometric Ga(2)O(3) within the metastable oxide matrix-in chemical terms by a disproportionation. This novel mechanism--an insulator-metal transition driven by a heterogeneous solid-state reaction--opens up a new route to achieve metallic behaviour in oxides that are expected to exist only as classic insulators.

Oxidation states of the transition metal cations in the highly nonstoichiometric perovskite-type oxide Ba0.1Sr0.9Co0.8Fe0.2O3−δ

The oxidation states of Fe and Co in the complex perovskite-type oxide Ba0.1Sr0.9Co0.8Fe0.2O3−δ were determined by means of X-ray absorption near edge structure (XANES) spectroscopy. Measurements performed in situ for 783 ≤ T/K ≤ 918 and 50 ≤ pO2/Pa ≤ 2 × 104 reveal that the electrons generated by reduction are not equally distributed between the Fe and Co cations. A comparison with thermogravimetric measurements indicates a discrepancy in the amount of oxygen lost from the sample. This discrepancy is an indication that not only are the transition metal cations reduced but the oxygen anions are, too. A model explaining the observed behaviour is proposed.

Collective and Tracer Diffusion via a Defect Cluster in LSGM

It was recently shown by Schulz and co-workers that in doped lanthanum gallate: La(1-x) SrxGa(1-y)MgyO(3-(x+y)/2) (LSGM) the three cations: La, Sr and Mg have tracer diffusivities that are nearly identical. To explain these findings, a bound defect cluster mechanism containing a cation vacancy from both the A- and the B- sublattices and an anion vacancy was proposed as the principal vehicle for cation diffusion in LSGM. In this paper, implications of this mechanism are considered for the first time. Sum-rule expressions for the collective correlation factors are derived and found to be in excellent agreement with Monte Carlo calculations. Expressions are also developed for the tracer correlation factors of lanthanum and gallium for diffusion via the cluster mechanism and tested by Monte Carlo computer simulation. Good agreement was found. The ratio of the tracer diffusivities of lanthanum and gallium are shown to be consistent with the cluster mechanism.

Correlation effects for cation diffusion via vacancy-pairs in fluorite-related oxides

Recent research has suggested that diffusion via vacancy-pairs could be an important contribution to cation diffusion in fluorite-related oxides, such as yttria-stabilized zirconia. In this paper, a combination of analytical development and Monte Carlo computer simulation is used to analyze various diffusion correlation effects for cation and oxygen ion diffusion via tightly bound vacancy-pairs in the fluorite structure. It is shown that the application of sum-rule relations provides exact expressions for the collective correlation functions. It is also shown that a formalism inspired by Mannings diffusion kinetics formalism gives accurate expressions for tracer correlation factors when tested against Monte Carlo simulation results. It is also shown that the tracer correlation factors follow the impurity-form, thereby simplifying an interpretation of the diffusion isotope effect.

Diffusion Kinetics Analysis of Cation Diffusion in YSZ and LSGM

In this paper, we discuss different possible cation diffusion mechanisms in YSZ and LSGM. Monte Carlo simulations are also reported of tracer diffusivities in LSGM for a postulated cluster mechanism. These simulations extend earlier simulations on un-doped material. The limits of the ratio of the diffusivities are consistent with experimental tracer diffusion findings over a wide range of cation-vacancy exchange frequencies. We also develop relationships between the phenomenological coefficients and use these relationships to predict possible demixing and interdiffusion experimental outcomes.

Analysis of the demixing of yttria-stabilized zirconia in an electric field for different diffusion mechanisms

Steady-state demixing of the cations in yttria-stabilized zirconia in an electric field is analyzed for diffusion via independent vacancies, by vacancy pairs and by vacancy triplets. We analyze two conditions, one (open system) where the vacancies are everywhere close to an internal surface, which allows the Schottky defect reaction to permit the equilibration of the concentration of the vacancies, the other (closed system) where such equilibration is only possible at the external surface. For diffusion via independent vacancies and vacancy pairs, critical values of the ratio of the yttrium to zirconium vacancy exchange frequencies causing yttrium enrichment at the cathode are determined. It is also shown that diffusion via the vacancy-triplet mechanism always leads to yttrium enrichment at the anode end. Using Monte Carlo simulation, we also verify the analysis of the situation where demixing occurs by independent cation vacancies in a closed system.

Cation Diffusion and Demixing in Yttria Stabilized Zirconia: Comparison of Assumptions of Constant Electric Field and Constant Current

In this paper, the phenomenon of steady-state cation demixing in yttria-stabilized zirconia in an electric field is examined for the case of an open system wherein the defects are everywhere close to internal surfaces which allow the Schottky-defect reaction to occur (very fine grain material). The effect of the constant electric field assumption as a substitute for the constant current condition is analyzed for diffusion by vacancy-pairs and by vacancy-triplets over a wide range of applied electric fields. It is shown that the assumption of a constant electric field provides only a moderate correction to the shape of the demixing concentration profile.

Contribution to the Theory of Demixing of Yttrium in Yttria- Stabilized-Zirconia in an Electric Field

In this paper, the nature of steady state demixing of yttria-stabilized zirconia in an electric field is examined for the case of open system conditions (very fine grain material) wherein the defects are everywhere close to internal surfaces which allow the Schottky defect reaction to occur. It is shown that under such conditions the applied force needs to be some 20 -25 times larger than for the case of closed system conditions (Schottky defect reaction occurs only at the external surfaces) in order to achieve the same degree of demixing of the cation components. The effect of the constant electric field assumption as a substitute for the constant current condition is also analyzed for a wide range of applied electric fields. It is shown that in most cases, the assumption of a constant electric field provides only a moderate correction to the shape of the concentration profile.