Simultaneous electrical impedance and optical absorption spectroscopy for rapid characterization of oxygen vacancies and small polarons in doped ceria

Mixed ionic-electronic conductors (MIECs) play a central role in emerging energy conversion and energy efficient computational technologies. However, it is both challenging and resource demanding to characterize MIECs over the broad range of experimental conditions of interest, thereby significantly limiting their study and applications. Here, a novel method of a simultaneous measurement of electrical conductivity and optical absorption of thin films in out-of-equilibrium state, i.e. during a reduction process, is employed for a comprehensive study of a MIEC oxide, Pr_xCe_1-xO_2-δ (PCO). It enables, orders of magnitude faster than by established techniques, characterization of the oxygen vacancy and small polaron formation and transport as a function of temperature (demonstrated here down to 200 °C), in a wide range of deviation from stoichiometry, δ. For instance, at 600 °C the PCO properties were obtained during a ten minute reduction process, in the pO₂ range from 1 to 10^-13 bar. The experimental results show that the oxygen vacancy mobility is constant while the small polaron mobility is linear in δ, in the whole pO₂ range, which yields the total conductivity quadratic in δ. Furthermore, the method was applied to study the modification of PCO's transport properties with composition change. It was shown that increasing x from 0.1 to 0.2 suppresses the ionic mobility and, at the same time, enhances the small polaron mobility. Finally, the optically determined δ was used to define an instantaneous oxygen activity in PCO that can be accessed in the out-of-equilibrium experiments. This work opens up new possibilities to study the effects of microstructure, strain and other applied external stimuli on the transport and thermodynamic properties of PCO and similar types of MIEC materials.

Coupling of strain, stress, and oxygen non-stoichiometry in thin film Pr0.1Ce0.9O2-δ

Stress and strain in thin films of Pr0.1Ce0.9O2-δ, supported on yttria stabilized zirconia (YSZ) and sapphire substrates, induced by large deviations from oxygen stoichiometry (δ = 0) were investigated by in situ high temperature X-ray diffraction and wafer curvature studies. The measured stresses and strains were correlated with change in δ, measured in situ using optical transmission spectroscopy of defect centers in the films and compared with prior chemical capacitance studies. The coefficient of chemical expansion and elastic modulus values for the films were found to be 18% less than, and 16% greater than in the bulk, respectively. Irreproducible stress and strain during cycling on YSZ substrates was observed and related to microstructural changes as observed by TEM. The enthalpy of defect formation was found to be similar for films supported on sapphire and YSZ, and appeared to decrease with tensile stress, and increase with compressive stress. Larger stresses observed for YSZ supported films as compared to sapphire supported films were found and accounted for by the difference in film orientations.

The electrical conductivity of thin film donor doped hematite: from insulator to semiconductor by defect modulation

Hematite or α-Fe2O3 has emerged as a highly promising photoanode candidate for photoelectrochemical cells. While significant improvements in its performance have recently been achieved, it remains unclear why the maximum photocurrents still remain well below their theoretical predictions. Here, we report, for the first time, a detailed correlation between the electrical conductivity of undoped and 1 atom% Ti doped hematite and the conditions under which it was annealed (20 ≤ T ≤ 800 °C and 10(-4) ≤ pO2 ≤ 1 atm). Hematite thin films grown by pulsed laser deposition onto sapphire single crystals were evaluated by impedance spectroscopy. Hematite's room temperature conductivity can be increased from ∼10(-11) S cm(-1) for undoped films by as much as nine orders of magnitude by doping with the Ti donor. Furthermore, by controlling the non-stoichiometry of Ti-doped hematite, one can tune its conductivity by up to five orders of magnitude. Depending on processing conditions, donor dopants in hematite may be compensated largely by electrons or by ionic defects (Fe vacancies). A defect model was derived to explain this phenomenon. In addition, a temperature independent value for the electron mobility of 0.01 cm(2) V(-1) s(-1) for a donor density of 4.0 × 10(20) cm(-3) (1% Ti) was derived. These results highlight the importance of carefully controlling photoanode processing conditions, even when operating within the material's extrinsic dopant regime, and more generally, provide a model for the electronic properties of semiconducting metal oxide photoanodes.

High-Temperature Properties of Langasite

Materials such as langasite (La3Ga5SiO14) and related compounds are promising candidates for piezoelectric applications at high temperatures. In particular, langasite does not exhibit phase transformations up to the melting point of 1470 °C. Langasite was investigated with respect to potential applications in high temperature resonator devices. In contrast to current resonator materials, we have observed bulk oscillations at temperatures of up to 750 °C in langasite devices. At 700 °C the mass load response for 0.78 mm thick resonators is approximately 0.10 µg/Hz.

At elevated temperatures, the bulk resistivity of the resonator devices cannot be neglected due to attenuation of the resonance signal. Therefore, the temperature dependence of the electrical properties of langasite resonator devices, including bulk resistivity, capacity and resonance frequency were measured and are presented. The electrical conductivity is characterized by an activation energy of 105 kJ/mol. In order to confirm langasites stability with respect to oxidation-reduction reactions, we examined the oxygen diffusivity by measuring 18O tracer profiles by SIMS. The diffusivity along the Y-axis is given by D = 5-10−5 exp(-140 kJ/mol / RT) cm2/s in the temperature range from 500 to 800 °C. Langasite shows low oxygen diffusion coefficients with respect to other materials which might be investigated using a langasite microbalance. This would, for example, enable oxygen diffusion kinetics to be examined in YBa2Cu3O6 at 600 °C by means of 18O/16O exchange.

Electrical Conductivity and Nonstoichiometry in Pr0.545Ce0.455O2-x

The oxygen nonstoichiometry of Pr0.545Ce0.455O2-x was studied by solid state coulometric titration and was found to be extensive (0 ≤ y ≤ 0.28) at temperatures of 400–600 °C. The Po2 - x curves showed evidence of the existence of a number of single and two phase regions. Electrical conductivity measurements, performed under similar conditions, exhibited a p-n transition for temperature of 500 °C and below. The activated p-type conductivity was modeled in terms of the small polaron hopping mechanism. The p-n transition was correlated with the onset of phase separation.

How Unique are the Microstructure and the Electrical Properties of Nanocrystalline Ceramics?

A literature overview on microstructural and electrical properties of nanocrystalline ceramics is given. Space charge effects in nanosystems, especially thin-films and colloidal particles, are discussed from a theoretical point of view. Some common features of nanocrystalline ceramics are pointed out: nanoparticles present few extended lattice imperfections and the densification process begins at distinctly lower temperatures than that of coarse-grained ceramics. A significant decrease of grain boundary resistance occurs due to grain-size dependent dopant segregation, which leads also to an important increase of the apparent solubility of dopants in nanocrystalline materials. A number of studies confirm the theoretically expected reduction of the point defect formation enthalpy at interface sites, giving rise to significantly larger nonstoichiometry and electronic conductivity of nanocrystalline materials. Increased ionic conductivity has been found only in a limited number of cases, some of which remain controversial.

Coulometric Titration Studies of Nonstoichiometric Nanocrystalline Ceria

Oxygen nonstoichiometry measurements in nanocrystalline ceria, x in CeO2-x, were performed using coulometric titration. The measurements reveal large apparent deviations from stoichiometry, of the order of 10−3 − 10−4 at T = 405 − 455 °C and Po2 = 0.21 − 10−5 atm, as compared to levels of ∼10−9 for coarsened materials under the same conditions. The level of nonstoichiometry is, however, larger then expected from previous electrical conductivity data of nanocrystalline ceria. In addition, x ∝ Po2−½ while Σ ∝po2−1/6. The observed dependence of x(Po2, T) can be explained by either the formation of neutral oxygen vacancies at or near the interface, or by surface adsorption.

Grain Boundary Dopant and Heat Treatment Effects on the Electrical Properties of Polycrystalline ZnO

Simplified varistor systems of bismuth- and cobalt-doped zinc oxide are studied. A prior study has shown that the distributions of bismuth segregation at the grain boundaries in such samples can be controlled by varying microstructure and heat treatment. Current-voltage and deep level transient spectroscopy measurements were done to evaluate the corresponding electrical properties. Low leakage and α values of ˜30 were attained, despite the nominal, twocomponent doping of these simplified varistors. Moreover, these samples show the signature defects that are found in many multi-dopant, commercial devices: two shallow bulk traps at ˜0.14 eV and ˜0.24 eV and one prominent interfacial trap at ˜1 eV.

Size-dependent optical properties of BaTiO3-SrTiO3 superlattices

Artificial BaTiO(3)-SrTiO(3) superlattices with stacking periodicity varying between 27 and 1670 A in separate films were grown on MgO substrates by pulsed laser deposition. Both the static and active optical properties were found to be sensitive on the stacking periodicity. Birefringence decreased with increasing individual layer thickness due to relaxation of the interface originated stress. The electro-optic response also showed a layer thickness dependence, reaching a maximum at an individual layer thickness of 13 unit cells.