Gabriela B. González

Defect structure studies of bulk and nano-indium-tin oxide

The defect structure of bulk and nano-indium-tin oxide was investigated by a combination of experimental techniques, including high-resolution synchrotron x-ray diffraction, extended x-ray absorption fine structure, and time-of-flight neutron diffraction on powder specimens. The structural results include atomic positions, cation distributions, and oxygen interstitial populations for oxidized and reduced materials. These structural parameters were correlated with theoretical calculations and in situ electrical conductivity and thermopower measurements as well as existing defect models, with special reference to the model of Frank and Kostlin [G. Frank and H. Kostlin, Appl. Phys. A 27, 197 (1982)].

Neutron diffraction study on the defect structure of indium--tin--oxide

The defect structure of undoped and Sn-doped In2O3 (ITO) materials was studied by preparing powders under different processing environments and performing neutron powder diffraction. The effect of tin doping and oxygen partial pressure was determined. Structural information was obtained by analyzing neutron powder diffraction data using the Rietveld method. The results include positions of the atoms, their thermal displacements, the fractional occupancy of the interstitial oxygen site, and the fractional occupancies of Sn on each of the two nonequivalent cation sites. The tin cations show a strong preference for the b site versus the d site. The measured electrical properties are correlated with the interstitial oxygen populations, which agree with the proposed models for reducible (2SnIn•Oi″)x and nonreducible (2SnIn•3OOOi″)x defect clusters.

Point defects and related properties of highly co-doped bixbyite In2O3

The method of co-doping has been employed to achieve and study the influence of high defect populations in bixbyite In2O3. Substantial metastable Sn-doping levels can be achieved in nanocrystalline In2O3 with associated co-doping by oxygen interstitials. The resulting electrical properties, diffraction data (X-ray and neutron), and EXAFS studies support the presence of 2 : 1 Sn-oxygen interstitial point defect clusters. Upon reduction, some of these clusters can be reduced to liberate donors and generate charge carriers. Extensive Cd/Sn co-substitution for indium in In2O3 has been achieved in equilibrium solid solutions. This self-compensated (isovalent) and relatively size-matched substitution reveals a tendency for off-stoichiometry in favor of donors, resulting in “self-doped” behavior irrespective of oxygen partial pressure. Ramifications of bixbyite defect structure for transparent electrode applications are discussed.

In situ studies on the kinetics of formation and crystal structure of In4Sn3O12 using high-energy x-ray diffraction

High-energy x-ray diffraction was utilized to study in situ the formation temperature and crystal structure of the rhombohedral phase In4Sn3O12. The kinetics of In4Sn3O12 formation from bixbyite In2O3 and tetragonal SnO2 nanopowders were investigated during isothermal annealing treatments ranging from 1335 to 1400 °C. The transformation data exhibited two regimes, well described with a two-exponent kinetics model. The first regime followed a Johnson–Mehl–Avrami–Kolmogorov (JMAK) behavior until 75% of the In4Sn3O12 phase formed and was modeled with a modified JMAK equation. The formation of the first grains of In4Sn3O12 at 1345 °C was observed in situ using diffraction two-dimensional images. Structural results obtained from Rietveld analysis include atomic positions, phase analysis compositions of the samples, and lattice parameters during heating, cooling, and isothermal conditions. Linear and volume coefficients of thermal expansion were determined for the In4Sn3O12 phase.

Impedance/Dielectric Spectroscopy of Nanoceramics

With proper attention to experimental design (i.e., electroding, cabling, stray apparatus imittances, etc.) impedance/dielectric spectroscopy is a powerful tool to study the electrical properties of nanoscale electroceramics. This study focuses on bulk non-ferroelectric materials (ZnO, CeO2, TiO2) and their frequency-dependent AC electrical properties, taken from a variety of literature sources. In particular, it is shown how to separate effective grain boundary and grain interior resistivities and also the effective capacitances associated with each region in the microstructure. This is possible even when Nyquist plots (-Z im vs. Z re ) without frequency markers are the only data supplied. A modified brick layer model (BLM) can be used to analyze the impedance/dielectric properties of nanoscale ceramics.