Reduced conductivity in poly(3,4-ethylenedioxythiophen)-poly(styrene sulfonate) and indium tin oxide nanocomposite for low indium tin oxide content J. Appl. Phys. 105, 054318 (2009); 10.1063/1.3080154 Impedance spectroscopy and optical characterization of polymethyl methacrylate/indium tin oxide nanocomposites with three-dimensional Voronoi microstructuresThe 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 Köstlin [G. Frank and H. Köstlin, Appl. Phys. A 27, 197 (1982)].

First-principles modeling combined with experimental methods were used to study hydroxyapatite in which Sr2+ is substituted for Ca2+. Detailed analyses of cation-oxygen bond distributions, cation-cation distances, and site 1-oxygen polyhedron twist angles were made in order to provide an atomic-scale interpretation of the observed structural modifications. Density functional theory periodic band-structure calculations indicate that the Ca2+ to Sr2+ substitution induces strong local distortion on the hydroxyapatite lattice: the nearest neighbor Sr-O bond structures in both cationic sites are comparable to pure SrHA, while Sr induces more distortion at site 2 than site 1. Infrared vibrational spectroscopy (FTIR) and extended X-ray absorption fine structure (EXAFS) analysis suggest increasing lattice disorder and loss of OH with increasing Sr content. Rietveld refinement of synchrotron X-ray diffraction patterns shows a preference for the Ca1 site at Sr concentrations below 1 at.%. The ideal statistical occupancy ratio Sr2/Sr1=1.5 is achieved for approximately 5 at.%; for higher Sr concentrations occupation of the Ca2 site is progressively preferred.

CuAlO 2 exhibits a unique defect structure that is dependent on the synthesis route. Hightemperature solid-state and low-temperature hydrothermal techniques are compared to illustrate how cation off-stoichiometry affects the electrical properties of CuAlO 2 . A defect complex of Al on a Cu-site stabilized by two bound oxygen interstitials (Al Cu •• 2O i ′′)′′ is proposed which serves as an acceptor dopant to set the hole concentration. Trapping of holes (small polarons) by such complexes with decreasing temperature is proposed to account for the decreasing carrier content with temperature. Hydrothermal samples exhibit approximately an order of magnitude increase in hole concentration relative to the solid-state synthesized samples; this is reflected in the electrical conductivity and may explain the variations in electrical properties reported for CuAlO 2 .

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.

Characterization of lead substitution for calcium in hydroxyapatite (CaHA) is carried out, using experimental techniques and Density Functional theoretical (DFT) analyses. Theoretical modeling is used to obtain information of the Pb chemical environment for occupancy at either Ca(I) or Ca(II) sites of CaHA. Effects of the larger ionic radius of Pb(+2) compared to Ca(+2) are apparent in embedded cluster calculations of local chemical bonding properties. DFT periodic planewave pseudopotential studies are used to provide first-principles predictions of local structural relaxation and site preference for Pb(x)Ca(10-x)HA over the composition range x< or = 6. General characteristics of the polycrystalline material are verified by X-ray diffraction and FTIR analysis, showing the presence of a single phase of CaHA structure. A short range structure around lead is proposed in order to interpret the Pb L-edge EXAFS spectrum of the solid solution Ca(6.6)Pb(3.4)HA. In this concentration we observe that lead mainly occupies Ca(II) sites; the EXAFS fit slightly favors Pb clustering, while theory indicates the importance of Pb-Pb avoidance on site (II).

Defect structures associated with tin doping of indium oxide, an optically transparent conductor, have been characterized by atomistic simulations and first‐principles density functional calculations. A comprehensive survey of defect clusters containing up to three tin dopants in the first and second cationic coordination shells of an oxygen interstitial has been conducted. The analysis of energetically favorable defects gives insights into the role and nature of defect clusters in the material. In particular, the origins of the experimentally postulated b‐site preference of tin dopants have been examined. Our results show that b‐site preference occurs only in defect clusters with oxygen interstitials and is not intrinsic to dopants. In contrast, in nearest coordination to an interstitial, a strong d‐site preference is found. Density functional calculations in the discrete variational‐embedded cluster approximation have been conducted on selected defect structures to illuminate the effect of clustering on partial atomic charges, bond‐orders, and 119Sn Mössbauer parameters.

The history, applications, and basic requirements of transparent conducting oxides (TCOs) are reviewed. Four basic families of TCOs are recognized, including n-type oxides with tetrahedrally-coordinated cations (e.g., ZnO), n-type oxides with octahedrally-coordinated cations (e.g., CdO, In 2 O 3 , SnO 2 , and related binary and ternary compounds), p-type oxides with linearly-coordinated cations (e.g., CuAlO 2 , Cu 2 SrO 2 , and related compounds), and n-type oxides with cage structures (e.g., 12CaO·7Al 2 O 3 ). TCO behavior is discussed with attention to structural and chemical factors, especially point defect chemistry, governing carrier generation and transport properties.

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