M. F. Smith

Charge distribution near bulk oxygen vacancies in cerium oxides

Understanding the electronic charge distribution around oxygen vacancies in transition metal and rare earth oxides is a scientific challenge of considerable technological importance. We show how significant information about the charge distribution around vacancies in cerium oxide can be gained from a study of high resolution crystal structures of higher order oxides which exhibit ordering of oxygen vacancies. Specifically, we consider the implications of a bond valence sum analysis of Ce₇O₁₂ and Ce₁₁O₂₀. To illuminate our analysis we show alternative representations of the crystal structures in terms of orderly arrays of coordination defects and in terms of fluorite-type modules. We found that in Ce₇O₁₂, the excess charge resulting from removal of an oxygen atom delocalizes among all three triclinic Ce sites closest to the O vacancy. In Ce₁₁O₂₀, the charge localizes on the next nearest neighbour Ce atoms. Our main result is that the charge prefers to distribute itself so that it is farthest away from the O vacancies. This contradicts the standard picture of charge localization which assumes that each of the two excess electrons localizes on one of the cerium ions nearest to the vacancy. This standard picture is assumed in most calculations based on density functional theory (DFT). Based on the known crystal structure of Pr₆O₁₁, we also predict that the charge in Ce₆O₁₁ will be found in the second coordination shell of the O vacancy. We also extend the analysis to the Magnéli phases of titanium and vanadium oxides (M(n)O(₂n₋₁), where M = Ti, V) and consider the problem of metal-insulator transitions (MIT) in these oxides. We found that the bond valence analysis may provide a useful predictive tool in structures where the MIT is accompanied by significant changes in the metal-oxygen bond lengths. Although this review focuses mainly on bulk cerium oxides with some extension to the Magnéli phases of titanium and vanadium, our approach to characterizing electronic properties of oxygen vacancies and the physical insights gained should also be relevant to surface defects and to other rare earth and transition metal oxides.

Identification of Mn site in Pb(Zr,Ti)O3 by synchrotron x-ray absorption near-edge structure: Theory and experiment

Synchrotron x-ray absorption near-edge structure (XANES) experiments are performed on Mn-doped PbZr1−xTixO3 samples (PZT) and compared with first-principles XANES simulations. The features of the measured Mn K-edge XANES are consistent with the first-principles XANES of Mn on the Ti∕Zr site and inconsistent with Mn on other sites. The clear agreement between measured and first-principles theoretical XANES spectra reported here is by far the strongest evidence of Mn substituting for Ti∕Zr in PZT. This work illustrates that a first-principles supercell framework, which is popularly used to study impurities in crystals, can be used in conjunction with XANES measurement in order to identify an impurity structure with a high degree of confidence. This approach may thus be broadly applicable to study impurities in other crystals.

Determination of phase ratio in polymorphic materials by x-ray absorption spectroscopy: The case of anatase and rutile phase mixture in TiO2

We demonstrate that x-ray absorption spectroscopy (XAS) can be used as an unconventional characterization technique to determine the proportions of different crystal phases in polymorphic samples. As an example, we show that ratios of anatase and rutile phases contained in the TiO2 samples obtained by XAS are in agreement with conventional x-ray diffraction (XRD) measurements to within a few percent. We suggest that XAS measurement is a useful and reliable technique that can be applied to study the phase composition of highly disordered or nanoparticle polymorphic materials, where traditional XRD technique might be difficult.

First principles study of Mn impurities in PbTiO3 and PbZrO3

A first principles study of an Mn impurity in PbTiO3 and PbZrO3 has been carried out to determine its favorable location and its electronic and magnetic properties. We find that it is energetically favorable for the Mn atom to substitute for Ti∕Zr (as opposed to substituting for Pb and O or to residing in an interstitial position) under all equilibrium crystal growth conditions. The Mn defect mainly occurs as neutral-charge Mn substitute Ti∕Zr, which has a total electron spin of 3∕2. When no other impurities are present, a small concentration of charged Mn impurities that also form tends to make the sample slightly p type (n type) in oxygen-rich (poor) equilibrium growth conditions.

Identification of bulk and surface sulfur impurities in TiO2 by synchrotron x-ray absorption near edge structure

Synchrotron x-ray absorption near edge structure (XANES) measurements of Ti and S K edges, combined with first principles simulations, are used to characterize S-doped TiO2 prepared by oxidative annealing of TiS2 at various temperatures. Ti-edge XANES and x-ray powder diffraction data indicate that samples annealed above 300°C have an anatase TiO2 crystal structure with no trace of TiS2 domains. S-edge XANES data reveal that the local structure seen by S atoms evolves gradually, from TiS2 to a qualitatively different structure, as the annealing temperature is increased from 200to500°C. For samples annealed at 500°C, the spectrum appears to have features that can be assigned to S on the surface in the form of SO4 and S defects in the bulk (most likely S interstitials) of TiO2.

Mixed valency in cerium oxide crystallographic phases: Valence of different cerium sites by the bond valence method

We have applied the bond valence method to cerium oxides to determine the oxidation states of the Ce ion at the various site symmetries of the crystals. The crystals studied include cerium dioxide and the two sesquioxides along with some selected intermediate phases which are crystallographically well characterized. Our results indicate that cerium dioxide has a mixed-valence ground state with an f-electron population on the Ce site of 0.27 while both the A- and C-sesquioxides have a nearly pure f1 configuration. The Ce sites in most of the intermediate oxides have nonintegral valences. Furthermore, many of these valences are different from the values predicted from a naive consideration of the stoichiometric valence of the compound.

Hand in Hand: Tools and techniques for understanding children's touch with a social robot

Robots that facilitate touch by children have special requirements in terms of safety and robustness, but little is known about how and when children actually use touch with robots. Tools and techniques are required to sense the variety of childrens touch and to interpret the volumes of data generated. This explorative user study investigated childrens patterns of touch during game play with a robot. We examined where the children touch the robot and their patterns of touch over time, using a raster-based visualisation of each childs time series of touches, recording patterns of touch across different games and children. We found that children readily engage with the robot, in particular spontaneously touching the robots hands more than any other area. This user study and the tools developed may aid future designs of robots to autonomously detect when they have been touched.

Children's Expectations and Strategies in Interacting with a Wizard of Oz Robot

This paper presents an analysis of childrens interactions with an early prototype of a robot that is being designed for deployment in early learning centres. 23 children aged 2-6 interacted with the prototype, consisting of a pair of tablets embedded in a flat and vaguely humanoid form. We used a Wizard of Oz (WoZ) technique to control a synthesized voice that delivered predefined statements and questions, and a tablet mounted as a head that displayed animated eyes. The childrens interactions with the robot and with the adult experimenter were video recorded and analysed in order to identify some of the childrens expectations of the robots behaviour and capabilities, and to observe their strategies for interacting with a speaking and minimally animated artificial agent. We found a surprising breadth in childrens reactions, expectations and strategies (as evidenced by their behaviour) and a noteworthy tolerance for the robots occasionally awkward behaviour.

Apparent violation of the Wiedemann-Franz law near a magnetic field tuned metal-antiferromagnetic quantum critical point.

The temperature dependences of the interlayer electrical and thermal resistivity in a layered metal are calculated for Fermi liquid quasiparticles which are scattered inelastically by two-dimensional antiferromagnetic spin fluctuations. Both resistivities have a linear temperature dependence over a broad temperature range. Extrapolations to zero temperature made from this linear-T range give values that appear to violate the Wiedemann-Franz law. However, below a low-temperature scale, which becomes small close to the critical point, a recovery of this law occurs. Our results describe recent measurements on CeCoIn5 near a magnetic field-induced quantum phase transition. Hence, the experiments do not necessarily imply a non-Fermi liquid ground state.

Response to “Comment on ‘Characterization of As-doped, p-type ZnO by x-ray absorption near-edge structure spectroscopy: Theory’ ” [Appl. Phys. Lett. 92, 236101 (2008)]

by x-ray absorption near-edge structure spectroscopy: Theory’ ” †Appl. Phys. Lett. 92, 236101 „2008...‡ S. Limpijumnong, M. F. Smith, and S. B. Zhang National Synchrotron Research Center, Nakhon Ratchasima 30000, Thailand School of Physics, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, New York 12180, USA