Charge States in Ceramics

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9973894SpenceThe properties of new functional ceramics are controlled by small changes in the charge state of the constituent ions. This state can now be determined by quantitative convergent-beam electron microdiffaction (QCBED). This project continues the PI's previous work (DMR9412146) on the measurement and use of charge state to model and predict ceramic properties. Three tasks are planned: (i) Charge-density measurement in transition metal monoxides for the purpose of testing ab-initio algorithms in difficult systems. (ii) Mapping of the three-dimensional charge-density and hole distribution in oxide superconductors using QCBED. Elastically filtered static diffuse scattering will also be used to determine the strain around dopant atoms. By comparing this measured ground-state charge density with ab-initio crystal computations, the most successful theoretical approximations can be identified and used to predict the properties of modified (e.g. differently doped) material. This work may readily be extended to the CMR manganites, where similar small polarons are important. In both cases we are interested in the relationship between the charge state of ions and the resulting lattice relaxation. (iii) Experimental tests of a new solution to the inversion problem of electron diffraction, which allows crystal charge density maps to be computed directly from experimental microdiffraction patterns. As in X-ray crystallography, these maps reveal the position and type of every atom in the unit cell, and may be obtained from nanometer sized regions of material. The spatial resolution of this diffraction method for solving microphase crystal structures is not limited by imaging lenses. Microphase identification is an important requirement for understanding the properties of intergranular phases, sintered or precipitation hardened materials and the new nanophase composites, amongst many other applications. All this work is based on the existing Leo/Zeiss Omega-filter electron microscope at ASU. A new CCD camera and liquid-helium cold stage will be acquired. %%%This work advances our knowledge of bonds between atoms (which control the properties of materials) and furthers the effort to design and synthesize new materials with wanted properties. ***