Ionic insulators

The muonium centers observed in the curpous halides (see Table II) are unusual in several respects compared with Mu in other semiconductors and insulators. Figure 12 shows the reduced hyperfine parameters for Mu in semiconductors and ionic insulators plotted as a function of the ionicity (Philips, 1970). The positive correlation is especially apparent for compounds composed of elements on the same row of the periodic table where the lattice constants and valence orbitals are similar (see solid points in Fig. 12). Note however that the Mu hyperfine parameters in cuprous halides lie well below the line and in fact are smaller than in any other semiconductor or insulator (Kiefl et al., 1986b). The reason for this unusual behaviour is still uncertain but may be related to other unusual properties of the cuprous halides. For example the upper valence band is believed... 

There are two times to consider, therefore, in order to characterise ionic conduction. One is the actual time, tj, taken to jump between sites this is of the order of 10 -10 s and is largely independent of the material. The other time is the site residence time, which is the time (on average) between successful hops. The site residence times can vary enormously, from nanoseconds in the good solid electrolytes to geological times in the ionic insulators. Ion hopping rates, cOp, are defined as the inverse of the site residence times, i.e. 

Ionic conductors are ionic compounds. Therefore it is appropriate to start with ions rather than atoms to construct the electronic energy level diagrams. Fig. 3.2 illustrates such a construction for the electronic and ionic insulator MgO. The energy levels and 0 correspond... 

Ample studies on pyrochlore oxide electrolytes have been carried out, particularly on Gd2Ti207- and Gd2Zr207-based conductors, where the Gd2Ti. Zrx07 solid solution is of great interest because the x=0 member is an ionic insulator whereas the x = 1 end member is a good oxide ion conductor [96,97],... 

Fig. 14.12 A material spot library and the electric connections to it with an 02 electrode probe and a battery test instrument. The material spot library contains 128 zinc spots spatially separated on a graphite plate. The surface of the graphite is coated with a plastic layer on the non-spot areas to keep ionic insulation between these material spots. The area of each spot is 0.196 cm2. (Reused with permission from Rongzhong Jiang and Deryn Chu.62 Copyright 2005, American Institute of Physics.)...

Summary. We have shown that ion transport in "Nafion" per-fluorinated membrane is controlled by percolation, which means that the connectivity of ion clusters is critical. This basically reflects the heterogeneous nature of a wet membrane. Although transport across a membrane is usually perceived as a one-dimensional process, our analysis suggests that it is distinctly three-dimensional in "Nafion". (Compare the experimental values of c and n with those listed in Table 7.) This is not totally unexpected since ion clusters are typically 5.0 nm, whereas a membrane is normally several mils thick. We have also uncovered an ionic insulator-to-conductor transition at 10 volume % of electrolyte uptake. Similar transitions are expected in other ion-containing polymers, and the Cluster-Network model may find useful application to ion transport in other ion containing polymers. Finally, our transport and current efficiency data are consistent with the Cluster-Network model, but not the conventional Donnan equilibrium. 

The model predicts a critical ionic-insulator-to-conductor transition, at which point the interconnection between the clusters is established and above which conductivity is proportional to void volume. Experimentally, the critical void volume is found at a 10% void-volume fraction. 

B. Atom-Multiphonon Scattering Time-of-Flight Scattering Instrument Clean Crystalline Surfaces Ionic Insulators A. Alkali Halides... 

In Sections V, VI, and VII some experimental results are presented. Since our laboratory has focused primarily on ionic insulator materials, this work is presented in much greater detail. For other materials, only a small sampling of results is presented to show the range of applicability of the HAS technique and the new insights into the physics of materials which it has provided in a relatively short span of time. 

The alkali halides are the prototypical ionic insulator materials. The ions all have closed shell configurations with essentially localized electronic dis-... 

The ionic insulators discussed in some detail in the previous section have closed shell electronic configurations similar to the noble gases and electronic distributions which are localized around the electronic core. The principal interactions are Coulombic, although their polarizabilities appear to influence greatly the response of the electronic distribution to surface lattice vibrations. For other materials, particularly metals and some layered compounds, the conduction and valence electrons are best thought of as somewhat delocalized if not entirely free. These electrons are what the helium atoms scatter from, and their states of motion are significantly modulated by the vibrations of the atomic cores. Thus, for these materials HAS is very... 

Because of its sensitivity to surface features and the gentleness of its interaction with the surface, HAS is an ideal probe for studies of interfaces, including the structures formed by adsorbate deposition, the dynamics of the interactions of adsorbates with substrates, and the dynamics of the formation of overlayers and films. In the Florida State University (FSU) laboratory we have focused on ionic insulator growth and more recently on organic films. 

It is difficult to treat the KBr/NaCl system by realistic models such as the Shell Model which works well for these ionic insulators because the unit cell is so large from the superstmcture found in the diffraction the real cell size is (7 x 7) NaCl s or (6 x 6) KBr s. For KBr/RbCl the unit cell is (1 X 1) and size poses no special computational difficulty. Schroder, and Bonart have undertaken the adaptation of the Shell Model to this overlayer/ substrate system [119]. 

To a good approximation, the more extensively studied azides are mostly ionic compounds with band gaps in excess of 3 eV, and they behave as insulators at room temperature. With such materials, it is not a simple matter to distinguish between contributions from ionic conductivity and electronic conductivity. Brief descriptions of the standard kinds of measurements appear below to point out some of the difficulties inherent in interpreting electrical experiments on ionic insulating materials. 

The second type of material that shows promise for passivation and electrical Isolation Is CaF2 [63]. This large-bandgap Ionic Insulator has a high static dielectric constant, can be readily evaporated from thermal sources in molecular form, and shows no tendency to modify the superconductor. As such. It may be useful as a dielectric layer In advanced devices. 

Biological Process ionic insulation of natrons by glial cells... 

Clusters of metal, semiconductor and non-ionic insulator species are more challenging, and require more subtle or complex models. Models for metal clusters span many levels of sophistication. The simplest are spherical representations of the atomic cores as smooth, continuous, positively-charged and rigid background in which the valence electrons dwell — the spherical jellium model — and more elaborate versions of the jellium... 

Particles of ionic insulators suspended in solution can have charged surfaces because of an excess there of ions of one type as a result, e.g., of adsorption from the solution. The surface charge depends on the concentration in the solution of ions that can adsorb. The sol-gel technique is discussed in the next section as an example of a synthesis of nonmetallic inorganic polycrystalline materials that makes use of surface charge control of colloidal particles. 

Chalcogenide R " ". The former being mostly ionic insulators, although there are exceptions, and the latter being mostly conductors. 

SIMS Secondary Ion Mass Spectrometry Ions Semi-quantitative elemental analysis. Useful only for conductors or Ionic insulators. Sample is severely damaged. 

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