Oxide lattice vibrations

Semiconductivity in oxide glasses involves polarons. An electron in a localized state distorts its surroundings to some extent, and this combination of the electron plus its distortion is called a polaron. As the electron moves, the distortion moves with it through the lattice. In oxide glasses the polarons are very localized, because of substantial electrostatic interactions between the electrons and the lattice. Conduction is assisted by electron-phonon coupling, ie, the lattice vibrations help transfer the charge carriers from one site to another. The polarons are said to "hop" between sites. 

The infrared spectra for various aluminum oxides and hydroxides are shown in Figure 3. Figure 3a is a-alumina (Harshaw A13980), ground to a fine powder with a surface area of 4 m /g. The absorption between 550 and 900 cm is due to two overlapping lattice modes, and the low frequency band at 400 cm is due to another set of lattice vibrations. These results are similar to those obtained by reflection measurements, except that the powder does not show as... 

The results obtained for the various aluminum oxides and hydroxides indicate that infrared photoacoustic spectroscopy may be useful in characterizing structural transformations in these species. Very clear differences between a-alumina and y-alumina were noted in the region of the lattice vibrations. The monohydrate, boehmite, showed a very distinct Al-OH stretching feature at 1070... 

The results presented here for silicas and aluminas illustrate that there is a wealth of structural information in the infrared spectra that has not previously been recognized. In particular, it was found that adsorbed water affects the lattice vibrations of silica, and that particle-particle Interactions affect the vibrations of surface species. In the case of alumina, it was found that aluminum oxides and hydroxides could be distinguished by their infrared spectra. The absence of spectral windows for photoacoustic spectroscopy allowed more complete band identification of adsorbed surface species, making distinctions between different structures easier. The ability to perform structural analyses by infrared spectroscopy clearly indicates the utility of photoacoustic spectroscopy. 

The lattice vibration at 938 cm , related to the oxidation state of the copper [17], indicates that after treatment in N2O or air the copper is in the 2+ oxidation state, while under CO it is reduced to +1 as is indicated by the shift to 963 cm. This reduction could also be achieved by treatment in N2 at 770 K. CO (5 kPa) strongly adsorbs at this catalyst at 450 K (figure 11). 

Mossbauer spectroscopy is one of the techniques that is relatively little used in catalysis. Nevertheless, it has yielded very useful information on a number of important catalysts, such as the iron catalyst for Fischer-Tropsch and ammonia synthesis, and the cobalt-molybdenum catalyst for hydrodesulfurization reactions. The technique is limited to those elements that exhibit the Mossbauer effect. Iron, tin, iridium, ruthenium, antimony, platinum and gold are the ones relevant for catalysis. Through the Mossbauer effect in iron, one can also obtain information on the state of cobalt. Mossbauer spectroscopy provides valuable information on oxidation states, magnetic fields, lattice symmetry and lattice vibrations. Several books on Mossbauer spectroscopy [1-3] and reviews on the application of the technique on catalysts [4—8] are available. 

What is the structure of this Co-Mo-S phase A model system, prepared by impregnating a MoS2 crystal with a dilute solution of cobalt ions, such that the model contains ppms of cobalt only, appears to have the same Mossbauer spectrum as the Co-Mo-S phase. It has the same isomer shift (characteristic of the oxidation state), recoilfree fraction (characteristic of lattice vibrations) and almost the same quadrupole splitting (characteristic of symmetry) at all temperatures between 4 and 600 K [71]. Thus, the cobalt species in the ppm Co/MoS2 system provides a convenient model for the active site in a Co-Mo hydrodesulfurization catalyst. 

Lewis, D.G. Cardde, C.M. (1989) Hydrolysis of Fe(III) solution to hydrous iron oxides. Aust. J. Soil Res. 27 103-115 Lewis, D.G. Farmer,V.C. (1986) Infrared absorption of surface hydroxyl groups and lattice vibrations in lepidocrodte (y-FeOOH) and boehmite (y-Al-OOH). Clay Min. 21 93-100... 

Onari, S. Arai,T. Kudo, K. (1977) Infrared lattice vibrations and dielectronic dispersion in a- Fe203. Phys. Rev. B16 1717 Onoda, G.Y. de Bruyn, P.L. (1966) Proton adsorption ot the ferric oxide/aqueous solution interface. I. A kinetic study of adsorption. Surface Sd. 4 48—63... 

The extinction curves for magnesium oxide particles (Fig. 11.2) and aluminum particles (Fig. 11.4) show the dominance of surface modes. The strong extinction by MgO particles near 0.07 eV( - 17 ju.m) is a surface mode associated with lattice vibrations. Even more striking is the extinction feature in aluminum that dominates the ultraviolet region near 8 eV no corresponding feature exists in the bulk solid. Magnesium oxide and aluminum particles will be treated in more detail, both theoretically and experimentally, in this chapter. 

The electrons in a solid interact both with one another and with the lattice vibrations. A theme of this book is the effect of the interaction between electrons in inducing magnetic moments and metal-insulator transitions. Interaction with phonons also has an important effect, particularly in some transitional-metal oxides. In this chapter both kinds of interaction are introduced. 

Whether the rhodium dicarbonyl was attached to the zeolite lattice or to an extra-framework anion such as OH, 0 or a labile ion, could be also decided upon using IR spectroscopy. Indeed lattice vibration between 1300 and 300 cm- characteristic of an NaY zeolite (16) are sensitive to the interaction of lattice oxide ions with cations. In particular, it was observed that an IR absorption band at 877 cm- grew simultaneously with the growth of CO absorptions at 2115-2048 characteristic of the dicarbonyl (13).This... 

Figure 47. As oxygen ions move towards each other on account of lattice vibrations, die activation energy for proton jump is lowered, and die proton changes partner. According to Ref..192. (Reprinted from K. D. Kreuer, W. Munch, U. Traub and J. Maier, On Proton Transport in Perovskite-Type Oxides and Plastic Hydroxides , Ber. Bunsenges. Phys Chem. 102, 552-559. Copyright 1998 with...

Akaogi, M., N. L. Ross, P. McMillan, and A. Navrotsky (1984). The Mg2Si04 polymorphs (olivine, modified spinel, spinel)—thermodynamic properties from oxide melt solution calorimetry, phase relations, and models of lattice vibrations. Amer. Mineral. 69, 499-512. 

Depending upon the relationship between the momentum in the initial and final states (which, in turn, depend on the profile of the parabolic energy valley ) direct or indirect transitions can occur, as shown in Figure 2.4, and this affects all the three terms Pn, vk and Uf. It must be noted that in transitions between indirect valleys (Figure 2.4B) momentum is conserved via interaction with a phonon (i.e. a quantum lattice vibration), which can be either emitted or adsorbed. Some additional detail on such transitions will be given in the section deaUng with semiconductor oxides. 

The cation-to-anion vibrations (lattice vibrations) are mainly located in the FIR region and their assignments, based on similar considerations to those reported for ionic oxides, are frequently difficult. 

Lattice dynamics in bulk perovskite oxide ferroelectrics has been investigated for several decades using neutron scattering [71-77], far infrared spectroscopy [78-83], and Raman scattering. Raman spectroscopy is one of the most powerful analytical techniques for studying the lattice vibrations and other elementary excitations in solids providing important information about the stmcture, composition, strain, defects, and phase transitions. This technique was successfully applied to many ferroelectric materials, such as bulk perovskite oxides barium titanate (BaTiOs), strontium titanate (SrTiOs), lead titanate (PbTiOs) [84-88], and others. 

As shown in Table I, lanthanum and lutetium oxides have Sq ground states and consequently their heat capacities should be attributed to lattice vibration. Data on these substances may be used to represent the lattice contribution to a first approximation for neighboring isostructural (and nearly so) sesquioxides. Cubic gadolinium oxide provides a midseries lattice heat capacity approximation at relatively high temperatures... 

State lifetimes and modes of energy transfer within the structure. Examples of this are photoluminescence of ZnS nanoparticles studied by Wu et al. (1994), and Mn doped ZnS nanoparticles by Bhargava et al. (1994). In the latter study, the doped nanocrystals were found to have higher quantum efficiency for fluorescence emission than bulk material, and a substantially smaller excited state lifetime. In the case of environmental nanoparticles of iron and manganese oxides, photoluminescence due to any activator dopant would be quenched by magnetic coupling and lattice vibrations. This reduces the utility of photoluminescence studies to excited state lifetimes due to particle-dopant coupling of various types. The fluorescence of uranyl ion sorbed onto iron oxides has been studied in this way, but not as a function of particle size. 

The heat capacity data of the actinide(IV) oxides, fluorides and chlorides have been analysed, and the data expressed as the sum of three contributions from the lattice vibrations, from /-electron excitation and from a residual term, probably arising from the interaction of ri-electrons. It is demonstrated that the latter contribution becomes zero around T = 500 to 600 K. A similar approach is given for the entropies, from which the standard entropies of a number of An(IV) compounds have been estimated. 

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