Defect rocksalt structure

The NaCl structure is also found in compounds like TiO, VO and NbO, possessing a high percentage of cation and anion vacancies. Ternary oxides of the type MggMn 08 crystallize in this structure with of the cation sites vacant. Solid solutions such as Li,j )Mg Cl (0 x 1) crystallize in the rocksalt structure stoichiometric MgCl may indeed be considered as having a defect rocksalt structure with 50% of ordered cation vacancies. 

It has been mentioned in the introduction that the condensed cluster halides of the rare earth metals based on the MgXi2-type cluster with an interstitial atom (or molecular unit) generally exhibit a defect rocksalt structure. Figure 10 provides clear evidence for this remark. The NaCl subcell in the structure of GdijInCg, marked by strong streaks is only weakly distorted (a = 6.07, b = 6.10, c = 5.92 A, a = y = 90°, p = 91°) by the ordering of I atoms and Cj units and occupation of all voids around the C2 units by Gd atoms. 

It should be noted that these types of spectra are expected only for quadrupolar nuclei of semiconductors in non-cubic axially-symmetric forms such as the WZ structure cubic forms such as ZB or rocksalt structures ideally lack any anisotropy, and the ST peaks overlap the CT peak. However, defects in such cubic structures can produce EFGs that have random orientations, and the resulting ST are spread out over a wide range. 

Figure 5.12 (a) Ordered defects in monoclinic TiOfTij gOj g) (b) ordered defects in (orthorhombic) nonstoichiometric TiOj l (c) coherent intergrowth of (a) and (b) along the (120) planes of rocksalt structure. Lines indicate unit cell faces of the superstructures. (After Anderson, 1984.)... 

Fig. 8.13. A model of the (100) surface of the rocksalt structure of MgO. Large circles are oxygen anions, small circles are Mg cations. A (100) step to another (100) terrace is shown, as also is a missing anion point defect.

This volume provides a view of some of the main areas of development and of recent progress in the study of well-characterised oxide surfaces. The first chapter by Henrich, one of the pioneers of modem surface studies of oxides, and co-author of the first text on the subject, provides an overview of the subject and relates the remaining chapters to this overview. Chapters 2 to 4, by Noguera, by Pacchioni and by Hermann and Witko, are concerned with the theory of oxides surfaces they cover a range of materials from simple rocksalt structures such as MgO through to the complexity of transition metal oxides, and also present some complementary methods of modelling and calculation. These theoretical studies also address the key issue of surface defects, and cover some aspects of adsorption at oxide surfaees. fri some ways oxide surfaces is a topic in which theory was, for some years, ahead of experiment, and hence unchallenged. This was especially tme in the predictions and... 

Open shell defects in rocksalt-structured matrices the observables of interest are in this case the hyperfine constants, measurable experimentally by EPR, which depend both on the defect structure and on the localisation of the defect states. 

Many R-chalcogenides (R = Sc, Y, Lu, La) are superconductors. They crystallize in the rocksalt structure. Table 10.3 shows some examples for recently reported Te s, or the ranges of temperature, where superconducting transitions were observed. The Sc, Y and Lu monochalcogenides are known to be defect structures which can exist with an appreciable anion or cation deficiency (Hulli ger and Hull, 1970 Moodenbaugh et al., 1974). The Te s depend strongly on composition and are therefore not well-defined. 

Lattice parameters of ZnO have been investigated over many decades [22-30]. The lattice parameters of a semiconductor usually depend on the following factors (i) free electron concentration acting via deformation potential of a conduction hand minimum occupied by these electrons, (u) concentration of foreign atoms and defects and their difference of ionic radii with respect to the substituted matrix ion, (iii) external strains (e.g., those induced by substrate), and (iv) temperature. The lattice parameters of any crystalline material are commonly and most accurately measured by high-resolution X-ray diffraction (HRXRD) using the Bond method [31] for a set of symmetrical and asymmetrical reflections. Table 1.2 tabulates measured and calculated lattice parameters, cja ratio, and u parameter reported by several groups for ZnO crystallized in wurtzite, zinc blende, and rocksalt structures for comparison. 

Actually, the defect structure in some wiistite compositions is even more complicated. In Feo 9O the vacancies and Fe ions cluster together in a specific arrangement, to be described later, giving a volumetric defect. The clusters themselves at low temperatures are spaced in a periodic array to form a supercell. In addition, within the vacancy clusters in highly nonstoichiometric wiistite some cations occupy tetrahedral sites rather than octahedrally coordinated sites as in the perfect rocksalt structure. It is probably mostly Fe ions which occupy tetrahedral sites the Fe ions are smaller than the Fe ions and the tetrahedral sites are smaller than the octahedral sites in wiistite. 

AIMD simulations of a similar composition of C-doped GeTe ((Geo.52Teo.4s)o.85 Co.is) have been performed [10]. It was found that in the simulated amorphous models, the C dopant atoms preferentially bond to Ge atoms and also to other C atoms, forming C-C-C chains. In addition, the addition of the C dopants was found to promote the transformation of (defective) octahedrally-coordinated Ge atoms (a structural motif characteristic of the metastable rocksalt c-phase) to tetrahedrally-bonded sites instead. It was speculated that the presence of the C chains, and the difficulty in breaking the strong C-C bonds, is responsible for the experimentally observed increase in the barrier for crystallization with C-doping. 

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