Rocksalt-structured oxides

Magnetic perovskite and rocksalt-structured oxides and fluorides. These are highly ionic compounds, in which the calculated coupling constants J indicate the degree of locaHsation of the unpaired electrons on the transition metal sites, and the range of the magnetic (spin-spin) interaction. 

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. 

Superexchange describes interaction between localized moments of ions in insulators that are too far apart to interact by direct exchange. It operates through the intermediary of a nonmagnetic ion. Superexchange arises from the fact that localized-electron states as described by the formal valences are stabilized by an admixture of excited states involving electron transfer between the cation and the anion. A typical example is the 180° cation-anion-cation interaction in oxides of rocksalt structure, where antiparallel orientation of spins on neighbouring cations is favoured by covalent... 

Periclase (MgO) has the rocksalt structure, which is face-centered cubic with each Mg-+ cation surrounded by six 0 anions in a regular octahedral arrangement, and each O " similarly coordinated to six cations at the corners of a regular octahedron. By far the most stable surface for oxides having the rocksalt structure is the (100), illustrated in Fig. 8.13... 

The rocksalt structure consists in two interpenetrating fee lattices of anions and cations, in which all atoms are in an octahedral environment. It is met in alkaline-earth oxides (MgO, CaO, SrO, BaO) and in some transition metal oxides like TiO, VO, MnO, FeO, CoO, NiO, etc, with cations in a 4-2 oxidation state. The non-polar surfaces of lowest Miller indices are the (100) and (110) surfaces they have neutral layers, with as many cations as oxygen ions, and their outermost atoms are 5- and 4-fold coordinated, respectively. Actually, planar surfaces can only be produced along the (100) orientation. The polar direction of lowest indices is (111) it has an hexagonal 2D unit cell, three-fold coordinated surface atoms and equidistant layers of either metal or oxygen composition. 

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... 

A number of factors can influence the behavior of ferroelectric thin films and multilayer stmctures with layer thickness at nanometer scale. One of the major factors is strain in epitaxial structures [15]. Recent demonstrations of huge strain effect on ferroelectric properties include changes in the phase diagram [16-22], dramatic enhancement of ferroelectric polarization, and increase of the ferroelectric phase transition temperature [23-27], induced ferroelectricity in non-ferroelectric materials like SrTi03 or KTa03 [28-33], or even simple rocksalt binary oxides like BaO ([34], theoretically predicted). 

Among the oxides and sulfides, only CdO adopts the octahedral rocksalt structure found with group 2 elements, although the solid is normally very deficient in oxygen and the electrons not used in bonding give rise to metallic properties. ZnO and ZnS are prototypes of the tetrahedrally coordinated wurtzite and zinc blende (or sphalerite) structures in fact, ZnS can adopt either structure, as can CdS and CdSe. HgO and HgS have chain structures with linear two-coordination of Hg. 

Copper oxide is the only transition-metal monoxide which does not have the rocksalt structure and the only one for which large deviations from cubic... 

Let us now consider the AF2 ground state of the late rocksalt structured transition metal oxides. Within a mean field (MF) approximation for Hisi g, a statistical treatment of the magnetic ordering indicates that the mean magnetisation, , of the cation lattice as a function of temperature (T) for the AF2 phase is given by... 

The structure of calcined dolomite is essentially that of periclase and calcium oxide, both of which have the rocksalt structure (Figure 35). 

Wiistite (FeO) Cubic rocksalt structure. Iron is in the 2+ oxidation state. 

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