Transition metal compounds defects

Conversely, the presence of some metal ions of lower oxidation state in the metal ion sublattice requires vacant anion sites to balance the charge. In some cases, the charge imbalance is caused by ions of some other element or, rarely, by multiple valence of the anions. In any event, the empirical formula of a recognizable solid transition metal compound may be variable over a certain range, with nonintegral atomic proportions. Such non-stoichiometric compounds may be regarded as providing extreme examples of impurity defects. 

In transition metal compounds where the transition metal is stable in more than one oxidation state, by nonstoichiometry. For example, in CaO and FeO (both of which possess the rock salt structure) Schottky defects are thermally formed. Additionally, however, FeO (wiistite) exhibits mixed valence and, accordingly, has Fe2+ and Fe3+ ions and vacancies distributed over the cation sublattice. 

Nonequihbrium concentrations of point defects can be introduced by materials processing (e.g. rapid quenching or irradiation treatment), in which case they are classified as extrinsic. Extrinsic defects can also be introduced chemically. Often times, nonstoichiometry results from extrinsic point defects, and its extent may be measmed by the defect concentration. Many transition metal compounds are nonstoichiometric because the transition metal is present in more than one oxidation state. For example, some of the metal ions may be oxidized to a higher valence state. This requires either the introduction of cation vacancies or the creation of anion interstitials in order to maintain charge neutrality. The possibility for mixed-valency is not a prerequisite for nonstoichiometry, however. In the alkah hahdes, extra alkah metal atoms can diffuse into the lattice, giving (5 metal atoms ionize and force an equal number... 

In addition to the binary catalysts from transition metal compounds and metal alkyls there 2ire an increasing number which are clearly of the same general type but which have very different structures. Several of these are crystalline in character, and have been subjected to an activation process which gives rise to lattice defects and catalytic activity. Thus, nickel and cobalt chlorides, which untreated are not catalysts, lose chlorine on irradiation and become active for the polymerization of butadiene to high cis 1,4-polymer [59]. Titanium dichloride, likewise not a catalyst, is transformed into an active catalyst (the activity of which is proportional to the Ti content) for the polymerization of ethylene [60]. In these the active sites evidently react with monomer to form organo-transition metal compounds which coordinate further monomer and initiate polymerization. 

Non-stoichiometric Defects. These often occur in transition-metal compounds, especially oxides and sulfides, because of the ability of the metal to exist in more than one oxidation state. A well-known case is FeO which consists of a cep array of oxide ions with all octahedral holes filled by Fe2 + ions. In reality, however, some of these sites are vacant, while others—sufficient to maintain electroneutrality—contain Fe3 + ions. Thus the actual stoichiometry is commonly about Fe0 950. Another good example is TiO which can readily be obtained with compositions ranging from Ti0 740 to Tij 670 depending on the pressure of oxygen gas used in preparing the material. 

Different new catalysts based on transition metal compounds of groups IVb, Vb, Vb, and Vlllb combined with reducing agents AlEtj and BuLi in toluene or silicone oil were used for the polymerization of acetylene [39-41]. By modifying the polymerization conditions, e.g, using silicone oil, acetylene can be polymerized at room temperature to yield PA with a reduced number of sp defects. The cis-isomer content of new PA film versus storage time (months) at 22°C under N2 decreases very slowly and is 78% after 3 months. The conductivities after doping of such stretched samples with are 2 x 10 S/cm (20-/xm-thick film) and 8 x 10 S/cm (0.1-)Ltm-thick film). 

Electron spin resonance (ESR) spectroscopy is also known as electron paramagnetic resonance (EPR). spectroscopy or electron magnetic resonance (EMR) spectroscopy. The main requirement for observation of an ESR response is the presence of unpaired electrons. Organic and inorganic free radicals and many transition metal compounds fulfil this condition, as do electronic triplet state molecules and biradicals, semicon-ductor impurities, electrons in unfilled conduction bands, and electrons trapped in radiation-damaged sites and crystal defect sites. 

The ESR g-factor is also known as the Lande 7-factor or spectroscopic splitting factor and depends on the resonance condition for ESR (Eq. 3) and is independent of both applied field and frequency. The 7-factor of a free electron is 2.002322, while the 7-factors of organic free radicals, defect centers, transition metals, etc. depend on their electronic. structure. The 7-factors for free radicals are close to the free electron value but may vary from 0 to 9 for transition metal compounds. The most comprehensive compilations of 7-factors are those published in [75], [76]. The magnetic moments and hence 7-faclors of nuclei in crystalline and molecular environments are anisotropic, that is the 7-factor (and hyperfine interactions) depend on the orientation of the sample. In general, three principal 7-factors are encountered whose orientation dependence is given by ... 

The unrestricted and restricted open-sheU Hartree-Fock Methods (UHF and ROHF) for crystals use a single-determinant wavefunction of type (4.40) introduced for molecules. The differences appearing are common with those examined for the RHF LCAO method use of Bloch functions for crystalline orbitals, the dependence of the Fock matrix elements on the lattice sums over the direct lattice and the Brillouin-zone summation in the density matrix calculation. The use of one-determinant approaches is the only possibility of the first-principles wavefunction-based calculations for crystals as the many-determinant wavefunction approach (used for molecules) is practically unrealizable for the periodic systems. The UHF LCAO method allowed calculation of the bulk properties of different transition-metal compounds (oxides, perovskites) the qrstems with open shells due to the transition-metal atom. We discuss the results of these calculations in Chap. 9. The point defects in crystals in many cases form the open-sheU systems and also are interesting objects for UHF LCAO calculations (see Chap. 10). 

An atom or a molecule with the total spin of the electrons S = 1 is said to be in a triplet state. The multiplicity of such a state is (2.S +1)=3. Triplet systems occur in both excited and ground state molecules, in some compounds containing transition metal ions, in radical pair systems, and in some defects in solids. 

The methods listed thus far can be used for the reliable prediction of NMR chemical shifts for small organic compounds in the gas phase, which are often reasonably close to the liquid-phase results. Heavy elements, such as transition metals and lanthanides, present a much more dilficult problem. Mass defect and spin-coupling terms have been found to be significant for the description of the NMR shielding tensors for these elements. Since NMR is a nuclear effect, core potentials should not be used. 

The formation of surface defects of a crystal lattice. It was observed while using crystal compounds of transition metals as catalysts [e.g. as was shown by Arlman (171, 173), for a TiCl3 surface defects appear on the lateral faces of the crystal]. In this case low surface concentration of the propagation centers should be expected, as is illustrated in the case of polymerization by titanium dichloride (158). The observed... 

In Chap. C, the thermodynamic and structural outlook of the bond, which had been the matter of discussion in Part A of this chapter, is further developed, and the model formalism, which takes advantage of the well known Friedel s model for d-transition metals but is inspired by the results of refined band calculations, is presented for metals and compounds. Also, a hint is given of the problems which are related to the nonstoichiometry of actinide oxides, such as clustering of defects. Actinide oxides present an almost purely ionic picture nevertheless, covalency is present in considerable extent, and is important for the defect structure. 

Non-stoichiometry is a very important property of actinide dioxides. Small departures from stoichiometric compositions, are due to point-defects in anion sublattice (vacancies for AnOa-x and interstitials for An02+x )- A lattice defect is a point perturbation of the periodicity of the perfect solid and, in an ionic picture, it constitutes a point charge with respect to the lattice, since it is a point of accumulation of electrons or electron holes. This point charge must be compensated, in order to preserve electroneutrality of the total lattice. Actinide ions having usually two or more oxidation states within a narrow range of stability, the neutralization of the point charges is achieved through a Redox process, i.e. oxidation or reduction of the cation. This is in fact the main reason for the existence of non-stoichiometry. In this respect, actinide compounds are similar to transition metals oxides and to some lanthanide dioxides. 

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