Transition metal monoxides

Terakura K, Williams A R, Qguchi T and Kubler J 1984 Transition-metal monoxides Band or Mott insulators Phys. Rev. Lett. 52 1830... 

Terakura K, Qguchi T, Williams A R and Kubler J 1984 Band theory of insulating transition-metal monoxides Band-structure calculations Phys. Rev. B 30 4734... 

If the holes are able to gain enough energy to move from a cation when illuminated, the materials are photoconducting. Thermal energy may also be able to liberate the holes and the solids are p-type semiconductors. The transition-metal monoxides NiO and CoO represent this behavior (Sections 1.11.4 and 4.3.2). 

An interesting group of non-stoichiometric oxides are the transition-metal monoxides (MO, where M = metal) for example, titanium monoxide (TiO) and vanadium monoxide (VO). These have a wide range of non-stoichiometric... 

As discussed in Chapter 4, unusually for a transition metal monoxide, TiCi.oo demonstrates metallic conductivity. The existence of the vacant sites within the TiO structure is thought to permit sufficient contraction of the lattice that the 3 c/ orbitals on titanium overlap, thus broadening the conduction band and allowing electronic conduction. 

Partial pressure of oxygen controls the nature of defects and nonstoichiometry in metal oxides. The defects responsible for nonstoichiometry and the corresponding oxidation or reduction of cations can be described in terms of quasichemical defect reactions. Let us consider the example of transition metal monoxides, M, 0 (M = Mn, Fe, Co, Ni), which exhibit metal-deficient nonstoichiometry. For the formation of metal vacancies in M, 0, the following equations can be written ... 

The above simple picture of solids is not universally true because we have a class of crystalline solids, known as Mott insulators, whose electronic properties radically contradict the elementary band theory. Typical examples of Mott insulators are MnO, CoO and NiO, possessing the rocksalt structure. Here the only states in the vicinity of the Fermi level would be the 3d states. The cation d orbitals in the rocksalt structure would be split into t g and eg sets by the octahedral crystal field of the anions. In the transition-metal monoxides, TiO-NiO (3d -3d% the d levels would be partly filled and hence the simple band theory predicts them to be metallic. The prediction is true in TiO... 

As mentioned previously (see Section 1.3.5) the binary M-X system shows a phase separation phenomenon in which the phase decomposes into two phases, having lower and higher concentrations of vacancies, below the critical temperature f, under the condition < 0, i.e. there is an attractive force between vacancies. In Section 1.3.5 it was not possible to refer to the details of those structures, because the model was less than simple. In any case, it can be safely said that if s < 0, vacancies cluster at low temperatures (from a thermodynamic point of view). Here let us briefly review the non-stoichiometry of 3d transition metal monoxides Mj- O, and then discuss the Fej system as a typical example of the clustering of vacancies in detail. 

As shown in Table 1.7, 3d transition metal monoxides with NaCl-type structure are classified into two groups one has the narrow non-stoichiometry seen in CoO and NiO, and the other has the wide non-stoichiometry seen... 

A systematic study to identify solid oxide catalysts for the oxidation of methane to methanol resulted in the development of a Ga203—M0O3 mixed metal oxide catalyst showing an increased methanol yield compared with the homogeneous gas-phase reaction.1080,1081 Fe-ZSM-5 after proper activation (pretreatment under vacuum at 800-900°C and activation with N20 at 250°C) shows high activity in the formation of methanol at 20°C.1082 Density functional theory studies were conducted for the reaction pathway of the methane to methanol conversion by first-row transition-metal monoxide cations (MO+).1083 These are key to the mechanistic aspects in methane hydroxylation, and CuO+ was found to be a likely excellent mediator for the reaction. A mixture of vanadate ions and pyrazine-2-carboxylic acid efficiently catalyzes the oxidation of methane with 02 and H202 to give methyl hydroperoxide and, as consecutive products, methanol and formaldehyde.1084 1085... 

Analogous compounds are obtained with the transition metal monoxides. Here again, our early results suggested a lower platinum content for the orthorhombic phases, but a recognition of the participation of Pt metal in the reaction led to a revision of the composition. Subsequent experiments at the stoichiometry given by Equation 2, together with chemical analyses of the purified products, have confirmed the MPt306... 

The archetypal examples are the 3d transition metal monoxides NiO, CoO, FeO, MnO, VO, and TiO. AU of these oxides possess the rock-salt stmcture (which makes both cation-cation and cation-anion-cation overlap important). One-electron band theory correctly predicts the metallic behavior observed in TiO, which is expected of a... 

Illustration 3 Transition Metal Monoxides with Edge-Sharing Octahedra... 

Now look at the transition metal monoxides with the rock-salt stmcture. Since the rock-salt structure is a three-dimensional network of edge-sharing MX(, octahedra, in which the metal may possess an incomplete d shell, it can be concluded that the Fermi level should reside in the metal t2g- or -block bands. 

Figure 5.16. The dx2-y2-dx2-y2 arid p-d interactions at T, X, and W, in the transition metal monoxides with the rock-salt structure, viewed down [00 1].

Figure 5.17. Dispersion curves for the d 2 y2 and d bands in the transition metal monoxides. Upper right calculated band dispersions for TiO. The Fermi level is indicated by the dashed line. Bottom right calculated band dispersions for VO.

CO. For that matter, in regards to predicting the type of electrical behavior, one has to be careful not to place excessive credence on actual electronic structure calculations that invoke the independent electron approximation. One-electron band theory predicts metallic behavior in all of the transition metal monoxides, although it is only observed in the case of TiO The other oxides, NiO, CoO, MnO, FeO, and VO, are aU insulating, despite the fact that the Fermi level falls in a partially hUed band. In the insulating phases, the Coulomb interaction energy is over 4 eV whereas the bandwidths have been found to be approximately 3 eV, that is, U > W. 

As alluded to in Section 5.5.5, the transition metal monoxides with the rock-salt strucmre are the archtypical examples of correlated systems. Of these oxides, only TiO is metallic. The others, NiO, CoO, MnO, FeO, and VO, are all insulating, despite the fact that the Fermi level falls within a partially filled band (in the independent electron picture). Direct electron transfer between two of the transition metal cations (in the rock-salt strucmre, d d interactions are important owing to the proximity of the cations), say, manganese, is equivalent to the disproportionation reaction ... 

Electronic. structure of the transition-metal monoxides. The nictal-ion configuration has a formula (Ar)(3r/)" Excitation energies (in eV) arc to a configuration (Ar)(3d)" 4.s- If At (Kq. 19-2) is negative, metallic conductivity is expected. 

With several other transition metal monoxides, covalent and special electronic effects that are not compatible with the ideal stmctnre manifest themselves. As examples, PdO and PtO display the characteristic tendencies of heavier d ions for sqnare-planar coordination by adopting different structures while a severe Jahn Teller distortion yields essentially fonrfold planar coordination in CuO. In the cases of TiO, VO, and NbO, formation of weak metal to metal bonds are seen as defect variants of the rock salt stmcture. 

Attention will then be turned to the major oxide minerals MgO, AljOj, and SiOj and the binary transition-metal oxides of Ti, Mn, and Fe, with some brief discussion of the series of transition-metal monoxides (MnO, FeO, CoO, NiO) and complex oxides (FeCr204, FeTiOj, etc.), and of the problem of the calculation of Mossbauer parameters in iron oxides (and other compounds). 

Fig. 4.34. Band picture of the transition-metal monoxides. The qualitative difference between this picture and the Mott insulator is the energy position of the empty d states (after Terakura et al., 1984a reproduced with the publisher s permission).

Table 4.24. The equilibrium lattice constant (a ) of CaO and some transition-metal monoxides. The experimental values are given in the second column. The theoretical values of in the nonmagnetic state are given in the third column. The theoretical values of are also calculated for the ferromagnetic and antiferromagnetic states...

Terakura, K., A. R. Williams, T. Oguchi, and J. Kiibler (1984a). Transition-metal monoxides band or Mott insulators. Phys. Rev. Lett. 52, 1830-33. 

Yamashita, J., and S. Asano (1983b). Cohesive properties of 3d transition-metal monoxides. J. Phys. Soc. Jpn. 52, 3514-19. 

The band structures of the transition metal monoxides including NiO have been a topic of considerable interest for many years, and study of spectra and transport properties continues in an effort to determine band widths, separations and electrostatic correlation energies. NiO is a Mott insulator (96) and the localized electron description assumed here is probably appropriate. Augmented plane wave band structure calculations have recently been made for NiO and other monoxides (97) and a localized electron multiple scattering Xa calculation for NiO (98). Neither type of calculation includes electron-electron correlation effects. 

This functional which is elfectively the PBE functional with 25% of exact Hartree-Fock exchange included, performed better than GGA + U. Alkauskas and Pasquarello found that PBEO increased the calculated band gap of a-quartz to 8.3 eV close to the experimental value of 9 eV and a great improvement on the value of 5.8 eV obtained with PBE. Blaha s group showed that the hybrid functionals PBEO and B3PW91 gave comparable or superior results to LDA + U for the transition metal monoxides MnO, FeO and CoO. 

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