Structural Alternatives Transition-metal Nitrides

Are there other quantum-chemical alternatives to the density-functional approach used here Yes, another choice for generating a reliable electronic-structure calculation for CaO would have been to use a Flartree-Fock-based strategy. Because CaO is nonmetallic, the method works well [262], and it yields theoretical lattice parameters of 4.864/4.940 A - depending on the in-clusion/exclusion of virtual 3d orbitals for Ca - which stand for a 1.1/2.7% overestimation of the experimental value, almost as good as the GGA result. 

2) Despite the fact that this section deals with transition-metal nitrides, we should also mention that rare-earth nitrides emerge as superior luminescent materials because of their very special properties see also Section 3.3. 

3) Because of the crystal field around the metal cation and the latter s antibonding interactions with the ligands, dll five formerly degenerate d orbitals become energetically destabilized within the octahedral environment, but the f2g set is less destabilized than 

The question of alternative structure can be answered by electronic-structure theory, and it turns out that a quantitative answer is slightly more complicated because different magnetic properties are calculated for the [NaCl] and [ZnS] types. Nonetheless, non-spin-polarized band-structure calculations are quite sufficient to supply us with a correct qualitative picture. This has been derived using the TB-LMTO-ASA method and the LDA functional, and they give the correct lattice parameters with lowest energies for both structure types [267], just as for the case of CaO. 

Transformations to the [ZnS] structure type considerably lessen the strength of these unfavorable Fe-Fe interactions. In the earlier members of the MN series (ScN, TiN, VN,. ..), the Fermi level lies low in the M-M COHP curve, not yet sampling so many antibonding states. For these electron counts, the sodium chloride structure, with much shorter M-M contacts, is favored. For higher electron counts, switching to the [ZnS] structure type reduces the antibonding M-M interactions by a significant amount. It also can be shown, numerically, that the Fe-N bond for the tetrahedral coordination is more covalent than for the octahedral coordination (see also below). 

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Nitrides can be sub-divided into ionic, covalent and interstitial types.An alternate general classification of nitrides, based on bonding classification, as ionic, covalent and metallic has also been applied. Ionic or salt-like nitrides are formed by electropositive elements such as Li, Mg, Ca, Sr, Ba, Cu, Zn, Cd and Hg and possess formulae which correspond to those expected on the basis of the combination of the metal ion with ions. A range of covalent nitrides are known and are exhibited by less electropositive elements such as B, S, P, C and Si. Interstitial nitrides are formed by some transition metals and refer to compounds which can be described in terms of the occupancy of interstitial sites in close packed metallic structures by nitrogen atoms. Oxygen can also be accommodated within these structures and a range of oxynitrides are known to... 

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