Metallic and insulating3dtransition-element monoxides and their stability

The binding properties of the $3d$ monoxides are studied in detail, and it is shown that the metallic character of TiO and VO is directly reflected in their heat of formation. The same holds true for NbO. From a stability analysis of the $3d$ monoxides versus decomposition, it is found that TiO, VO, and FeO are close to an instability. Further, it can be concluded that both ScO and CrO must be very near existence. The general occurrence of transition-metal monoxides is shown to be directly correlated with ionic properties of the transition elements. An investigation of the absorption edge in the $3d$ monoxides is also undertaken. The importance of the crystal-field splitting is noticed, and it is shown that the heat of formation of the monoxides can be used to derived the crystal-field parameter $\Delta$. The change from a delocalized to a localized behavior of the $d$ electrons in the $3d$ monoxides is compared with a similar change of the $f$ electrons in the actinides. Some similarities between these two series of materials are pointed out.