Intermetallic compounds with non-metallic properties

In the realm of intermetallic compounds, there is a small group of phases that demonstrate the properties of non-metals. These are intermetallics formed by elements from different blocks of the periodic table, viz., transition metals on the one hand and p-block metals (or metalloids) on the other hand. The interaction between transition metal d-orbitals and p-element s- and p-orbitals leads to the opening of a gap in the band structure, and the Fermi level may fall into the band gap at a certain valence electron concentration, thus providing the non-metallic properties. Despite the lack of a harmonious theory describing the electronic structure of the intermetallics in question, there are approaches that allow one to chemically modify them and thus to design novel thermoelectric materials through the interplay between metals and semiconductors. The review concerns the main families of intermetallic compounds with non-metallic properties. Specific features of their crystal and electronic structures, the reasons for the onset of non-metallic (semiconducting) state, and the prospects for application of these compounds as thermoelectric materials are analyzed.