Band Dispersion and Site Preference in Ternary Transition Metal Silicides, Germanides, and Stannides RTX ₂ (R = Rare Earth Metal or Ca, Sr, Ba, T = Transition Metal, X = Si, Ge, Sn)

More than 100 solid-state compounds of the formula RTX₂ (R = rare earth metal or Ca, Sr, Ba, T = Group 7-11 3<i>d</i>, 4<i>d</i>, and 5<i>d</i> transition metal, X = Si, Ge, Sn) are known. Most of them adopt the CeNiSi₂ or the related LaMnSi₂ structure. These two structural types are related, in containing square TX nets and polyacetylene-like X chains, but with a different disposition of the TX units in their nets. Using density functional computational tools, we have investigated the electronic origin of the site preference by the transition metal T and the main group atom X in these two crystal structures. Due to the larger band dispersion of the atomic orbitals at sites with closer neighbors and the stronger bonding feature in the lower half of the energy bands arising, and antibonding features of the upper half of the energy bands, the LaMnSi₂ type is preferred for low <i>d</i> electron count, and the CeNiSi₂ type for high <i>d</i> electron count. More generally, variation in dispersion (inherently greater in lattice sites with more nearby overlapping atoms) interacts with electron occupation, to explain structural preferences in these and other ternary compounds.