  Electron-counting rules were applied to understand the stability, structural
preference, and physical properties of metal disilicides. Following predictions
made by 14 electron counting rules, the ordered semiconductor ReGaSi, the first
ternary phase in this system, is proposed and successfully synthesized. It
crystallizes with a primitive tetragonal structure (space group P4/nmm) closely
related to that of MoSi2-type ReSi2, but with Ga and Si orderly distributed in
the unit cell. The band structure, density of states, and crystal orbital
calculations confirm the electron count hypothesis to predict new stable
compounds. Calculations, based on 14 electrons per ReGaSi units, show a small
indirect band gap of ~0.2 eV around Fermi level between full and empty
electronic states. Additionally, first-principles calculations confirm the site
preference of Ga and Si which is observed through the structural refinement.
Experimental magnetic measurements verified the predicted non-magnetic
properties of ReGaSi.
