ErSi crystallizes in the orthorhombic Cmcm space group. The structure is three-dimensional. Er is bonded in a 5-coordinate geometry to seven equivalent Si atoms. There are a spread of Er–Si bond distances ranging from 2.92–3.11 Å. Si is bonded in a 9-coordinate geometry to seven equivalent Er and two equivalent Si atoms. Both Si–Si bond lengths are 2.49 Å.