ErNi crystallizes in the orthorhombic Pnma space group. The structure is three-dimensional. Er is bonded in a 7-coordinate geometry to seven equivalent Ni atoms. There are a spread of Er–Ni bond distances ranging from 2.84–2.90 Å. Ni is bonded in a 9-coordinate geometry to seven equivalent Er and two equivalent Ni atoms. Both Ni–Ni bond lengths are 2.50 Å.