FeAs2 is zeta iron carbide structured and crystallizes in the orthorhombic Pnnm space group. The structure is three-dimensional. Fe3+ is bonded to six equivalent As+1.50- atoms to form a mixture of corner and edge-sharing FeAs6 octahedra. The corner-sharing octahedral tilt angles are 53°. There are two shorter (2.36 Å) and four longer (2.39 Å) Fe–As bond lengths. As+1.50- is bonded in a 4-coordinate geometry to three equivalent Fe3+ atoms.