Hf2NiS4 crystallizes in the monoclinic C2/m space group. Hf(1) is bonded to three equivalent S(1) and three equivalent S(2) atoms to form HfS6 octahedra that share corners with six equivalent Ni(1)S6 octahedra, edges with six equivalent Hf(1)S6 octahedra, and  a faceface with one Ni(1)S6 octahedra. The corner-sharing octahedral tilt angles range from 47-49°. Ni(1) is bonded to two equivalent S(1) and four equivalent S(2) atoms to form NiS6 octahedra that share corners with twelve equivalent Hf(1)S6 octahedra, edges with two equivalent Ni(1)S6 octahedra, and faces with two equivalent Hf(1)S6 octahedra. The corner-sharing octahedral tilt angles range from 47-49°. There are two inequivalent S sites. In the first S site, S(1) is bonded in a rectangular see-saw-like geometry to three equivalent Hf(1) and one Ni(1) atom. In the second S site, S(2) is bonded to three equivalent Hf(1) and two equivalent Ni(1) atoms to form a mixture of distorted edge and corner-sharing SHf3Ni2 square pyramids.
Hf2NiS4 crystallizes in the monoclinic C2/m space group. Hf(1) is bonded to three equivalent S(1) and three equivalent S(2) atoms to form HfS6 octahedra that share corners with six equivalent Ni(1)S6 octahedra, edges with six equivalent Hf(1)S6 octahedra, and  a faceface with one Ni(1)S6 octahedra. The corner-sharing octahedral tilt angles range from 47-49°. There are two shorter (2.54 Å) and one longer (2.56 Å) Hf(1)-S(1) bond length. There is one shorter (2.55 Å) and two longer (2.58 Å) Hf(1)-S(2) bond lengths. Ni(1) is bonded to two equivalent S(1) and four equivalent S(2) atoms to form NiS6 octahedra that share corners with twelve equivalent Hf(1)S6 octahedra, edges with two equivalent Ni(1)S6 octahedra, and faces with two equivalent Hf(1)S6 octahedra. The corner-sharing octahedral tilt angles range from 47-49°. Both Ni(1)-S(1) bond lengths are 2.39 Å. All Ni(1)-S(2) bond lengths are 2.46 Å. There are two inequivalent S sites. In the first S site, S(1) is bonded in a rectangular see-saw-like geometry to three equivalent Hf(1) and one Ni(1) atom. In the second S site, S(2) is bonded to three equivalent Hf(1) and two equivalent Ni(1) atoms to form a mixture of distorted edge and corner-sharing SHf3Ni2 square pyramids.