Crystallographic isomorphism in the structural type of α-HgI₂ by example of KHgI₃ · H₂O, β-Ag₂HgI₄, and β-Cu₂HgI₄

The structure of KHgI₃ · H₂O is assigned to the family of crystal structures having the three-layer cubic packing of iodine anions with cations in the tetrahedral voids (the structures of α-HgI₂, β-Ag₂HgI₄, and β-Cu₂HgI₄ among them). Crystallographic analysis shows that the nodes of the three-layer close packing are populated by iodine anions and K cations in the ratio 3/4: 1/4. Transformation of the structure of α-HgI₂ into the structure of KHgI₃ · H₂O can be formally represented as the replacement of (HgI)_n⁺ fragments by (KH₂O)_n⁺ fragments: (Hg₂I₄)–(HgI)⁺ + (KH₂O)_n⁺ = KHgI₃ · H₂O. Perforated layers of vertex-sharing HgI₄ tetrahedra break down into parallel isolated chains. Channels formed in place of I–Hg–I–Hg–fragments are occupied by–H₂O–K–-H₂-O-K-H₂O-chains weakly bound to neighbors.