K₄Cu²⁺ Cu²⁺ Cl₈ ∙2H₂O: a novel non-centrosymmetric mixed-valent copper compound and its relation to minerals

Kornyakov I. V., Krivovichev S. V.

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2021. V. 150. N 5. P. 103-114

https://doi.org/10.31857/S0869605521050026

Full text is available on eLIBRARY.RU

Language: English

Abstract

Reddish-brown crystals of  K₄Cu²⁺ Cu²⁺ Cl₈ ∙2H₂O were prepared by chemical transport reactions. The crystal structure was solved using single-crystal diffraction data (a = 9.0472(5), b = 11.5591(4), c = 9.1786(5) Å, β = 118.692(7)°, V = 842.01(9) Å3) and refined to R1 = = 0.039 for 4369 independent observed reflections. The crystal structure consists of the Cu²⁺ Cu ⁺₂Cl₈ anionic chains extended along the a axis and linked through the K+ ions and H₂O molecules. There are three Cu sites in the title compound. The Cu1 and Cu3 sites are occupied by monovalent Cu⁺ ions and coordinated tetrahedrally by four Cl atoms each with the Cu–Cl bond lengths in the range 2.309–2.441 Å. The (Cu1Cl₄) and (Cu3Cl₄) tetrahedra share Cl…Cl edge to form a (Cu₂Cl₆) dimer with relatively short Cu…Cu intermetallic distance of 2.585 Å. However, the theoretical analysis of the electron-density distribution shows the absence of bonding interaction between the adjacent Cu centers. The dimers are linked into 1D chains through the Cu₂ atoms in an octahedral coordination. According to its coordination geometry and bond-valence calculations, the Cu2 site is occupied by Cu²⁺ ions. The K1 and K2 sites are coordinated by seven Cl atoms each to form (KCl7) capped trigonal prisms. In contrast, the K3 and K4 sites have a bicapped trigonal prismatic coordination by six Cl atoms and two H₂O groups. The Cu²⁺ Cu⁺₂ Cl₈ chains are linked via K– Cl and K–H₂O interactions as well as by H₂O…Cl hydrogen bonds. The non-centrosymmetricity of the overall structure is the result of the shift of the adjacent chains relative to each other in the direction parallel to the a axis. The title compound is chemically close to avdoninite, K₂Cu₅Cl₈(OH)₄ · 2H₂O, mitscherlichite, K₂CuCl₄ · 2H₂O, and romanorlovite, K₁₁Cu₉Cl₂₅(OH)₄ 2H₂O. However, it differs from them in its mixed-valence characterThe title compound possesses neither unusually high or unusually low complexity and thus its formation as a secondary phase in fumaroles corresponds to the typical level of complexity observed in this geochemical environment.

Keywords: copper, mixed valence, non-centrosymmetric compounds, volcanic fumaroles, structural complexity