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THE CUPRITE-LIKE FRAMEWORK OF OCu₄ TETRAHEDRA IN THE CRYSTAL STRUCTURE OF SYNTHETIC MELANOTHALLITE, Cu₂OCl₂, AND ITS NEGATIVE THERMAL EXPANSION

Sergey V. Krivovichev^1,, Stanislav K. Filatov¹ and Peter C. Burns²

¹ Department of Crystallography, Faculty of Geology, St. Petersburg State University, University Emb., 7/9, St. Petersburg 199034, Russia
² Department of Civil Engineering and Geological Sciences, University of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, Indiana 46556-0767, U.S.A.

e-mail address: skrivovi{at}mail.ru

Crystals of synthetic melanothallite, Cu₂OCl₂, have been prepared by chemical transport reactions. Its crystal structure (orthorhombic, Fddd, a 7.4691(15), b 9.5969(19), c 9.700(2) Å, V 695.3(2) ų, Z = 8) has been refined to R₁ = 0.028 (wR = 0.078; S = 1.180) on the basis of 346 unique observed reflections (|F_o| 4_F). There is one symmetrically independent Cu atom in the structure, coordinated by two O and four Cl atoms. The mixed-ligand CuO₂Cl₄ octahedron shows strong Jahn–Teller (4 + 2)- distortion, with two O and two Cl atoms located in the equatorial plane and two Cl atoms in apical positions. The structure may be described as a three-dimensional framework formed by the cross-linking of chains of edge-sharing CuO₂Cl₂ squares. The chains are parallel to [110] and [10], and are linked to each other through the sharing of O atoms. The structure can also be described as an array of OCu₄ oxocentered tetrahedra linked by the sharing of Cu corners into a cuprite-like three-dimensional framework, with cavities in the framework containing Cl anions. As established previously, melanothallite shows negative thermal expansion along the b axis of its unit cell (_b = –26.7 x 10^–6 °C^–1), whereas thermal expansion along the a axis is positive and large ((_a = 50.6 x 10^–6 + 25.2 x 10^–9t °C^–1). This thermal behavior is explained on the basis of a cross-linking of chains of edge-sharing CuO₂Cl₂ squares. In the room-temperature structure, two chains are inclined to each other by ~76°. When the temperature increases, the angle tends toward 90°, and the change of the angle between the two chains is accompanied by an increase of the a unit-cell parameter and by a decrease in the b parameter. This hinge mechanism provides an explanation for the high anisotropy in the thermal behavior of melanothallite observed by X-ray powder-diffraction methods.

Keywords: melanothallite, crystal structure, cuprite, oxocentered tetrahedra..