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

Abstract

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) A, V 695.3(2) A 3 , Z = 8) has been refined to R1 = 0.028 (wR = 0.078; S = 1.180) on the basis of 346 unique observed reflections (|Fo| ≥ 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 [¯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 � 10 –6 °C –1 ), whereas thermal expansion along the a axis is positive and large (� a = 50.6 � 10 –6 + 25.2 � 10 –9 t °C –1 ). This thermal behavior is explained on the basis of a cross-linking of chains of edgesharing 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.