TY - JOUR

T1 - The cuprite-like framework of OCu4 tetrahedra in the crystal structure of synthetic melanothallite, Cu2OCl2, and its negative thermal expansion

AU - Krivovichev, Sergey V.

AU - Filatov, Stanislav K.

AU - Burns, Peter C.

PY - 2002/8

Y1 - 2002/8

N2 - 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) Å3, Z = 8) has been refined to R1 = 0.028 (wR = 0.078; S = 1.180) on the basis of 346 unique observed reflections (lFol ≥ 4σF). 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 [110], 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 (αb = -26.7 × 10-6 °C-1), whereas thermal expansion along the a axis is positive and large (αa = 50.6 × 10-6 + 25.2 × 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.

AB - 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) Å3, Z = 8) has been refined to R1 = 0.028 (wR = 0.078; S = 1.180) on the basis of 346 unique observed reflections (lFol ≥ 4σF). 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 [110], 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 (αb = -26.7 × 10-6 °C-1), whereas thermal expansion along the a axis is positive and large (αa = 50.6 × 10-6 + 25.2 × 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.

KW - Crystal structure

KW - Cuprite

KW - Melanothallite

KW - Oxocentered tetrahedra

UR - http://www.scopus.com/inward/record.url?scp=0036700980&partnerID=8YFLogxK

U2 - 10.2113/gscanmin.40.4.1185

DO - 10.2113/gscanmin.40.4.1185

M3 - Article

AN - SCOPUS:0036700980

VL - 40

SP - 1185

EP - 1190

JO - Canadian Mineralogist

JF - Canadian Mineralogist

SN - 0008-4476

IS - 4

ER -