Cu2Sc2Ge4O13, a novel germanate isotypic with the quasi-1D compound Cu2Fe2Ge4O13 between 100 and 298 K

The germanate compound Cu2Sc2Ge4O13 has been synthesized by solid-state ceramic sintering techniques between 1173 and 1423 K. The structure was solved from single-crystal data by Patterson methods. The title compound is monoclinic, a = 12.336(2) Angstrom, b = 8.7034(9) Angstrom, c = 4.8883(8) Angstrom, beta = 95.74(2), space group P2(1)/m, Z = 4. The compound is isotypic with Cu2Fe2Ge4O13, described very recently. The structure consists of crankshaft-like chains of edge-sharing ScO6 octahedra running parallel to the crystallographic b-axis. These chains are linked laterally by [Cu2O6](8-) dimers forming a sheet of metal-oxygen-polyhedra within the a-b plane. These sheets are separated along the c-axis by [Ge4O13](10-) units. Cooling to 100 K does not alter the crystallographic symmetry of Cu2Sc2Ge4O13. While the b, c lattice parameter and the unit cell volume show a positive linear thermal expansion (alpha = 6.4(2) x 10(-6), 5.0(2) x 10(-6) and 8.3(2) x 10(-6) K-1 respectively), the a lattice parameter exhibits a negative thermal expansion (alpha = -3.0(2) x 10(-6) K-1) for the complete T-range investigated. This negative thermal expansion of a is mainly due to the increase of the Cu-Cu interatomic distance, which is along the a-axis. Average bond lengths remain almost constant between 100 and 298 K, whereas individual ones partly show both significant shortages and lengthening. (C) 2004 Elsevier Inc. All rights reserved.