title: Compressed octahedral coordination in chain compounds containing divalent copper: structure and magnetic properties of CuFAsF6 and CsCuAlF6.
abstract: Crystal structures and magnetic investigations of CuFAsF6 and CsCuAlF6 are reported. Together with KCuAlF6, these appear to be the only examples of Jahn-Teller pure Cu(II) compounds containing only one type of ligand that exhibits a compressed octahedral coordination geometry. The Rietveld method has been used for refining the CsCuAlF6 structure based on neutron powder diffraction data at 4 K. The compound crystallizes in space group Pnma (no. 62) with a=7.055(1), b=7.112(1), c=10.153(1) A and Z=4 at 4 K. The structure is built from infinite [CuF5]n(3n-) chains of [CuF6]4- octahedra running along the [1 0 0] direction and (AlF6)3- octahedra connected by corners in the trans position, thus giving rise to chains oriented along the [0 1 0] direction. Single crystals of CuFAsF6 were prepared under solvothermal conditions in AsF5 above its critical temperature. The structure was determined from single-crystal data. CuFAsF6 crystallises in the orthorhombic space group Imma (No. 74) with a=10.732(5), b=6.941(3), c=6.814(3) A and Z=4 at 200 K. The structure can also be described in terms of one-dimensional infinite [CuF5]n(3n-) chains of tilted [CuF6](4-) octahedra linked by trans-vertices running along the b axis. The [CuF5]n(3n-) chains are connected through [AsF6]- units sharing joint vertices. The compressed octahedral coordination of CuII atoms in CuFAsF6 and CsCuAlF6 compounds at room temperature is confirmed by Cu K-edge EXAFS (extended x-ray absorption fine structure) analysis. For both compounds strong antiferromagnetic interactions within the [CuF5]n(3n-) chains were observed (theta(p)=-290+/-10 K and theta(p)=-390+/-10 K for CuFAsF6 and CsCuAlF6, respectively). The peculiar magnetic behaviour of chain compounds containing divalent copper at low temperature could be related to uncompensated magnetic moments in the one-dimensional network.
authors.affiliation: Department of Inorganic Chemistry and Technology, Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia. zoran.mazej@ijs.si <zoran.mazej@ijs.si>
authors.forename: Zoran
authors.initials: Z
authors.lastname: Mazej
authors.forename: Iztok
authors.initials: I
authors.lastname: Arcon
authors.forename: Primoz
authors.initials: P
authors.lastname: Benkic
authors.forename: Alojz
authors.initials: A
authors.lastname: Kodre
authors.forename: Alain
authors.initials: A
authors.lastname: Tressaud