Compressed octahedral coordination in chain compounds containing divalent copper: structure and magnetic properties of CuFAsF6 and CsCuAlF6.

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.