The crystal structure of chalcoaiumite: mechanisms of Jahn-Teller-driven distortion in ~[6]Cu~(2+)-containing oxysalts

摘要

The crystal structure of chalcoaiumite, ideally Cu~(2+)Al_4(S0_4)(OH)_(12)(H_2O)_3, monoclinic, P2_1 ,Z= 4: a 10.228(3), b 8.929(3), c 17.098(6) A, β 95.800(11)°, V 1553.6(1.5) A~3, has been refined to R_1= 3.08% for 4,022 unique observed (4σ) reflections collected on a Bruker D8 three-circle diffractometer equipped with a rotating-anode generator, multilayer optics and an APEX-II CCD.detector. In the structure of chalcoaiumite, there is one S site, tetrahedrally coordinated by four O anions, with = 1.472 A. There are four Al sites with site-scattering values in accord with occupancy by Al and distances of 1.898—1.919 A. There is one Cu site occupied by Cu~(2+) and coordinated by six anions in the [4 + 2] arrangement typical for octahedrally coordinated Cu~(2+). The short distance of 2.086 A is in accord with the low degree of bond-length distortion of the Cu octahedron. There are 19 anion sites: 4 sites are occupied by O atoms that are bonded to the S cation, 12 sites are occupied by (OH) groups that bond to all octahedrally coordinated cations, and 3 sites are occupied by (H_2O) groups that are held in the structure solely by hydrogen bonding. The structure of chalcoaiumite consists of interrupted sheets of edge-sharing Al and Cu octahedra of the form [Cu~(2+)Al_4(OH)_(12)]~(2+) that intercalate layers of (SO_4) tetrahedra and (H_2O) groups. Chalcoaiumite is a member of the nickelalumite group.Cu~(2+)ψ6 (ψ = O~(2-), (OH)-, (H_2O)~0) octahedra show a wide range of bond-length distortion away from the holosymmetric arrangement, driven by spontaneous symmetry-breaking of the degenerate electronic ground-state in holosymmetric octahedral coordination. Here, we examine the structural mechanisms that allow large octahedron distortions of this type. There are two mechanisms: (1) coupling of (usually parallel) octahedron distortions to a vibrational phonoh, inducing a (often ferroelastic) phase transition in M~(2+)-Cu~(2+) solid-solutions; (2) cooperative orientational disorder, where bond topology (polyhedron linkage) allows large differences in bond lengths within polyhedra to accord with the valence-sum rule of bond-valence theory.