The crystal structure of chalcoalumite: mechanisms of Jahn-Teller-driven distortion in <sup>[6]</sup>Cu<sup>2+</sup>-containing oxysalts
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Bibliographic record
Abstract
Abstract The crystal structure of chalcoalumite, ideally Cu 2+ Al 4 (SO 4 )(OH) 12 (H 2 O) 3 , monoclinic, P 2 1 /n , Z = 4: a 10.228(3), b 8.929(3), c 17.098(6) Å, β 95.800(11)°, V 1553.6(1.5) Å 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 chalcoalumite, there is one S site, tetrahedrally coordinated by four O anions, with < S –O> = 1.472 Å. There are four Al sites with site-scattering values in accord with occupancy by Al and < Al –O> distances of 1.898–1.919 Å. 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 <Cu–O> distance of 2.086 Å 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 2 O) groups that are held in the structure solely by hydrogen bonding. The structure of chalcoalumite 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 2 O) groups. Chalcoalumite is a member of the nickelalumite group. Cu 2+ ϕ 6 (ϕ = O 2– , (OH) – , (H 2 O) 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 phonon, 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.
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| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
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| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
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