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Layered Structures and Disordered Polyanionic Nets in the Cation-Poor Polar Intermetallics CsAu1.4Ga2.8 and CsAu2Ga2.6

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journal contribution
posted on 07.12.2016, 00:00 authored by Volodymyr Smetana, Simon Steinberg, Anja-Verena Mudring
Gold intermetallics are known for their unusual structures and bonding patterns. Two new compounds have been discovered in the cation-poor part of the Cs–Au–Ga system. Both compounds were obtained directly by heating the elements at elevated temperatures. Structure determinations based on single-crystal X-ray diffraction analyses revealed two structurally and compositionally related formations: CsAu1.4Ga2.8 (I) and CsAu2Ga2.6 (II) crystallize in their own structure types (I: R3̅, a = 11.160(2) Å, c = 21.706(4) Å, Z = 18; II: R3̅, a = 11.106(1) Å, Å, c = 77.243(9) Å, Z = 54) and contain hexagonal cationic layers of cesium. This is a unique structural motif, which has never been observed for the other (lighter) alkali metals in combination with Au and post transition elements. The polyanionic part is characterized in contrast by Au/Ga tetrahedral stars, a structural feature that is characteristic for light alkali metal representatives, and disordered sites with mixed Au/Ga occupancies that occur in both structures with a more significant disorder in the polyanionic component of CsAu2Ga2.6. Examinations of the electronic band structure for a model approximating the composition of CsAu1.4Ga2.8 have been completed using density-functional-theory-based methods and reveal a deep pseudogap at EF. Bonding analysis by evaluating the crystal orbital Hamilton populations show dominant heteroatomic Au–Ga bonds and only a negligible contribution from Cs pairs.