Structure-Composition Subtleties in NaZn13-type Derivatives of Sr/Ca(AuxAl1-x)(12-13)

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2020-10-14
Authors
Pham, Joyce
Palasyuk, Andriy
Miller, Gordon
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Ames Laboratory
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Chemistry
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Ames LaboratoryChemistry
Abstract

In the ternary (Sr/Ca)-Au-Al phase space for 5-8 at % Sr/Ca and nearly equimolar mixture of Au and Al, five new NaZn13-derivatives Sr/Ca(AuxAl1-x)(12-13) were discovered and their atomic site preferences and electronic structures were studied: (1) at higher Au content, cubic SrAuxAl13-x [7.24(2) >= x >= 6.68(2)] shows no obvious atomic site preferences; (2) at lower Au content, tetragonal SrAuxAl13-x [6.59(1) >= x >= 6.35(3)] shows icosahedra exclusively centered by Al and preferential atomic arrangements to maximize the number of Au-Al (or Al-rich) shortest distances; (3) a monoclinic SrAu6.10(3)Al6.40 compound was uncovered from studies of a single crystal specimen with problematic refinement issues but more detailed powder X-ray diffraction analysis which suggest even more pronounced geometric distortion from the NaZn13-type structure as well as preferential arrangements for Au-Al nearest contacts; (4) tetragonal SrAu5.75(2)Al6.25 with all empty icosahedra; and (5) orthorhombic CaAu6.09(2)Al6.01(1) with icosahedra that are half empty and half partially occupied by Al. Electronic DOS and COHP curves were used to rationalize the structural depiction of Sr/Ca(AuxAl1-x)(12-13) as icosahedra packing of (AuxAl1-x)(12-13) with voids filled by the electropositive Sr/Ca.

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