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Abstract
Bi8Te3 is a member of the tetradymite homologous series, previously shown to be compositionally and structurally distinct from hedleyite, Bi7Te3, yet inadequately characterized structurally. The phase is identified in a sample from the Hedley district, British Columbia, Canada. Compositions are documented by electron probe microanalysis and structures are directly imaged using high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM). Results confirm that Bi8Te3 has an 11-atom layer structure, in which three Bi-Bi pairs are placed adjacent to the five-atom sequence (Te-Bi-Te-Bi-Te). Bi8Te3 has trigonal symmetry (space group R3¯m) with unit cell dimensions of a = ~4.4 Å and c = ~63 Å calculated from measurements on representative electron diffraction patterns. The model is assessed by STEM simulations and EDS mapping, all displaying good agreement with the HAADF STEM imaging. Lattice-scale intergrowths are documented in phases replacing Bi8Te3, accounting for the rarity of this phase in nature. These results support prior predictions of crystal structures in the tetradymite homologous series from theoretical modeling and indicate that other phases are likely to exist for future discovery. Tetradymite homologues are mixed-layer compounds derived as one-dimensional superstructures of a basic rhombohedral sub-cell. Each member of the series has a discrete stoichiometric composition and unique crystal structure.
Highlights
Published: 9 September 2021The tetradymite homologous series (Bix Xy, where X = Te, Se, S) comprises phases derived from a simple 5-atom layer represented by tellurobismuthite, Bi2 Te3, and isostructural phases, including tetradymite, Bi2 Te2 S [1,2]
Are combined in various proportions: S0 (Bi2k X3 )·L0 (Bi2(k+1 )X3 ) (k ≥ 1; X = chalcogen; S0, L0 = number of short and long modules, respectively). This approach was introduced as a working model based on a high-resolution transmission electron microscopy (HR-TEM) study of phases in the extended compositional range Bi2 Te3 –Bi8 Te3 [2]
Bi8 Te3 occurs within disseminations of bismuth minerals in a hedenbergite skarn (Figures 1 and 2)
Summary
To provide a systematic homology for the series enabling prediction of the structural arrangements for any stoichiometry, Ciobanu et al [2] presented a model drawing on seminal work by many authors since the 1960s [3,4,5,6,7,8,9], in which constituent layer stacks of different size (5-, 7-, 9-, 11-atom, etc.). Are combined in various proportions: S0 (Bi2k X3 )·L0 (Bi2(k+1 )X3 ) (k ≥ 1; X = chalcogen; S0 , L0 = number of short and long modules, respectively). This approach was introduced as a working model based on a high-resolution transmission electron microscopy (HR-TEM) study of phases in the extended compositional range Bi2 Te3 –Bi8 Te3 [2]. Such displacements are quantifiable by fractional shifts between reflections in the derived and basic structures [2]
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