Characterization of PbBi4Te4S3 and inferences for ordered phases in the aleksite series

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Marco E. Ciriotti
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Iscritto il: ven 25 giu, 2004 11:31
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Characterization of PbBi4Te4S3 and inferences for ordered phases in the aleksite series

Messaggio da Marco E. Ciriotti » sab 12 ott, 2019 10:51

Referenza:
▪ Cook, N.J., Ciobanu, C.L., Liu, W., Slattery, A., Wade, B.P., Mills, S.J., Stanley, C.J. (2019): Polytypism and Polysomatism in Mixed-Layer Chalcogenides: Characterization of PbBi4Te4S3 and Inferences for Ordered Phases in the Aleksite Series. Minerals, 9, 628; https://doi.org/10.3390/min9100628

Abstract:
Bi-Pb-chalcogenides of the aleksite series represent homologous mixed-layer compounds derived from tetradymite (Bi2Te2S). Considering tetradymite as composed of five-atom (Bi2Te2S) layers, the named minerals of the aleksite homologous series, aleksite (PbBi2Te2S2) and saddlebackite, (Pb2Bi2Te2S3) have been considered as phases formed by regular stacking of longer seven- and nine-atom layers. High-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) imaging of thinned foils prepared in-situ on domains deemed homogeneous from electron probe microanalysis, STEM energy-dispersive X-ray spectrometry (EDS) element mapping and fast Fourier transforms (FFTs) from the images offer new insights into these structures. The hitherto-unnamed phase, PbBi4Te4S3, previously interpreted as regular intergrowths of five- and seven-atom layers, is characterized in terms of regular repeats of five- and seven-atom layers over distances of at least 350 nm, defining the (57), or 24H polytype. Imaging of other domains in the same lamella with identical composition at the electron microprobe scale reveals the presence of two additional polytypes: (5559), or 48H; and (557.559) or 72H. Unit cell dimensions for all three polytypes of PbBi4Te4S3 can be both measured and predicted from the HAADF STEM imaging and FFTs. STEM EDS mapping of each polytype confirm the internal structure of each layer. Lead and S occur within the centre of the layers, i.e., Te–Bi–S–Pb–S–Bi–Te in the seven-atom layer, Te–Bi–S–Pb–S–Pb–S–Bi–Te in the nine-atom layer, and so on. Polytypism is an intrinsic feature of the aleksite series, with each named mineral or unnamed phase, except the simple five-atom layer, defined by several alternative stacking sequences of different length, readily explaining the differing c values given in previous work. Homology is defined in terms of layer width and the stacking arrangement of those layers. Coherent structures of the same composition need not only be built of layers of adjacent size (i.e., five- and seven-atom layers) but, as exemplified by the (5559) polytype, may also contain non-adjacent layers, a finding not anticipated in previous work. New polysomes remain to be discovered in nature and each potentially exists as multiple polytypes. The present study further emphasizes the utility of HAADF STEM imaging and atomic-scale STEM EDS element mapping as an optimal tool for tracking stacking sequences and characterising structures in mixed-layer compounds.
Marco E. Ciriotti

«Things are interesting only in so far as they relate themselves to other things»

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Marco E. Ciriotti
Messaggi: 25986
Iscritto il: ven 25 giu, 2004 11:31
Località: via San Pietro, 55 I-10073 Devesi/Cirié TO - Italy
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Re: Characterization of PbBi4Te4S3 and inferences for ordered phases in the aleksite series

Messaggio da Marco E. Ciriotti » sab 12 ott, 2019 15:47

Hitachiite, poli-solfotelluride con formula Pb5Bi2Te2S6, è anch'essa una aleksite seires.

Probabilmente tutte le specie del "sottogruppo aleksite della tetradymite" [oppure "gruppo aleksite del supergruppo tetradymite"] sono trigonali.

Aleksite PbBi2Te2S2 hex (!) 4.238 4.238 79.76
Hitachiite Pb5Bi2Te2S6 trig P3m1 4.2200 4.2200 27.02
Saddlebackite Pb2Bi2Te2S3 hex (!) 4.230 4.230 33.43
Marco E. Ciriotti

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