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Structural anomalies in tobelite-2M2 explained by high resolution and analytical electron microscopy
No full textA transmission electron microscopy (TEM) investigation was undertaken in order to elucidate the nature of the structural disorder in a tobelite specimen from the sedimentary rocks of the Armorican sandstones (western France). This structural disorder may be the origin of residual electron-density maxima in difference-Fourier maps reported previously in single-crystal XRD studies of tobelite. The TEM investigation of tobelite confirmed that it is the 2M2 polytype in subfamily-B, but the ordered sequence is interrupted by numerous stacking faults parallel to (001), for which the stacking vectors belong to both subfamilies-A and -B of mica polytypes, with a prevalence of the latter. Chemical heterogeneity depending on the Si/Al ratio and Na and Mg concentration was observed at the nanoscale among different mica lamellae in a single crystal. The observed variations are consistent with a change in mica chemistry leading to interlayer vacancies which may cause shortening of the interlayer separation, as revealed by the single- crystal structure refinements
NEAR ATOMIC IMAGES OF TWIN BOUNDARIES AND STACKING FAULTS IN A 15R SiC SAMPLE: TWIN LAW AND GROWTH MECHANISM
No full text
TEM investigations of Ag- and Cu-free lillianite and heyrovskyite from Vulcano (Aeolian Islands, Italy)
No full textStructural Defects and Polytypism in Moissanite and Synthetic SiC Crystals
No full textIn recent years, defect characterisation of SiC polytypes has
been developed significantly, due to the increasing importance
of such polytypes for high temperature, high-power and highfrequency
electronic devices [1]. On the other side, moissanite-
SiC is a rare mineral that was discovered in 1905 within a
meteorite and only recently as a rock-forming mineral (8.4
vol%) in one unique specimen of a terrestrial volcanic rock [2].
Besides it has been found in kimberlitic pipes and can provide
some light on the redox conditions of the Earth Mantle. Finally
SiC is a particularly interesting species of presolar grain because
the polytype distribution can be related to different parameters
of the expanding stellar atmospheres of asymptotic giant
branch-AGB-stars.
In the frame of a research program in progress on SiC, growth
defects of bulky gem-crystals of 6H-SiC were studied by X-ray
Diffraction Topography-XRDT and it was found that the main
types of defects were dislocations parallel and perpendicular
to the growth directions, and micro-channels [3].
This piece of work is devoted to the study of structural defects
and degree of order of natural and synthetic moissanite.
Natural and synthetic samples have been investigated by XRD
Topography and TEM, focusing on the relationships between
defects and growth conditions. XRDT analyses of synthetic
6H-SiC allowed the characterization of dislocations and channels
to be made and other polytypes for further investigation by
TEM to be localized. All studied sample are characterized by
the presence of linear defects, dislocations and micro-channels,
uniformly distributed in the crystal. Moreover samples grown
by means of Physical Vapour Transport-PVT method, show the
same linear defects with different character, strictly related to
growth conditions.
TEM images and electron diffractions (EDs) strongly differentiate
natural from synthetic samples. ED patterns with [100]
incidence of natural crystals are consistent with the 6H polytype
and do not show streaks along the [001] stacking direction. This
result is confirmed by structure refinement from area detector
single crystal X-ray data [4]. Synthetic samples are comparatively
much more disordered. Conventional images show high
density of (001) faults, not observed in natural samples. Consistently,
ED patterns of the [100] zone are streaked along c*.
Atomic resolution imaging shows that synthetic samples mainly
consist of (001) stacking sequence described as (32)3 [5]. Locally
mixed stacking sequence described by notation 23(3233)5,
probably referred to a long period polytype, are present.
[1] Neudeck, P. G., Journal of Electronic Materials, 1995, 24, 283.
[2] Di Pierro S., Gnos E., Grobety B. H., Armbruster T., Bernasconi
S. M. and Ulmer P., Am. Mineral., 2003, 88, 1817.
[3] Agrosì G., Fregola R.A., Monno A., Scandale E., Tempesta G.,
Materials Science Forum, 2005, 483-485, 311.
[4] Capitani G.C.*, Tempesta G., Di Pierro S., Scandale E., 2006, This
congress.
[5] Zhdanov G.S., Minervina Z.V., Compt. Rend. Acad. Sci. URSS 1945,
48.
Industrial mineralogy
23rd European Crystallographic Meeting, ECM23, Leuven, 2006
Acta Cryst. (2006). A62, s64
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