Polymorphic phase transition in Superhydrous Phase B |
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| Authors: | M Koch-Müller P Dera Y Fei H Hellwig Z Liu J Van Orman R Wirth |
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| Institution: | (1) Department 4, Telegrafenberg, GeoForschungsZentrum Potsdam, 14473 Potsdam, Germany;(2) Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road NW, Washington DC, 20015, USA;(3) Department of Geology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA;(4) Department of Geological Sciences, Case Western Reserve University, Cleveland, OH 44106, USA |
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| Abstract: | We synthesized superhydrous phase B (shy-B) at 22 GPa and two different temperatures: 1200°C (LT) and 1400°C (HT) using a
multi-anvil apparatus. The samples were investigated by transmission electron microscopy (TEM), single crystal X-ray diffraction,
Raman and IR spectroscopy. The IR spectra were collected on polycrystalline thin-films and single crystals using synchrotron
radiation, as well as a conventional IR source at ambient conditions and in situ at various pressures (up to 15 GPa) and temperatures
(down to −180°C). Our studies show that shy-B exists in two polymorphic forms. As expected from crystal chemistry, the LT
polymorph crystallizes in a lower symmetry space group (Pnn2), whereas the HT polymorph assumes a higher symmetry space group (Pnnm). TEM shows that both modifications consist of nearly perfect crystals with almost no lattice defects or inclusions of additional
phases. IR spectra taken on polycrystalline thin films exhibit just one symmetric OH band and 29 lattice modes for the HT
polymorph in contrast to two intense but asymmetric OH stretching bands and at least 48 lattice modes for the LT sample. The
IR spectra differ not only in the number of bands, but also in the response of the bands to changes in pressure. The pressure
derivatives for the IR bands are higher for the HT polymorph indicating that the high symmetry form is more compressible than
the low symmetry form. Polarized, low-temperature single-crystal IR spectra indicate that in the LT-polymorph extensive ordering
occurs not only at the Mg sites but also at the hydrogen sites. |
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| Keywords: | Super hydrous phase B Phase transformation OH incorporation Infrared Synchrotron infrared radiation High pressure behaviour |
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