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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Acknowledgments
We thank C. Hadidiacos for the help with the electron microprobe analyses, B. Mysen and F. Schicks for access and help with the Raman spectrometer and U. Schade for support at the synchrotron beamline Bessy II. D. Rodny is thanked for preparation and IR measurements. C.T. Prewitt is thanked for helpful comments and discussions. The comments and suggestions by Dan Frost and an anonymous reviewer helped to improve the manuscript.
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An erratum to this article is available at http://dx.doi.org/10.1007/s00269-014-0697-6.
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Koch-Müller, M., Dera, P., Fei, Y. et al. Polymorphic phase transition in Superhydrous Phase B. Phys Chem Minerals 32, 349–361 (2005). https://doi.org/10.1007/s00269-005-0007-4
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DOI: https://doi.org/10.1007/s00269-005-0007-4