Abstract
Polarized reflection spectra between 4.000 and 20 cm−1 and absorption spectra (at temperatures of liquid helium) between 2.000 and 650 cm−1 have been measured of the isomorphous crystals NaN03, CaCO3, MgCO3 (D 63d ) and of CaMg (CO3)2 (C 23i ). The wavenumbers of the fundamental infrared transitions and of the two-phonon-combinations have been determined. The combinationbands show a structure, which allows some insight into the wavevector dispersion of the phonon branches of these crystals. Selection rules have been worked out for several points in the interior and on the border of the Brillouin zone. By comparison with the spectra they give evidence for the presence of critical points of the phonon-branches for wavevectors parallel to the trigonal axis of the crystal.
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Wir danken Herrn Prof. Dr. Paulitsch (Mineralogisches Institut, T.H. Darmstadt) für seine Hilfe bei der Orientierung einiger Kristalle sowie dem Rechenzentrum der T.H. Darmstadt für die Durchführung der numerischen Rechnungen. Die Stiftung Volkswagenwerk und die Deutsche Forschungsgemeinschaft haben dies Vorhaben durch die Überlassung von Geräten unterstützt.
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Hellwege, K.H., Lesch, W., Plihal, M. et al. Zwei-Phononen-Absorptionsspektren und Dispersion der Schwingungszweige in Kristallen der Kalkspatstruktur. Z. Physik 232, 61–86 (1970). https://doi.org/10.1007/BF01394946
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DOI: https://doi.org/10.1007/BF01394946