No abstract

The Raman spectrum of single-crystal BaTiO, has been observed in the Stokes region over the temperature range 4-475'K and becomes more complex as the temperature is lowered from the paraelectric state to the ferroelectric state, in which the crystal undergoes further first-

A model is presented which describes the optical modes of a mixed crystal. The model emphasizes a realistic use of the local electric field and is completely defined by the macroscopic parameters of the pure end members. The concentration dependence of the optical frequencies can be categorized into four distinct classes in contrast to the usual two classes, "one-mode" and "two-mode". We suggest that experimental evidence already exists which supports this new classification scheme.Es wird ein Modell behandelt, welches die langwelligen optischen Phonon-Moden von Mischkristallen beschreibt. Das Modell beinhaltet eine realistische Behandlung des lokalen elektrischen Feldes und ist vollstandig durch die makroskopischen Parameter der reinen Endkristalle definiert. Die Konzentrationsabhangigkeit der optischen Frequenzen zeigt eine Unterscheidung in vier unterschiedliche Klassen im Gegensatz zu den ublichen zwei Klassen, namlich den Ein-Moden-und Zwei-Moden-Mischkristallen. Es wird gezeigt, daB bereits experimentelle Evidenz fur das neue Klassifikations-Schema existiert.

The direct and indirect band gaps of Si1−x−yGexSny are inferred from the calculated energy-band structure of α-Sn and from the known structures of Ge and Si. Our assumptions are: that the energy-band shapes of the binaries Sn1−xGex, Ge1−ySiy and Si1−ySny change smoothly with x and y, and that the energy gap of SiGeSn can be estimated by interpolation from the gaps of SnGe, GeSi, and SiSn. The optical indices of refraction of SiGeSn are also estimated.