The lattice parameters of the hexagonal unit cell, the atomic parameters, and the shift of the Raman active vibrations are measured of p-Bi,Te, under uniaxial and hydrostatic pressure. The structural investigations are performed by neutron diffraction, and the lattice vibrations are studied by Raman scattering. The results of the diffraction experiments constitute a direct experimental confirmation of the weak bonding between the sandwiches in this narrow gap semiconductor with sandwich structure. As a consequence, some Raman active phonon modes show a rather strong dependence upon uniaxial pressure up to Aa/Ap = 1.9 x 10-8cm-l/Pa. Die Gitterparameter der hexagonalen Einheitszelle, die Atompositionen und die Verschiebung der Raman-aktiven Gitterschwingungen von p-Bi,Te, unter uniaxialem und hydrostatischem Druck werden gemessen. Die strukturellen Untersuchungen werden mittels Neutronenbeugung durchgefuhrt, die Gitterschwingungen mit Ramanstreuung untersucht. Die Ergebnisse der Diffraktionsexperimente konstatieren einen direkten experimentellen Beweis fur die schwachen Bindungen zwischen den Sandwiches in diesem Schmalband-Halbleiter mit Sandwichstruktur. Als Konsequenz zeigen einige Raman-aktive Gitterschwingungen eine relativ starke Druckabhangigkeit der Anregungsfrequenzen, unter uniaxialem Druck bis zu AG/Ap = 1,9 x 10-8 cm-l/Pa. l) Rontgenring 8, D-8700 Wurzburg, FRG. 2, Present address: Siemens AG, Zentralbereich Technik, D-8000 Munchen 83. 9. 132 W. KULLMANN et al. Te'

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The extremely anisotropic bonding properties of the narrow gap semiconductor bismuth telluride (Bi2Te3) with sandwich structure is studied by inelastic neutron scattering. In combination with lattice dynamical shell model calculations, new information is obtained on the complex bonding forces of this sandwich structured V–VI semiconductor. The sandwich character leads predominantely to three‐dimensional bonding behavior. No characteristic two‐dimensional lattice properties can be observed.

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The authors have measured the phonon dispersion curves of the fluoride perovskite materials KMnF3 and KZnF3 by means of coherent neutron inelastic scattering. The aim of this investigation was to provide the phonon spectrum of KMnF3 which undergoes structural phase transitions and which has been studied by various methods in the last decade and, for comparison, of KZnF3 in which no phase transitions occur. In addition they have found an anomalous temperature dependence of the R15' and M2 modes in KZnF3 which are the modes that soften near the phase transitions of KMnF3. This indicates that there is in general a tendency towards an instability against octahedra rotations in all fluoride perovskites. For the analysis of the phonon spectra, they have employed a lattice dynamical shell model under various conditions. In some of the models, they have successfully used interionic potentials which were derived from experimental data in other materials for some of the short-range interactions. Thus it could be shown that these potentials are also valid in the fluoride perovskites with different interionic distances. Furthermore, the experimental data are compared with the results of infrared spectroscopy and Raman studies at low temperatures as a test of the assignment of optically determined phonon frequencies to normal modes which are at the zone centre Gamma at low temperatures and at zone-boundary points in the cubic perovskite structure.