We present the results of room-and low-temperature measurements of second-order Raman scattering for perfect GaN and AlN crystals as well as the Raman-scattering data for strongly disordered samples. A complete group-theory analysis of phonon symmetry throughout the Brillouin zone and symmetry behavior of phonon branches, including the analysis of critical points, has been performed. The combined treatment of these results and the lattice dynamical calculations based on the phenomenological interatomic potential model allowed us to obtain the reliable data on the phonon dispersion curves and phonon density-of-states functions in bulk GaN and AlN. ͓S0163-1829͑98͒06840-4͔

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An interpretation of the Raman spectra of monoclinic ZrO2 and monoclinic HfO2 is made by analyzing the results of the zirconia–hafnia substitution jointly with a lattice dynamical treatment of both structures. The Raman spectra of tetragonal ZrO2 and tetragonal HfO2 are also interpreted. Emphasis is put on their relations to the spectrum of the parent cubic structure and on the position of the soft mode. The band assignment proposed earlier by other researchers is critically reconsidered.

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International audienceThe stress-induced zircon-scheelite phase transition is theoretically studied by means of ab initio and shellmodel calculations. It is shown that this phase transition may originate from shear elastic strains which bring together the structures of both phases and draw them unstable against the SiO4-tetrahedron rotations necessary for direct and inverse conversions. As a result, a structural catastrophe occurs within the ZrO8 polyhedrons: part of the Zr-O bonds disrupts and new ones form. Such a “bond switching” is crucial for the first-order reconstructive character of the transition which, according to this study, would proceed via the D4h 19−D2h 24 −C2h 6 −C4h 6 symmetry group succession

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