G. Raunio’s research while affiliated with Chalmers University of Technology and other places

Phonon dispersion relations in RbI have been obtained by inelastic scattering of thermal neutrons. Measurements were performed in the crystallographic symmetry directions Δ, Σ, A, and Z at 80 °K and some selected zone boundary phonons were also measured at 300 °K. A simplified version of the generalized shell model, involving a variable number of parameters was fitted to the experimental dispersion curves. From an eleven parameter model the frequency spectrum, the Debye temperature and other physical quantities have been calculated.Die Phononendispersionsbeziehungen in RbI wurden durch inelastische Streuung thermischer Neutronen erhalten. Die Messungen wurden in den kristallographischen Symmetrierichtungen Δ, Σ, A und Z bei 80 °K durchgeführt. Einige ausgewhlte Zonengrenzen-Phononen wurden auch bei 300 ° K untersucht. Mit einer vereinfachten Version des verallgemeinerten Schalenmodells, das eine variable Anzahl von Parametern enthielt, wurden die experimentellen Dispersionskurven angepaßt. Mittels eines Elf-Parameter-Modells wurden das Frequenzspektrum, die Debye-Temperatur und andere physikalische Größen berechnet.

Phonon frequencies in RbBr at 80 K have been obtained by inelastic scattering of thermal neutrons. The measurements were performed in the symmetry directions Delta , Sigma and Lambda and along the line Z. Some zone boundary phonons were also measured at 300 K. Four versions of the shell model were fitted to the experimental frequencies, and the elastic and dielectric constants were calculated from the model parameters. From an eleven parameter model the authors have also calculated the phonon frequency spectrum and the temperature variation of the Debye temperature.

Phonon dispersion relations have been obtained in the Delta , Sigma and Lambda symmetry directions using inelastic scattering of thermal neutrons. The results are used to calculate effective charges and effective compressibilities from the relations derived by Szigeti (1949 and 1950). Elastic constants are calculated from the slopes of the dispersion curves for small wave vectors and compared to those obtained in ultrasonic work. The dispersion curves are compared with calculations by Karo using the deformed-dipole-next-nearest- neighbours model. The Lyddane-Sachs-Teller (1941) relation is found to be satisfied in all instances except in RbCl at 80 K. Results from earlier work on NaCl and KCl are included for comparison.

Phonon frequencies in RbBr are determined at the three temperatures 80, 290, and 370 K in the symmetry directions, Δ, Σ, and A using neutron inelastic scattering. The experimental frequency shifts obtained are compared to theoretical mode Grüneisen parameters calculated by Jex. Die Phononenfrequenzen in RbBr werden bei drei Temperaturen 80, 290 und 370 K in den Symmetrierichtungen Δ, Σ und A mit inelastischer Neutronenstreuung bestimmt. Die erhaltenen experimentellen Frequenzverschiebungen werden mit theoretischen von Jex berechneten Grüneisenparametern verglichen.

The phonon dispersion curves of cesium bromide have been observed by inelastic neutron scattering in the symmetry directions Δ, Σ, and Λ and along the symmetry line T. The measurements were performed at 80 and 300 K using a three-axis crystal spectrometer. The observations have been fitted to various versions of the shell model, and the frequency distribution and the temperature dependence of the Debye temperature have been calculated from the model which gives the best fit to the experimental frequencies.

In a series of earlier papers, experimental results on phonon dispersion relations at 80 °K in NaCl, KCl, RbCl, and RbF have been reported. We now present calculations on these halides using the extended-shell-model approach with both ions polarizable and including next-nearest-neighbor interactions. The parameters obtained in a least-squares fit to the experimental points in the symmetry directions have been used to calculate the phonon frequencies in 512000 equally spaced points in an octant of the Brillouin zone, whereby, after sorting these into intervals of width Δω=2×1011 rad sec-1, the frequency spectrum was obtained. From these spectra the variation of the Debye temperature with temperature was also calculated. The agreement with results from specific-heat measurements for NaCl and KCl is quite satisfactory at low temperatures.

Phonon frequencies in KC1 and RbC1 have been determined by neutron scattering. The results are used to interpret the second-order Raman spectra in these solids. Assignments of two-phonon processes to the reported Raman peaks are made. It is found that most of these peaks can be accounted for.ZusammenfassungPhononenfrequenzen in KC1 und RbC1 wurden durch inelastische Neutronenstreuung gemessen. Die Ergebnisse werden verwendet um die Ramanspektra von zweiter Ordnung zu erklären. Zweiphononprozessen werden den Ramanspitzen zugeordnet. Fast alle von diesen Spitzen können erklärt werden.

The interpretation of second-order Raman spectra involves assignments of observed peaks to the frequencies of certain phonons. The peaks appear where the two-phonon density of states is high i.e. close to the zone boundaries and inside the zone in areas where the phonon dispersion curves exhibit maxima or minima. It is shown, with NaCl as an example, that a detailed knowledge of the dispersion curves is essential in making correct assignments.RésuméL'interprétation de spectres Raman du second degré entraîne des attributions de crêtes observées aux fréquences de certains phonons. Les crêtes ou valeurs de pointe apparaissent là où la densité à deux phonons des états est élevée, c'est à dire proche des limites de zone et à l'intérieur de la zone dans des régions où les courbes de diffusion des phonons font apparaître des maximum ou des minimum. On démontre, en prenant NaCl comme exemple, qu'une connaissance détaillée des courbes de diffusion est essentielle pour faire des attributions exactes.

A new feature of the spectrometer is the monochromator unit, which contains two parallel reflecting copper crystals and delivers a beam of monoenergetic neutrons through a fixed collimator which is parallel to the incident beam from the reactor. About half the neutrons reflected by the first crystal are reflected by the second crystal, though the fraction declines at the upper end of our energy scale (about 60 meV). The beam from the monochromator is free from fast neutrons, and the background extremely low. The material, thickness, and mosaic width appropriate to efficient reflection in a monochromator crystal are discussed. The sample and analyzer form a compact separate unit on a stationary base. All the angles of the spectrometer (including those of the monochromator) are continuously variable over wide ranges, with either manual control or automatic control from a programming tape. Two spectrometers which are mirror images of one another are mounted side by side at the same reactor beam hole. They are mainly intended for measurements on phonons with comparatively high resolution. An example of the accuracy attained is given.