Some light scattering experiments in solids

This thesis describes some Raman scattering experiments on solids in the temperature range 1.8 - 300°K. The three main problems studied were : (a) the phase transitions in PrAlO3; (b) the Jahn-teller phase transitions in the series of mixed crystals TbcGd1-cVO4; (c) the Two-Magnon excitations in the mixed crystals Mn1-cZncF2.
Chapter 2. reviews the theory of phonons, magnons (both ferromagnetic and anti-ferromagnetic) and electronic excitations in solids. The mechanisms which cause light scattering from these excitations are discussed, together with the selection rules for Raman scattering.
Chapter 3- describes the experimental techniques that were used. The spectra were excited using an Argon-ion laser. A 90° scattering geometry was employed, and the spectra analysed by a double-grating spectrometer. Detection was by means of a photomultiplier tube used in the pulse-counting mode. The sample was mounted in a conventional helium cryostat. Optical birefringence was used to study TbcGd1-cVO4; the experimental arrangement is described in this chapter. A superconducting magnet dewar was built in this laboratory for magneto-optical work, and is briefly described in Chapter 3.
In Chapter 4. some experiments on PrAlO3 are discussed, The Raman spectra were measured between 4.2 and 300°K and used to study the low temperature phase transitions at 146 and 196°K. The Hainan data were compared with the predictions of a theoretical model of PrAlO3, based on molecular field theory; this model was found to give a reasonable qualitative description of the observed behaviour. It was concluded that below 99°K PrAlO3 has only a very small deviation from tetragonal symmetry. Optical fluorescence and EPR experiments reinforce this conclusion. In the vicinity of the phase transition at 146°K the anomalous behaviour of two of the low frequency Raman lines was found to be due to coupling between an electronic level of Pr3+ and a phonon.
Chapter 5. deals with some Raman and optical birefringence studies on the series of mixed crystals TbcGd1-cVO4. The Jahn-Teller phase transition temperatures were measured usinf" the optical birefringence method and found to be in good agreement with the predictions of Molecular Field theory. In addition, this theory predicted a unique type of phase transition for a certain range of c, and this was observed in the crystals with c = 0.365. On cooling, these crystals underwent a Jahn-Teller phase transition to a lower symmetry at 8.5±0.5°K; further cooling resulted in a second phase transition at 2.3±0.1°K temperature at which the crystals reverted to their original high-temperature symmetry. These crystals therefore had a high and low- temperature phase of the same symmetry, with an intermediate phase of lower symmetry. Raman scattering was used to measure the Eg phonon splittings below the transition temperature in the mixed crystals as a function of temperature. Comparison of experiment with predictions of a Molecular Field theory showed that this theory gave an excellent description of the behaviour of the system. This indicated that the electron-phonon interaction in the mixed crystals was of long range.
Chapter 6. describes the results of some Two-Magnon Raman scattering experiments on Mn1-cZncF2 alloys. The peak-frequencies and linewidths of the Two-Magnon lines at helium temperature were measured over the whole concentration range. These were compared with the predictions of two different theories. The first theory involved calculation of the first three moments of the Magnon lineshape, while the second was a cluster-model theory based on the Ising model. Both theories gave reasonable agreement with the experimental results. The phonon frequencies were also measured at low temperature as a function of c. The Eg phonon exhibited two-mode behaviour for c<0.25, and one-mode behaviour for the rest of the concentration range. This anomaly was attributed to a local mode effect.
Appendix 1. gives a brief description of some attempts to subtract the background Rayleigh scattering from Magnon spectra in order to measure the true lineshape. The experiments failed, but had they been successful, would have enabled a direct comparison between experimentally and theoretically determined moments to be made.
Finally, Appendix 2. describes some phonon and electronic Raman scattering experiments on HoF3, and TbF3. The frequencies of all but two of the Reman active phonons were measured in both materials. At low temperatures electronic transitions between the levels of the ground-state multiplets of the Ho3+ and Tb3+ were observed. A level scheme for some of the electronic levels within the 5I8 multiplet of Ho3+ was derived from these results, but the results for Tb3+ have so far proved difficult to interpret.