Raman scattering and Mössbauer absorption spectroscopy studies on Fe2+ in AMX3 crystals

The electronic Raman scattering by Fe2+ ions doped in the hexagonal compounds CsCdBr3 , CsMgBr3 , CsMgCl3 and the layered compounds CdBr2 , CdCl2 have been successfully recorded at 20 K and in some cases, also at 2 K. By keeping the concentration of Fe2+ low, it has been possible to obtain sharp and well polarised spectral lines which are due to transitions from thermally populated states to any other higher lying states within the 5T2g multiplet. The 5T2g levels were successfully fitted using crystal field theory. The results indicated a strong, positive trigonal crystal field in all three of the hexagonal compounds consistent with a singlet ground state of the Fe2+ ion. Theoretical calculation incorporating the well known closure approximation gave good agreement with the observed intensity pattern. Further investigations using both Raman and Mössbauer spectroscopy were carried out on one of the above systems, namely CsMg1-xFexCl3. The Mössbauer results showed unexpected distinctions between the various sites for the Fe2+ ions. There is evidence of structural changes centered about the junction between the Mg and Fe ions. For one particular site, Qs increases with temperature. This is interpreted as increasing trigonal crystal field with rise in temperature. An usual line broadening phenomenon is also reported. The Raman spectrum for CsFeCl3 has been clarified. An unusual “exciton"-phonon coupling is reported. The controversial YX break-through of the Alg phonon mode observed in the Raman spectra for AMX3 crystals has been shown to be largely geometry-induced