Solid state spectroscopy : the spectra of Co²⁺ ions in AMX₃-type crystals
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
A Raman scattering and infrared absorption investigation of Co²⁺ ion electronic excitations in several diamagnetic AMX₃-type crystals is reported. Spectra arising from these excitations were measured over the range 0-1400 cm-¹ and at temperatures of 80K and 15K. Co²⁺ ions substitute for the divalent cation (M) in these crystals and experience a trigonal crystal field, which splits the four spin-orbit multiplets of the ⁴T₁g(⁴F) manifold into six Kramers doublets with an energy spacing of approximately 1400 cm-¹. Spectral features due to Co²⁺ ion electronic excitations were analysed and assigned to the appropriate symmetry species using Raman scattering and infrared absorption selection rules. Various models were used to describe the Co²⁺ ion electronic levels within the ⁴T₁g(⁴F) manifold. A complete 3d⁷ crystal-field calculation and an effective Hamiltonian model in second-order gave equally good agreement with experiment. The superposition model of crystal fields was used to interpret the associated crystal-field parameters. The crystal-field analyses also provided single ion wavefunctions which were used in the interpretation of the spectra of Co²⁺ ion pairs in these host crystals. Spectral features due to first-order zone-centre lattice phonons for several AMX₃-type crystals are reported and assigned to particular symmetry vibrations using a normal mode determination. An attempt to observe resonance Raman scattering is reported. The Raman scattering and optical absorption spectra of pure and transition-metal ion doped crystals of Cs₂ZnCl₄, CsCdCl₃, and MgAl₂O₄ were recorded at 290, 80 and 15K. Some indication of resonance behaviour was noted in the Raman spectrum of Cs₂ZnCl₄.