Date of Award

1989

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Physics

Keywords

Physics, Condensed Matter.

Rights

CC BY-NC-ND 4.0

Abstract

A zero-phonon assisted 4f-5d electronic transition of a trivalent rare earth in a fluorite host is accompanied by a vibronic sideband. This sideband resembles the single phonon density of states of the crystalline host with notable exceptions. A strong feature above the $\omega$$\sb{\rm LO}$(k = O) frequency of the host lattice is present, and is attributed to a local phonon mode feature of the defect crystal. The frequency of this local mode is weakly dependent on the trivalent rare earth and depends on the fluorite host chosen. The presence of this mode in the ultraviolet absorption spectrum of a XF$\sb{\rm 2}$:RE$\sp{\rm +3}$ system is due to the excited 4f$\sp{\rm n-1}$5d state of the trivalent rare earth ion interacting with the defect lattice to excite a local mode. Electric dipole vibronic transitions specify that it must be an even symmetry mode (assuming a predominantly cubic crystal field environment for the substitutional rare earth ion) and has been attributed to the symmetric breathing mode (A$\sb{\rm 1g}$). The host fluorite lattice is represented by a rigid ion model and the dynamics of the defect lattice due to a single substitutional impurity at one of the Ca$\sp{\rm +2}$ lattice sites is determined via the evaluation of a defect matrix which specifies the interaction of the defect ion with its neighboring ions. The direct effect of an interstitial charge compensating fluorine has been neglected, (ie., the remotely compensated case), allowing us to work in a defect subspace with O$\sb{\rm h}$ point group symmetry. The conditions required to sustain local phonon modes in the defect lattice with A$\sb{\rm 1g}$ and F$\sb{\rm 1u}$ symmetries are then determined. These modes are found to be insensitive to mass variations corresponding to the full range of RE$\sp{\rm +3}$ substitutional impurities, but require a large change in the original Ca-F and F-F force constant values of the host lattice about the Ca$\sp{\rm +2}$ substitutional impurity site. The change in the local mode frequency of both symmetries (A$\sb{\rm 1g}$ and F$\sb{\rm 1u}$) is sensitive to variations in the RE-F and F-F force constants connected to the impurity site. This suggests that the interaction between the 5d orbitals of the excited trivalent rare earth ion and the defect lattice is the most probable mechanism giving rise to the local modes. The force constants needed to sustain the F$\sb{\rm 1u}$ symmetry mode suggests that infrared absorption corresponding to the presence of an infrared active local mode may be possible in a CaF$\sb{\rm 2}$:Ce$\sp{\rm +3}$ system where the trivalent cerium has undergone a 4f-5d electronic transition. Infrared studies of BaF$\sb{\rm 2}$:Ce$\sp{\rm +3}$ and SrF$\sb{\rm 2}$:Er$\sp{\rm +3}$ with the rare earth ion in its ground state revealed no infrared absorption that could be assigned to any of the local phonon modes, lending credence to the excited state interaction picture.Dept. of Physics. Paper copy at Leddy Library: Theses & Major Papers - Basement, West Bldg. / Call Number: Thesis1989 .D458. Source: Dissertation Abstracts International, Volume: 50-03, Section: B, page: 1007. Thesis (Ph.D.)--University of Windsor (Canada), 1989.

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