Long Seminars (Abstracts)
Investigations of the vibronic spectra of ions in crystals can be used to study the lattice dynamics of the host crystal and the effects caused by the interaction between the lattice phonons and the transition electron on the impurity ion. In this seminar we review the results of such studies of ions in strontium titanate crystals. This is an interesting host to study because of the presence of “soft” phonon modes which are responsible for the interesting dielectric properties and structural phase transitions of the material. Our recent work on SrTiO4:Cr3+ will be presented in detail as an example of a research project on vibronics. Most of the structure in the low energy vibronic sideband at low temperature can be identified using selection rules derived from group theory and comparing the results with those obtained from infrared absorption, Raman and neutron scattering. An iteration process was used to obtain a computer fit to the data from which one-phonon, two-phonon, and multi-phonon contributions to the sideband were derived. It was found that quadratic coupling between an impurity-induced local mode and the lattice phonon modes had to be included in order to obtain a fit. A very simple model is used to obtain a phonon density of states from the one-phonon vibronic sideband and this is found to agree fairly well with the density of states obtained from neutron scattering. The high energy vibronic sideband is shown to be useful in observing the low frequency phonons including the soft modes. The temperature dependences of the widths and positions of both the zero-phonon lines and the impurity-induced local mode are discussed in terms of the theoretical fittings predicted using a Debye phonon distribution, the effective phonon distribution obtained from the vibronic sideband, and an Einstein phonon distribution describing coupling to only a soft phonon mode.
KeywordsStrontium Titanate Vibronic Spectrum Lead Azide Trivalent Actinide Soft Phonon Mode
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