Title

Optical studies at high pressure on chromium-doped ordered perovskite crystals

Date of Completion

January 1991

Keywords

Physics, General|Physics, Condensed Matter|Physics, Optics

Degree

Ph.D.

Abstract

Pressure dependent luminescence spectra were recorded on the chromium-doped elpasolite crystal $Cs\sb2NaYCl\sb6:Cr\sp{3+}$ at ambient and near liquid nitrogen temperatures in a diamond anvil cell. The broad $\sp4T\sb{2g}\to\ \sp4A\sb{2g}$ fluorescence band of the intrinsically low crystal field compound exhibited a rapid blue shift with increasing pressure. The low temperature measurements yielded enhanced resolution of a $\sp2E\sb{g}\to\ \sp4A\sb{2g}$ phosphorescence appearing at the higher pressures which was interpreted in terms of vibrational modes of the chromium-hexahalide complex. The blue shift of the broad band and the appearance of the structured emission were anticipated by their previous observation in uncorrected luminescence spectra of the chromium doped elpasolites $K\sb2NaGaF\sb6:Cr\sp{3+}$ and $K\sb2NaScF\sb6:Cr\sp{3+}$. The spectra of the three crystals were corrected for the spectral response of the corresponding detection system and a transformation was applied to permit interpretation in terms of theoretical lineshape curves.^ Pressure dependent Raman spectra were recorded on the three crystals. Values of the ratio of the Gruneisen parameter and the bulk modulus were derived for the Raman-active modes. Frequencies of defect modes were determined as a function of pressure in a sample of the $K\sb2NaScF\sb6:Cr\sp{3+}$ crystal with a higher dopant concentration.^ The photoluminescence lifetime of the chromium-doped chloride elpasolite was measured as a function of temperature, at elevated pressures. The onset of thermal quenching was shifted to higher temperatures with increasing pressure. This behavior was modeled by a pressure dependent activation energy having a rate of change of 1668 $\pm$ 52 cm$\sp{-1}$/GPa.^ Several derivations and calculations were performed. These included: The extraction of pressure-dependent local compressibilities of the chromium-hexahalide complex for the three crystals and the development (within the LCSCC model) of expressions for the pressure derivatives of the effective phonon energy, the Huang-Rhys factor, the Frank-Condon offset, and the Energy gap. An analogue was developed for the Mott formula activation energy within a two dimensional configuration coordinate space. Finally, an existing two-accepting-mode model for the pressure dependence of the luminescence lineshape and non-radiative transition rate of the chloride crystal was evaluated in terms of these results and deemed to be consistent with the available data. ^

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