Relationship of Electronic States to d -> f Emission Efficiency of Lanthanide Activated Phosphors

University Of Georgia Research Foundation Inc, Athens GA

Investigators

Abstract

This project addresses fundamental questions associated with the energy level structure and efficiency of blue, x-ray storage and photostimulable phosphor materials, divalent (Eu 2+ , Sm 2+ ) and trivalent (Ce 3+ ) rare earth ions in insulating oxide and sulfide lattices. These materials are currently being used or considered for applications that include tunable blue solid state lasers, the blue phosphor component of electroluminescent displays, x-ray storage phosphors for medical imaging, and optical memory based on electron trapping or spectral holeburning. Key materials issues relevant to the performance of the proposed materials for these applications include the dependence of 5d to ?4f luminescence transitions on host lattice structure and bonding, the relationship of excited 5d levels to the band structure of the host lattice, and mechanisms of excitation and stimulation processes in co-activated materials. The first of these issue requires the ability to predict effects of local coordination environment, defects, crystal field strength and bonding covalency on the energies and splittings of the 5d levels. The second issue concerns the position of the excited 5d levels relative to the conduction bandedge and its consequences for photoionization, thermalization, and electron transfer. The third issue involves charge migration, interactions between co-activators, and defect mediated phenomena. The approach is based on using high pressure to vary lattice structure, local coordination environment, 5d energy states, defect energy states and host lattice band structure in blue-UV emitting, x-ray storage and photostimulable phosphor materials. Through control of underlying electronic and structural factors responsible for determining phosphor properties, new insight is sought into excitation, emission, storage and stimulation mechanisms. The objectives of the proposed work include: understanding the dependence of the energy and intensity of 4f n-1 5d to ?4f n electronic transitions in Ce 3+ -, Eu 2+ -, and Sm 2+ -activated phosphors on local coordination environment; examining the influence of 5d electronic states on 4f to ?4f luminescence transitions; investigating the energy of 5d electronic states relative to the host lattice conduction band and its effect on luminescence, photoionization, and photostimulation processes in Ce 3+ -, Eu 2+ -, and Sm 2+ -activated phosphors; determining the relative importance of covalency and crystal field strength in establishing the energy of the emitting 5d level in blue and UV phosphors based on Ce 3+ and Eu 2+ ; elucidating the fundamental excitation, storage and photostimulation processes in Eu 2+ -activated mixed halide x-ray storage phosphors and Eu 2+ , Sm 3+ co-activated photostimulable phosphors; examining the effect of pressure on F-center formation and aggregation in Eu 2+ -activated mixed halide storage phosphors in the presence of x-ray and UV radiation; demonstrating the ability of high pressure studies to contribute to predictive capability at ambient conditions through correlations of phosphor properties with band structure, electronic states, covalency and crystal field effects. %%% The project addresses basic research issues in a topical area of materials science with high technological relevance. These studies are expected to improve fundamental understanding of factors limiting the efficiency of solid state laser materials and a variety of optical display applications. An important feature of the program is the integration of research and education through the training of students in a fundamentally and technologically significant area. ***