(Invited) Computational Design of Lanthanide-Doped Optical Materials By a Combined DFT and Crystal-Field Approach: Recent Developments and Applications

Abstract

In the last decades, lanthanide activated luminescent materials for lighting and display applications have been explored and optimized mainly by the cumbersome and costly trial-and-error experimental approach. Nowadays, the quick development of lanthanide spectroscopy theory together with an increase of the computational resources have made it possible to propose a systematical calculation strategy to understand the physics picture behind experiment and build up the link between the structural and optical properties of lanthanide-doped materials. It is well known that density functional theory (DFT) calculations employing the single-particle ground-state picture can provide very sufficiently accurate structural relaxation and the chemical trends that determine the emission energies and the luminescence mechanisms of optical dopants [1], whereas crystal-field (CF) theory calculations can very well reproduce experimentally observed optical spectra, which involve transitions between multi-electronic energy states of lanthanide ions [2]. Therefore, a combined theoretical scheme of modern DFT and conventional CF models has been proposed to give full play to their advantages [3]. In this talk, we will demonstrate how such a combined theoretical scheme supports the recent experimental studies of lanthanide phosphors. Ce3+ and Eu2+ ions will be paid more attention to due to their applications for white LED phosphors. The real reason resulting in the red or blue shift of the lowest 5d-4f emissions of Eu2+ or Ce3+ ions [4] will be analyzed by considering the combined effects of electronic structure and Stokes shift. Moreover, the misunderstanding about the position of the lowest 4f65d energy level in the 4f-5d excitation spectra of Eu2+ ions will be clarified, and thus the Stokes shift can be properly reproduced and further fed into understanding the electron-phonon coupling effect of Eu2+ ions with host lattices [5]. The successful application to the solid solution phosphors Ca2(Al1-x Mg x )(Al1-x Si1+x )O7: Eu2+will be shown in order to highlight the role of the theoretical work in the discovery of novel lanthanide phosphors [6]. Final, the recent developments and future challenges of the combined theoretical scheme will be given and commented, which include the first-principles structural determination of the 5d excited states of lanthanide ions and reproduction of the lowest charge transfer transition energy and its Stokes shift. Acknowledgements: The work is financially supported by National Natural Science Foundation of China (Grant No. 11204393), Natural Science Foundation Project of Chongqing (Grant No. CSTC2014JCYJA50034) and National Training Program of Innovation and Entrepreneurship for Undergraduates (Grant No. 201410617001).