First-principles study on luminescent properties of Bi3+-doped ALuGeO4 (A = Li, Na): Insights into effects of host cation on emission wavelength

Modifying the host composition is considered as an effective approach to tailoring luminescent properties of impurity-activated phosphors. However, it remains challenging to elucidate the underlying microscopic mechanism from experiments alone. Herein, we demonstrate a first-principles study on Bi 3+ -activated ALuGeO 4 phosphors, which were recently reported to display significant emission redshift upon changing the host cation from A = Li to Na, although the associated excitation spectra remained stationary. Here, hybrid density functional theory (DFT) calculations with spin-orbit coupling are first carried out to determine the local structural and electronic properties of Bi 3+ in its ground state and the lowest-energy excited state, and wave function-based multiconfigurational relativistic ab initio calculations are then performed to derive the electronic energy levels of the 6 s 2 and 6 s 1 6 p 1 configurations. The predicted transition energies between the levels of the two configurations are in good agreement with experiments, and the temperature dependence of the lowest-energy emission is rationalized on the basis of relative thermal populations of the (6 s 1 6 p 1 ) 3 P 0 and 3 P 1 levels. Comparative analysis of the energy levels derived at the excited-state structures reveals that the variation in the emission wavelength of ALuGeO 4 :Bi 3+ (A = Li, Na) is not due to the inductive effect of neighboring cations on centroid-energy difference, but rather a result of the difference in the crystal-field splitting of Bi 3+ (6 s 1 6 p 1 ) 3 P levels, caused by the different degrees of local structural distortions. Further calculations of Debye temperatures and vibrational frequencies show that the different distortions originate from a decrease of the local structural rigidity around Bi 3+ from A = Li to Na, as reflected by an overall smaller vibrational frequency of the modes associated with Na than with Li, which are neighbors of Bi 3+ in the second coordination shell. The insights from the present study will be helpful for the rational design and exploration of Bi 3+ -activated luminescent materials for practical applications. • Energy level structures were derived for Bi 3+ 6s 2 and 6s 1 6p 1 configurations in Li/NaLuGeO 4 . • Low-temperature excitation spectra were assigned based on the calculated energy levels. • Temperature-dependence of the lowest-energy emission was understood. • The emission redshift upon Na/Li replacement were elucidated in terms of local rigidity.