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 Bi3+-activated ALuGeO4 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. Hybrid density functional theory (DFT) calculations with spin-orbit coupling are first carried out to determine the local structural and electronic properties of Bi3+ 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 6s2 and 6s16p1 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 (6s16p1)3P0 and 3P1 levels. Comparative analysis of the energy levels derived at the excited-state structures reveals that the variation in the emission wavelength of ALuGeO4:Bi3+ (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 Bi3+ (6s16p1)3P 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 Bi3+ 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 Bi3+ in the second coordination shell. The insights from the present study will be helpful for the rational design and exploration of Bi3+-activated luminescent materials for practical applications.

Automated summary

In this study, a first-principles investigation was conducted on Bi3+-activated ALuGeO4 phosphors, revealing that the variation in emission wavelength is attributed to the different degrees of local structural distortions.