In the last decades, the local coordination environments and valence states of luminescent centers in optical materials have been investigated mainly by using the combination of the spectroscopy techniques, such as the EPR or optical spectra, and the simple and qualitative theoretical models, such as the crystal-field theory (CFT) ^[1]. However, the reliability of the obtained local structures and oxidation numbers of doped optical centers is still questionable due to the empirical character of those models. Nowadays, the quick development of quantum chemistry theory for strongly-correlated d- and f-elements ^[2], together with an increase of the computational resources, has made it possible to quantitatively predict the structural and electronic properties of lanthanide- and transition metal(TM)- doped materials and further provide a deeper insight into the measured spectroscopic data.

In this talk, we will employ three typical examples to demonstrate how the first-principles calculations model the local structures and valence states of impurities in optical materials. First, the loss of the local inversion symmetry of Eu³⁺ ions doped in disordered compound Ca₂La₃Sb₃O₁₄ will be theoretically confirmed and the loss mechanism obtained here may be used for other systems as well to enhance spectral intensity for an improvement of characteristics of phosphor materials ^[3]. Second, the first-principles calculations of Cr-doped ZrSiO₄ will show us the conversion mechanism from a pair of Cr⁴⁺ ions formally appearing in the chemistry concept to the combination of both Cr³⁺ and Cr⁵⁺ centers ^[4], which indicates the treatment of the valence states of TM ions should be so delicate as to need to be paid more attention. Third, a simple calculation scheme for the structural determination of the 5d lowest excited state of lanthanide ion is proposed and successfully applied to the case of Tb³⁺-doped Cs₂NaYF₆ ^[5]. In addition, a CFT-based methodology for solving the multiple minima problem in the HSE06 calculations of lanthanide-doped materials is suggested and applied to cubic elpasolites doped with Ln³⁺ ions (Ln=Ce-Tm), and the calculation results present a good agreement with empirical “zigzag” model of Dorenbos ^[6].

References:

[1] N.M. Avram, and M.G. Brik, Optical Properties of 3d-Ions in Crystals: Spectroscopy and Crystal Field Analysis, Springer and Tsinghua University Press, 2013.

[2] R. Gillen, S.J. Clark, and J. Robertson, Phys. Rev. B 87 (2013) 125116.

[3] D.X. Liu, C.-G. Ma, P.W. Hu, Z. Li, Y. Tian, P. Su, M.G. Brik, A.M. Srivastava, and S. Tanabe, J. Am. Ceram. Soc. DOI: 10.1111/jace.15406, 2018.

[4] M. Gaft, G. Boulon, G. Panczer, Y. Guyot, R. Reisfeld, S. Votyakov, and G. Bulka, J. Lumin. 87-89 (2000) 1118.

[5] C.K. Duan, P.A. Tanner, A. Meijerink, and V. Makhov, J. Phys. Chem. A 115 (2011) 9188.

[6] P. Dorenbos, Phy. Rev. B 85 (2012) 165107.