Energy Structure and Luminescence of CeF3 Crystals

Abstract

The results of the calculation of the energy band structure and luminescent research of CeF3 crystals are presented. The existence of two 5d1 and 5d2 subbands of the conduction band genetically derived from 5d states of Ce3+ ions with different effective electron masses of 4.9 me and 0.9 me, respectively, is revealed. The large electron effective mass in the 5d1 subband facilitates the localization of electronic excitations forming the 4f-5d cerium Frenkel self-trapped excitons responsible for the CeF3 luminescence. The structure of the excitation spectra of the exciton luminescence peaked at 290 nm, and the defect luminescence at 340 nm confirms the aforementioned calculated features of the conduction band of CeF3 crystals. The peculiarities of the excitation spectra of the luminescence of CaF2:Ce crystals dependent on the cerium concentration are considered with respect to the phase formation possibility of CeF3.

The Influence of Halide Ion Substitution on Energy Structure and Luminescence Efficiency in CeBr2I and CeBrI2 Crystals

“…The width of the 5d1 sub-band at Γ-point is the splitting of the 5d states of cerium ions by the crystal field of the matrix. Excitation of electrons in the 5d2 sub-band does not necessarily lead to luminescence, as the high mobility of electrons leads to their significant diffusion, which negatively affects the efficiency of their recombination with holes [ 31 , 32 ]. The energy gap between the top of the halide valence band and the 5d2 sub-band of the conduction band is considered as the value of the forbidden band gap ( E g ).…”

“…Transitions from 4f states to delocalized states in the 5d2 conduction sub-band do not exhibit any structure, and the excitation spectrum appears as a smooth (unstructured) curve. Such spectral characteristics (structured for 4f–5d1 and unstructured for 4f–5d2) are typical for the 5d–4f excitation spectrum in the luminescence of CeF 3 crystals within this family [ 32 , 40 ]. The structuring of the conduction band into localized states (5d1) and delocalized states (5d2) determines the efficiency of the luminescent process in the series of CeX 3 compounds.…”

“…Two approaches were considered to solve this problem: the use of the hybrid functional of the exchange-correlation interaction PBE0 [ 36 ] and Hubbard corrections in the DFT + U method [ 37 ]. The former works better for crystals with a high concentration of lanthanide ions where they are constitutional ions of the matrix [ 32 ]. On the other hand, the DFT + U method is accurate enough to describe the energy states of lanthanides when computational power is limited.…”

“…However, further increase in the iodine content resulted in a deterioration of the scintillation parameters. Based on our previous research on the energy structure of CeX 3 crystals (X = F, Cl, Br, I) [ 31 , 32 ], we aim to demonstrate that the decrease in the light output is caused by peculiarities in the band energy structure of self-activated cerium halide crystals. The conduction band in these crystals is formed by the 5d states of Ce 3+ ions.…”