The basic of the Ce4+ model, assume the energy levels of Ce4+ (4f and 5d states) ions is similar with that of Ce3+ in the band gap, which is not true from my side.

I expect that a large shift of VB-4f states of Ce4+ ion compared to that of Ce3+ ions.

This means that the energy levels of Ce4+ ions might be not inside the band gap.

Such VB-4f states of Ce4+ should be not possible determined from experiment.

while we need high-level computation, scGW, from theory part.

In my opinion, this is the reason that we don't have Ce4+-doped scintillator.

And, for the effect of enhancement effect of M2+ (Ca2+,Mg2+,Zn2+) ions, we need think more.

What do you think?

Ce4+ ion has not electrons in 4f-shell. It is similar to La3+. Therefore, Ce4+ cannot have 4f level in band gap. Usually, Ce4+ level in a schemes means energy of charge transfer transition and distance between VB and Ce3+.

Yes. this is true.

I also expect that the unoccupied 4f and 5d states of Ce4+ ion is similar with those of La3+, which might be above or resonance with CB. Roman Shendrik

Here is a recent reference involved:

Figure 5 in:

Article Role of Multiple Charge States of Ce in the Scintillation of...

Figure 5 in PHYS. REV. APPLIED 10, 024026 (2018) is very strange. Why does the "empty" 4f state of Ce4+ ion have higher energy than filled 4f Ce3+ state? After all, 4f level of Ce4+ ion capturing an electron should have energy higher than "empty" state and much higher than filled Ce3+ state but this is not true

I cannot catch your point.

What do you mean? Roman Shendrik

Figure 5 in PHYS. REV. APPLIED 10, 024026 (2018), these information is from calculation, and shows that the Ce4+ cannot be the luminescence center.

Sure, Yongchao Jia, but empty 4f level of Ce4+ should be located closer to valence band top than filled 4f level of Ce3+ ion

Oh, I expect an opposite position for the 4f state of Ce4+ ion.

I assume that such 4f state of Ce4+ ion around the CBM, or above.

Roman Shendrik

Do you have some references for the change of Ce4+-4f state, with respect to the Ce3+-4f state?

Yongchao Jia, it is sophisticate question, because we have no electrons in 4f level. The same problem where is La3+ empty 4f state located?

However, Pr4+ has an electron in 4f shell, where is its 4f ground state position in relation to 4f ground state of Pr3+ ion?

From the other hand, addition an electron to Pr3+ leads to the ground state of Pr2+ becomes closer to CBM, therefore, subtraction an electron can lead to lowering of Pr4+ ground state related to Pr3+.

Yes. The energy levels of Ce4+ ion is not easily determined.

I understand your meaning, such shift of 4f states also occur from Eu3+ to Eu2+.

However, for the case of Ce3+/Ce4+, I expect that it differs with Pr2+/Pr3+, Eu2+/Eu3+. Maybe it is another story.

I expect that the energy levels of Ce4+ may be similar with La3+.

From my previous study, even just based on DFT framework, the energy levels of La3+ ion is near CBM, or above. This is also the reason that mention:

I assume that such 4f state of Ce4+ ion around the CBM, or above.

Roman Shendrik

The discussion is interest. But from chemical point of view we have the additional questions: Why do Ce4+ should exist in different crystals (in garnets, for example, where all metal ions have stable valent state +3)? What concentration of Ce4+ ions should be in crystal for observing the effect?

The valence of the rare earth ions in inorganic solids should be determined from the fermi level position with respect to the 4f state. The concentration of Ce4+ can approximately determined from the intensity of CT bands. @Evgenij Vladimirovich Tret’yak

I talked about other reason. In garnets that are been formed by ions in +3 valence state (Y3+, Al3+, Gd3+, Ga3+) the Ce3+ should be in +3 state too. In this case we can say that the Ce3+ ions have been stabilized valently. It might been confirmed practically. You may carry out the YAG:Ce synthesis with using of Y2O3, Al2O3 and CeO2 as initial reactants on air and finally observe the luminescence for Ce3+ (Ce4+ to Ce3+ transition is take place on air). Of cource, in the case of real crystals we may expect the change of valence state for part of Ce3+ ions for compensating the existing defects. Typical calculated Ce3+ concentration in crystalls is 500-1000 ppm in respect with substituted ion. Thus, the comparison of possible defects concentrations at certain temperature with REE ion concentration in crystals looks like more suitable approach. Furthermore, M2+ ions may also either compensate specific kinds of defects in crystalls or create new. I suppose that real mechanism of M2+ ions more complex and most probably does not include the Ce4+ ions (or do it partially).

Yongchao Jia Yauhen Uladzimiravich Tratsiak Also presence of Ce4+ ions in crystals was obtained by XANES in Article Role of Ce4+ in the Scintillation Mechanism of Codoped Gd3Ga3Al2O12:Ce

Article Evidence and consequences of Ce4+ in LYSO:Ce,Ca and LYSO:Ce,...

Roman Shendrik Thank you for the interesting articles. If the proposed mechanism of involving the Ce4+ ions is real, we can return to initial question: Why there is no 100% Ce4+-doped scintillator?

It should be noted, that garnets have lattice with high tollerance to substitution. In accordance with that they may "forgive" to experimemtator some deviations in stoichiometry. But, the small discrepancies between real and planned composition always have place. For example, Gd2MgGa3AlSiO12:Ce sample may be obtained. For this example composition you may impact on excess of specific ions (Mg for example) by variating the ratio of initial components at synthesis and as a result, expect the change in Ce3+/Ce4+ ratio. Ideally, you can receive the sample with Ce4+ ions only, but it will not have the emission, because at decreasing the Ce3+ ions concentration the PL and RL intensities decrease also.

Glasses are more indicative in this situation. You may recieve colorless glass doped with Ce3+ ions and have normal RL sigmal. After few remelts in air the glass will be colored (from yelloywish to brown, in dependence of Ce concentration). And you will be surprised, because the absence of any or extrimely weak RL signall will be registered. Why?

Of course, Ce4+is a part of scintillation mechanism, but the proposed approach probably do not work for all cases or have a limitations (matrix, Ce concentration and etc). Effect of M2+ ions, evidently, related with thin modification of defects structure in material, that of course, impact on luminescent center. In proposed articles the influence of M2+ ions is more interesting and not so obvious.