Eu3+ is the stable form, but why do people have to go through the laborious task to reduce it to Eu2+ for phosphor application. And also kindly suggest a low temperature (not higher than 200oC) to reduce the same for doping.
It is not necessary to use Eu2+ for doping in phosphor/luminescence applications. Eu3+ could also serve as good doping agent in these application. You can go through my paper on this entitled :"High temperature stabilization of Y2Sn2O7:Eu luminescent nanoparticles - A facile synthesis".
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Eu is a lanthanide and general oxidation station of lanthanides is +3 but several times it forms +2 and +4 oxidation state when f7,f0 and f14 configuration is arising so Eu2+ will have f7 configuration which is a stable one as compare to Eu3+.
The purpose of Eu3+ doping in a host to get intense bright emission in the red region(613~616nm & 702 nm). Due to f to f orbital transition(forbidden) the sharp emission is purely host independent. But the scenario in case of Eu2+ is different as the emission here we obtained is d to f transition(allowed). This transition purely depends on surrounding crystal behavior, hence the emission from Eu2+ is host dependent. The emission from Eu2+ is always a broad spectrum (mainly 583-602 nm) compared to Eu3+'s narrow emission. Due to this broad spectrum, it's application in White LED fabrication by a blue emitting diode is vast.
The reduction procedure will depend on the host lattice synthesis technique. Normally a H2/N2 gas flow is used to reduce the Eu3+ to Eu2+.But reduction procedure is very much difficult to control uniformly on the whole sample area.
It may due to the facilitating lower oxidation state for better application performances. Hydrothermal below 200 deg C may work out if we optimize the reaction duration and temperature.
Eu3+ due to the nature of its transitions has a red emission spectrum that is hardly shiftable. If you want that red 610 nm or so emission it is your good candidate. But in applications you might want to shift emission and than Eu2+ is obviously better. Another thing is excitability. Again in applications you need to efficiently excite the material therefore you will use Eu2+ as it has allowed transitions. Eu3+ can also be efficiently excited via CT or host excitation. All depends on appl8cation. You cannot general8ze as there are real appl7cations where Eu3+ is used.
As a related topic to this discussion, I would like to ask whether it is possible to dop Eu2+ to an assembly containing Au and TiO2 where the doping mainly to the TiO2. In some papers it is shown that it is possible to do this. But there are also contradictory statements. Can someone provide me some insights?
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