You ask about upconversion luminescence? Probably structure levels and their splitting in a cubic crystal is different from the hexagonal gagarinite
It is show luminescence but hexagonal exhibits considerable enhanced upconversion emissions compared with the cubic. It might cause by distribution of cation and distance between them (hexagona is closerl than cubic).
- Heer, S. et al. Highly Efficient Multicolour Upconversion Emission in Transparent Colloids of Lanthanide-Doped NaYF4 Nanocrystals. Adv Mater 16, 2102–2105 (2004).
- Krämer, K. W. et al. Hexagonal Sodium Yttrium Fluoride Based Green and Blue Emitting Upconversion Phosphors. Chem Mater 16, 1244–1251 (2004).
Michael Päch
Sir,
The primary aim of RG is to encourage academic discussion where vote up/ vote down becomes secondary . So I take this opportunity to add a few lines for this query; no matter whether I come up to your expectatons or not. Moreover, she being a young scholar should be encouraged. Rgds.
It is an effort as the reasons may differ from one cubic structure of a compound to an other compound. Yet a few generalizations may be made as follows:
At low temperature, NaYF 4 is a meta stable cubic phase and due to the crystal symmetry, it has a naturally low up-conversion efficiency as explained below
[1] The origin of low luminescent intensity in found in concentration quenching which takes place as a consequence of excitation energy transfer to a killer or a quenching site. The structural defects produced and the presence of trace impurities introduced from the raw materials can serve as quenching centers.
[2] Even with a very low trap concentration, excited energy can be transferred to the traps via exchange interaction to the Eu^3+ or Yb^3+ and dipole- dipole interaction
[3] In a cubic system, the Eu^3+ or Yb^3+ occupy the CENTROSYMMETRIC sites. Only the MAGNETIC- DIPOLE transitions are allowed which have VERY LOW TRANSITION PROBABILITY
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