YAG, a well-known oxide of Aluminum and Yttrium with formula Y3Al5O12, is a luminescent crystalline solid which has a very high quantum efficiency, i.e. a high light intensity emitted per number of photons (UV) received during excitation. Commonly used in lasers and many LEDs, this luminescence depends very strongly on the crystalline structure of the YAG and especially on the number of defects it presents. Very stable compound, it is unfortunately very difficult to modify its chemical composition and therefore to modulate its defects.
The structure of this material is simple: the yttrium atoms occupy sites of coordination 8 (they are surrounded by 8 oxygen atoms) and the aluminum atoms occupy tetrahedral sites (60% aluminum atoms surrounded by 4 oxygens) and octahedral (40% surrounded by 6 oxygens). To modify this structure and thus modulate the luminescence properties, it has long been considered to induce defects by permuting the aluminum and yttrium atoms on the different crystallographic sites. But, given the stability of the compound, only small modifications might be made, such as the occupation of a tiny part of the crystallographic sites of aluminum by yttrium.
The scientists of laboratory “ Extreme conditions and materials: high temperature and irradiation” (CNRS), specialized in synthesis of new materials out of thermodynamic equilibrium, have succeeded in stabilizing new crystalline phases of YAG. By crystallization of the molten liquid at high temperature, they obtained compounds with a substitution at least ten times greater of Al atoms by Y! Using X-ray diffraction, transmission electron microscopy, solid-state NMR and EXAFS, they showed that excess yttrium atoms localized to octahedral sites previously occupied by aluminum and that the modifications structures induced by this substitution then made it possible to modulate the optical properties of YAG. These results, published in the journal Advanced Functional Materialsmake it possible to envisage new ranges of emission wavelength which have remained inaccessible until now.
