Realization of extreme nonstoichiometry in gadolinium aluminate garnet phosphors by nonequilibrium synthesis

Rare-earth aluminates with the cubic garnet structure are an important class of optical materials with a range of technological applications. When synthesized as ceramics or single crystals, these materials do not tolerate large deviations from ideal RE3Al5O12 stoichiometry, and their luminescence properties are typically controlled by dopant selection. Here, we use glass crystallization as a nonequilibrium synthesis route to a new family of highly nonstoichiometric gadolinium aluminate garnet (GAG) phosphor hosts Gd3+xAl5-xO12 with 0 ≤ x ≤ 0.60. In these materials, excess Gd3+ is accommodated on the octahedrally-coordinated Al3+ sublattice of the garnet structure. The most extreme composition Gd3.6Al4.4O12 has 30% of these Al3+ sites substituted by Gd3+, but retains the cubic garnet structure type despite the vast size contrast between the two cations. The accessible nonstoichiometry range for GAG extends far beyond that of nonstoichiometric YAGs (Y3+xAl5-xO12, 0 ≤ x ≤ 0.4), enabled by a broader glass-forming domain in the Gd2O3 – Al2O3 system. We investigate three model phosphor systems based on nonstoichiometric GAG, and determine the crystallographic distributions of the dopant ions where possible, to evaluate the response of upconversion and photoluminescence to extreme nonstoichiometry. In particular, upconversion from the small rare-earth activator Tm3+ is found to be sensitive to nonstoichiometry in GAG. These results demonstrate that highly nonstoichiometric garnet aluminates are not limited to small rare-earth hosts such as YAG and should be realizable across the full 4f series, highlighting the potential for color tuning of new upconversion phosphors by control of host stoichiometry and opening new opportunities for development of different garnet-based optical and magnetic materials.