Garnet compounds are very attractive host materials for optical applications such as scintillators, phosphors and lasers. The {A}₃(B)₂[C]₃O₁₂ garnets are oxide structures with cubic symmetry and the locations of the cations over the AO₈ dodecahedral {24c}-, BO₆ octahedral (16a)- and CO₄ tetrahedral [24d]- sites are potential research objects for the study of luminescence centers designed by substitution and defects. In particular, our group has recently reported that the highly nonstoichiometric Y_3+xAl_5-xO₁₂ (0 ≤ x ≤ 0.4) garnet can exhibit up to 20% of AlO₆ occupied by excess Y³⁺, breaking the conventional ≤ 1% deviation from the stoichiometric Y₃Al₅O₁₂ limit and contributing to the dopant distribution on the AO₈ and BO₆ sublattices, which has been determined to be responsible for the modulation of the luminescent colour.^[1] As a member of the garnet family, Gd₃Al₅O₁₂ (GAG) has been extensively investigated for potential applications in optical materials due to lower electronegativity and higher theoretical density of Gd³⁺, which is optically better than the well-known YAG. In addition, Gd³⁺ (0.938 Å) with larger ionic radii compared to Y³⁺ (0.90 Å) potentially offers possibilities for larger dopants to occupy the AlO6 site in nonstoichiometric GAG. Therefore, the structural study of highly nonstoichiometric GAG may provide a new idea for subsequent exploration of a range of non-stoichiometric garnet material for optical applications.
Here, the crystallization from glass synthesis method, using aerodynamic levitation (ADL) coupled with laser heating to prepared glass was used to synthesize a new series of highly non-stoichiometric Gd_3+xAl_5-xO₁₂ solid solutions with an unprecedented composition range of 0 ≤ x ≤ 0.6. Synchrotron powder diffraction (SPD) average structure and extended X-ray absorption fine structure (EXAFS), transmission electron microscope (TEM) local structure studies confirmed that about 30% six coordinated Al3+ in ns-GAG is replaced by excess Gd³⁺, which allows the dopant ion RE³⁺ (RE³⁺ = Tb³⁺, Er³⁺, Tm³⁺, Yb³⁺) to partially disorderly occupy the AlO₆ site, forming six coordinated REO6 luminescent centers. Furthermore, systematic spectroscopic studies have shown the differentiated sensitivity of dopant RE³⁺ (RE³⁺ = Ce³⁺, Tb³⁺, Er³⁺, Tm³⁺, Ho³⁺) to (1) inequivalent AO₈ sites around different BO₆ neighbours configurations and (2) BO₆ sites in non-stoichiometric GAG in terms of variations of emission spectra and luminescence colour. Hence, the highly non-stoichiometric GAG study provides a research basis for performance enhancement and luminescent colour modulation of commercially viable garnet-based optical materials. In addition, the further exploration and research of highly non-stoichiometric GGG frustrated magnetic materials and non-stoichiometric GAGG persistent materials are in process.
[1] Weiwei Cao, Ana Isabel Becerro, Victor Castaing, Xue Fang. et al. Advanced function materials, 2023. https://doi.org/10.1002/adfm.202213418.