The compound, therefore, offers multiple defect chemistry options, including Eu site substitution, non-Eu site substitution, and polymorph mixing. It also reveals the need for spatially-resolved characterization of QIS candidates. While stoichiometric rare-earth QIS candidates provide the promise of increasing optical center homogeneity, the extent to which synthesis procedures and resulting defects undermine this is understudied. EuAl 3 (BO 3 ) 4 can be utilized to understand that impact.