Depending on the flux quantity, crystal morphology varied. When using 70 wt% flux, many smaller and thin hexagonal crystals were produced (Figure ). Single crystal XRD showed that these smaller hexagons matched the C2/c polymorph previously observed in bulk single crystals of PrAl 3 (BO 3 ) 4 , NdAl 3 (BO 3 ) 4 , and SmAl 3 (BO 3 ) 4 . It is not surprising then that the structure type could accommodate Eu 3+ cations. Data from the single crystal XRD run are provided in Table . Refined atomic positions are in the Supporting Information (Table ). The cell parameters are close to those reported, with only a 0.6% volume increase. powder XRD pattern's additional peaks. The authors noticed "flattened" hexagonal crystals and associated the extra peaks with celldoubling due to Eu substitution onto Al sites. Each peak aligns well with one from the C2/c polymorph, including a peak that could not be indexed to the proposed R32 supercell. The Supporting Information contains a table of these peaks (Table ). Eu/Al substitution was already highly unlikely. The cations' ionic radii are significantly different, and for the R32 polymorph's Al 3+ site, bond valence analysis gives an unreasonably high sum of +9.7 for Eu 3+ (+3.1 for Al 3+ ), using empirical bond valence parameters and a published R32 structure. This result subverts the argument for Eu/Al site mixing, which will only briefly be discussed again in the photoluminescence mapping section.