High Fraction of Penta‐Coordinated Aluminum and Gallium in Lanthanum–Aluminum–Gallium Borates

This study is focused on structural changes induced by increasing treatment temperature of sol‐gel–derived La 2 O 3 ∙Al 2 O 3 ∙Ga 2 O 3 ∙5B 2 O 3 system. The structure of samples heated for 30 min up to 900°C was investigated by X‐ray diffraction ( XRD ), Fourier transform infrared ( FTIR ) spectroscopy, and magic‐angle spinning nuclear magnetic resonance ( MAS ‐ NMR ) analysis of 27 Al, 11 B, and 71 Ga nuclei. The vitreous structure is preserved inclusively after 800°C treatment, and starting with 850°C the only crystalline phase evidenced in XRD patterns is of LaAl 2.03 B 4 O 10.54 type, of La(Al,Ga) 2.03 B 4 O 10.54 composition. The FTIR results point out the presence of BO 3 , AlO 4 , and AlO 6 , and starting with 800°C treatment also of BO 4 and AlO 5 structural units, but more detailed information related to boron, aluminum, and gallium environments is obtained from the analysis of MAS ‐ NMR data. These data evidenced in both amorphous xerogels and in crystallized samples a high fraction of penta‐coordinated aluminum and gallium.