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 La2O3∙Al2O3∙Ga2O3∙5B2O3 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 27Al, 11B, and 71Ga 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 LaAl2.03B4O10.54 type, of La(Al,Ga)2.03B4O10.54 composition. The FTIR results point out the presence of BO3, AlO4, and AlO6, and starting with 800°C treatment also of BO4 and AlO5 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.