Structural studies in the BaO–B2O3–TiO2 system by XAS and 11B-NMR

The structure of barium–titanium–metaborate xBaO–xB2O3–yTiO2 (y=0%, 4%, 8%, 16% and x=50–y/2) amorphous and crystallized powders, obtained using a polymeric precursor method, was investigated by Ti and B K-edge X-ray absorption spectroscopy (XAS) and 11B-NMR high-resolution techniques. XANES study of amorphous samples shows that Ti4+ ions exist as [4]Ti species associated to [6]Ti and [5]Ti species in a practically equivalent amount. After crystallization, titanium environment is predominately composed by [6]Ti species. According to XANES results obtained at the B K-edge, the fraction of boron in tetrahedral sites ([4]B) reduces as the amount of TiO2 is increased from x=0% to 4%, with a consequent increase of boron in trigonal sites ([3]B). By a combination of 11B-NMR spin-echo and triple quantum magic angle spinning (3Q-MAS) techniques, the detailed borate speciation was determined as consisting in [4]B and two kind of trigonal sites, [3]BA and [3]BB, corresponding, respectively, to borates sharing three and two O atoms with other boron units. NMR results reveal not only the reduction in boron coordination also seen by XANES but also the simultaneous reduction in the condensation degree of trigonal units, when the Ti content is increased in the glass. In crystallized samples, β-BaB2O4 and BaTi(BO3)2 phases were identified and quantified by 11B-NMR.