Vibrational investigation of lithium metaborate-metaaluminate glasses and crystals

Lithium metaborate-metaaluminate glasses xLiAlO(2) .(1 - x)LiBO2, (0 less than or equal to x less than or equal to 0.42), and crystals (Li3B2AlO6,Li2BAlO4) have been prepared and studied by infrared and Raman spectroscopies. The two crystalline compounds have similar networks made of Alempty set(4)(-) and BO2O- units (O, O: bridging and terminal oxygen atoms, respectively) in arrangements which maximize the number of B-O-Al linkages. Both compounds decompose at high temperatures into alpha-LiBO2 and gamma-LiAlO2. Increasing x in glass induces the isomerization of B-4(-) tetrahedra to BO2O-triangles, while aluminum forms AlO4- units. This structural scheme is sufficient to account for the minimum in the glass transition temperature observed for x approximate to 0.25. There is no vibrational evidence for the presence of AlO5 or AlO6 polyhedra in glass, although their presence in small amounts cannot be excluded. Glasses and crystals of the same stoichiometry (x = 0.33) are differentiated by an excess of kinetically arrested B-O-B and Al-O-Al linkages in the former. (C) 1997 Elsevier Science B.V.