Boron Coordination in Rapidly Cooled and in Annealed Aluminum Borosilicate Glass Fibers

The fraction of four‐coordinated boron was observed to increase from 10% in a rapidly cooled aluminum borosilicate glass fiber to 16% in the same fiber after it was annealed; the refractive index also increased. The index of the annealed fiber agreed with that of the bulk glass. Crystallization was not observed in the annealed fiber. It is concluded that boron coordination is temperature dependent and associated with structural relaxation.