On the structure of trapped holes in borosilicates

Danburite (CaB2Si2O8) is a naturally occurring borosilicate mineral that is a crystalline analog of borosilicate glass. Using two-dimensional correlation electron spin echo envelope modulation spectroscopy, we determined the structure of the prevalent radiation-induced defect, boron oxygen hole center, in this mineral. The hole is trapped by nonbridging oxygen in a strongly distorted O3−B–O––BO3− linkage; the precursor of the hole center is a Ca2+ vacancy. These results and computer modeling suggest that in borosilicate glasses, the hole center is the same >B–O⋅ radical as in alkali borate glasses. We suggest that a >B–O− unit near a cation vacancy is the precursor of hole centers in both types of glasses.