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 correlat ion electron spin echo envelope modulation spectroscopy, we determined the structure of the prevalent radiation-induced defect, boron oxygen hole cent er, in this mineral. The hole is trapped by nonbridging oxygen in a strongl y distorted -O3B-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 gl asses. We suggest that a >B-O- unit near a cation vacancy is the precursor of hole centers in both types of glasses. (C) 2000 American Institute of Ph ysics. [S0021-9606(00)71347-X].