Based upon first-principles density-functional calculations the structure of, and

diffusion of boron in, amorphous and silica was studied. It was found that

incorporation of a boron atom into the amorphous silica matrix resulted in various

minimum-energy configurations; with and without oxygen deficient centers. Also,

the B-related defects could undergo interconversion at high temperatures. Whereas

the B atoms preferred to remain in the trigonal BO3 form in amorphous silica, the

present results showed that B diffusion might require transformation of the

immobile BO3 state into a mobile B state via the capture of oxygen-vacancy related

defects equivalent to an S center; which was a combination of an oxygen vacancy

and a trivalent Si defect with an unpaired electron. Upon considering the energy

cost for S-center creation, the predicted activation energy for B diffusion was in

good agreement with experiment. Such a defect-mediated diffusion model could

also explain the observed correlation between B diffusion and Si self-diffusion in

amorphous silica.

Structure and Diffusion of Boron in Amorphous Silica: Role of Oxygen Vacancy

Related Defects. C.L.Kuo, G.S.Hwang: Physical Review B, 2009, 79[16], 165201