Proof of the thermodynamical stability of the E′ center in SiO2

The El center is a paradigmatic radiation-induced defect in SiO2 whose peculiar EPR and hyperfine activity has been known for over 40 years. This center has been traditionally identified with a distorted, positively charged oxygen vacancy V-O(+). However, no direct proof of the stability of this defect has ever been provided, so that its identification is still largely incomplete. Here we prove directly that distorted V-O(+) is metastable and that it satisfies the key requirements for its identification as E ', such as thermal and optical response, and activation-deactivation mechanisms.