EPR study of the E’ defects in optical glasses and cristobalite R. I. Mashkovtsev, A. I. Nepomnyashchikh, A. P. Zhaboedov and A. S. Paklin EPL, 133 1 (2021) 14003 Published online: 05 March 2021 DOI: 10.1209/0295-5075/133/14003