ELECTRON-SPIN-RESONANCE AND ELECTRON NUCLEAR DOUBLE-RESONANCE INVESTIGATION OF A NEW CR-3+ DEFECT ON AN NB SITE IN LINBO3-MG-CR

For LiNbO3 crystals heavily doped with Mg and codoped with Cr a single isotropic ESR line with g = 1.971 +/- 0.002 and a minimal linewidth of 1.8 +/- 0.2 mT has been observed. With ENDOR it could be shown unambiguously that the new defect is Cr3+ substituting for Nb. The Cr-53 hyperfine and quadrupole constants are A parallel-to = 50.8 +/- 0.2 MHz, A perpendicular-to = 51.0 +/- 0.2 MHz and P = 0.10 +/- 0.05 MHz. The crystal field at the chromium site is smaller by more than an order of magnitude than for previously reported Cr3+ centres in LiNbO3, indicating that those centres are not on the Nb site.