O-17 HYPERFINE INTERACTION FOR THE [GEO4](I,II)(-) AND [GEO4/LI](A,C)(0) CENTERS IN AN ENRICHED CRYSTAL OF ALPHA-QUARTZ

Hyperfine splitting arising from O-16 for the Ge3+ (S = 1/2) uncompensated [GeO4]I,II- as well charge-compensated [GeO4/Li]A,C0 centers has been measured in an isotopically enriched a-quartz crystal. The EPR results indicate that the Geo4 tetrahedron is distorted for [GeO4]I- and [GeO4/Li]C0 so that no two oxygen ions are symmetry equivalent. In contrast, for [GeO4]II- and [GeO4/Li]A0 the expected local symmetry (C2) is present and only two types of oxygen atoms are observed. Inclusion of the oxygenic contribution brings the total measured spin population to 100% (within the margin of probable errors). Analysis of the O-17 hyperfine matrices revealed that the majority of the oxygenic spin density is found on only two of the four oxygen atoms, in 2p orbitals. For each center studied, the unique principal direction of the O-17 hyperfine matrix of the high-spin-density oxygen atoms lies along the O-O interatomic direction of these two oxygen atoms. The unique principal direction of the Ge-73 hyperfine matrix lies along the bisector of angle O-Ge-O for these same oxygen atoms. Comparison of the various O-17 hyperfine splitting patterns furnishes conclusive proof of an isomorphic relationship between renters [GeO4]I- and [GeO4/Li]C0 and also between [GeO4]II- and [GeO4/Li]A0. We conclude that the compensated centers basically have the same electronic structure as the corresponding uncompensated centers, with the compensating Li+ causing only a minor perturbation. The EPR data are consistent with a qualitative model which suggests that the MO of the unpaired electrons in electron-rich paramagnetic defects resembles the LUMO of a Si(OSi1/4)4 cluster model for quartz.