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) uncompens ated [GeO4]I,II- as well charge-compensated [GeO4/Li]A,C0 centers has been measured in an isotopically enriched a-quartz crystal. The EPR re sults indicate that the Geo4 tetrahedron is distorted for [GeO4]I- and [GeO4/Li]C0 so that no two oxygen ions are symmetry equivalent. In co ntrast, 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 populatio n to 100% (within the margin of probable errors). Analysis of the O-17 hyperfine matrices revealed that the majority of the oxygenic spin de nsity is found on only two of the four oxygen atoms, in 2p orbitals. F or each center studied, the unique principal direction of the O-17 hyp erfine 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 an gle O-Ge-O for these same oxygen atoms. Comparison of the various O-17 hyperfine splitting patterns furnishes conclusive proof of an isomorp hic relationship between renters [GeO4]I- and [GeO4/Li]C0 and also bet ween [GeO4]II- and [GeO4/Li]A0. We conclude that the compensated cente rs basically have the same electronic structure as the corresponding u ncompensated centers, with the compensating Li+ causing only a minor p erturbation. The EPR data are consistent with a qualitative model whic h suggests that the MO of the unpaired electrons in electron-rich para magnetic defects resembles the LUMO of a Si(OSi1/4)4 cluster model for quartz.