O17hyperfine interaction for the [GeO4

Abstract

Hyperfine splitting arising from "0 for the Ge'+ (S = -') uncompensated [Ge04], "aswell as chargecompensated [Ge04/Li]z c centers has been measured in an isotopically enriched a-quartz crystal. The EPR results indicate that the Ge04 tetrahedron is distorted for [Ge04], and [Ge04/Li]c so that no two oxygen ions are symmetry equivalent. In contrast, for [Ge04],, and [Ge04/Li]0"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 100go (within the margin of probable errors).Analysis of the ' 0 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 0 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 hyperfine matrix lies along the bisector of angle 0-Ge-0 for these same oxygen atoms. Comparison of the various "0 hyperfine splitting patterns furnishes conclusive proof of an isomorphic relationship between centers [Ge04], and [Ge04/Li]c and also between [Ge04]tt and [GeO~/Li]". 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(OSi&&4)4 cluster model for quartz.

Trapped-electron centers in pure and doped glassy silica: A review and synthesis

“…The orientations of the principal axes of the hyperfine matrices of the central germanium and oxygens 3 and 4 are illustrated in Fig. 4(II) [33]. This defect exhibits the same C 2 symmetry (rotations by π) as is the case when no electron is trapped.…”

“…2(a)). And while the full explanation will surely have to come from future theoretical calculations, the amazing study of 17 [33], certainly yields a major clue. These authors' results for Ge(I) were dramatically different from those for Ge(II):…”

A Minireview of the Natures of Radiation-Induced Point Defects in Pure and Doped Silica Glasses and Their Visible/Near-IR Absorption Bands, with Emphasis on Self-Trapped Holes and How They Can Be Controlled

“…The naïveté of my notion that they should match exactly is illustrated by the light dashed curves in Figure 12. My successful fits using substantially larger values of in my fractal-kinetic second-order decay solution (Equation (20) of [48]) are shown in Figure 12 as the small hollow circles. My fractal first-order-kinetic best-fit ("Kohlrausch best fit": small hollow triangles) is clearly inferior.…”

“…(A) Given that the Ge(1) center in Ge-doped silica glass [5][6][7]18] is characterized by virtually the same values and mean hyperfine coupling constant as those of the Ge(II) center in quartz [19,20], it must be concluded that (i) " -quartz-crystal-like" GeØ 4…”

“…(B) Given that the Ge(I) center in quartz [19,20] is characterized by values only slightly different from, and mean hyperfine coupling constant virtually identical to, those of the Ge(2) center in silica glass [5][6][7]18]-and it is known [19,20] that Ge(I) is an electron trapped in symmetry-breaking orbitals perpendicular to the twofold axis a GeØ 4 tetrahedron in the -quartz lattice (thus making it an energetically different state of the same defect responsible for Ge(II) [19])-it must be concluded that (i) Ge(I) is a "glass like" defect even though it occurs in perfect crystal and (ii) the "default" interpretations of Ge (1) and Ge (2) are that they are two energetically different states of the same defect in Ge-doped silica glass.…”