The magnetic properties of the oxygen–hole aluminum centers in crystalline SiO₂. I. [AlO₄]⁰

Abstract

The most prominent paramagnetic impurity center in single crystal alpha-quartz X-irradiated at room temperature is [AlO4]0, which contains an aluminum ion substituted for a silicon ion, with an electron removed from a neighboring oxygen by the ionizing radiation. A reinvestigation of this species at ca. 35 K using electron paramagnetic resonance has disclosed weak multiplets due to hyperfine interaction of the unpaired electron with an 17O nucleus, present in natural abundance (0.037%). The spin-Hamiltonian [Formula: see text] parameter matrix [Formula: see text] describing this interaction confirms the above model of [AlO4]0 and yields the coefficients [Formula: see text] and [Formula: see text] for the oxygen s- and p-orbitals, respectively, in the unpaired electron wavefunction ψ. Similarly, weak multiplets caused by 29Si (4.70% natural abundance) have been analyzed. The eigenvectors of [Formula: see text], in conjunction with eigenvectors of other parameter matrices (i.e., [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text]), indicate that the trapping oxygen is one which would have a bond of the "long" type to the silicon replaced by aluminum (there are two different Si–O bond lengths in quartz). These eigenvectors also show that the axis of the oxygen p-orbital component of ψ is almost perpendicular to the Al–O–Si plane. The complete oxygen component of ψ has been used to obtain estimates of the dipolar contributions to [Formula: see text] and [Formula: see text], which however do not agree too well with the respective observed anisotropic components.

Electronic structure of a neutral oxygen vacancy inSrTiO3

“…18 -20 One recent and illustrative example is that of the spin localization in an Al substitutional impurity in SiO 2 . Because of the selfinteraction problem, DFT calculations predict that the hole associated with a neutral ͓AlO 4 ͔ 0 center is delocalized over four oxygen atoms, contrary to accurate EPR experiments 21 and Hartree-Fock calculations that indicate full localization of the hole on a single bridging oxygen. 22 A similar problem can be found every time the electrons associated with a defect can be localized or delocalized on a given state of the system.…”

Hole Trapping at Al impurities in Silica: A Challenge for Density Functional Theories

“…The neutral substitutional Al impurity in a quartz (hereafter denoted as ͓AlO 4 ͔ 0 ) is one of the most well-studied states of Al in silica [10][11][12][13][14][15][16][17][18][19][20][21][22][23]. The state is observed in irradiated samples of natural or manufactured quartz and gives rise to a dark coloration ("smoky quartz").…”

“…Hence the center is often referred to as a "trapped hole species." Because of the presence of an unpaired electron the center can be detected with electron paramagnetic resonance spectroscopy, and this technique, together with dielectric relaxation and acoustic loss experiments, has yielded much information about the electronic structure of the impurity [10][11][12][13][15][16][17]. The picture emerging from these investigations is that of a hole localized on one of the four O neighbors of the Al impurity, with an accompanying distortion of the lattice structure.…”

The Structures of Two Hole Centers in the Mineral Brasilianite NaAl3(PO4)2(OH)4

“…The detailed determination of the structure of such radiation defects largely rests on the evaluation of the hyperfine splitting (hfs) of the neighboring atoms. The best known example of this kind is the smoky quartz center with dominating hfs due to an Al impurity [1][2][3]. In brazilianite hf interaction with the nucleus of at least one structural atom, either Al or P, can be expected, and in a previous study [4] an CT adjacent to one Al and one P was postulated suggesting substitution of the second Al by a divalent impurity.…”