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 [AlO 4 ] 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 17 O 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 [AlO 4 ] 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 29 Si (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.