Spin–lattice Interaction Experiments on Color Centers in Quartz

Abstract

The paramagnetism of the color centers in smoky quartz is caused by the electron which is missing from a nonbonding oxygen orbital near an aluminum impurity. The effects produced by externally introduced microwave phonons on the spin-resonance signal from these color centers have been studied in detail as a function of the relative phonon and spin-resonance frequencies, the phonon power, the spectrometer power, the magnetic field angle, and the phonon polarization. As has been noted previously, these interactions do not conform with those observed in other paramagnetic crystals. Effects of an applied electric field on the resonance lines and various further experiments on the color centers in smoky quartz are reported; two in particular suggest that the "hole", the missing electron, makes thermally excited transitions between two different sites adjacent to a given impurity atom. One of the experiments is a measurement of the cross relaxation which takes place between the various lines of the spectrum, while the other is a measurement of the low-frequency dielectric loss found at liquid helium temperatures. An attempt is made to discuss the spin-phonon results in terms of such transitions for the holes.

Dielectric relaxation processes in smoky quartz crystals at very low temperatures

“…The derivation of T values from tg5 versus T curves taken at constant w, which procedure is often regarded as useful and rather reliable, will therefore give wrong results in this case. This may explain why our results differ from those of Taylor and Farnell, who reported similar experiments as part of their work on spin lattice interaction in colour centres in quartz [5]. An explanation of this relaxational behaviour can be given in terms of a tunneling process by which the electron, trapped near a defect, may pass through potential barriers between those equivalent sites at which the electron may be localized more specifically.…”

Self-trapped holes in pure-silica glass: A history of their discovery and characterization and an example of their critical significance to industry

“…According to the best available evidence ca. 1978 (see my review [74]), the ground-state hole of the [Al] 0 center is indeed localized on the very oxygens possessing the peculiar dihedral angles that I impute to STH 2 [88] and the hole in fact hops back and forth between these two degenerate ground-state oxygens on each AlO 4 tetrahedron [89]. However, this hopping between these two sites is not rapid enough to result in an STH 2 -type spectrum.…”

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.…”