No abstract

10 GHz electron paramagnetic resonance studies of Fe3+ in synthetic brown α-quartz single crystals have been carried out at 295 and at 40 K. The spin-Hamiltonian parameters were determined including high-spin Zeeman terms of form BS3 and allowing (g,D) matrix noncoaxiality as well as nonuniaxial anisotropy of the g matrix. Evaluation of the results, in part using the pseudosymmetry technique of Michoulier and Gaite, suggests that the so-called interstitial model (I center) is incorrect, and that the Fe3+ ions in fact occur substitutionally at Si4+ sites. The new label [FeO4]− is proposed for the center.

No abstract

Centres [Formula: see text] formed in α-quartz by 77 K x irradiation and warming above ca. 165 K, both exhibit a reversible change in symmetry of the centres from C1 at 75 K to C2 at 295 K, as observed by X-band electronic paramagnetic resonance spectroscopy. Each centre contains a Ge3+ ion (S = 1/2) substituted for a Si4+ ion, with a nearby Na+ interstitial ion. Spin-Hamiltonian electronic Zeeman, 73Ge and 23Na nuclear hyperfine and quadrupole parameter matrices for these centres were measured at temperatures 75, 190, and 295 K. They show that the change is associated with the defect reorienting between sites related by a two-fold crystallographic axis a, with energy barrier 0.223(6) eV for A and 0.178(14) eV for C. In both centres, ca. 50% of the spin density is located in a Ge sp orbital; in A, the sp orbital is oriented close to a, while in C it is nearly parallel to an O–Si–O bond-angle bisector that is nearly normal to a. Theoretical hyperfine matrices indicate that the Na ions are located near a, in a c-axis channel near the Ge site. Above ca. 270 K, A and C are observably in equilibrium, and are thus deemed to be isomeric species.

No abstract