EPR study ofFe3+inα-quartz: Characterization of a new type of cation-compensated center

A new type of cation-compensated ${\mathrm{Fe}}^{3+}$ (S=(5/2) center designated [${\mathrm{FeO}}_4$/M${]}_{\mathrm{\beta }}^0$, with M=Li and H, has been found to exist in synthetic iron-doped α-quartz, through an X-band electron paramagnetic resonance study. The spin-Hamiltonian parameters at temperature approximately 20 K were determined for both centers, allowing for anisotropy of the g factor as well as g- and D-matrix noncoaxiality, and including high-spin terms of the form $S^4$. While the ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ hyperfine matrix was not obtainable, the EPR measurements did yield the proton hyperfine matrix for M=H. For M=Li, in contrast to the $C_2$ symmetry of the previously studied [${\mathrm{FeO}}_4$/Li${]}_{\mathrm{\alpha }}^0$, center β shows $C_1$ symmetry at low temperatures (20 K), but approaches $C_2$ symmetry at higher temperatures (approximately ≥160 K). The hydrogenic center, designated as [${\mathrm{FeO}}_4$/H${]}_{\mathrm{\beta }}^0$, shows $C_1$ symmetry up to room temperature. The analysis of the spin-Hamiltonian parameter matrices for both the α (the previously studied form) and the β form of the cation-compensated ${\mathrm{Fe}}^{3+}$ centers indicates that the (interstitial) M ion is located, respectively, at opposite sides of the slightly distorted [Si(Fe)${\mathrm{O}}_4$] ‘‘tetrahedron.’’