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

Abstract

A new type of cation-compensated Fe + (S = z) center designated [Fe04/M]&, with M =Li and H, has been found to exist in synthetic iron-doped a-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 gand 0-matrix noncoaxiality, and including high-spin terms of the form S . While the 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 C2 symmetry of the previously studied [Fe04/Li], center P shows C, symmetry at low temperatures (20 K), but approaches C2 symmetry at higher temperatures (approximatelỹ 160 K). The hydrogenic center, designated as [Fe04/H]0&, shows C& symmetry up to room temperature. The analysis of the spin-Hamiltonian parameter matrices for both the n (the previously studied form) and the P form of the cation-compensated Fe + centers indicates that the (interstitial) M ion is located, respectively, at opposite sides of the slightly distorted [Si(Fe)04] "tetrahedron. "

Characterisation of the Fe(III):H+ Defect Centre in Natural Amethyst

“…Apparently, a wide number of lines, most of which characterised by a very small width (less than 1 mT, in average) are present. The evidence of lines at very high and very low effective g values are in full agreement with previous experimental studies on synthetic and natural iron-bearing quartz samples, and attributed to Fe(III) ions undergoing an intense crystal field interaction [2,4,5,10]. The so large number of lines can be attributed to the occurrence of different Fe centres in the crystal and to the presence of inequivalent sites in the structure for most of them [1,2,4,5,10].…”

“…Extensive continuous wave EPR (cwEPR) characterisations of Fe-bearing quartz single crystals, where the presence of the S2 centre has been assessed, are reported in [5], for natural samples, and in [2,4] for synthetic samples. Remarkably, these studies enlighted the occurrence of two S2 centres, one more abundant (the socalled S2(D), with the proton connected to the distant oxygen) than the other (the S2(C) centre, with the proton linked to the close oxygen).…”

“…[4] and [5] refined the component of the SHF tensor. [2,4] obtained a best fit also including higher-order terms in the ZFS interaction, whereas [5] excluded this necessity. One relevant aspect is that [4,5] and [2] found all tensors (Zeeman, SHF, ZFS) partially misaligned with respect to the crystallographic reference frame.…”

“…Accordingly, the ion should be preferably small, suggesting a marked preference for Li + (S1 site), H + (S2 site) and Na + (S3 site). Most of this coupled Fe(III):M + centres were described in specific EPR spectroscopic studies devoted to the characterisation of a single centre on synthetic crystals [2][3][4][5][6][7][8].…”

Thermodynamic and magnetic properties of surface Fe3+ species on quartz: effects of gamma-ray irradiation and implications for aerosol–radiation interactions

“…This peak does not correspond to any known lattice-bound Fe 3? centers in quartz (Barry and Moore 1964;Matarrese et al 1969;Mombourquette et al 1986;Halliburton et al 1989;Minge et al 1989Minge et al , 1990Weil 1994;Balitsky et al 2000;Di Benedetto et al 2010) and was interpreted to represent surface Fe 3? species formed during sample preparation (SivaRamaiah et al 2011).…”