W C Tennant

Zircon EPR revisited: 10 K EPR of three low-symmetry centres in irradiated zircon (zircon silicate)

8.9 GHz 10 K EPR studies of three low-symmetry centres in X-irradiated zircon are reported. Precise spin-Hamiltonian parameters are given for a Zr3+ centre, including 91Zr hyperfine and 89Y superhyperfine matrices, where the ideal tetragonal symmetry of the Zr site is lowered by charge compensator(s) to Laue class 1. Precise g-values are given for two previously unreported electron-hole centres shown to be located respectively at short- and long-bonded O-ligand sites, each of Laue class 2/m, monoclinic.

Oxygenic-hole centres in x-irradiated zircon: 10 K EPR studies

A surprisingly large number of paramagnetic hole centres in zircon have been observed and characterized by EPR. The field is reviewed and a model proposed based on some ten precisely measured centres. EPR data suggest that the trapped hole, produced by x-irradiation, is normally located in the non-bonding pz orbital of an O- ion and is stabilized by a nearby impurity cation, often, but not always, a +3 ion. Two previously unobserved oxygenic-hole centres are here reported. In one the stabilizing ion is shown from observed 89 Y superhyperfine structure to be Y3+ and conforms to the above model. The second has the most anisotropic g values yet observed in an oxygenic-hole centre in zircon. Two g values are significantly below the free-electron value and a px ground state is inferred.

10 K EPR of an oxygen-hole aluminium centre, (AlO4)0, in X-irradiated zircon, ZrSiO4

Precise parameter interaction matrix determinations, including 27Al hyperfine and nuclear electric quadrupole matrices, are reported for an aluminium centre, with an electron removed from an adjacent oxygen and referred to as an oxygen-hole centre, in x-irradiated zircon based on 8.9 GHz EPR measurements at around 10 K. Each of the parameter interaction matrices exhibits monoclinic symmetry indicating sites of Laue class 2/m for both the hole-centre and the interacting 27Al nucleus. Consideration of earlier reported results for two single electron-hole centres in zircon, and comparison with results for a similar centre in alpha quartz, show that the hole-centre is trapped at a short bonded ligand O in the zircon structure and interacts with an aluminium nucleus replacing Si at an adjacent site; each of the O and Si sites of the zircon structure have point group symmetry m.

15 K EPR of an oxygen-hole boron centre, [BO4]0, in x-irradiated zircon

Precise interaction matrices are reported for a boron-stabilized oxygenic-hole centre in x-irradiated zircon as derived from x-band electron paramagnetic resonance (EPR) measurements at ca 15 K. The presence of boron has been identified unequivocally through the observation and spin Hamiltonian analysis of superhyperfine structures from both naturally occurring isotopes of boron. Each of the parameter matrices indicates that the centre has point-group symmetry m (Cs ), as is common in hole centres in zircon, where the hole centre is trapped on an oxygen atom within one of the crystal mirror planes. Analysis of the principal directions of these matrices indicates that the boron atom resides in a silicon position neighbouring the oxygenic electron vacancy.

An EPR/ENDOR investigation of a [ZrPO4]0 centre in x-irradiated zircon: the Zr(α) centre

The Zr(α) centre is the most prominent centre observed after low temperature x-irradiation of zirconium silicate (zircon). The centre has been identified as due to an electron trap in the form of a Zr3+ ion in the zircon lattice with the centre being stabilized by interaction with a 100% abundant I = 1/2 ion. Previous reports have discussed the Zr(α) centre in terms of an 89 Y3+ (I = 1/2) ion in a neighbouring Zr4+ site. Using both EPR and ENDOR it has now been possible to show unequivocally that the I = 1/2 nuclear interaction is due to the presence of a 31P ion. Detailed analysis of the g and hyperfine matrices for 91Zr, 31P and 29Si is used to develop a model of the centre as an electron trapped on zirconium as a Zr3+ ion with a P5+ ion in the nearest Si4+ site on the mirror plane. The defect is now best described as a [ZrPO4]0 centre.

X-band electron paramagnetic resonance of a new centre in x-irradiated zircon

A new centre formed by x-irradiation of a synthetic zircon crystal at 77 K has been found and studied by electron paramagnetic resonance at 10 K. The impurity ion is believed to occupy a substitutional site, as was the case for the earlier reported B centre. Interaction with an nucleus in a nearest-neighbour site lowers the site symmetry from (Laue class, 4/mmm) tetragonal to m (Laue class, 2/m) monoclinic.

Two Ti3+ centres studied by X-band electron paramagnetic resonance at 10 K in zircon

Two previously unreported Ti3+ paramagnetic centres have been observed in zircon (zirconium silicate, ZrSiO4) and studied by X-band electron paramagnetic resonance at 10 K. The first of these is shown to arise from a Ti3+ electron-trap centre where Ti substitutes into one of the Si4+ sites, point group symmetry 2m (D2d), of the tetragonal crystal. A full spin-Hamiltonian analysis of the centre is given. The second centre arises from Ti3+ substituting for Zr4+ in the lattice and interacting with a 100% spin-I = ½ nucleus, probably 31P, in a nearby lattice site, thought to be a next-nearest-neighbour Si site. The point group symmetry of the site of this centre is 2 (C2).

Anisotropic mean-squared-displacement tensor in cubic almandine garnet: a single crystal 57Fe Mössbauer study

This paper describes single-crystal measurements on a crystal plate cut from a naturally-occurring almandine-rich single crystal (Alm69Pyr19Spe8Gro4) from Wrangell Alaska. The objective was to measure the mean-squared-displacement (msd) tensor precisely using Mössbauer spectroscopy. Parallel quantum-mechanical calculations based on X-ray determined atomic displacement parameters and Mössbauer parameters from polycrystalline measurements indicated that the msd tensor should display significant anisotropy, easily measurable within the precision of the Mössbauer experiment. For each single-crystal orientation the observed Mössbauer spectrum represents a macroscopic quantity that is the average over six symmetry-related (local) dodecahedral sites in which the high-spin Fe2+ ions reside. The anisotropy in the measured msd tensor is, nevertheless, unequivocal. Furthermore, the magnitudes of the Mössbauer-determined msd principal values exceed those of the corresponding X-ray-determined quantities by a factor 3.7. The equivalent recoilless fractions are also anisotropic and consequently one observes the Gol’danskii–Karyagin Effect (GKE), as manifested by an asymmetric quadrupole doublet in polycrystalline absorbers. Moreover, the line widths of the two quadrupole lines are markedly different but angle invariant. This is interpreted as implying that, in addition to anisotropy in the msd tensor, differential spin–spin relaxation is present in the

X-band EPR of Fe3+/CaWO4 at 10K : Evidence for large magnitude high spin Zeeman interactions

m = \pm 3/2 \leftrightarrow \pm 1/2