10 K electron paramagnetic resonance of a d¹Ti³⁺centre in X-irradiated zircon (zirconium silicate)

Abstract

A d ' Ti3+ centre formed by x-irradiation of a synthetic zircon single crystal at 77 K has been studied by electton paramagneticpsonance (EPR) at 10 K. The Ti3t ion occupies n substitutional site of point group symmetry 42m (Dd. Absence of both site splitting in genenl crystal orientations and of superhyperhne structue from a +I ion indicated that a nearby charge compensator was not involved. Comparison with EPR measurements on Ti3t far quartz centres, point charge calculations and 29Si hyperfine spliuings indicated that the Ti3'' centre results from electron czpNre by Ti4+ occupying a &+ site in the crystal: the designation [TiOBI-is suggested. Precise spin-Hamiltonian parameters. including "Ti (I = z, 7.4%) and 49Ti (I = 1 2 ' 5.4%) hyperfine. nuclear electric quadrupole and nuclear Zeeman parameter matrices. together with high-spin nuclear term of dimension 14, 13S, IsS. Bf' and B f s were determined. The set of term B I , Bf' and B I s were found to be crucial in obtaining a good fit to the many 'forbidden' hyperfine lines observed. This resulted in determination of nuclear quadrupole and n u c l w Zeeman panmeters with precisions not hitherto attained in conventional single-crystal EPR measurements.

Large-Magnitude High-Spin Nuclear Parameters in a Ti3+Center from X-band EPR Measurements at 10 K

“…The main features of the B(Ti 3ϩ ) spectra have been detailed (1). The new results presented here are those obtained in the ab crystal plane: complex spectra of around 64 lines arising from 47,49 Ti lines of B(Ti 3ϩ ) and at least two other Ti 3ϩ centers were observed (linewidths ranged from 0.02 mT for B c to 0.08 mT for B Ќ c).…”

“…Initially four sets of data, those from the (1 0 0) and (1 1 0) planes used by Claridge et al (1) and the current data from (1 1 0) and (0 0 1) planes, were refined simultaneously. All four sets could be fitted by a single parameter set which, within fitting errors, was close to that given in Ref.…”

“…Earlier measurements (1) suggested that the (1 1 0) crystal plane is the most favorable plane from the point of view of obtaining a wide selection of both allowed and forbidden lines with reasonable intensities; this plane contains both the parallel (c axis) and perpendicular orientations of the uniaxial B(Ti 3ϩ ) center. Data were collected in this plane over a 180°angular range in 5°intervals and in the perpendicular, (0 0 1), plane over a 210°angular range in 10°intervals.…”

“…That such precision and such unexpectedly large-magnitude HSNZ parameters resulted from a largely conventional EPR experiment elicited some skepticism from the referees of our earlier paper (1). It was conceded that the evidence is statistical and could not, at that time, be established unequivocally.…”

“…A recent paper (1) gave a detailed description of a 10 K X-band EPR study of a d 1 Ti 3ϩ (S ϭ 1/2) center in zircon (zirconium silicate) in which it was claimed that high-spin nuclear Zeeman (HSNZ) parameters of dimension BI 3 and BI 5 are present with sizes several orders of magnitude larger than had been observed previously for first-row transition ions. As a result of including such terms in the spin-Hamiltonian (SH) analysis of the center, the ratio of the nuclear quadrupole moments, 47 Q/ 49 Q, for the two Ti isotopes involved was determined in good agreement with, but apparently more precisely than by, direct nuclear methods (2).…”