Electron-spin-resonance studies have been conducted at 9.2 and 23.1 GHz on synthetic calcium hydroxyapatite single crystals after exposure to ionizing radiation. The major paramagnetic defect is a spin-1/2 holelike center which is observable at 92 K but not at room temperature. It exhibits hyperfine interaction with a hydrogen nucleus as proved by deuterium substitution. The center is believed to be an 0 ion which results from removal of hydrogen from an OH ion. The hyper6ne interaction is w1th the hydI'ogen of an adjacent OH Ion. In first approximation the defect 1s axially symmetric with respect to the pseudohexagonal c axis; the spin-Hamiltonian parameters are g~I --2.0018~0.0002, g~= 2.0683~0.0002, A~~---5.6~0.2 G, and A j = +5.9~0.2 G. "Forbidden" transitions -+ -+ EM, =~1 cause complex unresolved spectra for orientations intermediate between H j[c nnd H i c. To Gt computer-simulated spectra to the experimental spectra, it was necessary to postulate that the hypefflne interaction is with an OH ion tilted 6 or 7 relative to c in six geometrically inequivalent directions. The isotropic hyperfine interaction was calculated using an approximate wave function constructed by a linear combination of orbitals from the 0 and OH ions. This establishes that the hole is strongly localized on the 0 and gives the absolute signs of the hyperfine components. the investigations of Shcherbakova et aE ' and of Gilinskaya et aE. ' on mineral fluorapatite crystals. The latter study is of particular interest since it reports the presence of the CO, and COS centers substituted for PO& in the apatite structure. This substitution is of interest because of its relation to carbonate impurity in biological crystals. Some similar studies on synthetic single crystals of chlorapatite, Ca, (POt), C1, have been reported. Banks et aE. '7 have studied by ESR the substitution of CrO~for PO', in chlorapatite single crystals. ESR investigations of Mna' substituted at the calcium sites were done by Piper' and Piper and I rener. " Knotterus et aE. 19 described an optical and ESR study of several electx'on-excess and bole-excess centex'8 1Q 1x'1Rd1Rtedchlorapatite. Hovrever, the models that these authors ploposed fox' two of the holellke centex'8 where disputed by Roufosse, Stapelbroek, Bartram, and Gilliam, " whose computer-simulated fit of the spectrum and agxeement between observed and calculated hyperfine parameters for one of the defects
ESR studies of radiation-induced defects have been conducted on synthetic calcium hydroxyapatite single crystals. For a room temperature x-irradiation a major defect (labeled A) was reported to be an O− ion. X irradiation at 6 K shows defect A, trapped atomic hydrogen, and a nonaxial holelike center (labeled I). These new centers are stable at 6 K but anneal near 77 K. Observations at 9 and 35 GHz indicate that the I center is a spin-1/2 defect located in six inequivalent sites. In the ab plane, spectra exhibit an isotropic hyperfine doublet (∼13 G splitting) and an anisotropic doublet (17–27 G splitting) in three symmetry-related sites. For other orientations additional site splitting and ’’forbidden transitions’’ make the spectra very complex. The hyperfine coupling tensor for the anisotropic doublet has diagonal elements −29.5, −19.2, and +3.11 G. The g tensor for this defect has diagonal elements 2.0068, 2.0032, and 2.0148. The sets of directional cosines associated with the minimum g value and the intermediate A value each indicate a direction corresponding approximately to that of the vector from an OH oxygen to a neighboring PO43− oxygen. The model proposed for the I center is a hole trapped by both an OH− and a neighboring PO43−. The anisotropic doublet is accounted for with 65% of the spin density on OH−. The remaining spin density is on a phosphate oxygen. This creates a PO2−4 defect with the isotropic coupling arising from hyperfine interaction with the 31P nucleus.
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