Low Temperature Intrinsic Defects in X-Irradiated Hydroxyapatite Synthetic Single Crystals

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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 PO 43-. 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 PO42- defect with the isotropic coupling arising from hyperfine interaction with the 31P nucleus.