Analysis of electron-excess and electron-deficient centers in x-ray-irradiated tricalcium phosphates by electron spin resonance spectroscopy
After X-ray irradiation, new paramagnetic centers were observed in electron spin resonance (ESR) spectra of amorphous calcium phosphate (ACP) and Mg-containing tricalcium phosphate (Mg-TCP) when they were heated at 900°C for 1 h before irradiation. Heated ACP shows a single resonance signal atg=2.0005 in place of the hyperfine doublet of the PO 4 2− ion radical, which is the dominant species in the ESR spectrum of unheated ACP. A similar signal is observed in the heated Mg-TCP. Treatment with electron scavengers (Pb2+ and Cd2+) and hole scavengers (Ce3+) suggests that the single resonance signal can be ascribed to a trapped electron. Comparison between the ESR spectra before and after annealing indicates that in the case of the heated ACP such a signal is most likely connected with a localized unpaired electron close to the calcium; on the other hand, in the case of the heated Mg-TCP the signal is connected with the localized electron close to the magnesium.
In addition, new sets of signals are observed in those heated samples. Their hyperfine splittings are 612 and 713 gauss for the heated ACP, and 589 and 683 gauss for the heated Mg-TCP. Those new sets of signals are found to be ascribed to the trapped holes close to two magnetically distinct sites of phosphorus nuclei in the crystal lattice ofβ-tricalcium phosphate.
Treatment with deuterium oxide strongly indicates that hydrogen atoms originate from adsorbed water molecules, although their half-life is a matter of months even at room temperature.
Key wordsESR Tricalcium phosphates Electron and hole centers
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