Paramagnetic centers localized on a phosphorus ion in gamma-irradiated metaphosphate glasses

Radiation-induced paramagnetic centers whose unpaired electron is localized on a phosphorus ion were investigated with ESR in glassy and polycrystalline metaphosphates. Hyperfine doublet absorptions due to a 31P nucleus with a separation of 70-120 mT were analyzed. The mean values and fluctuation widths of ESR parameters for the centers in the glasses were determined by the lineshape simulation of the observed spectra. The results obtained are summarized as follows: (1) A P4+ center having fourfold coordination was observed in the as-irradiated (77K) glassy and polycrystalline hosts. This center is considered to be derived from a PO42- unit on capturing an electron. (2) On thermal annealing of as-irradiated glasses, the above center is converted into a P4+ center having threefold coordination of Csub(3v) symmetry, detaching an oxygen from its coordination sphere. The resulting defect center is an analogue of Si E'-center, the unpaired electron occupying a non-bonding level which is exclusively composed of sp3 hybrid orbital of phosphorus. (3) The formation of the fourfold-coordinated center and its thermal relaxation into the threefold-coordinated one are identical with the results of As(V) which has an electronic configuration analogous to P(V). It is suggested that these results are the common characteristics of NWF's which belong to the V group on the periodic table. This fact may be ascribed to that these NWF's provide a coulombic potential for an electron trapping because their valence is greater than its coordination number. (author)