Electron Spin Relaxation Rates of Radicals in Irradiated Boron Oxides

The boron–oxygen hole center (BOHC) that is formed by irradiation of boron oxides has previously been characterized extensively by continuous wave and pulsed electron paramagnetic resonance. We now report that the electron spin relaxation rates for the BOHC in irradiated high purity B₂O₃, practical grade B₂O₃, and sodium tetraborate Na₂B₄O₇ exhibit substantial sample dependence. Because of the low magnetic moments for the boron nuclei, the spin echo dephasing is dominated by electron–electron interaction (T₂) instead of the nuclear spin diffusion that dominates dephasing for organic radicals in lattices with high proton concentrations. The higher local concentration of defects in a sample of practical grade B₂O₃ than in a sample of reagent grade B₂O₃, shortens T_m (spin echo dephasing) and causes extensive cross relaxation contributions to T₁ (spin lattice relaxation) at 10 K. At temperatures below about 60 K T₁ is shorter for the BOHC in B₂O₃ than in sodium tetraborate or for the radical formed by irradiation of calcium metaborate. T₁ for the BOHC and the radical in irradiated calcium metaborate are shorter than for other irradiated solids including glycylglycine, l-alanine and the E´ center in quartz. The temperature dependence of T₁ for the BOHC in B₂O₃ is dominated by the Raman process with a lower Debye temperature than for the radical formed by irradiation of calcium metaborate.