Electron spin resonance studies of chlorine centres in irradiated chlorine-doped Na2SO4 single crystals

The nature of paramagnetic centres trapped in X-irradiated Na₂SO₄ single crystals containing chlorine as impurity has been determined from E.S.R. measurements both at room and liquid nitrogen temperatures. ClO₃, ClO₂, SO₄ ^- and O₃ ^- are the paramagnetic centres identified. The g and A parameters of ClO₃ change considerably between 298 K and 77 K. Thus the parameters of ClO₃ obtained at 298 K are g_xx = 2·0123, g_yy = 2·0143, g_zz = 2·0206 G and those of the A-tensor are A_xx = 97, A_yy = 100 and A_zz = 112 G. The parameters obtained for the same centre at 77 K are g_xx = 2·0132, g_yy = 2·0173, g_zz = 2·0073 G and those of the A-tensor are A_xx = 130·6, A_yy = 146·3 and A_zz = 181·2 G. The temperature variation of the principal values are determined in the temperature range between 298 K and 77 K. These data can be interpreted on the assumption that ClO₃ undergoes libration at room temperature, which is frozen out reversibly on cooling. The structure and mechanism of these defects are discussed further.