Structure and reactivity of the oxyanions of transition metals. Part XVIII. A study of gamma-irradiated potassium chromate by electron spin resonance spectroscopy

Abstract

γ-Irradiation of K₂CrO₄ at 77°K produces two main paramagnetic species which disappear on warming, though the e.s.r. spectra are complicated by spurious lines thought to be from impurities. The first of the two centres is believed to be the electron-deficient species CrO₄^–, and the following e.s.r. parameters have been obtained for this ion: g_xx= 2·0162, g_yy= 2·0142, g_zz= 2·0080, and A_x [graphic omitted] 3G. A single-crystal study of this ion has indicated four magnetically distinct sites in the unit cell for an arbitrary orientation of the external field, with g_xx approximately parallel to the c-axis of the crystal. The electron hole is thought to be in a non-bonding orbital on oxygen, hyperfine coupling with the Cr nucleus occurring via spin-polarisation of the Cr–O bonds. The second of the two centres is thought to be CrO₄^3– which gives an e.s.r. spectrum which can only be measured below about 10°K. The g-values obtained for this ion are: g_xx= 1·9776, g_yy= 1·9539, g_zz= 1·9281. The reaction sequence leading to the formation of these centres, CrO₄^2–+γ→ CrO₄^–+ e^–; CrO₄^2–+ e^–→ CrO₄^3–, is apparently reversed on warming.