Knowledge Bank

Single crystals of $\gamma$- and X-irradiated calcite $(CaCO3)$ studied from $4.2\circ K$ to $300\circ K$ by ESR reveal a number of paramagnetic defect centers two of which have been tentatively identified as OH and $CO3$ radicals. The OH radical spectrum at $77\circ K$ consists of six doublets, each being associated with one of six distinct orientations in the crystal. At temperatures above $100\circ K$ the spectrum collapses into one axially symmetric doublet. This spectral behavior as temperature increases along with line narrowing is taken as evidence that the OH radical undergoes some form of periodic motion whose mean direction is parallel to the crystal (111) axis. The g and A tensors both exhibit symmetry about the crystal (111) axis with principal values $g\ast \ast =2.0120,g\perp =2.0055$ and $A\ast \ast =18.50oe,A\perp =19.37oe$ at $150\circ K$. The $CO3-$ molecule-ion spectrum also exhibits symmetry about the crystal (111) axis with principal g-values given by $g\parallel =2.0051$ and $g\perp =2.0162$. The hyperfine splittings obtained for the 1.1% naturally abundant $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}CO3-$ molecule-ion are $A\parallel =10$ oe and $A\perp =8$ oe. The decay of the $CO3-$ molecule-ion spectrum above $90\circ K$ and simultaneous growth of the OH radical spectrum suggests that a process of the kind $OH-+CO3-\to OH+CO3-$ may be occurring.