EPR of CO2− Defects in Calcite: Motional and Nonsecular Contributions

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Abstract
The electron paramagnetic resonance (EPR) of substitutional defect CO2− radicals in single-crystal calcite has been observed between 150 and 500°K. Thermally activated CO2− hindered rotations among three equilibrium positions related by rotations of ±2π / 3 about the calcite trigonal axis average the g-tensor splittings, which are well resolved at low temperature. The EPR spectra up to 380°K, where a single, motionally narrowed line is observed at X band, are reproduced quantitatively by the modified Bloch equations if 2ωe, the CO2− reorientation rate, is given by ωe = 7 × 1010exp (−2500 / T) Hz. Above 380° nonsecular broadening due primarily to motional modulation of the g tensors was observed and identified through its angular behavior. Defect CO2− radicals in calcite provide, both experimentally and theoretically, an ideal system for analyzing the effects of molecular motions on magnetic resonance spectra.