Stochastic theory of line shape. II. Nonsecular effects in the EPR spectrum of CO2−in calcite

The random-phase-approximation result derived in the preceding paper is used to study the EPR line shape of C${\mathrm{O}}_2^{-}$ defects in CaC${\mathrm{O}}_3$. Relaxation is caused by a thermally activated mechanism due to which the oxygen ions hop between three equivalent sites in the (111) plane. As a result, the Zeeman Hamiltonian jumps at random between three different forms with equal probability. The calculated spectra for different values of the relaxation rate agree well with the experimental results of Hughes and Soos. At high temperatures, an additional spin-relaxation mechanism is shown to account for the observed residual linewidth when the external magnetic field is along the crystal trigonal axis.