Reorientation of Nitrogen in Type-$\mathrm{I}b$ Diamond by Thermal Excitation and Tunneling

The rate of anneal of stress-induced ordering of isolated substitutional nitrogen impurities in diamond, measured in the temperature range $78 \mathrm{K}<T<\mathrm{}200\mathrm{} \mathrm{K}$, shows large deviations from Arrhenius-type behavior. It is concluded that in the temperature range considered, reorientation of the centers occurs by tunneling between thermally populated excited vibrational states. Two parameters serve to describe reorientations by thermal activation, by tunneling, and by the combined process.