EPR-revealed new highly symmetric radiation-induced structural defect in diamond

A structural center formed in Ib-type synthetic diamond after irradiation with $3$ MeV electrons and annealing at $1200^\circ$C for $30$ min has been studied initially using electron paramagnetic resonance. It is similar to the R5–R12 paramagnetic centers in the conditions of the formation and characteristics, but unlike them, the studied center is more symmetric: having the same spin $S = 1$, it has an almost isotropic (within the experimental and simulation accuracy) $g$-factor $g = 2.018(2)$, significant zero-field splitting $D = 2886(9)$ MHz, and $\langle 111\rangle$ ($C_{3v}$) symmetry. This center may be considered as a striking example of the tendency of structural centers in diamond to increase their local symmetry with the energy and, in particular, with the temperature of subsequent annealing.