Quadrupole interaction and static Jahn-Teller effect in the EPR spectra ofIr2+in MgO and CaO

The electron-paramagnetic-resonance spectra of ${\mathrm{Ir}}^{2+}$ were studied in single crystals of MgO and CaO at $X$- band frequencies. The spectra exhibit a strong quadrupole interaction and a static Jahn-Teller effect. The electric field gradient required for the quadrupole interaction is caused here by Jahn-Teller distortions. At high temperatures the spectrum is isotropic, at low temperatures it consists of a superposition of three tetragonal spectra. The unusually high transition temperatures ($T_t>120\mathrm{}$ K) at which the transitions from the high-temperature to the low-temperature spectra occur, are consistent with a strong Jahn-Teller coupling. It could be concluded for both hosts that $\frac{\overline{\delta }}{3\Gamma }>10\mathrm{}$ and that the first excited vibronic level is a singlet $A_1$. The quadrupole interaction was found to be much larger than the hyperfine interaction in MgO ($\frac{Q}{A_1}\sim 2$) and smaller in CaO ($\frac{Q}{A_1}\sim 0.25$). The anomalous effects in the EPR spectra caused by the strong quadrupole interaction were interpreted by an exact diagonalization of the spin Hamiltonian using a FORTRAN IV version of the magnspec program.