Quadrupole interaction and static Jahn-Teller effect in the EPR spectra of Ir2+ in MgO and CaO

The electron-paramagnetic-resonance spectra of Ir2+ 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 (Tt>120 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 δ̄3Γ>10 and that the first excited vibronic level is a singlet A1. The quadrupole interaction was found to be much larger than the hyperfine interaction in MgO (QA1∼2) and smaller in CaO (QA1∼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.