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

Abstract

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 (${T}_{t}g120$ 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 $\frac{\overline{\ensuremath{\delta}}}{3\ensuremath{\Gamma}}g10$ 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 ($\frac{Q}{{A}_{1}}\ensuremath{\sim}2$) and smaller in CaO ($\frac{Q}{{A}_{1}}\ensuremath{\sim}0.25$). 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.