The dynamic Jahn-Teller effect in Cu(II) doped MgO.

The electron paramagnetic resonance spectra of Cu(II) doped MgO single crystals have been re-examined in detail within the framework of a dynamic Jahn-Teller effect. The experimental 1.8 K X-band spectra can be modeled in terms of a cubic spin Hamiltonian operating within the set of four Kramers doublets corresponding to the lowest vibronic energy levels of an Emultiply sign in circlee Jahn-Teller problem. This "four state" model must also include vibronic (Ham) reduction factors and a random distribution of the crystal strain. It was found to be important to treat the Zeeman, hyperfine, quadrupole, tunneling, and strain terms without recourse to perturbation theory or other approximations and this has been carried out using the eigenfield method. We find that the first excited singlet is of A(2) symmetry, indicating that the CuO(6) center has the expected Emultiply sign in circlee Jahn-Teller potential energy surface with three equivalent minima at tetragonally elongated octahedral geometries. Small random crystal strains have a dominant influence on the spectra and we find that the major features can be reproduced by averaging over the strain in the angular direction phi(s) with a small magnitude centered about zero. Details of the strain broadening require a distribution of strains centered at zero with a larger spread; however, the use of a single intrinsic linewidth could not account for all linewidth features. Our analysis also differs from that of previous workers in that different hyperfine values (A(1)=-20.0 x 10(-4) and A(2)=-86.0 x 10(-4) cm(-1)) are required as well as a nuclear quadrupole term (P(2)=+8.75 x 10(-4) cm(-1)) to account for the observed structure and the angular dependence. The transitions within the lowest excited singlet are observed directly, giving an estimate of the tunneling splitting as approximately 4 cm(-1). These parameter values are related to the intrinsic Jahn-Teller coupling parameters of the potential energy surface. We conclude that the Cu(II)/MgO system can be described as an almost pure dynamic Jahn-Teller case, with most spectral features accounted for by using a single isolated Gamma(8)((2)E) vibronic state.