Orbit-lattice interaction in Γ₈quartets. I. Determination of the coupling parameters for Er³⁺:MgO and Dy³⁺:CaF₂from EPR under uniaxial stress

The effect of uniaxial stress has been measured upon the EPR of the Gamma 8 states of MgO:Er 3+ and CaF 2 :Dy 3+ , and also the Gamma 7 state of the latter. From these data it has been possible for the first time to deduce all of the coefficients of the orbit-lattice Hamiltonian H OL for Gamma 3 and Gamma 5 strains, and all but one for Gamma 1 strains. Hence all but one of the parameters in H OL have been determined. This evaluation of so many parameters, as opposed to only a few obtained in previous investigations, has shown that most of the assumptions previously made about these parameters are erroneous. The experimentally determined parameters are used in a discussion of the EPR linewidths and spin-lattice relaxation rates.