EPR ofHo2+in SrCl2under uniaxial stress: A parametrization of the orbit-lattice coupling

The uniaxial induced shifts of the EPR lines of the ground doublet and of the first-excited double of ${\mathrm{Ho}}^{2+}$ in Sr${\mathrm{Cl}}_2$ have been measured for various applied stress directions. Although the strong hyperfine structure of ${\mathrm{Ho}}^{2+}$ complicates considerably the calculation, it was possible to show that the results are consistent within the description by a dynamic spin Hamiltonian adapted to each doublet. From these results and from the lifetime of the excited doublet states deduced from the EPR linewidths, five bits of experimental information on the orbit-lattice coupling have been obtained. Various models proposed for a parametrization of this coupling are tested with these results. Beside the validity of the approximations made for the building up of these models, other possibilities can explain their incompatibility with the experiments: (i) local changes in the elastic constants; (ii) existence of displacements in the strained crystal which are not predicted by the elasticity but are due to the particularity of the crystal structure. Relaxation-time measurements have also been made and some comments on their correlation with uniaxial-stress results and lifetimes are given.