Effect of uniaxial stress upon the EPR of Tm2+and Yb3+in alkaline earth fluorides: correlation with spin-lattice relaxation

The shift of the EPR lines from sites of cubic symmetry of Tm 2+ in CaF 2 and SrF 2 and Yb 3+ in CaF 2 has been measured for uniaxial stress applied both in the (100) and (111) directions. From these shifts parameters of the spin-lattice hamiltonian have been deduced, and spin-lattice relaxation rates have been calculated which are in excellent agreement with previous results. The results show dramatic changes in the parameters between Tm 2+ and Yb 3+ in CaF 2 , and between different alkaline earth fluorides for Tm 2+ . Electrostatic models of the orbit-lattice interaction are shown to be inadequate. A satisfactory model which includes overlap and covalency has been constructed, in which it was found necessary to include those distortions of the three atom basis of the FCC structure of the fluorite lattice which are allowed by the symmetry.