A b s l m L Both the temperature and the pressure dependences of the b: spin-Hamiltonian parameter for RbCaF,, CScaF,, R K d F 3 and ncdF, doped with Gd3+ have k e n reported. On the wumptian thal b! depends on the metal-ligand distance according to a law b: -P* the following values of k were obtained -20.9, -27.1, -24.9 and -29.3, mplively. It was found thal the lemperalurc dependence of b! is predominantly caused by spin-phonon interactions which were estimated to mntribure a b u t 68% m the total dependence observed. I h e effectiveness of =me theoretical models predicling the role of vibmnic mnlnbutions is also discussed.

Parametrization of phonon induced contributions to the experimentally determined temperature dependence of the b 0 4 spin-Hamiltonian parameter for Gd3+ doped RbCaF3, CsCaF 3 , RbCdF3 and TlCdF3 single crystals is discussed. Three physically different parametrization schemes are considered, i.e. that of Einstein, Debye and Huang. It is found that the model of Huang, which predicts that the phonon induced contributions should be proportional to the Debye-Waller factor, describes best the experimental data. The method allowing determination of the Debye temperature from EPR data is also suggested.

2+In our previous work on CaF2? SrF2, and BaF2 doped with Eu /1/ we have shown that if it is assumed that the superposition model /2, 3/ is applicable, one can deduce the metal-ligand distance from measured changes of spinHamiltonian parameters under uniaxial s t r e s s . Among the isomorphous group of europium doped fluoride type crystals there is the interesting case of CdF2. A s shown by Baberschke /4/ and Baker /5/, the Eu2+-F-bond length in CdF2:Eu t o the CaF2:Eu2+ system although the situation is reversed for the cation-Fbond length in pure lattices. The purpose of this communication is to extend