TEMPERATURE DEPENDENCE OF HYPERFINE COUPLING OF RARE-EARTH S-STATE IONS IN CUBIN CRYSTALS

Measurements of the hyperfine coupling constant A of CaF2: Eu2+ were made for temperatures between 4.2 and 715°K by means of EPR techniques. It was found that |A| decreased in magnitude by approximately 2% over this temperature range. A calculation of the temperature dependence of A following imnek and Orbach was also performed. The orbit-lattice interaction for a molecular cluster with eightfold cubic coordination was expanded in third-order spherical harmonics and normal coordinates which are antisymmetrical under inversion through the origin. This interaction was used to admix excited ns states into the ground 4f orbitals so as to give rise to a temperature-dependent hyperfine coupling constant given by the relation A(T)=A(0)[1-CT60tx5(ex-1)-1dx] Good agreement was obtained between the temperature dependence predicted by this expression and our experimental data on CaF2: Eu2+. However, the strength of the constant C, calculated on the basis of a nearest-neighbor point-charge model and the long-wavelength approximation, was smaller than the experimentally determined value by approximately one order of magnitude. © 1968 The American Physical Society.