The electron-paramagnetic-resonance spectra of the systems CaF₂: ¹⁵¹Eu²⁺, SrF₂: ¹⁵¹Eu²⁺, BaF₂: ¹⁵¹Eu²⁺ have been studied at a pressure of 26 kbar. These spectra were interpreted in terms of the usual phenomenological spin Hamiltonian with parameters g (spectroscopic splitting factor), b₄ and b₆ (crystalline-field parameters), and A (hyperfine splitting factor). The variations of b₄ and b₆ with pressure were compared with their variations with cation, and a consistent interpretation of the differences was found in terms of local lattice distortions about the impurity europium ions. This model leads to the relationships b₄∝δ^-8.8, b₆∝δ^-13.6 between the crystalline-field parameters and the nearest-neighbor distance δ.
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Hurren et al. (1969) studied this question.