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The variation in the electron paramagnetic resonance (EPR) spectrum of ruby as a function of pressure has been observed to beyond 70 kbar for a magnetic field orientation parallel to the crystalline c axis. The data can be interpreted in terms of the usual spin Hamiltonian: H=g||βHzSz−12δ(Sz2−13S2), where the spectroscopic splitting factor g‖ has the same value as at ambient pressures, but where the zero-field splitting δ increases in a linear fashion from ∼0.38 to ∼0.43 cm−1. The experimental apparatus has been described previously, although important improvements in the pressure seal and in the pressure calibration have been made and are described in an Appendix.

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The electron-paramagnetic-resonance spectra of the systems CaF 2 : 151 Eu 2+ , Srl ? 2: 151 Eu 2+ , BaF 2 : 151 Eu 2+ 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), h and #6 (crystalline-field parameters), and A (hyperfine splitting factor). The variations of & 4 and h 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 ft^S -8 -8 , b%^5~1 3 -6 between the crystalline-field parameters and the nearest-neighbor distance 8.

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