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Single crystals of iron-doped KMgF3 show a strong axial EPR spectrum with partially resolved hyperfine structure. The spectrum arises from Fe'+ in a crystalline field of [100] axial symmetry. An electron-nuclear-double-resonance study of the hyperfine interaction gives positive support to the model previously proposed. The iron ion substitutes for a magnesium ion, and one of the surrounding six-nearest-neighbor Auorine ions is either missing or replaced by a charge-compensating impurity such as 0, thus giving rise to an axial distortion. The measured values for the spin-Hamiltonian parameters are D = 0.352 + 0.005 cm ', g~--2.0042+ 0.0005, and g, = 2.0120+ 0.0005, where g is axially symmetric about the symmetric axis of the crystalline-field parameter D. The nearest-neighbor hyperftne interaction constants are A, gh = 104.6+ 0.1 MHz and Ar/h = 55.7+ 0.1 MHz for the four "F ions in a (100) plane perpendicular to the symmetry axis of D, and A~(h = 43.2+ 0.5 MHz, A"/h = 24.6 + 0.1 MHz for the single ' F ion along the symmetry axis of D. It is assumed that the hyperfine interaction tensors are axially symmetric about their corresponding Fe-F bond directions.

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