We have found that a positive muon (μ⁺) implanted into LiF, NaF, CaF₂, or BaF₂ pulls two F^- ions together in a strong "hydrogen bond" until the ¹⁹F nuclei are separated by roughly twice the nominal F^- ionic radius, with the μ⁺ midway between. The resultant "FμF" center is easily observed via the distinctive behavior of the collinear ¹⁹F:μ⁺:¹⁹F spin system (coupled by dipole-dipole interactions between the muon and the fluorine nuclei) in both transverse-field muon-spin rotation and zero-field muon-spin relaxation experiments. We speculate that implanted H⁺ ions may initially form similar hydrogen bonds between adjacent F^- ions in many metal fluoride crystals.
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Brewer et al. (1986) studied this question.
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