Observation of muon-fluorine "hydrogen bonding" in ionic crystals

Abstract

We have found that a positive muon (${\mu }^{+}$) implanted into LiF, NaF, Ca${\mathrm{F}}_2$, or Ba${\mathrm{F}}_2$ pulls two ${\mathrm{F}}^{-}$ ions together in a strong "hydrogen bond" until the ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ nuclei are separated by roughly twice the nominal ${\mathrm{F}}^{-}$ ionic radius, with the ${\mu }^{+}$ midway between. The resultant "$\mathrm{F}\mu \mathrm{F}$" center is easily observed via the distinctive behavior of the collinear ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}:{\mu }^{+}:{}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ 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 ${\mathrm{H}}^{+}$ ions may initially form similar hydrogen bonds between adjacent ${\mathrm{F}}^{-}$ ions in many metal fluoride crystals.