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Atomic hydrogen centres in LiF–H− and LiF–H−, Mg2+ are studied by optical and ESR techniques. X‐irradiation of these crystals at 80 K leads to the formation of the H io‐centres which are thermally stable up to 110 K. After thermal annealing of the H io‐centres in LiF–H−, Mg2+ crystals a new atomic hydrogen centre is observed which is interpreted as an interstitial hydrogen atom stabilized by a complex of the Mg2+ ion and a positive ion vacancy (H io(Mg)‐centre). Thermal decay of the (H io(Mg)‐centres occurs at 430 K and leads to the formation of a hydrogen atom in the cation vacancy (H s,co(Mg)‐centre).

The H;-centre in LiF is studied by optical and ESR techniques. The centre is produced by reaction between HP and H;-centres. The H;-centre ESR spectrum shows three broad lines which are due t o the hyperfine interaction of an unpaired electron with the two protons. The resolved superhyperfine structure (as in case of F-centre) arises from the interaction with the lithium nuclei in a first shell and the fluoride nuclei in a second shell. Optical absorption band of the H;-centre a t 5.05 eV is similar to that of the F-band, only slightly shifted to higher energies. The centre has the character of molecule-ion as well as of F-centre.

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