Abstract: While most complexes in fluorite-type lattices (CaF2, SrF2, CdF2) containing Mn2+ impurities remain cubic at all temperatures, electron paramagnetic resonance (EPR) measurements have shown that the one in BaF2 is tetrahedral below 50K. This surprising behaviour is intrinsic to the centre and not associated to any close defect or pure lattice phase-transition. Through the use of density functional (DFT) calculations we show that the distortion is linked to the unexpected low force constant along the tetrahedral distortion mode with a2u symmetry in these complexes and the large metal-ligand distance existing in BaF2:Mn2+. Ultimately, these facts reflect that, for substitutional impurities in fluorite-type crystals, the ligand-ligand interaction is dominant over the metal-ligand one.

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