Large crystalline fields and anisotropic vibrational modes of ¹⁵¹Eu²⁺ in metallic EuRu₂Ge₂

Mössbauer studies of ¹⁵¹Eu²⁺ in EuRu₂Ge₂ at temperatures 4.1 to 600 K reveal the largest crystalline electric field gradient ever observed in a metallic rare earth compound, A₂〈r²〉=425 K. The associated large quadropole interaction, eqQ=-440Mc/s, enabled a detailed study of the anisotropy in recoil free fraction, Goldanskii-Karyagin effect. While at 85 K the recoil free fractions along the crystallographic c-axis and perpendicular to it are 0.95 and 0.74, at 570 K they are 0.64 and 0.10, respectively. Within a Debye model this corresponds to Debye temperatures of θ_|=450 K and θ_⊥=180 K. This is the first observation of an anisotropic recoil free fraction in a metallic rare earth compound.