Direct Experimental Evidence for Atomic Tunneling of Europium in Crystalline Eu₈Ga₁₆Ge₃₀

Mössbauer-effect and microwave absorption experimental evidence unambiguously demonstrates the presence of slow, ∼450  MHz, tunneling of magnetic europium between four equivalent sites in Eu₈Ga₁₆Ge₃₀, a stoichiometric clathrate. Remarkably, six of the eight europium atoms, or 11% of the constituents in this solid, tunnel between these four sites separated by 0.55 Å. The off centering of the atoms or ions in crystalline clathrates appears to be a promising route for producing Rabi oscillators in solid-state materials.