Structural disorder and magnetism of the semiconducting clathrate Eu₈Ga₁₆Ge₃₀

The temperature dependence of the atomic displacement parameters for an isotopically enriched Eu₈Ga₁₆Ge₃₀ crystal determined from refinements of neutron diffraction data show the anomalously large values for the Eu atom in the large cage persist from 295 to 12 K. The Eu site in the large cage is modeled by splitting it into four fractionally occupied positions, displaced 0.4 Å from the cage center. Fourier maps of this Eu site support this view in showing residual nuclear density with distinct peaks in the directions of the split-atom positions, and lesser density at the cage center position. The Ga and Ge atoms appear to be fully disordered on the three distinct framework sites. In comparing the isostructural X₈Ga₁₆Ge₃₀ (X=Eu, Sr, Ba) compounds, the positional disorder around the large cage site increases progressively with decreasing size of the caged atom. Below 35 K, Eu₈Ga₁₆Ge₃₀ orders ferromagnetically, with the preferred direction of the Eu moments along (100). The projected saturation value of the Eu moment, 7μ_B, equals the free ion value.