Isolation of the new cubic phases RE₄FeGa_12-xGe_x (RE = Sm, Tb; x = 2.5) from Molten gallium: Single-crystal neutron diffraction study of the Ga/Ge distribution

The compounds RE₄FeGa_12-xGe_x (RE = Sm, Tb) were discovered in reactions employing molten Ga as a solvent at 850 °C. However, the isostructural Y₄FeGa_12-xGe_x was prepared from a direct combination reaction. The crystal structure is cubic with space group Im3m, Z = 2, and a = 8.657(4) Å and 8.5620(9) Å for the Sm and Tb analogues, respectively. Structure refinement based on full-matrix least squares on F₀² resulted in R₁ = 1.47% and wR₂ = 4.13% [1 > 2σ(1)] for RE = Sm and R₁ = 2.29% and wR₂ = 7.12% [1 > 2σ(1)] for RE = Tb. The compounds crystallize in the U₄Re₇Si₆ structure type, where the RE atoms are located on 8c (1/4, 1/4, 1/4) sites and the Fe atoms on 2a (0, 0, 0) sites. The distribution of Ga and Ge in the structure, investigated with single-crystal neutron diffraction on the Tb analogue, revealed that these atoms are disordered over the 12d (1/4, 0, 1/2) and 12e (x, 0, 0) sites. The amount of Ga/Ge occupying the 12d and 12e sites refined to 89(4)/11 and 70(4)/30%, respectively. Transport property measurements indicate that these compounds are metallic conductors. Magnetic susceptibility measurements and Mössbauer spectroscopy performed on the Tb analogue show a nonmagnetic state for Fe, while the Tb atoms carry a magnetic moment corresponding to a μ_elf of 9.25 μ_B.