Isolation of the new cubic phases RE4FeGa12-xGex (RE = Sm, Tb; x=2.5) from molten gallium:: Single-crystal neutron diffraction study of the Ga/Ge distribution

The compounds RE4FeGa12-xGex (RE = Sm, Tb) were discovered in reactions employing molten Ga as a solvent at 850 degreesC. However, the isostructural Y4FeGa12-xGex was prepared from a direct combination reaction. The crystal structure is cubic with space group Im (3) over barm, Z = 2, and a = 8.657(4) Angstrom and 8.5620(9) Angstrom for the Sm and Tb analogues, respectively. Structure refinement based on full-matrix least squares on F-0(2) resulted in R-1 = 1.47% and wR(2) = 4.13% [1 > 2sigma(1)] for RE = Sm and R-1 = 2.29% and wR(2) = 7.12% [1 > 2sigma(l)] 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 Mossbauer spectroscopy performed on the Tb analogue show a nonmagnetic state for Fe, while the Tb atoms carry a magnetic moment corresponding to a mu(eff) of 9.25 mu(B).