Ternary rare-earth bismuthides RE5SiBi2 and RE5GeBi2 (RE=La–Nd, Gd–Er): Stabilization of the β-Yb5Sb3-type structure through tetrel substitution

Ternary bismuthides RE5TtBi2 containing rare-earth (RE=La–Nd, Gd–Er) and tetrel (Tt=Si, Ge) atoms have been prepared by arc-melting of the elements followed by annealing at 800 °C. They adopt the β-Yb5Sb3-type structure (Pearson symbol oP32, space group Pnma, Z=4), as revealed through analysis by single-crystal X-ray diffraction on Ce5Si0.869(4)Bi2.131(4) and powder X-ray diffraction on Ce5GeBi2. Cell parameters for the entire series lie in the ranges of a=12.8–11.8 Å, b=9.6–9.0 Å, and c=8.4–7.9 Å. Solid solubility in Ce5SixBi3−x and Pr5SixBi3−x (approximately 0.9≤x≤1.2, depending on the RE member) is much more limited compared to the antimonides, consistent with a highly ordered structure in which the two possible anion sites are essentially segregated into a smaller one occupied by Tt atoms (CN7) and a larger one occupied by Bi atoms (CN9). Band structure calculations on La5SiBi2 confirm the importance of La–La bonding interactions near the Fermi level. X-ray photoelectron spectra support the presence of partially anionic Bi atoms, as indicated by a small negative binding energy shift relative to elemental Bi. The Ce and Pr members undergo magnetic transitions at low temperatures, possibly involving ferromagnetic interactions, that are strongly influenced by the nature of the Tt atom.