Valence Compounds versus Metals. Synthesis, Characterization, and Electronic Structures of Cubic Ae4Pn3 Phases in the Systems Ae = Ca, Sr, Ba, Eu; Pn = As, Sb, Bi

摘要

The isostructural compounds Sr4Bi3, Ba4Bi3, and Ba4As∼2.60 were prepared by direct reactions of the corresponding elements and their structures determined from single-crystal X-ray diffraction data as anti-Th3P4 type in the cubic space group I4̄3d, Z = 4 (a = 10.101(1) Å, 10.550(1) Å, 9.973 (1) Å, respectively). The two bismuth compounds are stoichiometric, and the arsenide refines as Ba4As2.60(2). Only unrelated phases are obtained for all binary combinations among the title components for either Ca or Sb. The magnetic susceptibility and resistivities of Ba4Bi3 and Eu4Bi3 show that they are good metallic conductors (∼40 μΩ·cm at 298 K), whereas Ba4As2.60 exhibits ρ150 u3e 1000 μΩ·cm. The electronic structures of Sr4Bi3, Ba4Bi3, and Ba4As3 were calculated by TB-LMTO-ASA methods. Mixing of cation d states into somewhat disperse valence p bands on Bi results in empty bands at EF and metallic behavior, whereas the narrower valence band in the electron-deficient Ba4As3 leads to vacancies in about 11% of the anion sites and a valence compound.