Synthesis, crystal and electronic structures of new narrow-band-gap semiconducting antimonide oxides RE₃SbO₃ and RE ₈Sb_3-δO₈, with RE = La, Sm, Gd, and Ho

In the search for high-temperature thermoelectric materials, two families of novel, narrow-band-gap semiconducting antimonide oxides with the compositions RE₃SbO₃ and RE₈Sb_3-δO ₈ (RE = La, Sm, Gd, Ho) have been discovered. Their synthesis was motivated by attempts to open a band gap in the semimetallic RESb binaries through a chemical fusion of RESb and corresponding insulating RE ₂O₃. Temperatures of 1350 °C or higher are required to obtain these phases. Both RE₃SbO₃ and RE ₈Sb_3-δO₈ adopt new monoclinic structures with the C2/m space group and feature similar REO frameworks composed of "RE₄O" tetrahedral units. In both structures, the Sb atoms occupy the empty channels within the REO sublattice. High-purity bulk Sm and Ho samples were prepared and subjected to electrical resistivity measurements. Both the RE₃SbO₃ and RE₈Sb _3-δO₈ (RE = Sm, Ho) phases exhibit a semiconductor-type electrical behavior. While a small band gap in RE ₃SbO₃ results from the separation of the valence and conduction bands, a band gap in RE₈Sb_3-δO ₈ appears to result from the Anderson localization of electrons. The relationship among the composition, crystal structures, and electrical resistivity is analyzed using electronic structure calculations.