Synthesis, Crystal Structure, and Electronic Properties of the CaRE₃SbO₄ and Ca₂RE₈Sb₃O₁₀ phases (RE = Rare-Earth Metal)

Abstract

Through high temperature synthesis at 1300 °C and above, our group has discovered and characterized the novel CaRE₃SbO₄ and Ca₂RE₈Sb₃O₁₀ phases (RE = Ce–Nd, Sm–Dy for CaRE₃SbO₄, RE = La–Nd, Sm–Dy for Ca₂RE₈Sb₃O₁₀). This result was motivated by the idea of opening a band gap and introducing structural complexity in the rare-earth antimonide framework by incorporation of rare-earth oxide and calcium oxide. The CaRE₃SbO₄ phases adopt the tetragonal <i>I</i>4/<i>m</i> symmetry while the Ca₂RE₈Sb₃O₁₀ ones adopt the monoclinic <i>C</i>2/<i>m</i> symmetry. These structures show many similarities to the other RE–Sb–O phases discovered recently, particularly to the RE₃SbO₃ and RE₈Sb₃O₈ phases, in which a prolonged heat treatment results in one structure converting to another by elongation of the rare-earth oxide slabs. Electrical resistivity measurements yielded semiconducting properties for both series, despite the unbalanced electron count for Ca₂RE₈Sb₃O₁₀ and electronic structure calculations that support metallic-type conduction. This unusual behavior is attributed to Anderson-type localization of Sb p states near the Fermi level, which arises from the highly disordered Sb layers in the structure. This Sb disorder was shown to be tunable with respect to the size of the rare-earth used, improving the electrical resistivity by approximately 1 order of magnitude for each rare-earth in the series