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

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 I4/m symmetry while the Ca2RE8Sb3O10 ones adopt the monoclinic C2/m 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.