Bi₃Cr_2.91O₁₁: A Ferromagnetic Insulator from Cr⁴⁺/Cr⁵⁺ Mixing

The search for materials with ferromagnetic and semiconducting/insulating properties has intensified recently because of their potential use in spintronics. However, the number of materials is rather limited because of conflicting requirements needed for the appearance of ferromagnetic and insulating properties. Here we show that Bi₃Cr_2.91O₁₁ belongs to the scarce family of ferromagnetic insulators. Bi₃Cr_2.91O₁₁ was synthesized at high pressure of 6 GPa and high temperature of 1570 K. Its crystal structure and properties were studied using single crystals. It crystallizes in the KSbO₃-type structure with space group <i>Pn</i>3̅ and the lattice parameter <i>a</i> = 9.2181­(2) Å. Bi₃Cr_2.91O₁₁ has almost a 1:1 mixture of Cr⁴⁺ and Cr⁵⁺ ions distributed in one octahedral crystallographic site. Bi₃Cr_2.91O₁₁ is a rare example of oxides having chromium ions in unusual oxidation states. The presence of Cr⁴⁺ and Cr⁵⁺ results in ferromagnetic properties with ferromagnetic Curie temperature <i>T</i>_C = 220 K

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 <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

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 <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