Synthesis, Structure, and Basic Magnetic and Thermoelectric Properties of the Light Lanthanide Aurobismuthides

We report the crystal structures and elementary properties of the new aurobismuthides La₃Au₃Bi₄, Ce₃Au₃Bi₄, Pr₃Au₃Bi₄, Nd₃Au₃Bi₄, Sm₃Au₃Bi₄, and Gd₃Au₃Bi₄. These ternary compounds are found only for the large lanthanides and crystallize in the cubic Y₃Au₃Sb₄ structure type, which is a stuffed Th₃P₄-type derivative. The compounds are electron-precise, leading to semiconducting behavior, and display magnetic properties arising from localized lanthanide <i>f</i> states. Resistivity data, Seebeck coefficient measurements, and electronic structure calculations suggest that these phases are heavily doped, p-type semiconductors. Nd₃Au₃Bi₄ and Sm₃Au₃Bi₄ have Seebeck coefficients of 105 and 190 μV/K at 350 K, respectively, making them worthy of further thermoelectric studies

Structure and Magnetic Properties of the Spin-1/2-Based Honeycomb NaNi₂BiO_6‑δ and Its Hydrate NaNi₂BiO_6‑δ·1.7H₂O

We present the structure and magnetic properties of the honeycomb anhydrate NaNi₂BiO_6‑δ and its monolayer hydrate NaNi₂BiO_6‑δ·1.7H₂O, synthesized by deintercalation of the layered α-NaFeO₂-type honeycomb compound Na₃Ni₂BiO₆. The anhydrate adopts ABAB-type oxygen packing and a one-layer hexagonal unit cell, whereas the hydrate adopts an oxygen packing sequence based on a three-layer rhombohedral subcell. The metal-oxide layer separations are 5.7 Å in the anhydrate and 7.1 Å in the hydrate, making the hydrate a quasi 2-D honeycomb system. The compounds were characterized through single crystal diffraction, powder X-ray diffraction, thermogravimetric analysis, and elemental analysis. Temperature-dependent magnetic susceptibility measurements show both to have negative Weiss temperatures (−18.5 and −14.6 K, respectively) and similar magnetic moments (2.21 and 2.26 μ_B/Ni, respectively), though the field-dependent magnetization and heat capacity data suggest subtle differences in their magnetic behavior. The magnetic moments per Ni are relatively high, which we suggest is due to the presence of a mixture of Ni²⁺ and Ni³⁺ caused by oxygen vacancies

Structure and Magnetic Properties of the α‑NaFeO₂‑Type Honeycomb Compound Na₃Ni₂BiO₆

We present the structure and magnetic properties of Na₃Ni₂BiO₆, which is an ordered variant of the α-NaFeO₂ structure type. This layered compound has a 2:1 ordering of (Ni²⁺/Bi⁵⁺)­O₆ octahedra within the <i>a-b</i> plane and sodium in octahedra between the layers. The structure is presented in the space group <i>C</i>2/<i>m</i>, determined through a combination of single crystal X-ray, powder neutron, and powder X-ray diffraction. Temperature dependent magnetic susceptibility measurements show Na₃Ni₂BiO₆ to display long-range antiferromagnetic ordering below 11 K, despite the dominance of ferromagnetic interactions above <i>T</i>_N as indicated by a positive Weiss constant. Heat capacity measurements and low-temperature neutron diffraction support the magnetic ordering and are consistent with a <i>T</i>_N of 10.4 K. A magnetic phase can be refined with (010) antiferromagnetic ordering along the <i>b</i>-axis in the honeycomb layer and moments aligned parallel to <i>c</i>. The compounds Na₃Mg₂BiO₆ and Na₃Zn₂BiO₆, synthesized as nonmagnetic analogues of Na₃Ni₂BiO₆, are briefly described

Gold–Gold Bonding: The Key to Stabilizing the 19-Electron Ternary Phases <i>Ln</i>AuSb (<i>Ln</i> = La–Nd and Sm)

We report a new family of ternary 111 hexagonal <i>Ln</i>AuSb (<i>Ln</i> = La–Nd, Sm) compounds that, with a 19 valence electron count, has one extra electron compared to all other known <i>Ln</i>AuZ compounds. LaAuSb, CeAuSb, PrAuSb, NdAuSb, and SmAuSb crystallize in the YPtAs-type structure, and have a doubled unit cell compared to other <i>Ln</i>AuZ phases as a result of the buckling of the Au–Sb honeycomb layers to create interlayer Au–Au dimers. The dimers accommodate the one excess electron per Au and thus these new phases can be considered <i>Ln</i>₂³⁺(Au–Au)⁰Sb₂^3–. Band structure, density of states, and crystal orbital calculations confirm this picture, which results in a nearly complete band gap between full and empty electronic states and stable compounds; we can thus present a structural stability phase diagram for the <i>Ln</i>Au<i>Z</i> (Z = Ge, As, Sn, Sb, Pb, Bi) family of phases. Those calculations also show that LaAuSb has a bulk Dirac cone below the Fermi level. The YPtAs-type <i>Ln</i>AuSb family reported here is an example of the uniqueness of gold chemistry applied to a rigidly closed shell system in an unconventional way

Trivalent Iridium Oxides: Layered Triangular Lattice Iridate K_0.75Na_0.25IrO₂ and Oxyhydroxide IrOOH

Solid oxides with transition-metal ions in unusual oxidation states attract enormous attention due to their electronic, magnetic, and catalytic properties. Yet, no crystalline oxide compounds based on purely trivalent iridium have been characterized to date. Here, we present the synthesis and thorough investigation of the properties of the compounds K_0.75Na_0.25IrO₂ and IrOOH, both containing trivalent iridium on a triangular lattice in layers of [IrO₂]⁻. K_0.75Na_0.25IrO₂ crystallizes in a P2-structure with the space group <i>P</i>6₃/<i>mmc</i>, while the crystal structure of IrOOH adopts the direct maximal subgroup <i>P</i>3̅<i>m</i>1. The trivalent state of the iridium ion is discussed with regards to the iridium–oxygen bond length from X-ray diffraction, the chemical composition, the electronic and magnetic behavior of the material, and X-ray photoemission spectroscopy. The discovery of a new valence state in ternary crystalline iridium oxides is not only of interest from a fundamental perspective, but also has far-reaching implications for such diverse fields as electrochromism, solid-state magnetism, and especially heterogeneous catalysis

Differences in Chemical Doping Matter: Superconductivity in Ti_1–<i>x</i>Ta_<i>x</i>Se₂ but Not in Ti_1–<i>x</i>Nb_<i>x</i>Se₂

We report that 1T-TiSe₂, an archetypical layered transition metal dichalcogenide, becomes superconducting when Ta is substituted for Ti but not when Nb is substituted for Ti. This is unexpected because Nb and Ta should be chemically equivalent electron donors. Superconductivity emerges near <i>x</i> = 0.02 for Ti_1–<i>x</i>Ta_<i>x</i>Se₂, while, for Ti_1–<i>x</i>Nb_<i>x</i>Se₂, no superconducting transitions are observed above 0.4 K. The equivalent chemical nature of the dopants is confirmed by X-ray photoelectron spectroscopy. ARPES and Raman scattering studies show similarities and differences between the two systems, but the fundamental reasons why the Nb and Ta dopants yield such different behavior are unknown. We present a comparison of the electronic phase diagrams of many electron-doped 1T-TiSe₂ systems, showing that they behave quite differently, which may have broad implications in the search for new superconductors. We propose that superconducting Ti_0.8Ta_0.2Se₂ will be suitable for devices and other studies based on exfoliated crystal flakes