Sb and Se Substitution in CsBi₄Te₆: The Semiconductors CsM₄Q₆ (M = Bi, Sb; Q = Te, Se), Cs₂Bi₁₀Q₁₅, and CsBi₅Q₈

The solid solutions of CsBi₄Te₆, a high ZT material at a low temperature region, with Sb and Se were synthesized with general formulas CsBi_4‑<i>x</i>Sb_<i>x</i>Te₆ and CsBi₄Te_6‑<i>y</i>Se_<i>y</i>. The introduction of Sb and Se in the lattice of CsBi₄Te₆ is possible but only to a limited extent. The Sb and Se atoms substituted are not uniformly distributed over all crystallographic sites but display particular site preferences. The structure of new Sb/Bi solid solutions retains the original framework of CsBi₄Te₆ composed of NaCl-type Bi/Te slabs interconnected by characteristic Bi–Bi bonds and Cs atoms located in the interlayer space. A structurally modified phase in Se/Te solid solutions was found from the reactions targeted for 0.2 < <i>y</i> < 2.4 with the formula of CsBi₅Te_{7.5‑<i>y</i>}Se_<i>y</i> (or Cs₂Bi₁₀Q₁₅, (Q = Se, Te)). The new structure is constructed by the same structural motif with an extended Bi/Te slab (29 Å) compared to that in CsBi₄Te₆ (23 Å). The CsBi₅Te_{7.5‑<i>y</i>}Se_<i>y</i> possesses Bi/Te slabs that extend by an additional “Bi₂Te₃” unit compared to the structure of CsBi₄Te₆, which implies the existence of a phase homology of compounds with the adjustable parameter being the width of the Bi/Q slab. In the reactions targeted for the compounds with higher <i>y</i>, a new phase CsBi₅Te_3.6Se_4.4 with a different type of framework was found. The electrical conductivity and thermopower for the selected samples show p-type conduction with metallic behavior. The room temperature values measured are in the range of 300–1100 S/cm and 100–150 μV/K for Sb-substituted samples and 20–500 S/cm and 70–140 μV/K for Se-substituted samples, respectively. Thermal conductivities of these samples are in the range of 0.9–1.2 W/m·K at room temperature. Tailoring the transport behavior of these materials for thermoelectric applications may be achieved by doping, as is possible for the parent compound CsBi₄Te₆

Superconductivity in the Narrow-Gap Semiconductor CsBi₄Te₆

Superconductivity was discovered in the narrow-gap semiconductor CsBi₄Te₆. A superconducting transition around 4.4 K was observed for p-type samples in temperature-dependent resistivity and magnetic susceptibility data. Stoichiometric CsBi₄Te₆ is not a superconductor. A remarkably high critical field of ∼10 T was estimated from the field-dependent resistivity data. The strongly anisotropic CsBi₄Te₆ system is monoclinic and the first member of a larger homologous series Cs₄[Bi_2<i>n</i>+4Te_3<i>n</i>+6] that exhibits unconventional superconductivity, suggesting that proper doping of the homologous series may create a novel class of superconductors from semiconductors

Metallic Borides, La₂Re₃B₇ and La₃Re₂B₅, Featuring Extensive Boron–Boron Bonding

La₂Re₃B₇ and La₃Re₂B₅ have been synthesized in single-crystalline form from a molten La/Ni eutectic at 1000 °C in the first example of the flux crystal growth of ternary rare-earth rhenium borides. Both compounds crystallize in their own orthorhombic structure types, with La₂Re₃B₇ (space group <i>Pcca</i>) having lattice parameters <i>a</i> = 7.657(2) Å, <i>b</i> = 6.755(1) Å, and <i>c</i> = 11.617(2) Å, and La₃Re₂B₅ (space group <i>Pmma</i>) having lattice parameters <i>a</i> = 10.809(2) Å, <i>b</i> = 5.287(1) Å, and <i>c</i> = 5.747(1) Å. The compounds possess three-dimensional framework structures that are built up from rhenium boride polyhedra and boron–boron bonding. La₃Re₂B₅ features fairly common B₂ dumbbells, whereas La₂Re₃B₇ has unique one-dimensional subunits composed of alternating triangular B₃ and trans-B₄ zigzag chain fragments. Also observed in La₃Re₂B₅ is an unusual coordination of B by an octahedron of La atoms. Electronic band structure calculations predict that La₂Re₃B₇ is a semimetal, which is observed in the electrical resistivity data as measured on single crystals, with behavior obeying the Bloch–Grüneisen model and a room-temperature resistivity ρ_300 K of ∼375 μΩ cm. The electronic band structure calculations also suggest that La₃Re₂B₅ is a regular metal

Superconductivity in the Narrow Gap Semiconductor RbBi_11/3Te₆

Superconductivity was discovered in the layered compound RbBi_11/3Te₆, featuring Bi vacancies and a narrow band gap of 0.25(2) eV at room temperature. A sharp superconducting transition at ∼3.2 K was observed in polycrystalline ingots. The superconducting volume fraction of oriented single crystals is almost 100%, confirming bulk superconductivity. Systematic Se and Sb substitutions in RbBi_{11/3–<i>y</i>}Sb_<i>y</i>Se_<i>x</i>Te_6–<i>x</i> revealed a dependence of the superconducting transition on composition that can increase the <i>T</i>_c up to ∼10%. The RbBi_11/3Te₆ system is the first member of the new homologous series Rb­[Bi_{2<i>n</i>+11/3}Te_3<i>n</i>+6] with infinite Bi₂Te₃-like layers. The large degree of chemical tunability of the electronic structure of the homology via doping and/or substitution gives rise to a new family of superconductors

Superconductivity in the Narrow-Gap Semiconductor CsBi₄Te₆

Superconductivity was discovered in the narrow-gap semiconductor CsBi₄Te₆. A superconducting transition around 4.4 K was observed for p-type samples in temperature-dependent resistivity and magnetic susceptibility data. Stoichiometric CsBi₄Te₆ is not a superconductor. A remarkably high critical field of ∼10 T was estimated from the field-dependent resistivity data. The strongly anisotropic CsBi₄Te₆ system is monoclinic and the first member of a larger homologous series Cs₄[Bi_2<i>n</i>+4Te_3<i>n</i>+6] that exhibits unconventional superconductivity, suggesting that proper doping of the homologous series may create a novel class of superconductors from semiconductors

TlHgInS₃: An Indirect-Band-Gap Semiconductor with X‑ray Photoconductivity Response

The quaternary compound TlHgInS₃ crystallizes in a new structure type of space group, <i>C</i>2/<i>c</i>, with cell parameters <i>a</i> = 13.916(3) Å, <i>b</i> = 3.9132(8) Å, <i>c</i> = 21.403(4) Å, β = 104.16(3)°, <i>V</i> = 1130.1(8) ų, and ρ = 7.241 g/cm³. The structure is a unique three-dimensional framework with parallel tunnels, which is formed by _∞¹[InS₃^3–] infinite chains bridged by linearly coordinated Hg²⁺ ions. TlHgInS₃ is a semiconductor with a band gap of 1.74 eV and a resistivity of ∼4.32 GΩ cm. TlHgInS₃ single crystals exhibit photocurrent response when exposed to Ag X-rays. The mobility-lifetime product (μ<i>τ</i>) of the electrons and holes estimated from the photocurrent measurements are (<i>μτ</i>)_e ≈ 3.6 × 10^–4 cm²/V and (<i>μτ</i>)_h ≈ 2.0 × 10^–4 cm²/V. Electronic structure calculations at the density functional theory level indicate an indirect band gap and a relatively small effective mass for both electrons and holes. Based on the photoconductivity data, TlHgInS₃ is a potential material for radiation detection applications

NaCu₆Se₄: A Layered Compound with Mixed Valency and Metallic Properties

A new ternary compound NaCu₆Se₄ was synthesized from the reaction of Cu in a molten sodium polyselenide flux. The compound crystallizes in trigonal space group <i>R</i>3̅<i>m</i> with <i>a</i> = 4.0465(3) Å and <i>c</i> = 41.493(5) Å. The crystal structure contains flat two-dimensional slabs of ¹/_∞[Cu₆Se₄] with a unique structural arrangement, separated by Na cations. The compound contains mixed valency and has a high conductivity of ∼3 × 10³ S cm^–1 at room temperature, and exhibits increasing conductivity with decreasing temperature, indicating metallic behavior. A small positive thermopower (4–11 μV K^–1 from 300 to 500 K) and Hall effect measurements indicate p-type transport with a carrier concentration of ∼2.8(3) × 10²¹ cm^–3 and a hole mobility of ∼8.75 cm² V^–1 s^–1 at 300 K. NaCu₆Se₄ exhibits temperature-independent Pauli paramagnetism

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Mixed-Valent NaCu₄Se₃: A Two-Dimensional Metal

The new ternary copper selenide NaCu₄Se₃ crystallizes in the RbCd₄As₃ structure type with the trigonal space group <i>R</i>3̅<i>m</i> and lattice constants <i>a</i> = 4.0316(4) Å and <i>c</i> = 31.438(8) Å. Its structure is built from two-dimensional slabs of ²/_∞[Cu₄Se₃] separated by Na⁺ cations. The compound is formally mixed-valent with Se^2–/Se^– atoms and exhibits metallic properties. It is a hole conductor with an electrical conductivity of ∼300 S cm^–1 at room temperature and a thermopower of ∼10 μV K^–1. Hall effect measurements indicate holes as the dominant carrier with a concentration of ∼6.12(1) × 10²¹ cm^–3 at 300 K. Density functional theory electronic structure calculations indicate p-type metallic behavior for the ²/_∞[Cu₄Se₃] framework, which is in a good agreement with the experimental metallic conductivity and Pauli paramagnetism

Mixed-Valent NaCu₄Se₃: A Two-Dimensional Metal

The new ternary copper selenide NaCu₄Se₃ crystallizes in the RbCd₄As₃ structure type with the trigonal space group <i>R</i>3̅<i>m</i> and lattice constants <i>a</i> = 4.0316(4) Å and <i>c</i> = 31.438(8) Å. Its structure is built from two-dimensional slabs of ²/_∞[Cu₄Se₃] separated by Na⁺ cations. The compound is formally mixed-valent with Se^2–/Se^– atoms and exhibits metallic properties. It is a hole conductor with an electrical conductivity of ∼300 S cm^–1 at room temperature and a thermopower of ∼10 μV K^–1. Hall effect measurements indicate holes as the dominant carrier with a concentration of ∼6.12(1) × 10²¹ cm^–3 at 300 K. Density functional theory electronic structure calculations indicate p-type metallic behavior for the ²/_∞[Cu₄Se₃] framework, which is in a good agreement with the experimental metallic conductivity and Pauli paramagnetism