45 research outputs found
Sb and Se Substitution in CsBi<sub>4</sub>Te<sub>6</sub>: The Semiconductors CsM<sub>4</sub>Q<sub>6</sub> (M = Bi, Sb; Q = Te, Se), Cs<sub>2</sub>Bi<sub>10</sub>Q<sub>15</sub>, and CsBi<sub>5</sub>Q<sub>8</sub>
The solid solutions of CsBi<sub>4</sub>Te<sub>6</sub>, a high ZT material at a low temperature region, with Sb and Se
were synthesized with general formulas CsBi<sub>4‑<i>x</i></sub>Sb<sub><i>x</i></sub>Te<sub>6</sub> and CsBi<sub>4</sub>Te<sub>6‑<i>y</i></sub>Se<sub><i>y</i></sub>. The introduction of Sb and Se in the lattice of CsBi<sub>4</sub>Te<sub>6</sub> 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 CsBi<sub>4</sub>Te<sub>6</sub> 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 CsBi<sub>5</sub>Te<sub>7.5‑<i>y</i></sub>Se<sub><i>y</i></sub> (or Cs<sub>2</sub>Bi<sub>10</sub>Q<sub>15</sub>, (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 CsBi<sub>4</sub>Te<sub>6</sub> (23
Å). The CsBi<sub>5</sub>Te<sub>7.5‑<i>y</i></sub>Se<sub><i>y</i></sub> possesses Bi/Te slabs that extend
by an additional “Bi<sub>2</sub>Te<sub>3</sub>” unit
compared to the structure of CsBi<sub>4</sub>Te<sub>6</sub>, 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 CsBi<sub>5</sub>Te<sub>3.6</sub>Se<sub>4.4</sub> 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
CsBi<sub>4</sub>Te<sub>6</sub>
Superconductivity in the Narrow-Gap Semiconductor CsBi<sub>4</sub>Te<sub>6</sub>
Superconductivity was discovered
in the narrow-gap semiconductor
CsBi<sub>4</sub>Te<sub>6</sub>. A superconducting transition around
4.4 K was observed for p-type samples in temperature-dependent resistivity
and magnetic susceptibility data. Stoichiometric CsBi<sub>4</sub>Te<sub>6</sub> is not a superconductor. A remarkably high critical field
of ∼10 T was estimated from the field-dependent resistivity
data. The strongly anisotropic CsBi<sub>4</sub>Te<sub>6</sub> system
is monoclinic and the first member of a larger homologous series Cs<sub>4</sub>[Bi<sub>2<i>n</i>+4</sub>Te<sub>3<i>n</i>+6</sub>] that exhibits unconventional superconductivity, suggesting
that proper doping of the homologous series may create a novel class
of superconductors from semiconductors
Metallic Borides, La<sub>2</sub>Re<sub>3</sub>B<sub>7</sub> and La<sub>3</sub>Re<sub>2</sub>B<sub>5</sub>, Featuring Extensive Boron–Boron Bonding
La<sub>2</sub>Re<sub>3</sub>B<sub>7</sub> and La<sub>3</sub>Re<sub>2</sub>B<sub>5</sub> 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 La<sub>2</sub>Re<sub>3</sub>B<sub>7</sub> (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 La<sub>3</sub>Re<sub>2</sub>B<sub>5</sub> (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.
La<sub>3</sub>Re<sub>2</sub>B<sub>5</sub> features fairly common B<sub>2</sub> dumbbells, whereas La<sub>2</sub>Re<sub>3</sub>B<sub>7</sub> has unique one-dimensional subunits composed of alternating triangular
B<sub>3</sub> and trans-B<sub>4</sub> zigzag chain fragments. Also
observed in La<sub>3</sub>Re<sub>2</sub>B<sub>5</sub> is an unusual
coordination of B by an octahedron of La atoms. Electronic band structure
calculations predict that La<sub>2</sub>Re<sub>3</sub>B<sub>7</sub> 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<sub>300 K</sub> of ∼375 μΩ cm. The electronic band structure
calculations also suggest that La<sub>3</sub>Re<sub>2</sub>B<sub>5</sub> is a regular metal
Superconductivity in the Narrow Gap Semiconductor RbBi<sub>11/3</sub>Te<sub>6</sub>
Superconductivity
was discovered in the layered compound RbBi<sub>11/3</sub>Te<sub>6</sub>, 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<sub>11/3–<i>y</i></sub>Sb<sub><i>y</i></sub>Se<sub><i>x</i></sub>Te<sub>6–<i>x</i></sub> revealed a dependence of the superconducting transition on
composition that can increase the <i>T</i><sub>c</sub> up
to ∼10%. The RbBi<sub>11/3</sub>Te<sub>6</sub> system is the
first member of the new homologous series Rb[Bi<sub>2<i>n</i>+11/3</sub>Te<sub>3<i>n</i>+6</sub>] with infinite Bi<sub>2</sub>Te<sub>3</sub>-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 CsBi<sub>4</sub>Te<sub>6</sub>
Superconductivity was discovered
in the narrow-gap semiconductor
CsBi<sub>4</sub>Te<sub>6</sub>. A superconducting transition around
4.4 K was observed for p-type samples in temperature-dependent resistivity
and magnetic susceptibility data. Stoichiometric CsBi<sub>4</sub>Te<sub>6</sub> is not a superconductor. A remarkably high critical field
of ∼10 T was estimated from the field-dependent resistivity
data. The strongly anisotropic CsBi<sub>4</sub>Te<sub>6</sub> system
is monoclinic and the first member of a larger homologous series Cs<sub>4</sub>[Bi<sub>2<i>n</i>+4</sub>Te<sub>3<i>n</i>+6</sub>] that exhibits unconventional superconductivity, suggesting
that proper doping of the homologous series may create a novel class
of superconductors from semiconductors
TlHgInS<sub>3</sub>: An Indirect-Band-Gap Semiconductor with X‑ray Photoconductivity Response
The quaternary compound TlHgInS<sub>3</sub> 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) Å<sup>3</sup>, and ρ = 7.241 g/cm<sup>3</sup>. The structure is a
unique three-dimensional framework with parallel tunnels, which is
formed by <sub>∞</sub><sup>1</sup>[InS<sub>3</sub><sup>3–</sup>] infinite chains bridged
by linearly coordinated Hg<sup>2+</sup> ions. TlHgInS<sub>3</sub> is
a semiconductor with a band gap of 1.74 eV and a resistivity of ∼4.32
GΩ cm. TlHgInS<sub>3</sub> 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>)<sub>e</sub> ≈ 3.6 × 10<sup>–4</sup> cm<sup>2</sup>/V and (<i>μτ</i>)<sub>h</sub> ≈ 2.0
× 10<sup>–4</sup> cm<sup>2</sup>/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, TlHgInS<sub>3</sub> is a potential material for radiation detection applications
NaCu<sub>6</sub>Se<sub>4</sub>: A Layered Compound with Mixed Valency and Metallic Properties
A new
ternary compound NaCu<sub>6</sub>Se<sub>4</sub> 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 <sup>1</sup>/<sub>∞</sub>[Cu<sub>6</sub>Se<sub>4</sub>] with a unique structural arrangement, separated by
Na cations. The compound contains mixed valency and has a high conductivity
of ∼3 × 10<sup>3</sup> S cm<sup>–1</sup> at room
temperature, and exhibits increasing conductivity with decreasing
temperature, indicating metallic behavior. A small positive thermopower
(4–11 μV K<sup>–1</sup> from 300 to 500 K) and
Hall effect measurements indicate p-type transport with a carrier
concentration of ∼2.8(3) × 10<sup>21</sup> cm<sup>–3</sup> and a hole mobility of ∼8.75 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> at 300 K. NaCu<sub>6</sub>Se<sub>4</sub> exhibits
temperature-independent Pauli paramagnetism
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Mixed-Valent NaCu<sub>4</sub>Se<sub>3</sub>: A Two-Dimensional Metal
The
new ternary copper selenide NaCu<sub>4</sub>Se<sub>3</sub> crystallizes
in the RbCd<sub>4</sub>As<sub>3</sub> 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 <sup>2</sup>/<sub>∞</sub>[Cu<sub>4</sub>Se<sub>3</sub>] separated by Na<sup>+</sup> cations. The compound is formally mixed-valent
with Se<sup>2–</sup>/Se<sup>–</sup> atoms and exhibits
metallic properties. It is a hole conductor with an electrical conductivity
of ∼300 S cm<sup>–1</sup> at room temperature and a
thermopower of ∼10 μV K<sup>–1</sup>. Hall effect
measurements indicate holes as the dominant carrier with a concentration
of ∼6.12(1) × 10<sup>21</sup> cm<sup>–3</sup> at
300 K. Density functional theory electronic structure calculations
indicate p-type metallic behavior for the <sup>2</sup>/<sub>∞</sub>[Cu<sub>4</sub>Se<sub>3</sub>] framework, which is in a good agreement
with the experimental metallic conductivity and Pauli paramagnetism
Mixed-Valent NaCu<sub>4</sub>Se<sub>3</sub>: A Two-Dimensional Metal
The
new ternary copper selenide NaCu<sub>4</sub>Se<sub>3</sub> crystallizes
in the RbCd<sub>4</sub>As<sub>3</sub> 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 <sup>2</sup>/<sub>∞</sub>[Cu<sub>4</sub>Se<sub>3</sub>] separated by Na<sup>+</sup> cations. The compound is formally mixed-valent
with Se<sup>2–</sup>/Se<sup>–</sup> atoms and exhibits
metallic properties. It is a hole conductor with an electrical conductivity
of ∼300 S cm<sup>–1</sup> at room temperature and a
thermopower of ∼10 μV K<sup>–1</sup>. Hall effect
measurements indicate holes as the dominant carrier with a concentration
of ∼6.12(1) × 10<sup>21</sup> cm<sup>–3</sup> at
300 K. Density functional theory electronic structure calculations
indicate p-type metallic behavior for the <sup>2</sup>/<sub>∞</sub>[Cu<sub>4</sub>Se<sub>3</sub>] framework, which is in a good agreement
with the experimental metallic conductivity and Pauli paramagnetism