5 research outputs found
Synthesis, Structure, and Electrical Properties of the Mo<sub>12</sub> Cluster Sulfide Hg<sub>∼2.8</sub>KMo<sub>12</sub>S<sub>14</sub> Containing Mercury Chains
The
new compound Hg<sub>∼2.8</sub>KMo<sub>12</sub>S<sub>14</sub> was synthesized by diffusing mercury into the metastable KMo<sub>12</sub>S<sub>14</sub> compound at 350 °C. Its crystallographic
structure, solved from single-crystal X-ray diffraction, shows that
the Mo–S framework is maintained during the synthesis. It is
based on Mo<sub>12</sub>S<sub>14</sub>S<sub>6</sub> units interlinked
via Mo–S bonds as in the parent compound. The mercury forms
linear chains with Hg–Hg distances of about 2.72 Å in
the tunnels delimited by the Mo<sub>12</sub>S<sub>14</sub>S<sub>6</sub> units. Superconductivity was observed below 2.5 K by electrical
resistivity and magnetic susceptibility measurements
Cu Insertion Into the Mo<sub>12</sub> Cluster Compound Cs<sub>2</sub>Mo<sub>12</sub>Se<sub>14</sub>: Synthesis, Crystal and Electronic Structures, and Physical Properties
Mo-based cluster compounds are promising
materials for high-temperature thermoelectric applications due to
their intrinsic, extremely low thermal conductivity values. In this
study, polycrystalline cluster compounds Cs<sub>2</sub>Cu<sub><i>x</i></sub>Mo<sub>12</sub>Se<sub>14</sub> were prepared for
a wide range of Cu contents (0 ≤ <i>x</i> ≤
2). All samples crystallize isostructurally in the trigonal space
group <i>R</i>3̅. The position of the Cu atoms in
the unit cell was determined by X-ray diffraction on a single-crystalline
specimen indicating that these atoms fill the empty space between
the Mo–Se clusters. Density functional theory calculations
predict a metallic ground state for all compositions, in good agreement
with the experimental findings. Magnetization measurements indicate
a rapid suppression of the superconducting state that develops in
the <i>x</i> = 0.0 sample upon Cu insertion. Transport properties
measurements, performed in a wide temperature range (2–630
K) on the two end-member compounds <i>x</i> = 0 and <i>x</i> = 2, revealed a multiband electrical conduction as shown
by sign reversal of the thermopower as a function of temperature
Cu Insertion Into the Mo<sub>12</sub> Cluster Compound Cs<sub>2</sub>Mo<sub>12</sub>Se<sub>14</sub>: Synthesis, Crystal and Electronic Structures, and Physical Properties
Mo-based cluster compounds are promising
materials for high-temperature thermoelectric applications due to
their intrinsic, extremely low thermal conductivity values. In this
study, polycrystalline cluster compounds Cs<sub>2</sub>Cu<sub><i>x</i></sub>Mo<sub>12</sub>Se<sub>14</sub> were prepared for
a wide range of Cu contents (0 ≤ <i>x</i> ≤
2). All samples crystallize isostructurally in the trigonal space
group <i>R</i>3̅. The position of the Cu atoms in
the unit cell was determined by X-ray diffraction on a single-crystalline
specimen indicating that these atoms fill the empty space between
the Mo–Se clusters. Density functional theory calculations
predict a metallic ground state for all compositions, in good agreement
with the experimental findings. Magnetization measurements indicate
a rapid suppression of the superconducting state that develops in
the <i>x</i> = 0.0 sample upon Cu insertion. Transport properties
measurements, performed in a wide temperature range (2–630
K) on the two end-member compounds <i>x</i> = 0 and <i>x</i> = 2, revealed a multiband electrical conduction as shown
by sign reversal of the thermopower as a function of temperature
X‑ray Characterization, Electronic Band Structure, and Thermoelectric Properties of the Cluster Compound Ag<sub>2</sub>Tl<sub>2</sub>Mo<sub>9</sub>Se<sub>11</sub>
We report on a detailed investigation
of the crystal and electronic band structures and of the transport
and thermodynamic properties of the Mo-based cluster compound Ag<sub>2</sub>Tl<sub>2</sub>Mo<sub>9</sub>Se<sub>11</sub>. This novel structure
type crystallizes in the trigonal space group <i>R</i>3̅<i>c</i> and is built of a three-dimensional network of interconnected
Mo<sub>9</sub>Se<sub>11</sub> units. Single-crystal X-ray diffraction
indicates that the Ag and Tl atoms are distributed in the voids of
the cluster framework, both of which show unusually large anisotropic
thermal ellipsoids indicative of strong local disorder. First-principles
calculations show a weakly dispersive band structure around the Fermi
level as well as a semiconducting ground state. The former feature
naturally explains the presence of both hole-like and electron-like
signals observed in Hall effect. Of particular interest is the very
low thermal conductivity that remains quasi-constant between 150 and
800 K at a value of approximately 0.6 W·m<sup>–1</sup>·K<sup>–1</sup>. The lattice thermal conductivity is
close to its minimum possible value, that is, in a regime where the
phonon mean free path nears the mean interatomic distance. Such extremely
low values likely originate from the disorder induced by the Ag and
Tl atoms giving rise to strong anharmonicity of the lattice vibrations.
The strongly limited ability of this compound to transport heat is
the key feature that leads to a dimensionless thermoelectric figure
of merit <i>ZT</i> of 0.6 at 800 K
Synthesis, Crystal and Electronic Structures, and Thermoelectric Properties of the Novel Cluster Compound Ag<sub>3</sub>In<sub>2</sub>Mo<sub>15</sub>Se<sub>19</sub>
Polycrystalline samples and single crystals of the new
compound
Ag<sub>3</sub>In<sub>2</sub>Mo<sub>15</sub>Se<sub>19</sub> were synthesized
by solid-state reaction in a sealed molybdenum crucible at 1300 °C.
Its crystal structure (space group <i>R</i>3̅<i>c</i>, <i>a</i> = 9.9755(1) Å, <i>c</i> = 57.2943(9) Å, and <i>Z</i> = 6) was determined
from single-crystal X-ray diffraction data and constitutes an Ag-filled
variant of the In<sub>2</sub>Mo<sub>15</sub>Se<sub>19</sub> structure-type
containing octahedral Mo<sub>6</sub> and bioctahedral Mo<sub>9</sub> clusters in a 1:1 ratio. The increase of the cationic charge transfer
due to the Ag insertion induces a modification of the Mo–Mo
distances within the Mo clusters that is discussed with regard to
the electronic structure. Transport properties were measured in a
broad temperature range (2–1000 K) to assess the thermoelectric
potential of this compound. The transport data indicate an electrical
conduction dominated by electrons below 25 K and by holes above this
temperature. The metallic character of the transport properties in
this material is consistent with electronic band structure calculations
carried out using the linear muffin-tin orbital (LMTO) method. The
complex unit cell, together with the cagelike structure of this material,
results in very low thermal conductivity values (0.9 W m<sup>–1</sup> K<sup>–1</sup> at 300 K), leading to a maximum estimated
thermoelectric figure of merit (<i>ZT</i>) of 0.45 at 1100
K