3 research outputs found
Dumbbells of Five-Connected Ge Atoms and Superconductivity in CaGe<sub>3</sub>
CaGe<sub>3</sub> has been synthesized at high-pressure,
high-temperature
conditions. The atomic pattern comprises intricate germanium layers
of condensed moleculelike dimers. Below <i>T</i><sub>c</sub> = 6.8 K, type II superconductivity with moderately strong electron–phonon
coupling is observed
Cluster Formation in the Superconducting Complex Intermetallic Compound Be<sub>21</sub>Pt<sub>5</sub>
ConspectusMaterials with the crystal structure of γ-brass type (Cu<sub>5</sub>Zn<sub>8</sub> type) are typical representatives of intermetallic
compounds. From the electronic point of view, they are often interpreted
using the valence electron concentration approach of Hume–Rothery,
developed previously for transition metals. The γ-brass-type
phases of the main-group elements are rather rare. The intermetallic
compound Be<sub>21</sub>Pt<sub>5</sub>, a new member of this family,
was synthesized, and its crystal structure, chemical bonding, and
physical properties were characterized.Be<sub>21</sub>Pt<sub>5</sub> crystallizes in the cubic space group <i>F</i>4Ì…3<i>m</i> with lattice parameter <i>a</i> = 15.90417(3)
Ã… and 416 atoms per unit cell. From
the crystallographic point of view, the binary substance represents
a special family of intermetallic compounds called complex metallic
alloys (CMA). The crystal structure was solved by a combination of
synchrotron and neutron powder diffraction data. Besides the large
difference in the scattering power of the components, the structure
solution was hampered by the systematic presence of very weak reflections
mimicking wrong symmetry. The structural motif of Be<sub>21</sub>Pt<sub>5</sub> is described as a 2 × 2 × 2 superstructure of the
γ-brass structure (Cu<sub>5</sub>Zn<sub>8</sub> type) or 6 ×
6 × 6 superstructure of the simple bcc structural pattern with
distinct distribution of defects. The main building elements of the
crystal structure are four types of nested polyhedral units (clusters)
with the compositions Be<sub>22</sub>Pt<sub>4</sub> and Be<sub>20</sub>Pt<sub>6</sub>. Each cluster contains four shells (4 + 4 + 6 + 12
atoms). Clusters with different compositions reveal various occupation
of the shells by platinum and beryllium. Polyhedral nested units with
the same composition differ by the distance of the shell atoms to
the cluster center.Analysis of chemical bonding was made applying
the electron localizability
approach, a quantum chemical technique operating in real space that
is proven to be especially efficient for intermetallic compounds.
Evaluations of the calculated electron density and electron localizability
indicator (ELI-D) revealed multicenter bonding, being in accordance
with the low valence electron count per atom in Be<sub>21</sub>Pt<sub>5</sub>. A new type of atomic interactions in intermetallic compounds,
cluster bonds involving 8 or even 14 atoms, is found in the clusters
with shorter distances between the shell atoms and the cluster centers.
In the remaining clusters, four- and five-center bonds characterize
the atomic interactions. Multicluster interactions within the polyhedral
nested units and three-center polar intercluster bonds result in a
three-dimensional framework resembling the structural pattern of NaCl.
Be<sub>21</sub>Pt<sub>5</sub> is a diamagnetic metal and one of rather
rare CMA compounds revealing superconductivity (<i>T</i><sub>c</sub> = 2.06 K)
Dumbbells of Five-Connected Silicon Atoms and Superconductivity in the Binary Silicides MSi<sub>3</sub> (M = Ca, Y, Lu)
The new metastable binary silicides MSi<sub>3</sub> (M
= Ca, Y,
Lu) have been synthesized by high-pressure, high-temperature reactions
at pressures between 12(2) and 15(2) GPa and temperatures from 900(100)
to 1400(150) K. The atomic patterns comprise intricate silicon layers
of condensed molecule-like Si<sub>2</sub> dimers. The alkaline-earth
element adopts the oxidation state +2, while the rare-earth and transition
metals realize +3. All of the compounds exhibit BCS-type superconductivity
with weak electron–phonon coupling below critical temperatures
of up to 7 K