Single crystals of Sc 5 Rh 6 Sn 18 were grown from Sn-flux. The crystal structure (SG: I4 1 /acd, a = 13.5529(2) Å, c = 27.0976(7) Å) was studied by high-resolution X-ray diffraction on powder and single crystal material as well as by TEM. All methods confirm it to crystallize with Sc 5 Ir 6 Sn 18 (space group I4 1 /acd) type of structure. The performed structural studies suggest also the presence of local domains with broken average translational symmetry. Analysis of the chemical bonding situation reveal highly polar Sc2-Sn1, Sn-Rh and Sc2-Rh bonds, two-and three-center bonds involving Sn-atoms as well as ionic nature of Sc1 bonding. The thermopower of Sc 5 Rh 6 Sn 18 is isotropic, small and negative (i.e. dominance of electron-like charge carriers). Due to structural disorder, the thermal conductivity is lowered in comparison with regular metallic systems.
Hf
2
B
2–2δ
Ir
5+δ
crystallizes
with a new type of structure: space group
Pbam
,
a
= 5.6300(3) Å,
b
= 11.2599(5)
Å, and
c
= 3.8328(2) Å. Nearly 5% of the
boron pairs are randomly replaced by single iridium atoms (Ir
5+δ
B
2–2δ
). From an analysis of
the chemical bonding, the crystal structure can be understood as a
three-dimensional framework stabilized by covalent two-atom B–B
and Ir–Ir as well as three-atom Ir–Ir–B and Ir–Ir–Ir
interactions. The hafnium atoms center 14-atom cavities and transfer
a significant amount of charge to the polyanionic boron–iridium
framework. This refractory boride displays moderate hardness and is
a Pauli paramagnet with metallic electrical resistivity, Seebeck coefficient,
and thermal conductivity. The metallic character of this system is
also confirmed by electronic structure calculations revealing 5.8
states eV
–1
fu
–1
at the Fermi
level. Zr
2
B
2–2δ
Ir
5+δ
is found to be isotypic with Hf
2
B
2–2δ
Ir
5+δ
, and both form a continuous solid solution.
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