We
present the crystallographic analysis, superconducting characterization
and theoretical modeling of LiBi, that contains the lightest and the
heaviest nonradioactive metal. The compound crystallizes in a tetragonal
(CuAu-type) crystal structure with Bi square nets separated by Li
planes (parameters
a
= 3.3636(1) Å and
c
= 4.2459(2) Å, c/a = 1.26). Superconducting state
was studied in detail by magnetic susceptibility and heat capacity
measurements. The results reveal that LiBi is a moderately coupled
type-I superconductor (λ
e-p
= 0.66) with
T
c
= 2.48 K and a thermodynamic critical field
H
c
(0) = 157 Oe. Theoretical studies show that bismuth square
net is responsible for superconductivity in this compound, but the
coupling between the Li planes and Bi planes makes a significant contribution
to the superconductivity.
Polycrystalline sample of superconducting ThIr 3 was obtained by arc-melting Th and Ir metals. Powder x-ray diffraction revealed that the compound crystalizes in a rhombohedral crystal structure (R-3m, s.g. #166) with the lattice parameters: a = 5.3414(2) Å and c = 26.432(1) Å. Normal and superconducting states were studied by magnetic susceptibility, electrical resistivity and heat capacity measurements. The results showed that ThIr 3 is a type II superconductor (Ginzburg-Landau parameter κ = 33) with the critical temperature T c = 4.41 K. The heat capacity data yielded the Sommerfeld coefficient γ = 17.6 mJ mol -1 K -2 and the Debye temperature Θ D = 169 K. The ratio ΔC/(γT c ) = 1.6, where ΔC stands for the specific heat jump at T c , and the electron-phonon coupling constant λ e-p = 0.74 suggest that ThIr 3 is a moderate-strength superconductor. The experimental studies were supplemented by band structure calculations, which indicated that the superconductivity in ThIr 3 is governed mainly by 5d states of iridium. The significantly smaller band-structure value of Sommerfeld coefficient as well as the experimentally observed quadratic temperature dependence of resistivity and enhanced magnetic susceptibility suggest presence of electronic interactions in the system, which compete with superconductivity.
Superconducting properties of two bismuthide intermetallic compounds, RbBi2 and CsBi2, were studied by means of experimental measurements and ab initio calculations. We show that in both compounds the superconductivity emerges from the pyrochlore Bi lattice and its formation is heavily influenced by relativistic effects. Based on our analysis of the effect of spin-orbit coupling on the electron-phonon coupling we suggest a possible criterion for finding new superconducting materials by looking for structures featuring relativistically-stabilized hypervalent networks of heavy p-block elements.
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