A polycrystalline sample of superconducting $\mathrm{ThI}{\mathrm{r}}_3$ 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: $a=5.3394\left(1\right)\AA$ and $c=26.4228\left(8\right)\AA$. Normal and superconducting states were studied by magnetic susceptibility, electrical resistivity, and heat capacity measurements. The results showed that $\mathrm{ThI}{\mathrm{r}}_3$ is a type-II superconductor (Ginzburg-Landau parameter $\kappa =38)$ with the critical temperature $T_{\mathrm{c}}=4.41\mathrm{K}$. The heat capacity data yielded the Sommerfeld coefficient $\gamma =17.6\mathrm{mJ}/\left(\mathrm{mo}\mathrm{l}{\mathrm{K}}^2\right)$ and the Debye temperature ${\mathrm{\Theta }}_{\mathrm{D}}=169\mathrm{K}$. The ratio $\mathrm{\Delta }C/\left(\gamma T_{\mathrm{c}}\right)=1.6$, where $\mathrm{\Delta }C$ stands for the specific heat jump at $T_{\mathrm{c}}$, and the electron–phonon coupling constant ${\lambda }_{e-p}=0.74$ suggest that $\mathrm{ThI}{\mathrm{r}}_3$ is a moderate-strength superconductor. The experimental studies were supplemented by band structure calculations, which indicated that the superconductivity in $\mathrm{ThI}{\mathrm{r}}_3$ 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 the presence of electronic interactions in the system, which compete with superconductivity.

• Received 24 April 2019
• Revised 12 October 2019

DOI:https://doi.org/10.1103/PhysRevB.100.214514