Conventional isotropic s -wave superconductivity with strong electron-phonon coupling in Sc5Rh6Sn18

${\mathrm{Sc}}₅{\mathrm{Rh}}₆{\mathrm{Sn}}₁₈$ is a diamagnetic metallic compound (space group: $I{4}₁/acd, a\ensuremath{\approx}\phantom{\rule{0.28em}{0ex}}13.5\phantom{\rule{0.28em}{0ex}}\AA{}$, $c\ensuremath{\approx}\phantom{\rule{0.28em}{0ex}}27.1\phantom{\rule{0.28em}{0ex}}\AA{}$) which becomes superconducting (sc) at ${T}_{\mathrm{c}}=5.15\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The sc state was studied on single-crystal samples by temperature- and field-dependent electrical resistivity, specific heat, and muon-spin rotation ($\ensuremath{\mu}\mathrm{SR}$) spectroscopy. No anisotropy of the upper critical field ${B}_{\mathrm{c}2}=7.75\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ is observed. Analyses of specific heat indicate enhanced values of the jump $[\mathrm{\ensuremath{\Delta}}{c}_p/({\ensuremath{\gamma}}_{\mathrm{tot}}{T}_{\mathrm{c}})=1.58]$ and of the energy gap ratio $[\mathrm{\ensuremath{\Delta}}(0)/({k}_{\mathrm{B}}{T}_{\mathrm{c}})=2.39]$ as well as an exponential decrease of the electronic contribution below ${T}_{\mathrm{c}}/2$. Together with the saturation of the superfluid density for $T<3\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ observed by $\ensuremath{\mu}\mathrm{SR}$ spectroscopy these findings lead us to classify ${\mathrm{Sc}}₅{\mathrm{Rh}}₆{\mathrm{Sn}}₁₈$ as a moderately strong coupled isotropic $s$-wave superconductor. However, above ${T}_{\mathrm{c}}$, the crystals reveal a surprisingly anisotropic and large resistivity of $2.5--4.5\phantom{\rule{0.28em}{0ex}}\mathrm{m}\mathrm{\ensuremath{\Omega}}$ m with metalliclike behavior perpendicular to the $c$ axis and by a factor of $\ensuremath{\sim}2$ larger parallel to $c$, which decreases upon increasing temperature. These observations are discussed in connection to a characteristic intrinsic disorder in the crystal structure of ${\mathrm{Sc}}₅{\mathrm{Rh}}₆{\mathrm{Sn}}₁₈$.