Enhanced high-pressure superconductivity and local structure of theBa8Si46clathrate

Abstract

The high-pressure superconducting properties and the local structure of the ${\mathrm{Ba}}_{8}{\mathrm{Si}}_{46}$ clathrate have been studied using electrical resistance and x-ray absorption spectroscopy measurements up to more than 20 GPa. At pressures above 10--13 GPa, corresponding to a well-known volume collapse phase transformation, a sudden increase in the critical temperature leads to a maximum value of the superconducting critical temperature ${T}_{c}$ of $\ensuremath{\sim}8.5--9$ K at 20 GPa. In the low-pressure clathrate phase, the superconducting critical temperature decreases as pressure is applied with an electron-phonon coupling constant $\ensuremath{\lambda}=1.1$ derived from the temperature evolution of the electrical resistance at different pressures. A progressive disorder in the Ba-Si correlations in the ${\mathrm{Ba}@\mathrm{Si}}_{24}$ cages of the structure is observed for pressures beyond $\ensuremath{\sim}5$ GPa. These observations exclude a structural homothecy at the volume collapse transition. The high-pressure collapsed phase of ${\mathrm{Ba}}_{8}{\mathrm{Si}}_{46}$ is then associated with local structural changes and shows enhanced superconducting properties.