Enhanced high-pressure superconductivity and local structure of theBa8Si46clathrate

The high-pressure superconducting properties and the local structure of the Ba8Si46 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 Tc of ∼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 λ=1.1 derived from the temperature evolution of the electrical resistance at different pressures. A progressive disorder in the Ba-Si correlations in the Ba@Si24 cages of the structure is observed for pressures beyond ∼5 GPa. These observations exclude a structural homothecy at the volume collapse transition. The high-pressure collapsed phase of Ba8Si46 is then associated with local structural changes and shows enhanced superconducting properties.