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.
Highlights
Group-IV clathrate compounds are nanocage sp3-based structures allowing for endohedral guest atom intercalation
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
We have shown that through the Landau theory of a Fermi liquid treatment, the superconducting critical temperature Tc scales with the quadratic term of the resistance dependence with temperature [48]
Summary
Group-IV clathrate compounds are nanocage sp3-based structures allowing for endohedral guest atom intercalation. The situation is totally different for heavier guest atoms [25] as in Ba8Si46 [24,26,27,28,29,30], K8Si46 [31,32], I8Si44I2, or Rb8Si46 [33] Their high-pressure evolution is first characterized by the preservation of the tetrahedral silicon clathrate structure for pressures up to more than 4 times the stability domain of the diamond phase and, second, by a progressive volume collapse in a rather narrow pressure domain [25]. The local structure studied by XAS shows a clear evolution from 5 GPa. The correlation between the two studies allows one to obtain better insight on the electron-phonon coupling mechanism driving the superconducting properties of Si clathrates, to discuss the possible role of Ba substoichiometry on the first high-pressure transition at 5–7 GPa, and to restrict the scenario on the nature of the collapse transition
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