Pressure-induced order-disorder phase transition in superconducting CaC6

TL;DR

This study investigates how high pressure affects the crystal structure of superconducting CaC6, revealing an order-disorder phase transition near 9 GPa that explains the observed decrease in critical temperature.

Contribution

It provides the first evidence of a pressure-induced order-disorder phase transition in CaC6, linking structural changes to superconducting properties.

Findings

No change in space group symmetry up to 13 GPa

Order-disorder transition at 9 GPa associated with Ca sublattice

Reversible effects upon depressurization

Abstract

By means of synchrotron X-ray diffraction, we studied the effect of high pressure, P, up to 13 GPa on the room temperature crystal structure of superconducting CaC6. In this P range, no change of the pristine space group symmetry, \textit{R\=3m}, is found. However, at 9 GPa, i.e. close to the critical value at which a large T_c reduction was reported recently, we observed a compressibility jump concomitant to a large broadening of Bragg peaks. The reversibility of both effects upon depressurization and symmetry arguments give evidence of an order-disorder phase transition of second order, presumably associated with the Ca sublattice, which provides a full account for the above Tc reduction.