Molecular thermal contraction of the Ar@C${}_{60}$ endohedral fullerene

Extended x-ray absorption fine structure (EXAFS) of argon at the K edge as a function of temperature for the Ar@C${}_{60}$ compound shows a negative molecular thermal expansion of the fullerene cage. The results indicate a possible buckling of the fullerene cage at low temperature.

Figure 1

X-ray absorption spectrum at the Ar K edge for Ar@C${}_{60}$: 80 K (red) and 300 K (black), fluorescence mode. In the inset are shown three EXAFS data.

Figure 2

Radial distribution functions (solid lines) and fitted values (dotted) for the different temperatures.

Figure 3

Calculated average Ar-C distance as a function of temperature with the error bars given by the fitting procedure.

Figure 4

Debye-Waller term for the Ar-C distance as a function of temperature. The red line is the fit using the Einstein model.