Analysis of thermodynamic properties of fullerite C60

Abstract

The thermal expansion coefficients of pure fullerite are determined on the basis of powder x-ray studies in the temperature range 30–293 K. The obtained results are in good agreement with dilatometric and neutron-scattering data. The data on thermal expansion are used to analyze the heat capacity at constant volume CV. The intramolecular component of CV is analyzed consistently and accurately taking into account the complete set of temperature-dependent intramolecular eigenfrequencies. The rotational component of heat capacity is obtained by subtracting the intramolecular and phonon contributions from the total CV. The phonon component is evaluated on the basis of the Debye model using the Debye temperature (ΘD(0)=55.4 K) calculated from the known sound velocities. The general and partial Grüneisen parameters are calculated as functions of temperature. The results obtained for the high-temperature phase indicate that rotations of C60 molecules are strongly hindered and intercorrelated.

Hysteretic phenomena in Xe-doped C60 from x-ray diffraction

“…Of course, since the orientational phase transition is a first-order one, it is quite natural that it occurs with a hysteresis. We understand but well that because the distribution of Xe atoms inside the crystallites should be, most likely, highly inhomogeneous, the span of the hysteresis could be much larger than in pure C 60 [22]. However, first, the temperature span is too broad and, second, the high-temperature bifurcation point is (contrary to expectations) above T c in the pure material.…”

“…The cool-down curve is steeper than the warm-up curve at the high-temperature end of (-). The lower solid curve is for pure fullerite C 60 [22]. the hysteresis loop.…”

“…Indirect evidence from our findings on the C 60 -He system [26,27], also suggests is a linear function, whereas for Ar the span of X Ar is too narrow to make a sound judgment at this stage. The [6] and the empty overturned triangle is from this study for Xe; the solid rhombs [3] for Ne; the empty rhombs [6] for Kr; the solid [5,6] and empty (this work, see text) squares for Ar; the tilted cross [26,27] for He; the open circle for pure C 60 [22].…”

Intercalation of fullerite C60 with N2 molecules. An investigation by x-ray powder diffraction

“…On cooling over this temperature interval the lattice parameter of our (N 2 ) 0.85 C 60 solution changes by 8a = 0.129 4 C, whereas the ( N 2 ) 0.6 C 60 solution of Renker et al [7] shows a somewhat smaller change of 8a = 0.126 9 C, though comparable within experimental error. Both of the values, however, slightly exceed the lattice parameter change typically observed for cooling of pure C 60 over a similar temperature change, namely 8a = 0.119 0 C, for [26]. This means that the thermal expansion of nitrogen-doped C 60 within the temperature range studied is 8.5% larger compared to pristine fullerite.…”

“…Using this procedure, we processed results of several experiments and obtained the average lattice parameter a = 14.1906 ± ± 0.003 C for the (N 2 ) x C 60 solution. This exceeds the value for pure C 60 by 0.029 ±0.003 C [25,26]. Figure 2 reveals considerable scatter of the calculated a hkl -values about the linear fit.…”