The specific heat of C(70) fullerene, nanotubes, photo-polymerized C(60) fullerene, potassium doped fullerenes, and rubidium doped fullerenes.

摘要

{dollar}rm Csb{lcub}70{rcub}{dollar} fullerene is the second most abundant fullerene after {dollar}rm Csb{lcub}60{rcub}{dollar}. To understand the low temperature vibrational modes of {dollar}rm Csb{lcub}70{rcub}{dollar}, we measured the specific heat of a polycrystalline {dollar}rm Csb{lcub}70{rcub}{dollar} sample from 0.3 to 78 K. Solid {dollar}rm Csb{lcub}70{rcub}{dollar} fullerene exhibits many structural phases at different temperatures, and below 276 K it crystallizes in a monoclinic structure with a basis of 4 molecules per primitive cell. The polycrystalline sample has spectroscopic purity of 99.5+%. The measurement was extended down to 0.3 K by use of a {dollar}sp3{dollar}He cryostat especially made for low temperature measurements. Analysis of the specific heat data shows that {dollar}rm Csb{lcub}70{rcub}{dollar} is a non-metallic material with a relatively low Debye temperature. However, the data can only be fitted by use of a model that contain various contributions to the specific heat in addition to the Debye contribution. These contributions include Einstein terms that account for two contributions made by librational and rotational frequencies in this system. It is most important to note that in {dollar}rm Csb{lcub}70{rcub}{dollar} the specific heat peaks past the Dulong-Petit limit of 6R (Equipartition Theorem) at the low temperature of 30 K. We show that this excess specific heat can not be accounted for by the intramolecular modes, as previously thought. We attribute this excess to "orientational defects" that originate in {dollar}rm Csb{lcub}70{rcub}{dollar} due to its oblate structure. No such excess specific heat is observed in C{dollar}sb{lcub}60{rcub}{dollar}.; We have measured the specific heat of fullerene nanotubes from 0.6 to 210 K. The specific heat curve is similar to graphite, and shows characteristics of a two-dimensional structure throughout the whole temperature range. The data also show the absence of any metallic behaviour, as was thought to exist in a certain percentage of the tubes. We found that the characteristic Debye temperature for the nanotubes is 1400 K, which is intermediate in temperature between the values of 950 and 2500 K found for the characteristic temperature in graphite.; We photo-polymerized C{dollar}sb{lcub}60{rcub}{dollar}, (C{dollar}sb{lcub}60{rcub})sb{lcub}n{rcub}{dollar}, from its powder form. Evidence for photopolymerization is shown in the additional specific heat in comparison with pristine C{dollar}sb{lcub}60{rcub}{dollar}. We also measured the specific heat of alkali-doped C{dollar}sb{lcub}60{rcub}{dollar} from 3 K to 210 K. The specific heat of (C{dollar}sb{lcub}60{rcub})sb{lcub}n{rcub},{dollar} matches that of KC{dollar}sb{lcub}60{rcub}{dollar}, a well known polymer. The AC{dollar}sb{lcub}60{rcub}{dollar} compounds are isostructural at high temperature. When they polymerize they exhibit varied behaviours, from their n value (degree of polymerization) to their electronic properties. The linear temperature coefficients for the AC{dollar}sb{lcub}60{rcub}{dollar} are high, indicating a one-dimensional behaviour in RbC{dollar}sb{lcub}60{rcub}{dollar}, and metallicity and disorder in KC{dollar}sb{lcub}60{rcub}{dollar}.