Crystalline Phases

The A₁C₆₀ phase (hereafter abbreviated A₁) has the single interstitial alkali ion (A = K, Rb or Cs) in the O-sites of the high temperature FCC phase fullerite[57] (Fig.1.2). This cubic phase, however, undergoes a structural phase transition to a slightly distorted orthorhombic phase. The critical temperature of this transition is about 350K, but there is a broad range of temperatures over which the two phases co-exist (250K - 350K for Cs and 295K - 350K for Rb[58]). Extensive Reitveld analysis of x-ray diffraction profiles indicates[59] that the orthorhombic phase (o-A₁) has chains of C₆₀, that are spaced much more closely than in the corresponding FCC phase. The direction of these chains is along a face diagonal of the FCC structure (e.g. $\langle 1,-1,0 \rangle$ with respect to the FCC lattice). It is now well accepted that in this phase the C₆₀ units are polymerized within the chains by the same ``2+2 cycloaddition'' mechanism suggested for the fullerite polymer (section 1.2.1). The model proposed to explain the x-ray diffraction has, along the polymer axis, a short C₆₀-C₆₀ spacing of about 9.1Å. It also involves a stretching distortion of the C₆₀ along the chain axis, yielding a nearest C-C distance of about 1.4Å which is typical of carbon bond lengths. The distortion of the C₆₀ molecule has been corroborated recently by magic angle spinning NMR, which resolves the broadened ¹³C resonance into several lines corresponding to inequivalent carbon atoms[60]. The array of chains in a plane containing the nearest neighbour chains (e.g. $\langle 1, 1, -1 \rangle$) is shown in Figure 1.4. In addition, there is a metastable structural phase that exists if an A₁ material is quenched rapidly from the FCC phase. This phase involves C₆₀ dimers. No experiments on the dimer phase of A₁ are reported in this thesis, so it will not be discussed further. The interested reader is directed to [61].