Crystalline Structure of Fullerene Solids

In this chapter we discuss the crystalline structure of C60 (§7.1), and the various phases that have been identified as a function of pressure (§7.3) and temperature (§7.4). Also included is a review of the polymeric phase formed by photopolymerization and the conditions under which this phase is formed (§7.5). Although less is known about the crystalline structure of C70, an approximate picture of the temperature dependence of the various structural phases has already emerged (§7.2). A summary of the status of knowledge about the even less known crystal structure of higher fullerenes is also given (§7.2). The crystalline forms of fullerenes are often called fullerites in the literature. Knowledge of the crystal structure of any fullerite is, of course, necessary to gain a microscopic understanding of their solidstate physical properties that are discussed in subsequent chapters.

7.1. Crystalline C60

In this section the structure of crystalline C60 is reviewed, including the structure at room temperature, phase transitions occurring as a function of temperature, group theoretical considerations for the crystalline phases, and merohedral disorder.

In the solid state, the Cgg molecules crystallize into a cubic structure with a lattice constant of 14.17 A, a nearest-neighbor C60-C60 distance of 10.02 A [7.1] and a density of 1.72 g/cm3 (corresponding to 1.44 xlO21 C60 molecules/cm3) (see Table 7.1). Whereas the C60 molecules themselves are almost incompressible, the molecular solid is quite compressible. Thus under hydrostatic pressure, it is expected that the mass density of solid

Table 7.1

Physical constants for crystalline C60 in the solid state.

Table 7.1

Physical constants for crystalline C60 in the solid state.

Quantity

Value

Reference

fee Lattice constant

14.17 Â

[7.2]

C60-Ci0 distance

10.02À

[7.1]

Qo-Q,, cohesive energy

1.6 eV

[7.3]

Tetrahedral interstitial site radius

1.12 Â

[7.1]

Octahedral interstitial site radius

2.07 Â

[7.1]

Mass density

1.72 g/cm3

[7.1]

Molecular density

1.44 x liF'/cm3

[7.1]

Compressibility (-d In V/dP)

6.9xl0"12 cm2/dyn

[7.4]

Bulk modulus"

6.8, 8.8 GPa

[7.5,6]

Young's modulus6

15.9 GPa

[7.7]

Transition temperature (Tm)':

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