Info

[8.141]

"The location of the K ion in K4C60 is at (0.21, 0.5, 0), close to the ideal location (0.25, 0.5, 0).

''The primitive unit cell volume V is found by V = a2c/2, and the C^-Qo distance by [2(a/2)2 + (c/2)2]1'2.

"The location of the K ion in K4C60 is at (0.21, 0.5, 0), close to the ideal location (0.25, 0.5, 0).

''The primitive unit cell volume V is found by V = a2c/2, and the C^-Qo distance by [2(a/2)2 + (c/2)2]1'2.

V — 7.89 x 10~22 cm3 [8.141]. In the lower-symmetry body-centered tetragonal phase [8.90], the C60 molecules are believed to occupy sites with C2v symmetry. Orientational disorder is expected for the C60 anions relative to the alkali metal ions residing in distorted tetrahedral sites, and attempts have been made to account for this disorder on the basis of merohedral disorder [8.142]. Orientational rearrangement of the C60 anions in the M4C60 alkali metal compounds is expected to occur rather easily [8.143].

The alkali metal-saturated compound M6C60 for M = K, Rb, Cs has a body-centered cubic (bcc) structure, as shown in Fig. 8.1(g), and corresponds to the space group or Im3. For this semiconductor, six electrons are transferred from the M ions to the C60 molecule to produce a anion with a filled -derived band [8.84] (see §12.7.3). In the M6C60 structure, the alkali metal ions all occupy equivalent distorted tetrahedral sites, which are surrounded by four C60 anions, two with adjacent pentagonal faces and two with adjacent hexagonal faces. In contrast to other crystalline M^Cfi0 compounds, the C60 anions in the bcc M6C60 alkali metal compounds are orientationally ordered as shown in Fig. 8.10 [8.112, 142], and this orientational ordering is maintained up to at least 520 K. The point group for the Cgg anion site in this structure is Th (see §7.1.2). In this structure each C60 anion is surrounded by 24 M cations. Values for some of the structural parameters for the KgCgo, Rb6C60, and Cs6C60 crystal phases obtained from Rietveld refinement of x-ray diffraction data are given in Table 8.5. The C-M distances in Table 8.5 are all larger than the sum of the van der Waals carbon radii and the alkali metal radii r, in Table 8.1. Also the M-M nearest-neighbor distances are all significantly larger than their respective ionic radii, as shown in Table 8.5. For the case of Rb6C60, the nearest-neighbor C60-C60 distance is 9.99 A, which is less than 10.03 A for undoped C60, while the closest Cg0 approach to a Rb+ cation is 3.42 A, which can be contrasted to the C-M distance of 3.25 A in Table 8.5; the volume per primitive unit cell for Rb6C60 is 7.68 x 10~22 cm3 which can be contrasted with the corresponding value of 7.89 x 10 ~22 cm3 in Rb4C60, given in Table 8.4 [8.141],

8.5.3. NaxC60 Structures

Since the ionic radii of the alkali metal ions Na+ and Li+ are both much smaller than the K+, Rb+, and Cs+ ions, the repulsive term in the orientational potential [Fig. 8.6(b)] is much smaller for the small Na+ and Li+ ions, and consequently, a number of unusual structural phenomena might be expected and are indeed observed. First, a stable compound Na2C60 is formed [see Fig. 8.1(b)], where the dopants fill the tetrahedral sites, leaving the octahedral sites unoccupied [8.83]. Second, it is possible to fit more

Was this article helpful?

0 0

Post a comment