Z

circles denote C atoms, not to scale. In this bcc structure, an equivalent C60 molecule is centered at (5,5,5) (not shown). The Cs coordinates (clockwise from the top) obtained from x-ray data are (0, 1 - S), (0, ± + S, §), (0, ±, S), and (0, {-&,{) where S = 0.28 with respect to an origin at the center of the anion at the bottom left corner. Faces normal to the y or z axes may be visualized by appropriately rotating the diagram by ±90° [8.112].

circles denote C atoms, not to scale. In this bcc structure, an equivalent C60 molecule is centered at (5,5,5) (not shown). The Cs coordinates (clockwise from the top) obtained from x-ray data are (0, 1 - S), (0, ± + S, §), (0, ±, S), and (0, {-&,{) where S = 0.28 with respect to an origin at the center of the anion at the bottom left corner. Faces normal to the y or z axes may be visualized by appropriately rotating the diagram by ±90° [8.112].

than one Na+ in an octahedral site [8.83,114], For example, the fee structure can be preserved upon adding six Na atoms, with two Na ions going into tetrahedral sites and four Na atoms (not ions) into octahedral sites [see Fig. 8.1(h)] to form Na6C60 as a stable fee phase. This is in contrast to M6C60 (M = K, Rb, Cs), which crystallizes in a bcc structure [see Fig. 8.1(g)] and where complete charge transfer is thought to lead to the formation of the hexanions C®q . For Na6C60 only about two electrons are donated by the total of six Na atoms, because of the large electrostatic energy that would be needed to stabilize four Na+ ions in close proximity at a single octahedral site. The maximum stability for four Na atoms on an octahedral site is at the corners of a regular tetrahedron with respect to the site center [8.83,144], Only scant evidence is available for a stable Na3C60 phase [8.109]. Most authors who have prepared materials with this stoichiometry report that Na3C60 tends to phase separate (disproportionate) into Na2C60 [Fig. 8.1(c)] and Na6C60 [Fig. 8.1(h)] [8.83,109,144], The small size of the Na atom also

Table 8.5

Some structural parameters for the bcc M6C60 phases obtained from Rietveld refinement of x-ray data. C-M and M-M denote the near-neighbor carbon-metal and metal-metal distances, respectively. Values of ionic radii are taken from Table 8.1, pm denotes the mass density, and V is the volume of the primitive unit cell [8.142].

Some structural parameters for the bcc M6C60 phases obtained from Rietveld refinement of x-ray data. C-M and M-M denote the near-neighbor carbon-metal and metal-metal distances, respectively. Values of ionic radii are taken from Table 8.1, pm denotes the mass density, and V is the volume of the primitive unit cell [8.142].

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