X-Ray Diffraction Evidence for Nonstoichiometric Rubidium-C ₆₀ Intercalation Compounds

Abstract

Powder x-ray diffraction at 300 K on equilibrated samples of several nominal compositions chi in Rb(chi)C(60) is reported. In addition to the face-centered cubic (fcc) (chi = 3, superconducting), body-centered tetragonal (chi = 4), and body-centered cubic (bcc) (chi = 6) stoichiometric phases, direct evidence for a dilute fcc doped phase, 0 x c </= 1, and for a substoichiometric bcc phase, chi approximately 5, is presented. In contrast, chi = 3 and chi = 4 appear to be line phases with nearly zero solubility of Rb vacancies and interstitials at 300 K. These results are summarized in a provisional binary phase diagram.

Structure and properties of a fullerideSm6C60

“…Meansquare thermal amplitudes of the isotropic Debye-Waller factors were refined to 0.016 and 0.103Å 2 for C and Sm, respectively, and all samarium sites were found to be occupied. The refined lattice constant, a=10.890Å, however, is significantly smaller than measured values for K 6 C 60 and Cs 6 C 60 , 6 Ba 6 C 60 , 10 and Rb 6 C 60 , 17 and suggests a 9.43Å nearest-neighbor separation for the molecules. The atomic coordinates of C1, C2, C3 and Sm are nearly the same as that of A 6 C 60 (A=K, Rb, Cs, and Ba).…”

“…In contrast to the RE 2.75 C 60 , the highly Sm-doped Sm 6 C 60 is isostructural with A 6 C 60 (A=K, Rb, Cs, and Ba). 6,10,17 It suggests that further intercalation of Sm into C 60 solids leads to disappearance of cation-vacancy ordering and formation of a cation-disordered phase. The number of interstitial sites increases from three per C 60 in a faced-centred cubic (fcc) structure to six in the bcc structure.…”

Phenomenological model of anisotropic peak broadening in powder diffraction

“…Anisotropic broadening of powder diffraction peaks in the alkali fulleride superconductor Rb 3 C 60 was ®rst noted by Zhu et al (1991). Cox (1994) had developed a model of the anisotropic variation of line widths which has a signi®cant effect on the quality of ®ts attainable, and which allows the conclusion that the material is actually non-stoichometric, with a few percent cation vacancies (Fischer et al, 1995;Bendele et al, 1998).…”

Fullerenes

“…12 which was experimentally determined for Rb-doped C 6 o, but is believed to be broadly applicable to the other heavy alkali metal M^Qo compounds (M = K, Cs). 103 In Fig. 12 fcc(i) refers to the C 60 structure discussed in Sec.…”