Structural phase transition in the ammoniated alkaliC60compound(NH3<

Recent Advances in Fullerene Superconductivity

“…One feature of the structural data is the large isotropic temperature factor found for the octahedral ion (B16 ( A 2 ), implying the existence of substantial static and/or dynamic disorder (13). In addition, while the crystal structure is consistent with the presence of only two inequivalent alkali ions, 39 K and 87 Rb NMR measurements in K 3 C 60 and Rb 3 C 60 reveal that the peak associated with the tetrahedrally coordinated ions splits into two components (T and T 0 ) at low temperature (14). Possibilities to account for this behavior include the existence of local misorientations of C 3À 60 ions, displacement of the octahedral ions off the center of the interstices, molecular Jahn-Teller distortions, structural instabilities like charge density waves, or a small concentration d of tetrahedral vacancies.…”

“…Indeed, it is reminiscent of the complex orbital, charge and magnetic order relationship encountered in strongly correlated transition metal oxides, like the CMR manganites (38). Interestingly, a structural phase transition has been observed in (NH 3 ÞK 3 C 60 below 150 K to an orthorhombic structure (space group Fddd) derived by doubling the lattice constants of the high temperature phase (space group Fmmm) along all three axes (39). The K þ -NH 3 pairs orient along the /1 1 0S direction in an antiferroelectric fashion, while the C 60 units adopt two distinct orientations related by 901 about c and are antiferrorotatively ordered along a and b (Fig.…”

Evidence for Insulating Behavior in the Electric Conduction of(NH3)K3C60Kitano1, Matsuo2, Miwa3 et al. 2002Phys. Rev. Lett.382331View full textAdd to dashboardCite

“…Because of these difficulties, the dc resistivity and the optical reflectivity have not yet been measured. A further complication seems to arise from the orientational order transition of the K-NH 3 pairs at T S ͑ 150 K͒ [14].In this Letter, we report the first direct study of the electric conduction of ͑NH 3 ͒K 32x Rb x C 60 (x 0, 2, 3) by using a microwave cavity perturbation technique. By comparing these results with our previous results from K 3 C 60 and ͑NH 3 ͒ x NaRb 2 C 60 (x 0.8, 0.9) [15], and also with the results from other reference powders such as Pb, V 2 O 3 , and C 60 , we concluded that the electric conduction of ͑NH 3 ͒K 32x Rb x C 60 is insulating between 4.5 and 250 K, without any MIT below 250 K. These results are in sharp contrast to the metallic nature of superconducting ͑NH 3 ͒ x NaRb 2 C 60 [15].…”

“…Because of these difficulties, the dc resistivity and the optical reflectivity have not yet been measured. A further complication seems to arise from the orientational order transition of the K-NH 3 pairs at T S ͑ 150 K͒ [14].…”

Structural differences in two polymorphs of tetra-kis-(dimethylamino)-ethylene-C60: An x-ray diffraction study

“…Similar "orbital" ordering, which greatly controls the magnetism, has been observed in the ammoniated alkali C 60 salt NH 3 K 3 C 60 . [17][18][19] It is proposed that the antiferrorotative ͑ferrorotative͒ ordering leads to the ferromagnetic ͑antiferromagnetic͒ interaction between the spins on C 60 . Structurally, at least two inequivalent C 60 sites should exist in order to introduce the ferromagnetic intermolecular interactions.…”