Evidence of a structural anomaly at 14 K in polymerised CsC ₆₀

Abstract

We report the results of a high-resolution synchrotron X-ray powder diffraction study of polymerised CsC 60 in the temperature range 4 to 40 K. Its crystal structure is monoclinic (space group I2/m), isostructural with RbC 60 . Below 14 K, a spontaneous thermal contraction is observed along both the polymer chain axis, a and the interchain separation along [111], d 1 . This structural anomaly could trigger the occurrence of the spin-singlet ground state, observed by NMR at the same temperature.PACS.61.48+c Fullerenes and fullerene-related materials. PACS.61.10Nz Single crystal and powder diffraction.

Crystal structures of polymerized fullerides AC60, A=K, Rb, Cs, and alkali-mediated interactions

“…14͒ and CsC 60 . 13 However our calculations show that the indirect chain-chain interaction mediated by the displacements of the alkalis is relatively weak and is not able to account for the structural difference between KC 60 and Rb-, CsC 60 . The quadrupolar polarizability mediated interaction is the decisive mechanism.…”

Theoretical Model for the Structural Phase Transition and the Metal–Insulator Transition in Polymerized KC60

“…The structure of both polymerized RbC 60 [5] and CsC 60 [6] is monoclinic with space group I2/m [ Fig. 1(b)].…”

“…[6] Bearing in mind the structural differences of the polymer phases of KC 60 and Rb-and CsC 60 , the presence/absence of a second structural phase transition in the AC 60 compounds seems to be related to the alternating or non-alternating polymer chain orientations. Here, we propose a periodicity doubling mechanism which accounts for the observed structural transition in polymerized KC 60 and for the absence of it in polymerized Rb-and CsC 60 .…”

“…We recall that for KC 60 , the space group is Pmnn, and for Rband CsC 60 , I2/m. Since the monoclinic angle of RbC 60 [9] and CsC 60 [6] is very close to 908, we treat them as having an orthorhombic Bravais lattice. The centers of mass of the N C 60 monomers are then located on the lattice points of a body-centered orthorhombic lattice for all three compounds.…”

“…Therefore, our model accounts for the experimentally observed periodicity doubling in polymerized KC 60 and the absence of a similar transition in Rb-and CsC 60 . [6,7] …”