Metal-Insulator and Structural Phase Transition Observed by ESR Spectroscopy and X-Ray Diffraction inKC60

Abstract

We have performed electron spin resonance (ESR) spectroscopy and x-ray diffraction experiments at low temperature on KC60 single crystals. ESR data reveal the occurrence of a metal-insulator phase transition at about 50 K. In the same temperature range, we observe the stabilization of a superstructure which doubles the volume of the unit cell. We suggest that displacements of the K atoms and a modulation of the C60 charge may be involved in the mechanism of this phase transition. These results shed new light on the subtle interplay of structure, dimensionality, and electronic properties in the AC60 fullerides.

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

“…Recently, Coulon et al [7] have performed a combined study of the structural and electronic properties of KC 60 : single crystal x-ray diffraction studies have revealed a structural phase transition at 60 K T c 65 K, while ESR measurements have shown the existence of a metal -insulator transition at around 50 K. Below the structural critical temperature T c , the crystal changes to a (ã a þc c,b b,ã a Àc c) superstructure, implying a doubling of the unit cell volume. The superstructure can be viewed as a doubling of the periodicity alongã a andc c, while the periodicity alongb b remains the same.…”

“…The superstructure can be viewed as a doubling of the periodicity alongã a andc c, while the periodicity alongb b remains the same. Concerning the possible nature of this doubling mechanism, Coulon et al [7] already pointed out (i) that displacements of the C 60 centers of mass can be ruled out and (ii) that a combination of a charge density wave (CDW) on the C 60 molecules and large correlated K displacements is an appealing hypothesis.…”

“…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] …”

Theoretical model for the structural phase transition at the metal-insulator transition in polymerizedKC60

“…Recently, a combined study of the electronic and structural properties of KC 60 has been carried out by Coulon et al: 11 x-ray diffraction studies have revealed a structural phase transition in polymerized KC 60 at 60 KрT c р65 K, while ESR measurements have shown the existence of a metal-insulator transition at TϷ50 K, which stresses once again the close connection between structural and electronic properties in the AC 60 alkali-metal fullerides. Above the structural critical temperature T c , the Bravais lattice of polymerized KC 60 is orthorhombic; at T c , the crystal changes to a (a ជ ϩc ជ ,b ជ ,a ជ Ϫc ជ ) superstructure, which can be viewed as a doubling of the basis vectors a ជ and c ជ .…”

“…Above the structural critical temperature T c , the Bravais lattice of polymerized KC 60 is orthorhombic; at T c , the crystal changes to a (a ជ ϩc ជ ,b ជ ,a ជ Ϫc ជ ) superstructure, which can be viewed as a doubling of the basis vectors a ជ and c ជ . Concerning the possible nature of this structural phase transition ͑i.e., the doubling mechanism͒, it was pointed out by Coulon et al 11 ͑i͒ that displacements of the C 60 centers of mass can be ruled out and ͑ii͒ that a combination of a charge modulation on the C 60 monomers and large correlated K displacements is an appealing hypothesis. We note that such a combined structural and metalinsulator phase transition has not yet been observed in RbC 60 .…”