Infrared signatures of the dynamic Jahn-Teller effect in fullerene-based materials

Temperature‐dependent vibrational spectra show the evolution of dynamical Jahn‐Teller states in A(2)C(60) and A(4)C(60) fulleride salts. Whereas at low temperature the external field of the cations determines the local symmetry, at high temperature the fullerene balls distort into a D(3d) or D(5d) symmetry, independent of the surrounding lattice. The average structure is preserved while the molecules show pseudorotation between possible potential minima

Distortion and orientation of fulleride ions in A(4)C(60)

A(4)C(60) compounds (A = K, Rb, Cs) are good candidates to exhibit the Mott-Jahn-Teller insulating state. We present near-IR and neutron scattering data to reflect molecular and crystal stucture changes with temperature. We show how the size of the cation affects the structural and electronic properties of these compounds

Metallicity in fullerides

Metallic salts formed from fullerenes became popular because of their superconducting properties with a relatively high transition temperature, and were initially regarded as conventional metals and superconductors. Recently, owing to improved synthetic methods and a renewed interest in the study of their physical properties, many of them were found to exhibit exotic metallic and superconducting phases. In this paper, we summarize earlier results on unconventional metallic fulleride phases as well as the newly discovered expanded fulleride superconductors. The proximity of the Mott transition, a typical solid-state effect, results in molecular crystals, where molecular spectroscopic methods prove very successful. We concentrate on infrared and optical spectroscopy which is very well suited to follow metallicity and phase transitions in this class of substances

Distortions of C-60(4-) studied by infrared spectroscopy

The Jahn-Teller effect plays a crucial role in the explanation of the insulating character of A(4)C(60) (A = K, Rb, Cs). To detect possible phase transitions arising from the interplay between the molecular Jahn-Teller distortion and the distorting potential field of the counterions, we measured the mid-IR spectra of A(4)C(60) compounds in the temperature range 90 - 300 K and found significant spectral changes with temperature in all three compounds. We also compare these spectra to that of Na(4)C(60) in its room-temperature polymeric phase, where the distortion is more pronounced and evident from the structure

Ordered low-temperature structure in K4C60 detected by infrared spectroscopy

Infrared spectra of a K4C60 single-phase thin film have been measured between room temperature and 20 K. At low temperatures, the two high-frequency T1u modes appear as triplets, indicating a static D2h crystal-field stabilized Jahn-Teller distortion of the (C60)4- anions. The T1u(4) mode changes into the known doublet above 250 K, a pattern which could have three origins: a dynamic Jahn-Teller effect, static disorder between "staggered" anions, or a phase transition from an orientationally-ordered phase to one where molecular motion is significant.Comment: 4 pages, 2 figures submitted to Phys. Rev.

Jahn-Teller distortion in Cs4C60 studied by vibrational spectroscopy

We have measured the infrared spectra of Cs(4)C(60) in the temperature range 220 - 450 K. Two anomalies in the low-frequency modes at 270 K and 400 K point to changes in molecular or crystal structure. The most probable explanation is a rotator phase above 400 K and a fully ordered phase below 220 K; the intermediate structure is one where molecular Jahn-Teller distortions compete with crystal field effects

Nanosegregation in Na2C60

There is continuous interest in the nature of alkali metal fullerides containing C(4)(60) and C(2)(60), because these compounds are believed to be nonmagnetic Mott–Jahn–Teller insulators. This idea could be verified in the case of A(4)C(60), but Na(2)C(60) is more controversial. By comparing the results of infrared spectroscopy and X-ray diffraction, we found that Na(2)C(60) is segregated into 3-10 nm large regions. The two main phases of the material are insulating C(60) and metallic Na(3)C(60). We found by neutron scattering that the diffusion of sodium ions becomes faster on heating. Above 470 K Na(2)C(60) is homogeneous and we show IR spectroscopic evidence of a Jahn–Teller distorted C(2)(60) anion

Giant microwave absorption in fine powders of superconductors

Enhanced microwave absorption, larger than that in the normal state, is observed in fine grains of type-II superconductors (MgB$_2$ and K$_3$C$_{60}$) for magnetic fields as small as a few $\%$ of the upper critical field. The effect is predicted by the theory of vortex motion in type-II superconductors, however its direct observation has been elusive due to skin-depth limitations; conventional microwave absorption studies employ larger samples where the microwave magnetic field exclusion significantly lowers the absorption. We show that the enhancement is observable in grains smaller than the penetration depth. A quantitative analysis on K$_3$C$_{60}$ in the framework of the Coffey--Clem (CC) theory explains well the temperature dependence of the microwave absorption and also allows to determine the vortex pinning force constant

Static and dynamic Jahn-Teller effect in the alkali metal fulleride salts A4C60 (A = K, Rb, Cs)

We report the temperature dependent mid- and near-infrared spectra of K4C60, Rb4C60 and Cs4C60. The splitting of the vibrational and electronic transitions indicates a molecular symmetry change of C604- which brings the fulleride anion from D2h to either a D3d or a D5d distortion. In contrast to Cs4C60, low temperature neutron diffraction measurements did not reveal a structural phase transition in either K4C60 and Rb4C60. This proves that the molecular transition is driven by the molecular Jahn-Teller effect, which overrides the distorting potential field of the surrounding cations at high temperature. In K4C60 and Rb4C60 we suggest a transition from a static to a dynamic Jahn-Teller state without changing the average structure. We studied the librations of these two fullerides by temperature dependent inelastic neutron scattering and conclude that both pseudorotation and jump reorientation are present in the dynamic Jahn-Teller state.Comment: 13 pages, 10 figures, to be published in Phys. Rev.

Structure and properties of the stable two-dimensional conducting polymer Mg5C60

We present a study on the structural, spectroscopic, conducting, and magnetic properties of Mg5C60, which is a two-dimensional (2D) fulleride polymer. The polymer phase is stable up to the exceptionally high temperature of 823 K. The infrared and Raman studies suggest the formation of single bonds between the fulleride ions and possibly Mg-C-60 covalent bonds. Mg5C60 is a metal at ambient temperature, as shown by electron spin resonance and microwave conductivity measurements. The smooth transition from a metallic to a paramagnetic insulator state below 200 K is attributed to Anderson localization driven by structural disorder