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

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

Far-infrared vibrational properties of tetragonal C60 polymer

We report high-resolution far-infrared transmittance measurements and quantum-molecular-dynamics calculations of the two-dimensional tetragonal (7) high-temperature/high-pressure C-60 polymer, as a complement to our previous work on the C-60 dimer, and the one-dimensional orthorhombic (O) and two-dimensional rhombohedral (R) C-60 Polymers [V. C. Long et at., Phys. Rev. B 61, 13 191 (2000)]. The spectral features are assigned as intramolecular modes according to our quantum-molecular-dynamics calculations. In addition, we determine the I-h C-60 parent symmetry of each polymer vibrational mode by expanding the calculated polymer eigenvectors in terms of our calculated eigenvectors for I-h C-60. We find that many of the T-polymer vibrational modes are derived from more than one I-h C-60 parent symmetry, confirming that a weak perturbation model is inadequate for these covalently bonded C-60 balls. In particular, strongly infrared-active T-polymer modes with frequencies of 606 and 610 cm(-1) are found to be derived from a linear combination of three or more I-h C-60 parent modes. As in the O and R polymers, modes of the T polymer with substantial T-1u(2) character, which are polarized in the stretched directions, are found to have large downshifts. Finally, in our comparison of theory with experiment, we find indications that the in-plane lattice of the T polymer may not actually be square

Isolation and structure of fullerene photodimer, C-120

Photopolymerization of C60 was performed on a gram scale via a new monomer transport method. After subsequent extractions of the raw polymer in organic solvents, lOOmg dimer and lOmg mixture of soluble oligomers were yielded. The structures of the soluble fractions and the insoluble polymer were studied by optical spectroscopy and x-ray diffraction methods. According to IR and UV-VIS spectroscopic data, the obtained photodimer is identical to the (2+2) cycloadduct C12o prepared previously. In contrast to previous crystallization studies, a fast precipitation of the dimer from toluene gives rise to the formation of a solvent-free microcrystalline solid. The crystal structure of the pure dimer is fee (a= 14.05 A) consisting of closely packed balls with the cycloadduct bonds oriented randomly along the 12 nearest neighbor directions. A linear relationship is found to exist between the fee lattice parameter (a) and the average number of bonded neighbours (nB) in various photopolymerized structures. For insoluble polymers nB«2 is estimated, supporting previous photopolymer models of small ring or branched clusters

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

Synthesis, structural and magnetic properties of TM22+[MOIV(CN)(8)]center dot nH(2)O

Octacyanomolybdates (OCMs) TM2 divided by[Mo(CN)(8)]center dot nHO, where TM is Mn, Fe, Co, Ni, Cu were synthesised and characterised by IR spectroscopy and UV-VIS spectroscopy. The UV-VIS spectrum showed the intervalence charge transfer (IVCT) band between Mo-IV-CN-Cu-II and Mo-V-CN-Cu-I around 510 nm. Studied OCMs adopt tetragonal crystal structure. The H-1 NMR signals reflect the magnetic moment of the TM2- ions (mu(P)). The decay rates of free induction decay (FID) signals increase as pp and the applied static rf-field increases. The spin-lattice relaxation times at 27.7 MHz vary from 0.0187 ins (Mn) up to 0.45 ms (Cu). Magnetization measurements indicate long-range magnetic ordering of Mn- and Co- OCMs with the Curie temperature T-c = 4 K. The remaining OCMs (TM is Fe, Ni, Cu) are paramagnetic down to T = 1.9 K

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