Rotational dynamics of C60 in superconducting K3Ba3C60
Introduction
The study of the intercalation compounds of solid C60 with electron donors has been an active research field in recent years. Prominent among these systems have been the alkali fullerides, A3C60 (A= alkali metal) which adopt face-centred cubic (fcc) structures and exhibit superconductivity with Tc as high as 33 K at ambient pressure [1]. Intercalation of solid C60 to saturation leads to compositions A6C60 with body-centred cubic (bcc) structures, which are insulators, as the conduction band, arising from the lowest unoccupied molecular orbital (LUMO) of C60 of t1u symmetry is full. Doping of C60 to even higher charge state (n > 6) can be achieved when divalent alkaline earth metals are used as intercalants 2, 3. In such cases, the conduction band derives from the next unoccupied molecular orbital (LUMO+1) of C60 of t1g symmetry and superconductivity is found for the orthorhombic compositions AE4C60 (AE= Ba, Sr), in which the conduction band is not half-filled [4]. This is in contrast to the A4C60 fullerides, which are insulating. Half-filling of the t1g band can be formally achieved for the mixed alkali-alkaline earth fullerides, A3Ba3C60 (A = K, Rb, Cs) 5, 6. This family also displays superconductivity but in marked difference to its alkali antecedents, Tc decreases with interfullerene separation. The origin of this behaviour is not as yet understood but it may be related to the presence of strong interaction between Ba and C60 and hybridisation of Ba 5d and C60t1g orbitals 4, 7, 8. The crystal structure of K3Ba3C60 has been determined as bcc (a= 11.21661(7) Å at 10 K, space group Im 3̄), isostructural with that of the A6C60 fullerides. The K+ and Ba2+ cations are disordered in the same distorted tetrahedral interstitial sites, (0, , +δ). However, they are displaced by a different distance from the centre of the site (δ= 0.034(2) and δ= 0.0265(8) for K+ and Ba2+, respectively), resulting in different coordination environments with the C60 units [7]. Short Ba–C and K–C distances (∼3.08 Å), close to the sum of the ionic radii and the van der Waals radius of C are also noticeable.
Low-energy neutron inelastic scattering (NIS) measurements have been extensively used to probe the rotational dynamics of pristine C60[9]as well as of C603− and C606− ions in a variety of fullerides 10, 11, 12, 13, 14, 15, 16. In all cases, the excitations observed at low temperatures at non-zero energy transfer are due to fullerene molecules, librating about their equilibrium orientations. It is of particular interest to see how the introduction of the Ba2+ ions in the lattice and the consequent increase in the charge of the C60 units modify the interfullerene orientational potential. For this reason, we performed neutron inelastic scattering (NIS) measurements of the low-energy rotational excitations in K3Ba3C60 between 150 and 320 K. The momentum-transfer, Q dependence of the intensity of the broad low-energy excitations in K3Ba3C60 leads to their assignment as librational modes whose energies are substantially higher than those in the parent material K3C60[10], indicating a considerable change in the orientational potential between the two fullerides. Softening of the librational peaks is observed on heating, while their substantially increased widths indicate a very anisotropic rotational potential and/or rotation-translation coupling. Considerable similarities with the behaviour of isostructural Rb6C60[10]are also encountered.
Section snippets
Experimental details
The K3Ba3C60 sample used in the present work was synthesized by intercalation of K into preformed Ba3C605, 6. Stoichiometric amounts of potassium metal and Ba3C60 powder were loaded in a tantalum cell sealed under 500 mmHg of helium in a Pyrex tube and heated at 260°C for seven days. Phase purity was confirmed by X-ray diffraction with a Siemens D5000 diffractometer. SQUID magnetometer measurements established the onset of superconductivity below Tc= 5.4 K.
The neutron scattering measurements
Results and discussion
NIS measurements for K3Ba3C60 were performed as a function of Q and ω at several temperatures. Fig. 1 shows representative spectra at temperatures between 150 and 320 K at a scattering vector, Q= 5.5 Å−1. The solid circles are the corrected experimental data and the lines are the fits described in the previous section. Well-defined peaks are observed at non-zero energy transfer at all temperatures and may be assigned to librational modes of the C609− ions from the dependence of their integrated
Conclusions
In conclusion, we have measured the low-energy NIS spectra of K3Ba3C60 as a function of the scattering vector, Q at temperatures between 150 and 320 K. The fulleride units are found to undergo small-amplitude librations about their equilibrium position, giving rise to librational peaks near 6 meV. The librational energy is larger than in the parent K3C60 salt and the peaks are much broader, reflecting a stronger and more anisotropic orientational potential. While there are similarities in the
Acknowledgements
We acknowledge financial support by the Royal Society and the NEDO Frontier Carbon Technology programme.
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Cited by (1)
Inelastic neutron scattering study of the intermolecular vibrational modes of Ba<inf>4</inf>C<inf>60</inf>
2003, Chemical Physics LettersCitation Excerpt :As these low energy excitations are harder and broader than those measured in K3C60 and Rb3C60, they reflect the existence of stronger and more anisotropic (due to the different coordination environment of the C60 units) interactions in the present system. Their energy is comparable with those in the body-centred cubic fullerides K3Ba3C60 and Rb6C60 in which small amplitude librations of C609− and C606− ions have been observed at ∼5.7 and 5.5 meV, respectively [12,13]. From the above considerations it is clear that the energy of the librational modes increases as the ionicity of the C60 unit increases and the strong Coulomb forces between the charged ions dominate the rotational potential.