High-field ESR in the metallic fullerides RbC60 and CsC60

We discuss field-dependent electron spin resonance (ESR) experiments on the orthorhombic phase of RbC₆₀ and CsC₆₀. X-and W-band measurements have been performed to determine electronic scattering rates in the metallic temperature regime from 50 to 350 K. The insulating low-temperature phase (T < 50 K) has been investigated by X-, Q-, and W-band ESR, i.e., at 9, 34 and 95 GHz, in order to clarify whether collective resonance modes of a possibly magnetic ground state can be observed.     

Muon spin relaxation measurements of CsC60, RbC60, KC60

Muon spin relaxation (μ⁺SR) measurements of CsC₆₀ and RbC₆₀ have revealed magnetic ordering at 20 K and 15 K respectively, with static internal fields at the muon site of \sim7 G for both samples. No magnetic ordering is seen in KC₆₀ down to 2.5 K. An apparent small increase in muon spin relaxation which had been seen at 70 K in RbC₆₀ is ascribed to an extrinsic effect, originating in muons which stopped in the aluminum sample holder. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thermal conductivity of pressure polymerized C60

60 polymerization in the temperature interval at pressures below by measurements of the time dependence of the thermal conductivity. It has been found at that the polymerization process at is slower than the reverse transformation from “polymeric” to “monomeric” phase at . The thermal conductivity of polymerized C₆₀ was measured in the temperature range and found to increase with increasing temperature, which reflects strong phonon scattering. Both the presence of non-bonded C₆₀ molecules and a high degree of structural disorder in the crystalline lattice of the polymeric phase might be responsible for the behaviour of . The results for are qualitatively similar to those reported previously for C₆₀ polymerized at higher , but an order of magnitude smaller. Received: 20 September 1996/Accepted: 11 November 1996     

87Rb NMR investigations on binary and ternary fullerene compounds: Rb3C60, K2RbC60 and Rb2CsC60

Results of¹³C MAS NMR and⁸⁷Rb NMR measurements on Rb₃C₆₀, K₂RbC₆₀ and Rb₂CsC₆₀ are presented. The⁸⁷Rb NMR spectra show generally the same characteristics as already reported by other authors. For the ternary compound the degree of site preferential order of the alkali metal ions agrees with the analysis of X-ray absorption data. Furthermore, in the room temperature⁸⁷Rb NMR spectra of all samples an additional broadened line is detected indicating that at least for the samples prepared from sublimed C₆₀ a considerable amount of the Rb ions is located on sites with symmetry lower than cubic.     

Spectroscopic characterization of pressure modified C60

60 are reported. The material associated with the mixture of rhombohedral and tetragonal phases was synthesized under pressure of when treatment temperature was only a little lower than the high-temperature limit of C₆₀ stability. The substance exhibits very rich infrared and Raman spectra as well as luminescence spectra of an unusual shape. They show that vibrational transitions in the prepared carbon architecture substantially deviate from icosahedral symmetry of buckyballs and exhibit a similarity with lattice phonons in graphite. It may originate from mixing of C₆₀ modes and vibrations of a layer structure on deformed and weakly bound fullerene molecules. The luminescence spectra reveal three distinct electronic states located below . One of them responsible for the emission peak at is very characteristic for the pressure modified fullerene. The data should be useful for the accurate determination of structural changes in C₆₀ induced by pressure. Received: 6 September 1996/Accepted: 10 October 1996     

Coupled (localized+delocalized) electron spins dynamics in polymer phase of RbC60 fulleride

The transverse response (transverse dynamic susceptibility) of coupled localized (s) and delocalized (e) electron spins of a metal paramagnet as well as the longitudinal dynamic response of such a system (to be registered by a longitudinal coil) to the modulation of microwave power saturating electron spin resonance (ESR) are calculated. The ESR spectrum is analytically decomposed into two Lorentzians with normal resonance frequencies and decay rates of the coupled localized +delocalized electron spin system. In the case of relaxationally coupled s- and e-spins the longitudinal response is decomposed into two “relaxational” Lorentzians squared with amplitudes containing ESR lineshapes with above-mentioned shifted frequencies and linewidths as well as enhancement-suppression coefficients of magnetization evolution. These results are essential for the interpretation of experiments on longitudinal response in metal paramagnets, the latter being the source of important information on longitudinal electron relaxation; in particular, for extraction of information from longitudinal response experiments in the polymer phase of RbC₆₀ fullende, where the obtained results describe the observed form of the ESR spectrum and the main features of the longitudinal response.     

Site selectivities of alkali ions in CsRb2C60 and Cs2RbC60 superconductors

The temperature-dependent site selectivities of Cs and Rb ions in CsRb₂C₆₀ and Cs₂RbC₆₀ superconductors are computed using the free energy obtained from the configuration entropy and the total energy calculated using the ab initio pseudopotential density-functional theory. It is found that in CsRb₂C₆₀ the smaller Rb ions can occupy a considerable number of large octahedral interstitial sites at high temperatures, however, the transition to random occupation never takes place. For Cs₂RbC₆₀ the Cs occupancy of the large octahedral interstitial sites is almost always 100%, and, interestingly, the two tetrahedral interstitial sites are randomly occupied by Cs and Rb ions even at very low temperatures.     

Temperature dependence of infrared and Raman modes in polymeric RbC60

Temperature dependence of the intra-molecular vibrational modes of C₆₀ in the quasi-1D polymeric RbC₆₀, across the low temperature transition at ∼50 K, has been probed through infrared (IR) and Raman spectroscopies. With the lowering of temperature, the split IR modes of RbC₆₀ are seen to harden but below 50 K a small but definitive signature of an anomalous softening is observed. In addition, the background IR transmission shows an increase below 50 K with the opening of a well defined gap in the electronic spectrum. The implications of these results, along with those of Raman measurements, are discussed in terms of the interaction of intra-molecular phonons with electrons and spin excitations in the system.     

Transport and structural properties of polymerized AC60 (A = K, Rb) under zero and high pressure conditions

60 display transport properties vastly different from their unpolymerized counterparts. We here describe structural (X-ray diffraction and transmission electron microscopy) and dc resistivity measurements of polymerized AC₆₀ (A = K, Rb) at zero and high pressures. The structural and electronic properties are rich, and evidence is found for unusual phase transitions in both materials. Received: 28 October 1996/Accepted: 13 December 1996     

Is the ground state of AC60 (A = Rb, Cs) antiferromagnetic?

Electron paramagnetic resonance measurements in the metallic and the low-temperature orthorhombic phases of RbC₆₀ and CsC₆₀ powder samples and a RbC₆₀ powder sample aligned by uniaxial pressure have been performed at 34 and 94 GHz. A detailed analysis of the low-temperature signal of all samples allows the assignment of several line components to paramagnetic defects, leaving one broad line component unaccounted for. Although the behavior of this component gives strong indications of magnetic correlations, we do not find typical characteristics of antiferromagnetic resonance that have been reported by other groups at higher fields. We ascribe this to a high degree of structural disorder in the samples which inhibits the development of a long-range antiferromagnetic order. Therefore, we interpret the signal behavior as being due to magnetically coupled spin clusters.     

Coupling of orientational and translational modes in solid C60 and C70

The orientational phase transitions in solid C₆₀ and C₇₀ are accompanied by quite different anomalies in the crystalline strains. In solid C₆₀ the phase transition Fm3m→Pa3 is primarily an orientational effect (antiferro-rotational), which is driven by the condensation of orientational modes belonging to X₅ ⁺ irreducible representation (irreps) of Fm3m. These modes are the primary order parameters (oops) and their number is equal to the number of irreps of T_2g and T_1g symmetry within the manifolds under consideration. Taking into account irreps up to the manifold 1=12, we have studied the rotation-rotation-translation (RRT) coupling between the oops and the lattice displacements. We have investigated the resulting lattice contraction and the change of the elastic constant c₁₁ at the phase transition. In solid C₇₀ (fcc-phase) we investigate the bilinear coupling of orientational fluctuations of T_2g symmetry to transverse acoustic lattice displacements. This coupling is the driving mechanism for the ferroelastic phase transition Fm3m → R3m. Finally we investigate the transition from the rhombohedral phase to a low temperature monoclinic phase. This transition in antiferro-rotational.     

Intermolecular vibrations in pure and doped C60

We discuss the results of inelastic neutron time-of-flight spectroscopy on powder samples of C₆₀, K₃C₆₀ and Rb₃C₆₀. A well structured density of states is obtained from the raw data where the details can be attributed to translational acoustic/optic and librational modes. The intermolecular excitation spectra for all three compounds can be well understood within the frame-work of lattice dynamics of a molecular crystal. We show that a particular sample preparation is necessary to avoid impurity contributions in particular within the gap region between inter- and intramolecular vibrations. There are indications for a weak anomalous temperature dependence of low energy phonons in K₃C₆₀ atT _c .     

Electronic properties of ortho-polymer A1C60 (A = K,Rb) and related phases

1 C₆₀. Its singular structural, magnetic and electrical properties greatly enhanced the study of the AC₆₀ phases (A = Rb, K and \chem{Cs}). We report our continuous wave (cw) ESR results on the structural phase transformations, phase instabilities and the related kinetics of K₁C₆₀. We quantitatively explain the origin of the difference between Rb₁C₆₀ and K₁C₆₀ phase diagrams. The stability of OP-Rb₁C₆₀ together with the narrowness of its homogenous cw-ESR linewidth in the temperature range allow to perform pulsed ESR experiments. Pulsed ESR experiments measure precisely both the electronic spin-spin () and spin-lattice () relaxation times. Above , the equality of and , which are the only contributions to the cw-ESR linewidth, constitutes a convincing evidence of the metallic nature of OP-Rb₁C₆₀. The drastic increase (decrease) of the linewidth () below \valunit{40}{K} is interpreted in terms of an \qut{electronic dimerization}. Received: 12 September 1996/Accepted: 14 November 1996     

A Raman study of polymerised C60

60 are reported. Although prepared according to different routes the Raman spectra of the two polymeric phases of C₆₀ show a quantitative agreement with respect to mode positions and intensity. We conclude from this that both materials have the same structure at least in the short range order, i.e. the same type of bonding and co-ordination between neighbouring C₆₀ molecules. An investigation of the time dependence of the thermal decomposition of high pressure polymerised C₆₀ is also presented. The rate of decomposition of the polymeric phase is found to be multi-exponential at all temperatures investigated. From an Arrhenius-type analysis of the short time data and the long time data, respectively, the activation energy for thermal dissociation of polymeric bonds was found to increase with time. Received: 20 September 1996/Accepted: 11 November 1996     

Complete pentagon orientational ordering in C60 fullerite charged with NO

Solid C₆₀ was stored in NO under high pressure, and the gas molecules NO were found to diffuse into the octahedral interstitial sites in its fcc crystal lattice. Its ¹³C NMR MAS spectra are composed of a primary resonance at 143.7 ppm accompanied by two minor peaks shifted 0.4 and 0.8 ppm downfield, respectively. The dopant was found to depress its phase transition temperature at 260 K in pure C₆₀ and to substantially reduce the drop Δ?′ at the phase transition temperature. Furthermore, the spectral features associated with relaxation during glass transition at lower temperature, as observed in impedance spectra, were smeared. The fraction of P-orientation below T _c was calculated to be larger than 11/12. These results show that a completely P-oriented phase occurred in (NO)_0.1C₆₀ and that this phase is favored by a negative pressure on the C₆₀ lattice exerted by NO, as well as by the electrostatic interaction between the two.     

Luminescence detection of phase transitions,local environment and nanoparticle inclusions

Luminescence data offer delicate probes of changes in structure and local environment in insulating materials. Therefore they have long been employed in studies of imperfections and characterisation of lattice distortions. Luminescence techniques are inherently very sensitive so respond to small concentrations of intrinsic defects and impurities, and intentionally added probe ions, such as rare earth or chromium ions, are particularly effective at displaying responsive changes linked to modified structural environments. Parameters of interest are variations in luminescence efficiency, the details of the emission spectra, polarisation, temperature dependence and changes in excited state lifetimes. These are suitable properties to monitor variations in both short and long range lattice structure, composition, pressure and temperature. For modern photonic materials such probes are powerful tools to follow changes introduced by processing to make waveguides, surface layers and new materials. The use of different excitation conditions for the luminescence can resolve differences between near surface and bulk features. A less common approach is to use luminescence as a route to detect phase transitions. This is particularly valuable for rapid survey studies of new materials which are of relevance to modern optics. One of the more surprising results is the observation that phase changes of very small inclusions, such as phase precipitates or impurity nanoparticles, can totally dominate the luminescence response of the bulk material. Specific examples include the frequent observation of trapped nanoparticles of water, N₂, O₂ and CO₂. Further, the excitation techniques, and ion implantation or surface stresses can all induce surface relaxations which, in materials such as SrTiO₃ or ZnO, result in phase changes propagating throughout the entire crystal. This review rapidly recalls the methods used in the older studies of imperfections and then uses a range of examples to show how luminescence studies can be effective in detecting and confirming the presence of phase transitions. Interestingly the examples include not only transparent insulating materials but also fullerenes and superconductors.     

Phase transitions and the structure of the C60 crystal doped with lithium by electrodiffusion

The structure and phase transitions of C₆₀ crystals doped with lithium by injecting metal ions from the superionic crystal-C₆₀ single crystal heterojunction under electrodiffusion conditions are reported. The sample experienced irreversible transitions resulting in the virtually complete disappearance of EPR signals and MW conduction in the temperature range 320–370 K. In this temperature interval, a new C₆₀ phase was formed; the phase contained polymeric chains of C₆₀ molecules along the crystallographic c axis and lithium clusters. The structure of this phase was determined. Annealing at 620 K restored the EPR signal and, according to the X-ray data, the initial cubic structure of pure C₆₀. The X-ray pattern, however, contained additional diffraction peaks, which was evidence of the presence of one more phase with a structure yet unknown.     

Mechanism of three-dimensional polymerization of fullerite C60 at high pressures

A series of polycrystalline phases corresponding to different stages of three-dimensional polymerization and destruction of C₆₀ molecules has been synthesized by heating fullerite C₆₀ under a pressure P=12.5 GPa. The structure of the phases can be identified as fcc, and the lattice period decreases with increasing heating temperature. A model of three-dimensional polymerization in which the lattice parameter is a continuous function of the fraction of covalently bonded molecules is proposed. The model makes it possible to estimate the number of atoms in the sp ³ state. The hardness of the polymerized fcc phases is studied on the basis of percolation of rigidity. It is shown experimentally that the period a≈13.8 Å is the threshold for the formation of a three-dimensionally rigid C₆₀ polymer. It is found that the thermal stability of the strongly and weakly polymerized phases is qualitatively different. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 11, 755–759 (10 December 1996)     

A close look at the motion of C60 on gold

In this paper, we have studied the motion of buckminsterfullerene (C₆₀) on a gold surface by analyzing its potential energy and using classical molecular dynamics method. The results can be employed to investigate the motion of C₆₀-based nanocars which have been made in recent years. For this purpose, we have studied the translational and rotational motions of C₆₀ molecule independently. First, we have calculated the potential energy of a C₆₀ molecule on a gold surface in different orientations and positions and employed this data to predict fullerene motion by examining its potential energy. Then we have simulated the motion of C₆₀ at different temperatures using classical molecular dynamics methods. Specifying the regime of the motion at different temperatures is one of main goals of this paper. We have found that the rotational motion of C₆₀ molecule on the gold substrate, was easier than its sliding (translational) motion. Also, the regime of motion of fullerene depended on temperature. The results demonstrate that three different regimes of motion, dependent on temperature, could be observed: rare jumps to adjacent cells, frequent jumps, and continuous motion. Employing the results of this paper not only helps to understand the C₆₀ motion on the gold surface but also provides an appropriate tool for realizing motion of the thermally-driven fullerene-based nanocars.     

Low-frequency critical dynamics in (CnH2n+1NH3)2SnCl6 model biomembrane systems

Nuclear magnetic resonance has been employed as a probe for the collective hydrocarbon chain dynamics in the organic–inorganic model biomembranes (C_nH_2n+1NH₃)₂SnCl₆, undergoing order–disorder and conformational phase transitions. No anomalies were observed in the laboratory-frame spin–lattice relaxation measurements at the order–disorder phase transitions, whereas a discontinuity was manifest at the conformational phase transitions characteristic of a first-order phase transition. On the other hand, our rotating frame spin–lattice relaxation measurements revealed a low-frequency critical collective chain dynamics in the kilohertz regime associated with the order–disorder phase transition.