Structural features of the polymer-like phase of AC60 fullerides

A new approach is proposed for describing the deformation of the C₆₀ molecule in the polymer-like phase of the fullerides AC₆₀ (A=K, Rb). A discrete basis of orientational states of the molecule is introduced, and orientational ordering is described on the basis of a phenomenological theory. A symmetry analysis of the molecular vibrations in the Fm3m phase is performed. It is shown that small rotations of the C₆₀ can occur at a phase transition. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 2, 110–115 (25 July 1996)     

Effect of low-temperature compression on the parameters of the phase transition at 250–260 K in C60 single crystals

A thermodynamic transition in C₆₀ crystals, subjected beforehand to uniaxial compression at various temperatures and to thermal action, was investigated by differential scanning calorimetry. It was established that compression of the crystal at temperatures below the phase transition has a much smaller effect on the transition itself than a similar or even much weaker action at a temperature above the phase transition. A “quenching” effect was also found. This effect is probably due to the orientational order of individual fullerene molecules. A correlation was established between the magnitude of the effect and the preliminary deformation of C₆₀ crystals. Fiz. Tverd. Tela (St. Petersburg) 41, 550–553 (March 1999)     

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.     

Experimental study of the Grüneisen parameter of a fullerite C60 single crystal near phase transitions at 90 and 260 K by the photoacoustic method

The behavior of the Grüneisen parameter of single-crystal fullerite C₆₀ has been studied experimentally near orientational phase transitions at 90 and 260 K. The measurements have been performed by the photoacoustic method using an automated photoacoustic device with laser excitation (the intensity was modulated by a harmonic law) and shadow piezoelectric detection. The temperature dependence of the relative change in the Grüneisen parameter in the fullerite C₆₀ single crystal has been measured near the phase transitions at 90 and 260 K. The results have been analyzed.     

Laser Raman study of phase transformations in [N(CH3)4]2ZnCI4 single crystals

We report temperature-dependent Raman studies on single crystals of [N(CH₃)₄]₂ZnCI₄ from 300 to 10 K. The observed spectral features suggest that both the N(CH₃)₄ ⁺ and ZnCl^2- ₄ ions are distorted from their regular tetrahedral structure and occupy sites of C_s symmetry in the lattice at room temperature. From the variation of line width of some selected Raman bands and other spectral changes as a function of temperature, it is inferred that both the ZnCl^2- ₄ and—CH₃ groups have high motional freedom at room temperature and the different phase transitions up to 160 K are triggered by the gradual freezing-in of orientational freedom of these groups, while the N—C₄ tetrahedra do not play any significant role in these phase transitions. The monoclinic to orthorhombic superlattice phase transitions at 159 K is triggered by freezing-in of the orientational motions of both the ZnCl^2- ₄ and N(CH₃)⁺ ₄ groups in the lattice.     

Analysis of the parameters of a smeared orientational transition at 250–260 K in C60 crystals

The orientational structural transformation in C₆₀ crystals at temperatures of 250–260 K is investigated within the theory of smeared first-order phase transitions. The parameters of this transformation are analyzed using the experimental temperature dependences of the heat capacity and the inelastic (ferroelastic) strain rate in the phase transition range. The elementary volume of the transformation (11–83 nm³) in a correlated motion of C₆₀ molecules and the spontaneous shear strain of the lattice (2.4×10^?2) upon its transformation from the simple cubic to the face-centered cubic structure are determined.     

C60分子晶体中的类非晶散射研究

根据Ｘ射线衍射基本理论，计算了Ｃ₆₀分子晶体中的各种堆积方式下的Ｘ射线衍射花样，对Ｃ₆₀分子晶体中的类非晶散射产生的原因进行了探讨，指出在Ｃ₆₀分子晶体中，Ｃ₆₀分子有两种运动方式．非晶散射是Ｃ₆₀分子晶体固有结构特征所造成的 关键词：     

Electronic properties of C60 single crystals doped with lithium by electrodiffusion

Diffusion of lithium cations in C₆₀ single crystals driven by electric field has been detected and studied. A novel technique for fullerene crystal doping based on injection of ions through a “superionic crystal/C₆₀ single crystal” heterojunction has been suggested. It has been found that lithium doping of C₆₀ single crystals brings about an ESR signal, and this signal as a function of time has been investigated. The electronic conductivity in Li_xC₆₀ crystals has a nonmetallic nature. Reflection spectra measured in the IR band have shown that the reflectivity due to free electrons gradually decreases with time, which correlates with the evolution of signals due to ESR and microwave conductivity. Lithium doping of crystals increases the oscillator strength of the T _1u (4) vibrational mode and shifts it to lower frequencies (from 1429 cm⁻¹ to 1413 cm⁻¹), which indicates that one electron is present at the C₆₀ molecule, and this fact may be treated as evidence that the LiC₆₀ phase is generated in a C₆₀ crystal. Zh. éksp. Teor. Fiz. 116, 1706–1722 (November 1999)     

Ab initio calculations of supramolecular complexes of fullerene C60 with CdTe and CdS

This paper presents the results of ab initio quantum-chemical calculations of supramolecular complexes C₆₀CdHal, [C₆₀]₄CdHal, and [C₆₀]₆CdHal (Hal = S, Te), which simulate the defects forming in fullerite during the absorption or adsorption of cadmium telluride (sulfide). Calculations of the electronic structure of complexes with inclusion of their relaxation to the equilibrium state have been performed in terms of the density functional theory with the B3LYP hybrid functional. The obtained enthalpies of formation of complexes show that their formation leads to the energy gain of the order of 0.5–1.5 eV depending on the complex type. It has been shown that the formation of tetrahedral complexes [C₆₀]₄CdTe with the intercalated CdTe molecule is possible only with a considerable distortion of the tetrahedral void. The energy spectrum of low-lying excited electron states for the linear and octahedral complexes has been calculated. It has been found that a decrease in symmetry with the formation of complexes leads to the appearance of excited states of allowed singlet transitions in the electron spectrum, which are forbidden in optical spectra of initial components.     

Crystal-field induced mixing of electron states in C60 crystals at high pressure

Optical absorption spectra of thin fullerene (C₆₀) crystals in the range 1.7 to 3.8 eV have been measured at T=300 K and at pressures up to 2.5 GPa. The spectrum shifts toward the red with pressure, and the electron absorption intensity is redistributed among its bands. The intensity of the band associated with the lowest direct electron interband transition monotonically increases with pressure, whereas the intensity of the upper interband feature decreases. Bands related to weak edge absorption in the range between 1.7 and 2.2 eV gradually merge with the band associated with the lowest interband transition, whose intensity rises with pressure. A similar redistribution of intensity among electron transition bands has been observed when comparing the spectrum of an isolated C₆₀ molecule and that of a C₆₀ crystal. The results indicate that the crystal-field induced mixing of electron states is present in solid C₆₀, and they can be discussed in terms of the Craig-McClure model, which was suggested to describe crystal-field induced mixing of electron states in anthracene and naphthalene molecular crystals. Zh. éksp. Teor. Fiz. 113, 313–322 (January 1998)     

Permutation-inversion symmetry of C70 fullerite in the high-temperature phase

The permutation-inversion symmetry group of C₇₀ fullerite in its high-temperature phase is constructed with allowance for the rotation of its constituent molecules, and the local symmetry group of a rotating molecule in the crystal is identified. Irreducible representations of these groups are constructed that are compatible with the principle of wave-function symmetry with respect to permutations of identical nuclei. A group-theoretic classification is made of the quantum states of a rotating molecule and of the crystal in the high-temperature phase of C₇₀ fullerite. Selection rules are derived for electronic, vibrational, and rotational spectra in terms of irreducible representations of the permutation-inversion symmetry group of the crystal. Fiz. Tverd. Tela (St. Petersburg) 39, 1895–1901 (October 1997)     

X-ray scattering study of the phase transitions in crystalline fullerene-biphenyl C 60 [(C 6 H 5 ) 2 ]

The phase diagram of the newly synthesized mixed crystal C₆₀-biphenyl is investigated as a function of temperature by single-crystal X-ray scattering. Diffuse scattering investigations evidencing complex disorder and local order effects are presented. Two phase transitions leading to two different doublings of the high temperature unit cell are observed, at 212 K and 147 K. The first transition is attributed to the ordering of twisted biphenyls, which couples to the orientational ordering of the C₆₀ molecules as the temperature decreases. Full ordering of the C₆₀ molecules is achieved below 100 K only, in the low temperature phase. The rich phase diagram of C₆₀-biphenyl is due to the interplay between fullerene and biphenyl ordering phenomena. Received 31 August 2001 and Received in final form 4 December 2001     

Fluctuation interaction of Ising subsystems

A phase diagram of two Ising subsystems σ and s has been constructed on a Bethe lattice with a coordination number 4 (a simplified analog of a square lattice). In contrast to the known Ashkin-Teller model, the interaction between these two subsystems is of a purely fluctuational nature; i.e., it does not manifest itself in the ground state and nullifies the sums of the products of average spins 〈σ〉〈s〉 (interactions of this type are realized in lattice-type adsorbed systems with dipolelike intermolecular interactions and strong azimuthal angular dependence of the adsorption potential of symmetry C₄). Apart from conventional states, i.e., a high-temperature disordered state (〈σ〉=〈s〉=0) and a low-temperature ordered state (〈σ〉 and 〈s〉 =? 0), this system can also exist in a correlated state (〈σs〉 =? 0 at 〈σ〉=〈s〉=0). In the theory of orientational phase transitions, this state corresponds to a fundamentally different, intermediate (on the temperature axis) phase in which a preferred direction of long molecule axes arises in the absence of spontaneous polarization. The results of Monte Carlo simulation on a square lattice agree with the conclusions obtained on a Bethe lattice. The characteristics of the orientational phase transition in a 2 × 1 monolayer of CO molecules adsorbed on the NaCl(100) surface are discussed.     

Long-lived excited states and photoluminescence excitation spectra in single crystals of fullerene C60

Photoluminescence, optical absorption spectra, and photoluminescence excitation spectra were measured on large (2–3 mm), very pure crystals of fullerene C₆₀ at 5 K. It is shown that the main contribution to the photoluminescence of these crystals is from singlet and triplet excitons captured on crystal defects. The concentration of these defects does not exceed 10¹⁸ cm⁻²³, and the lifetime of triplet excitons on these defects is about 3 ms. It is shown that the symmetry distortion of the C₆₀ molecules at the defects is rather large and causes the oscillator strength of the zero-phonon optical transitions to be comparable to the most intense optical transitions with the participation of intramolecular vibrations. Zh. éksp. Teor. Fiz. 113, 734–746 (February 1998)     

High-pressure synthesis, structural and Raman studies of a two-dimensional polymer crystal of

Two-dimensional polymerisation of a C₆₀ single crystal has been obtained under high-pressure high temperature conditions (700 K - 2 GPa). Crystalline order is preserved but the crystal splits into variants (orientational domains). The analysis of X-ray diffraction and Raman spectroscopy data reveals that the polymer crystal is primarily tetragonal with some admixture of rhombohedral phase. Furthermore, Raman spectroscopy gives evidence for additional C₆₀-C₆₀ dimers, which are probably disordered. For the tetragonal phase, it is shown that successive polymer layers are rotated by about the stacking axis, according to the P4₂/mmc space group symmetry. The structure of the rhombohedral phase is also clarified. The role of the interlayer interactions in stabilising the two-dimensional polymer phases of C₆₀ is discussed. Received 8 October 1999     

Excitons in monoclinic zinc diphosphide. Longitudinal exciton and the mixed mode

Exciton absorption spectra in high-quality β-ZnP₂ single crystals have been investigated at T=1.7 K for various directions of the wave vector and various polarization states of the radiation. It has been unambiguously established that the additional high-energy A series, which in some works has been called a D series and ascribed to ZnP₂ crystals, of so-called “rhombic” symmetry,^1,8,10,11 is an intrinsic exciton of the β-ZnP₂ series. A mixed mode has been detected for the first time, and the energy of the longitudinal exciton has been determined. The selection rules for the exciton transitions have been analyzed by a group-theoretical approach, and the symmetry of the nS states of the single exciton has been established on the basis of the experimental data — Γ ₂ ⁻ (z). Fiz. Tverd. Tela (St. Petersburg) 41, 193–202 (February 1999)     

Relaxation contribution to shear moduli of the low-temperature phase of solid C60

The relaxation contribution of the molecular reorientation in an elastic field of an acoustic wave to the effective shear moduli is calculated in the framework of the phenomenological two-level model of orientational states in the low-temperature phase of solid C₆₀. The polarity of the rotation axis of C₆₀ molecules and the possible existence of orientational domains in the structure of the low-temperature phase are taken into consideration. The estimates obtained are compared with the available experimental data.     

Jahn-Teller transitions in Yb<Emphasis Type="Italic">X</Emphasis>O<Subscript>4</Subscript> (<Emphasis Type="Italic">X</Emphasis>=V,P) stimulated by a strong magnetic field

The effect of an external magnetic field directed along various symmetry axes of a crystal on Jahn-Teller-type structural phase transitions (quadrupole ordering) is studied in YbPO₄ and YbVO₄ crystals with zircon structure. In the absence of a magnetic field, the crystals are in a precritical state and do not exhibit a spontaneous quadrupole ordering. It is shown that, in a field H ∥ [110], the strain susceptibility χ_γ increases with the field and, at a sufficiently high field strength, an orthorhombic lattice deformation along the [100] axis arises in the crystals under study; i.e., a stimulated Jahn-Teller phase transition of γ symmetry occurs. Using interaction constants determined from independent experiments, we calculated phase diagrams and anomalies in the magnetic and magnetoelastic properties of the YbPO₄ and YbVO₄ crystals near the stimulated phase transitions, investigated the effect of various pairwise interactions on them, and analyzed possible experimental observations of the predicted effects.     

Molecular dynamics simulations of a flexible liquid crystal

Molecular dynamics simulations have been performed for a liquid crystal composed of a Gay—Berne core site with two alkyl chains of different length (C₇ and C₃) at either end of the molecule. Calculations have been carried out for 512 molecules in the NVT ensemble for simulation times of up to 8.0ns at two distinct densities. The liquid crystal phases of the material have been fully characterized by measurements of orientational order parameters and radial distribution functions in each phase. Results are also presented for conformational distributions and effective torsional potentials of the system. We conclude that models of this nature represent a powerful approach to the study of flexibility in mesogenic systems and open up possibilities for predicting both the phase behaviour and bulk properties of liquid crystals based solely on a prior knowledge of intermolecular interactions.     

Quadratic stark effect in the fullerene C60 at low symmetry orientation in the field

Results of numeric simulation of the influence of the electric field E = 0 ? 1 V/Å on the electronic structure of the neutral fullerene C₆₀ taking into account orientational deformation of its carbon cage at arbitrary orientations in the electric field including low symmetry orientations are presented. Splitting of the frontier t _1u - and h _u -levels of the molecule due to the quadratic Stark effect has been investigated. Dependencies of the effective electron work function and the energy gap between the lowest unoccupied and highest occupied molecular orbitals on the strengths of the electric field have been determined.