Pressure-induced transformations in two-dimensional polymeric phases of C<Subscript>60</Subscript>

The structural stability of the tetragonal and rhombohedral two-dimensional (2D) polymeric phases of C₆₀ was studied under pressures up to 27 GPa at room temperature by means of in situ Raman scattering spectroscopy. The results show that the tetragonal 2D phase undergoes an irreversible transformation in the region of 20 GPa while no pressure-induced transitions were observed for the rhombohedral 2D phase. The obtained data are discussed within the framework of recent numerical calculations, which predict the pressure-induced transformation of the 2D polymeric phases of C₆₀ into three-dimensional (3D) polymers in the pressure range 14–20 GPa.     

Structure of C<Subscript>60</Subscript> polymer coatings deposited via electron-beam dispersion of fullerite

The structure and surface of thin coatings deposited via electron-beam dispersion of the C₆₀ fullerite have been investigated using IR and Raman spectroscopy, mass spectroscopy, and atomic-force microscopy. It has been demonstrated that layers with different contents of the polymerized phase, crystals of the tetragonal polymer phase, and three-dimensional polymeric forms of the C₆₀ fullerene are formed under the conditions providing for irradiation by secondary electrons in vacuum at a substrate temperature of 300 K.     

Spectroscopic study of phase transformations between orthorhombic and tetragonal C60 polymers

We present a detailed study of transformations between the orthorhombic and tetragonal polymeric states of C₆₀. The transformations are characterised by Raman spectroscopy and X-ray diffraction. We show that the transformation from the orthorhombic (O) phase to the tetragonal (T) phase is very fast and our results indicate that the transformation goes via an intermediate dimer (D) state in a two-stage process, O↦D and, D↦T transformations, where the second process is slower than the first. On the other hand, the transformation from the tetragonal to the orthorhombic phase is significantly slower, indicating a high-energy threshold to break the polymer bonds at the temperatures used. The results also support earlier suggestions that the tetragonal phase contains disordered dimers that can be viewed as lattice defects in the formation of higher polymers.     

三方和四方相PbZr0.5Ti0.5O3 的结构稳定性和电子结构的第一性原理研究

采用基于密度泛函理论的第一性原理超原胞方法和虚晶近似方法, 在局域密度近似和广义梯度近似下系统研究了三方相和四方相 PbZr_0.5Ti_0.5O₃的能量稳定性、原子结构以及电子结构. 计算结果表明三方相的能量比四方相低, 说明三方相结构更加稳定, 并且发现利用广义梯度近似计算的结构参数与实验值符合得更好. 电子结构表明, 两种相的Ti/Zr的3d电子和O的2p电子间存在明显的轨道杂化, 并且Ti-O之间的作用比Zr-O作用更强;Pb的6s和5d电子与O的2s和2p电子也分别存在轨道杂化. 而三方相中Pb的5d电子与O的2s电子杂化比四方相更强, 进一步说明三方相比四方相结构更加稳定.     

Structural aspects of two-dimensional polymers: Li4C60, Na4C60 and tetragonal C60. Raman spectroscopy and X-ray diffraction

We present an analysis of three different two-dimensional polymers, tetragonal C₆₀, Li₄C₆₀, and Na₄C₆₀. Based on X-ray diffraction and Raman spectroscopy, we conclude that Li₄C₆₀ forms a tetragonal structure with intermolecular bonds formed by 2+2 cycloaddition, in the same way as for tetragonal C₆₀. Na₄C₆₀, on the other hand, forms a monoclinic structure with single C–C bonds between the molecules. Our Raman spectroscopy results can be interpreted in two ways: either the charge transfer to the C₆₀ molecules is the same in both doped compounds with four electrons/molecule or the electron charge transfer is smaller from the Li ions than from the Na ions.     

Properties of C60 polymerized under high pressure and temperature

60 polymers. Pure and mixed phase polymeric samples were synthesized by simultaneously subjecting microcrystalline C₆₀ powder or pellets to various pressures () and temperatures (). The optical spectra of the orthorhombic, tetragonal, and rhombohedral C₆₀ polymer phases are observed to be quite distinct and rich. These spectra exhibit numerous lines and an overall downshift in frequency relative to C₆₀ is observed, consistent with a loss of double bonds from the fullerene cage. The LDMS spectra of a sample synthesized at under hydrostatic conditions and , exhibited a succession of clear peaks at mass numbers corresponding to , similar to the LDMS data on the C₆₀ photopolymer. This is taken as further evidence for interfullerene bonds in these high-pressure polymers. The XRD pattern of this sample indicates the presence of a strong texture in the sample. Received: 14 November 1996/Accepted: 8 January 1997     

Pressure-induced transformations and optical properties of the two-dimensional tetragonal polymer of C60 at pressures up to 30 GPa

The Raman spectra of the two-dimensional tetragonal (2D(T)) polymeric phase of C₆₀ have been studied in situ at pressures up to 30 GPa and room temperature. The pressure dependence of the phonon modes shows an irreversible transformation of the material near 20 GPa into a new phase, most probably associated with the covalent bonding between the 2D polymeric sheets. The Raman spectrum of the high-pressure phase is intense and well resolved, and the majority of modes are related to the fullerene molecular cage. The sample recovered at ambient conditions is in a metastable phase and transforms violently under laser irradiation: the transformed material contains mainly dimers and monomers of C₆₀ and small inclusions of the diamond-like carbon phase. The photoluminescence spectra of the 2D(T) polymer of C₆₀ were measured at room temperature and pressure up to 4 GPa. The intensity distribution and the pressure-induced shift of the photoluminescence spectrum drastically differ from those of the C₆₀ monomer. The deformation potential and the Grüneisen parameters of the 2D(T) polymeric phase of C₆₀ have been determined and compared with those of the pristine material.     

A polarized micro-Raman study of a 0.65PbMg1/3Nb2/3O3 - 0.35PbTiO3 single crystal

Polarized micro-Raman spectra of a 0.65PbMg_1/3Nb_2/3O₃-0.35PbTiO₃ (0.65PMN-0.35PT) single crystal poled in the [001] direction are obtained in a wide frequency range (50-2000 cm^-1) at different temperatures. The best fit to the Raman spectrum at 77 K is achieved using 17 Lorenzians to convolute into it, and this is proved to be a reasonable fit. According to the group theory and selection rules of overtone and combinational modes, apart from the seven Raman modes that are from first-order Raman scattering, the remaining ones are attributed to being from second-order Raman scattering. A comparison between the experimental results and theoretical predictions shows that they are in satisfactory agreement with each other. Our results indicate that at 77 K the sample belongs to the rhombohedral symmetry with the C_3v⁵ (R3m) space group (Z=1). In our study, on heating, the 0.65PMN-0.35PT single crystal undergoes a rhombohedral to tetragonal to cubic phase transition sequence. The two phase transitions occur at 340 and 440 K, which correspond to the disappearance of the soft mode near 106 cm^-1 recorded in VV polarization and the vanishing of the band around 780 cm^-1 in VH polarization, respectively.     

Phase transformation in (0.90−x)Pb(Mg1/3Nb2/3)O3-xPbTiO3-0.10PbZrO3 piezoelectric ceramic: X-ray diffraction and Raman investigation

Piezoelectric ceramics with compositions of (0.90−x)Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃-0.10PbZrO₃, x=0.28, 0.31, 0.34, 0.37, 0.40 and 0.43, were prepared using the conventional columbite precursor method, and their structural phase transformation and piezoelectric behaviors near the morphotropic phase boundary (MPB) have been systematically investigated as a function of PbTiO₃ content. X-ray diffraction (XRD) results demonstrate that the structure of the ceramics experiences a gradual transition process from rhombohedral phase to tetragonal phase with the increasing of PbTiO₃ content, and that compositions with x=0.34-0.40 lie in the MPB region of this ternary system. A Raman spectra investigation of the ceramic samples testified to the transformation process of rhombohedral phase to tetragonal phase by comparing the relative intensities of tetragonal E(2TO₁) mode and rhombohedral phase R_h mode. The structure information was also correlated to the parabola change of the piezoelectric constant; the maximum piezoelectric constants were obtained near the MPB region.     

Raman spectroscopy study of lattice dynamics of macro-, micro-, and nanostructured barium titanates

The temperature dependences of the components A ₁(2TO) and E(1TO) of the soft ferroelectric mode during phase transitions in single crystals, ceramics, polycrystalline and epitaxial thin films of barium titanate, as well as a superlattice consisting of alternating layers of barium and strontium titanates, have been studied using the Raman spectroscopy method. Abrupt changes in soft mode frequencies have been observed in the single crystal during phase transitions between tetragonal, orthorhombic, and rhombohedral phases. Smoothing of the temperature dependences of soft modes and the coexistence of phases have been observed in ceramics and polycrystalline films. In the epitaxial film, the sequence of structural transformations fundamentally differs from that observed in the single crystal; in the superlattice, the ferroelectric phase is stable to 550 K.     

Electronic interactions in two-dimensional polymers of the C60 fullerene

The electronic structure of tetragonal and rhombohedral polymers of the C₆₀ fullerene is investigated using x-ray emission spectroscopy. It is found that, compared to the C₆₀ molecular crystals, the formation of intermolecular covalent bonds in two-dimensional layers of the C₆₀ fullerene polymers leads to a broadening of the maxima in the CK _α x-ray emission spectra, a decrease in the density of high-energy states, and an increase in the width of the valence band of the polymer. The experimental data are interpreted by analyzing the results of the calculations performed within the density functional theory for the C₆₀ fullerene cage forming eight and twelve covalent bonds. It is shown that the electronic interactions between C₆₀ molecules in the polymerized layers are provided by two types of molecular orbitals located at energies 0.5–3.0 and ∼5.0 eV below the energy of the Fermi level.     

2D polymerization and doping of fullerenes under pressure

Abstract

Tetragonal polymeric C₆₀ has been studied by Raman spectroscopy and other methods. Attempts have been made to transform samples from the tetragonal to the orthorhom-bic phase and vice versa. The results suggest that the transformation is direct with no intermediate stage with free molecules. Tetragonal C₆₀ has also been intercalated by potassium metal.     

Phase transition behavior and proton conduction mechanism in cesium hydrogen sulfate/silica composite

Proton conduction and crystal structure in CsHSO₄/SiO₂ composite composed of polycrystalline CsHSO₄ and mesoporous silica particles were investigated based on conductivity measurement and characterizations using Raman spectroscopy, XRD, and differential thermal analysis. The conductivity of pure CsHSO₄ abruptly changes at around 414 K (superprotonic phase transition), being accompanied with the structural transformation from a monoclinic phase to a tetragonal phase, while the conductivity of CsHSO₄/SiO₂ composite is significantly larger by over three orders of magnitude than that of pure CsHSO₄ below the critical temperature of the superprotonic phase transition (353-414 K). Raman spectroscopy and XRD indicate that this remarkable conductivity-enhancement in the composite is not due to the stabilization of the tetragonal phase (superprotonic phase) below its critical temperature. The line-broadening of the internal modes in the Raman spectra suggests that the rapid reorientational motion of the HSO₄⁻ ion, which leads to superprotonic conduction, is induced in the composite even below the critical temperature. The reorientational motion of the HSO₄⁻ ion below the critical temperature will occur at the interfacial phase which is structurally disordered and forms between CsHSO₄ and SiO₂ in the mesopores and/or on the surfaces of silica particles. Proton transfer will be accelerated via the interfacial conduction-pathway in the composite.     

Photo-and pressure-induced transformations in the linear orthorhombic polymer of C<Subscript>60</Subscript>

Stability of the linear orthorhombic polymer of C₆₀ under pressure and laser irradiation is studied by Raman scattering and X-ray diffraction measurements. The Raman spectrum at ambient pressure remains unchanged, in the time scale of the experiment, up to an intensity of 3200 W/cm² of the 514.5 nm line of an Ar⁺ laser, but irreversible changes are observed at higher intensities. The Raman spectra recorded at increased pressure show similar irreversible changes even at the laser intensity as low as 470 W/cm². The X-ray diffraction and Raman measurements of the pressure-treated samples, performed after pressure release, show that the nonirradiated material does not exhibit any changes in the crystal structure and phonon spectra. This behavior indicates a pressure-enhanced photo-induced transformation to a new polymeric phase characterized by a Raman spectrum that differs from those of the other known polymeric phases of C₆₀. The Raman spectra of the phototransformed linear orthorhombic polymer of C₆₀ were measured at a pressure of up to 29 GPa. The pressure dependence of the Raman mode frequencies show singularities near 4 GPa and 15 GPa, respectively, related to a reversible phase transition and an irreversible transformation to a metastable disordered phase. The diffuse Raman spectrum of the disordered phase does not exhibit substantial changes with an increase in pressure up to 29 GPa. The high-pressure phase transforms to a mixture of pristine and dimerized C₆₀, after pressure release and exposure to ambient conditions for 30 h. The text was submitted by the authors in English.     

Electric-field-induced tetragonal phase in [Pb(Mg1/3Nb2/3)O3]0.68–[PbTiO3]0.32 by Raman scattering

ABSTRACT

The paper describes investigation of the electric-field-induced phase transition in PMN-xPT single crystals with x = 0.32 by means of detailed polarized Raman scattering measurements. Field-induced tetragonal state was reached by applying an electric field up to 30 kV/cm along pseudo-cubic direction at a temperature slightly below that of the zero-field rhombohedral–tetragonal phase transition (T_RT). In this field-induced tetragonal state, the angular dependencies of the polarized Raman spectra were recorded and compared with the similar data available for the rhombohedral single-domain and multidomain states.     

铁掺杂四方、三方两相共存PZN-PZT陶瓷的Raman散射分析

Raman散射常用于研究温度变化时铁电体的各种相变,如铁电-顺电相变等,但是,利用Raman散射研究掺杂诱导的相变并不常见。本文采用Raman散射研究了铁掺杂Pb(Zn1/3Nb2/3)0.2(Zr0.5Ti0.5)0.8O3(PZN-PZT)铁电陶瓷中三方-四方共存现象。通过分析四方相E(3TO)和A1(3TO)模式与三方相Rl模式,确定了铁掺杂对PZN-PZT陶瓷中三方-四方共存结构的影响。这一结果,得到了XRD相分析的验证。因此,通过对Raman散射中三方-四方振动模式的分析,可以表征掺杂诱导的PZT基陶瓷中三方-四方相结构变化趋势。     

Determination of the phase transitions in BaTiO3 type crystals by using the theory of symmetry breaking: Etude des transitions de phase dans les cristaux du type BaTiO3 par la theorie des symetries brisees

Crystaks of the BaTiO₃ family showing the following sequence of phase transitions : tetragonal, orthorhombic, rhombohedral, are investigated. From the MASON-MATHIAS model based on a TiO₆ octahedron in which a Ti ion can be displaced along the different pseudo-cubic axes, these different polar structures are found by considering the possible lowering of the O_h symmetry associated with the non polar phase. By the appropriate orientation of the basis of the vector representation connected with the occurence of the polarisation $\text{P}$, the order-parameter of the phase transitions are determined for three cases (O_h → C_4v, C_2v, C_3v). The order of the transition is discussed. These parameters can be used for a thermodynamical investigation of the crystal based on an expansion of its free energy.     

Raman spectra of samarium-fullerene intercalation compounds

Samarium-fullerene intercalation compounds of nominal composition Sm_xC₆₀ (x=1,2,…,6) have been synthesized by a solid-state reaction method. We obtain a Sm_2.75C₆₀ superconducting phase with orthogonal structure and a Sm₆C₆₀ phase with body-centered cubic structure. The broadening and weakening of Raman peaks of the Sm_xC₆₀ compounds are due to the distortion of C₆₀ and electron-phonon interaction. The Raman measurements reveal that the distortion of C₆₀ decrease in Sm_xC₆₀ (x=3,4,5) exposed to air, although the fulleride solids have transformed into an amorphous state. The Raman results also show that the distortion of C₆₀ is still very large in the Sm₆C₆₀ exposed to air, or the C₆₀ molecules have been destroyed and become some amorphous carbide.     

High-temperature X-ray diffraction and Raman scattering studies of Ba-doped (Na0.5Bi0.5)TiO3 Pb-free piezoceramics

The structural changes in (100 ? x)Na_0.5Bi_0.5TiO₃–xBaTiO₃ (0 ≤ x ≤ 10) ceramics were investigated as a function of composition and temperature by X-ray diffraction and Raman spectroscopy. As Ba concentration increases, the structure changed from rhombohedral to tetragonal (x ≥ 6.5) across a morphotropic phase boundary like phase coexistence at x ～ 5.5, which is further evidenced by phonon anomalies observed in composition-dependent Raman spectra. On heating, the disappearance of peak splits in {111} (x ≤ 5) and {200} (x ≥ 6.5) Bragg peaks and the changes in their 2θ-positions indicated temperature-driven structural changes: ferroelectric to antiferroelectric (≈T_d, depolarization temperature) at 220 °C and antiferroelectric to paraelectric (rhombohedral to tetragonal) at 320 °C. In addition, Raman spectral analysis suggested that at elevated temperatures, two tetragonal phases with slightly different space groups coexisted at x ≥ 6.5 and most of the phase transition temperatures shifted towards left with increasing x.