Structural phase transition in ferroelectric fluorine polymers: X-ray diffraction and infrared/Raman spectroscopic study

The structural phase transition has been investigated by X-ray diffraction and infrared and Raman spectroscopic measurements for ferroelectric fluorine polymers, including poly(vinylidene fluoride) and its copolymers with trifluoroethylene or tetrafluoroethylene. One of the most characteristic features of this ferroelectric transition is the large conformational change of the molecular chains between the trans and gauche rotational isomers, quite different from the structural change observed generally in the usual ionic ferroelectric materials. The crystallization and transition behaviors depend sensitively on the monomer composition in the copolymers as well as on the sample preparation conditions. The roles of the optic and acoustic phonons in the ferroelectric phase transition have been discussed based on the temperature dependences of the far-infrared spectra and the ultrasonic velocity.     

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.     

Pressure-induced phase transformation of CsPbI3 by X-ray diffraction and Raman spectroscopy

The structural transformation of cesium lead iodine (CsPbI₃) has been investigated in diamond anvil cells up to ～15 GPa at room temperature by employing synchrotron radiation X-ray diffraction and Raman spectroscopy. One reversible transformation from orthorhombic (Pnma) to monoclinic (P2₁/m) phase has been observed at 3.9 GPa. Isothermal pressure–volume relationship of orthorhombic CsPbI₃ is well fitted by the third-order Birch–Murnaghan equation of state with K₀ = 14(3) GPa, K′₀ = 6(2) and V₀ = 891(7) ų. The ultralow value of bulk modulus K₀ demonstrates the high compressible nature of CsPbI₃, similar to those of organic–inorganic metal halide perovskites. The present results provide essential information on the intrinsic properties and stability of CsPbI₃, which may be applied in photovoltaic devices.     

High-pressure in-situ X-ray diffraction and Raman spectroscopy of Ca2AlFeO5 brownmillerite

The behavior of Ca₂AlFeO₅ brownmillerite was studied by in situ synchrotron X-ray diffraction and Raman spectroscopy at 300?K with pressures up to 26.5 and 32.1 GPa, respectively. A reversible structural phase transition was observed. The P–V data were fitted by a third-order Birch–Murnaghan equation of state, and the isothermal bulk modulus was obtained as K₀?=?181.9(76) GPa with K₀′_?=?4.4(17). If K₀′ was fixed to 4, K₀ was obtained as 183.8(20) GPa. Ca₂AlFeO₅ brownmillerite shows an axial elastic anisotropy since the b-axis is more compressible than a- and c-axis. Combined with previous results, the isothermal bulk modulus and axial compressibility of Ca₂AlFeO₅ brownmillerite increase with more Al incorporated in the structure. The Raman spectra of Ca₂AlFeO₅ brownmillerite were analyzed and the pressure coefficients vary from 2.23 to 4.90?cm^?1/GPa. The isothermal mode Grüneisen parameters range from 0.83 to 1.77 and the thermal Grüneisen parameter is determined as 1.08(11).     

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.     

Study of the P-T phase diagram of pyridinium perchlorate by X-ray diffraction and Raman spectroscopy

The crystal structure and vibrational spectra of deuterated pyridinium perchlorate (d ₅PyH)ClO₄ (C₅D₅NHClO₄) are studied by means of neutron diffraction in ambient conditions, X-ray diffraction at high pressures up to 3.5 GPa in the temperature range 297–420 K, and Raman spectroscopy at high pressures up to 5.7 GPa. Deuterated pyridinium perchlorate at ambient conditions has rhombohedral structure with the R3m symmetry (paraelectric phase I). Over the pressure range of 0.5–1.2 GPa, the phase II with monoclinic symmetry Cm exists. At pressure P ～ 1.2 GPa, the phase transition to monoclinic phase III with the Pm symmetry is observed at ambient temperature. The lattice parameters, unit cell volume, and frequencies of internal vibrational modes as functions of pressure are obtained for different phases of deuterated pyridinium perchlorate. The P-T phase diagram of (d ₅PyH)ClO₄ over the extended pressure and temperature range is discussed.     

Studies on Im-3-type KSbO3 using high pressure X-ray diffraction and Raman spectroscopy

In situ X-ray diffraction and Raman scattering experiments using a diamond anvil cell revealed that Im-3-type KSbO₃ remains stable up to 40.5?GPa with a bulk modulus K₀?=?101.6 (7)?GPa. Rietveld structure refinements and mode Grüneisen parameters suggested that the stability mechanism of this three-dimensional cubic tunnel structure was attributed to the isotropic compression for all types of Sb–O bonding in the unit of SbO₆ octahedron. Isotropic structure adjustment with external pressure reflected the nature that Im-3-type KSbO₃ model structure has a high ionic tolerance with a change in the chemical pressure in the isomorphous substitutions.     

The structure of solvated halide ions in dimethyl sulfoxide studied by Raman spectroscopy and X-ray diffraction

The solvation structure of chloride, bromide, and iodide ions, X⁻, in dimethyl sulfoxide (DMSO) has been investigated by using Raman spectroscopy and large angle X-ray diffraction under ambient conditions. The positively charged sulfur atom in DMSO interacts with X⁻ and slightly positively charged methyl groups in the coordinating molecules also interact with X⁻. The X⁻---S, X⁻···C, and X⁻···O distances are determined as follows: Cl⁻---S: 416, Cl⁻···C: 363, Cl⁻···O: 543, Br⁻---S: 433, Br⁻···C: 372, Br⁻···O: 544, I⁻---S: 437, I⁻···C: 374 and I⁻···O: 520 pm. The coordination numbers of DMSO molecules around the anions are six, seven and eight for Cl⁻, Br⁻ and I⁻ ions, respectively, with the uncertainty of ±1. Rather large uncertainties in the measured solvation numbers suggest large fluctuations in the solvation structure of the anions.     

High-pressure effect on inverse spinel LiCuVO4:X-ray diffraction and Raman scattering

The effect of external quasi-hydrostatic pressure on the inverse spinel structure of LiCuVO 4 was studied in this paper. High-pressure synchrotron X-ray diffraction and Raman spectroscopy measurements were carried out at room temperature up to 35.7 and 40.3 GPa, respectively. At a pressure of about 20 GPa, both Raman spectra and X-ray diffraction results indicate that LiCuVO4 was transformed into a monoclinic phase, which remained stable up to at least 35.7 GPa. Upon release of pressure, the high-pressure phase returned to the initial phase. The pressure dependence of the volume of low pressure orthorhombic phase and high-pressure monoclinic phase were described by a second-order Birch-Murnaghan equation of state, which yielded bulk modulus values of B 0 = 197(5) and 232(8) GPa, respectively. The results support the empirical suggestion that the oxide spinels have similar bulk modulus around 200 GPa.     

Cu-Zn disorder in Cu2ZnGeSe4: A complementary neutron diffraction and Raman spectroscopy study

The crystal structure of the quaternary compound semiconductor Cu₂ZnGeSe₄ (CZGSe) was investigated by the complementary use of neutron diffraction, and Raman spectroscopy. The powder sample, which resulting from wavelength dispersive X-ray spectroscopy (WDX) turned out to be single phase Cu-rich CZGSe, was synthesized by solid state reaction of the pure elements in an evacuated silica tube at 700 °C. Raman spectroscopy confirmed the homogeneity and phase purity of the sample, in addition, the kesterite type structure was suggested. Rietveld analysis of the neutron diffraction data confirmed that the compound crystallizes in the tetragonal kesterite type structure. The refined site occupancy factors were used to determine the average neutron scattering lengths of the cation sites, giving insights into cation distribution and finally point defect types. Thus it has been shown, that additional to the Cu_Zn-Zn_Cu anti-site defects in the lattice planes at z=¼ and ¾ (Cu-Zn disorder) also the off-stoichiometry type related point defects like Cu_i and Cu_Zn occur in Cu-rich CZGSe.     

X-ray photoelectron spectroscopy and near-edge X-ray-absorption fine structure of C60 polymer films

We report core-level and valence-band X-ray photoelectron spectroscopy (XPS) and carbon [ _]K near-edge X-ray-absorptionfine structure spectroscopy (NEXAFS) results of plasma-polymerized C₆₀. In comparison with evaporated C₆₀ the C 1s peak is broader and asymmetric for the C₆₀ polymer and its shake-up satellites diminished. Furthermore, the features of the valence-band as well as the features of the π^* antibonding orbitals of the C₆₀ polymer are broader and reduced in intensity. Changes in the electronic structure are attributed to the polymerization of C₆₀, the post-plasma functionalization of the surface by oxygen after exposure to atmosphere, and the occurrence of amorphous carbon. Received: 28 May 1999 / Accepted: 31 August 1999 / Published online: 8 March 2000     

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.     

Ferroelastic domains in lead phosphate-arsenate: An afm,X-ray diffraction,tem and Raman study

Ferroelastic domain patterns in lead phosphate-type crystals depend on their chemical composition. The intersection of ferroelastic W domain walls with the (100) surface of lead phosphate-arsenate mixed crystals has been imaged using tapping mode atomic force microscopy. Dilution of the strain leads to characteristic surface deformations which deviate from those in pure lead phosphate. In highly twinned lead phosphate-arsenate X-ray diffraction was used to show renormalization effects and scattering phenomena stemming from the twin walls. Raman-active hard modes show wall-related profile changes and low-frequency tails. Symmetry mode analysis is used to describe the displacive modes characterizing the structural distortion in lead phosphate-arsenate for the sequence K m → P2₁/c.     

Investigation of porous InP by X-ray diffraction,IR spectroscopy,USXES, XANES spectroscopy,and photoluminescence

Complex investigation of the layers of porous indium phosphide (por-InP), obtained by anodic pulsed electrochemical etching of single-crystal n-InP(100) wafers (n ～ 10¹⁸), has been performed using X-ray diffraction, IR spectroscopy, ultrasoft X-ray spectroscopy, X-ray absorption near-edge structure spectroscopy, and photoluminescence. The data obtained indicate that the surface layers of por-InP have a cluster structure and contain InP quasi-molecules.