D. Palles

Unusual polymerization in the Li4C60 fulleride

Li4C60, one of the best representatives of lithium intercalated fullerides, features a novel type of 2D polymerization. Extensive investigations, including laboratory x-ray and synchrotron radiation diffraction, 13C NMR, MAS and Raman spectroscopy, show a monoclinic I2/m structure, characterized by chains of [2+2]-cycloaddicted fullerenes, sideways connected by single C-C bonds. This leads to the formation of polymeric layers, whose insulating nature, deduced from the NMR and Raman spectra, denotes the complete localization of the electrons involved in the covalent bonds.

On the nature of the laser irradiation induced reversible softening of phonon modes in C60 single crystals

Abstract The average temperature rise in the laser excitation spot of the C60 single crystal has been determined using the Stokes to anti-Stokes integrated peak intensity ratio for the Hg(1) phonon mode. The reversible softening of the Ag(2) pentagon pinch mode was found to be due to the heating of the sample caused by the laser irradiation, in agreement with experimental results obtained for uniformly heated samples. These findings are in quantitative agreement with results obtained by numerical calculations of the local temperature rise, which indicates the highly non-uniform temperature distribution in the laser excitation spot associated with the small thermal conductivity of solid C60.

Transition and post-transition metal ions in borate glasses: Borate ligand speciation, cluster formation, and their effect on glass transition and mechanical properties

A series of transition and post-transition metal ion (Mn, Cu, Zn, Pb, Bi) binary borate glasses was studied with special consideration of the cations impact on the borate structure, the cations cross-linking capacity, and more generally, structure-property correlations. Infrared (IR) and Raman spectroscopies were used for the structural characterization. These complementary techniques are sensitive to the short-range order as in the differentiation of tetrahedral and trigonal borate units or regarding the number of non-bridging oxygen ions per unit. Moreover, vibrational spectroscopy is also sensitive to the intermediate-range order and to the presence of superstructural units, such as rings and chains, or the combination of rings. In order to clarify band assignments for the various borate entities, examples are given from pure vitreous B2O3 to meta-, pyro-, ortho-, and even overmodified borate glass compositions. For binary metaborate glasses, the impact of the modifier cation on the borate speciation is shown. High field strength cations such as Zn2+ enhance the disproportionation of metaborate to polyborate and pyroborate units. Pb2+ and Bi3+ induce cluster formation, resulting in PbOn- and BiOn-pseudophases. Both lead and bismuth borate glasses show also a tendency to stabilize very large superstructural units in the form of diborate polyanions. Far-IR spectra reflect on the bonding states of modifier cations in glasses. The frequency of the measured cation-site vibration band was used to obtain the average force constant for the metal-oxygen bonding, FM-O. A linear correlation between glass transition temperature (Tg) and FM-O was shown for the metaborate glass series. The mechanical properties of the glasses also correlate with the force constant FM-O, though for cations of similar force constant the fraction of tetrahedral borate units (N4) strongly affects the thermal and mechanical properties. For paramagnetic Cu- and Mn-borate glasses, N4 was determined from the IR spectra after deducing the relative absorption coefficient of boron tetrahedral versus boron trigonal units, α = α4/α3, using NMR literature data of the diamagnetic glasses.

Structural study, resistivity, magnetization and Raman measurements for the HTc superconducting compounds SmBa2−xSrxCu3O6+y (x = 0.0, 0.25, 0.5, 0.75, 1.0 and 1.25)

Abstract The structure and the physical properties of the oxygenated and deoxygenated compounds of the series SmBa 2− x Sr x Cu 3 O 6+ y ( x = 0.0, 0.25, 0.5, 0.75, 1.0 and 1.25) are studied. In the XRD spectra of the oxygenated compounds a structure of tetragonal symmetry is revealed beyond the value of x = 0.75 and the upper limit for substitution of Sr for Ba is x = 1.0. Resistivity and magnetization measurements of the fully oxygenated compounds show that the T c values decrease with increasing Sr content and this is discussed in relation to the observed structural changes deduced by the Rietveld analysis of the XRD spectra. The bond length changes that occur in the coordination sphere of Cu(1), Cu(2) and (Ba,Sr) atoms are consistent with the “charge redistribution” model, i.e., electrons are transferred from Cu(2) to Cu(1) atoms and the number of holes on the “Cu(2) superconducting planes” increases beyond the optimum value inducing a reduction in the T c . The Raman measurements support the charge redistribution and the stress relaxation models as probable explanations for the reduction of T c upon Sr substitution.

Structural Stability, Vibrational Properties, and Photoluminescence in CsSnI3 Perovskite upon the Addition of SnF2

The CsSnI3 perovskite and the corresponding SnF2-containing material with nominal composition CsSnI2.95F0.05 were synthesized by solid-state reactions and structurally characterized by powder X-ray diffraction. Both materials undergo rapid phase transformation upon exposure to air from the black orthorhombic phase (B-γ-CsSnI3) to the yellow orthorhombic phase (Y-CsSnI3), followed by irreversible oxidation into Cs2SnI6 within several hours. The phase transition occurs at a significantly lower rate in the SnF2-containing material rather than in the pure perovskite. The high hole-carrier concentration of the materials prohibits the detection of Raman signals for B-γ-CsSnI3 and induces a very strong plasmonic reflectance in the far-IR. In contrast, far-IR phonon bands and a rich Raman spectrum are observed for the Y-CsSnI3 modification below 140 cm-1 with weak frequency shift gradients versus temperatures between -95 and +170 °C. Above 170 °C, the signal is lost due to B-α-CsSnI3 re-formation. The photoluminescence spectra exhibit residual blue shifts and broadening as a sign of structural transformation initiation.

Structure and Properties of Orthoborate Glasses in the Eu2O3–(Sr,Eu)O–B2O3 Quaternary

The structure and properties of melt-quenched glasses and partially crystallized samples from the borate series (1-2x)Eu2O3-x((Eu,Sr)O-B2O3) were investigated in the supermodified regime of x < 0.5, using Raman, infrared (IR), electron spin resonance (ESR), and UV-vis absorption and fluorescence spectroscopic techniques. ESR and optical spectroscopy showed that, despite the strongly reducing synthesis conditions, the Eu(2+)/Eu(3+) equilibrium remained shifted to the side of trivalent Eu(3+). Stable and transparent overmodified borate glasses were produced for compositions with x ≥ 0.36. Higher europium oxide concentrations resulted in precipitation of crystalline Eu2Sr3(BO3)4 and EuBO3 phases, as traced by X-ray diffraction. Raman and IR spectroscopy showed that the metaborate configuration which is present at x = 0.46 transforms gradually, with increasing Eu2O3 levels, into orthoborate [BO3](3-) triangular units. At higher europium oxide content (x ≤ 0.36), the presence of Eu(3+) supports the formation of orthoborate [BØ2O2](3-) tetrahedral species. These units organize into [B3O9](9-) rings, which exist in equilibrium with [BO3](3-) triangles. As a consequence, distinct variations can be observed also in the macroscopic properties such as density, glass transition temperature, refractive index, optical basicity, and oxygen polarizability. This observation confirms previous findings on manganese-strontium borates with high modification levels.

Phase separation, microstructure and superconductivity in the Y1−xPrxBa2Cu3Oy compounds

High resolution synchrotron x-ray powder diffraction combined with micro-Raman spectroscopy are used to investigate the effect of Pr substitution for Y in optimally doped or overdoped polycrystalline YBa2Cu3Oy (Y123) compounds. The spectral analysis of the Raman-active B1g-symmetry mode indicates a phase separation into coexisting nanoscopic environments consisting of pure Y123 and Pr123 and a mixed (Y‐Pr)123 phase of almost the nominal amount of Pr. The Y123 phase disappears at x ≈ 0.6 where superconductivity is suppressed, while the formation of the pure Pr123 phase is correlated with the increase of local lattice distortions at the Cu and Ba sites, the presence of crystal defects and the increase of microstrains, as obtained by analyzing the anisotropic XRD peak broadening. The comparison of the Ba Ag-symmetry phonon shift for the Y1−x Prx Ba2Cu3Oy and YBa2−zPrzCu3Oy compounds as well as lattice dynamic calculations proves that, when Pr substitutes for Y, it also occupies an amount of Ba sites. The data from Pr, La or Ca substitution for Y indicate that loss of superconductivity is correlated with the substitution of Pr, La for Ba and Ca for Y, though the underlying effects may not be the hole filling by these occupancies. (Some figures in this article are in colour only in the electronic version)

Evaluation of the strains in charge-ordered Pr1−xCaxMnO3 thin films using Raman spectroscopy

Thin films of Pr1−xCaxMnO3 (x=0.5,0.6) deposited on LaAlO3 and SrTiO3 substrates have been studied by Raman spectroscopy at low temperatures in order to investigate the effect of strains from the Ca doping or the substrate. A detailed assignment of the observed bands is suggested based on the present observations and published results on manganites. We assign the low frequency bands to modes involving only displacements of the A-site ions from their mass dependence by the Ca substitution for Pr (Pr/Ca–O modes). The Ag(2) mode, which is related to the tilting angle of the MnO6 octahedra, appears strongly coupled with the carriers and is very sensitive to the strain effects. Based on the Raman data obtained from the film cross sectional area, we extract the strain distribution across the film. Besides, we calculate in the pseudocubic approximation the phonon deformation potentials, the Gruneisen parameter, and the bulk modulus of the film, which are in good agreement with the ultrasonic measurements.

XRD and micro Raman characterization of epitaxial Bi-2201, Bi-2212 and Bi-2223 thin films

Copyright (c) 1997 Elsevier Science B.V. All rights reserved. Micro Raman characterization is performed on high quality thin films of Bi 2 Sr 2 CuO 6+x (2201), Bi 2 Sr 2 CaCu 2 O 8+x (2212), Bi 2 Sr 2 Ca 2 Cu 3 O 10+x (2223) made by dc-sputtering. Single crystal X-ray measurements reveal the full epitaxy of the films, which allows for polarized Raman spectra to be obtained.

The effect of Ca substitution on the structure and the Raman active phonons in

Micro-Raman measurements and Rietveld refinements of x-ray data have been carried out in the superconducting system with , to delineate the effect of the excess doping on the Raman active phonons and especially on the characteristics of the plane oxygen modes. The increase of the Cu(2)-O(4) bond length with increasing Ca concentration induces a much smaller decrease in the frequency of the phonon of the apical O(4) atom than the rare earth substitutions for Y. The frequency of the in-phase vibrations of the plane oxygens decreases slightly with increasing Ca content, but remains well above the lowest value that has been reported for the oxygen overdoped pure compound, suggesting that the softening of this mode observed in those samples may not be attributed to the addition of carriers. The frequency of the out-of-phase -like vibrations of the plane oxygens is not affected by the substitution, but the width of the corresponding phonon increases considerably. This increase is due to the appearance of another mode of symmetry that could correspond to a hypothetical phase separated from the pure YBCO phase.