Effect of Temperature and X-Ray Illumination on the Oxygen Ordering in La2CuO4.1 Superconductor

Structural properties of La₂CuO_4.1 single crystal are studied by high-resolution synchrotron X-ray diffraction as a function of temperature and of X-ray fluence. Superstructures with a periodicity 2, along the c axis, due to a 3D long-range oxygen ordering, have been observed. The temperature-dependent study has allowed us to distinguish two order–disorder phase transitions, at 350 and 375 K for two different ordered phases, respectively. After rapid quenching from 380 to 100 K we were able to induce disorder-to-order-like transition because of a 2D rearrangement of the excess oxygen atoms in the temperature range of 130–180 K. The oxygen ordering could also be produced by X-ray beam illumination; a clear signature of X-ray-photo-induced phase transition has been found by placing the sample under high X-ray flux at 300 and 220 K.     

Activation Energy of the Photo Induced Q2 Oxygen Ordered Phase in the La2CuO4.08 Superconductor

The interstitial oxygen ordering in a La₂CuO_{4+ y} (y = 0.08) single crystal with a T _c = 38 K has been investigated. We have studied the growth dynamics of the three dimensional (3D) ordered phase, called Q2, at different temperatures under X-ray illumination. The critical temperature T ₀～334 K of the order-disorder transition for the Q2 phase has been determined. We have found that the illumination of the sample with a continuous polarized synchrotron radiation X-ray flux stimulates the oxygen ordering kinetics. The photoinduced ordering process shows a threshold characteristic of cooperative phenomena and its kinetics shows a temperature dependence that is controlled by the activation energy E _A = 100 meV.     

Activation Energy of the Photo Induced Q2 Oxygen Ordered Phase in the La2CuO4.08 Superconductor

The interstitial oxygen ordering in a La₂CuO_{4+ y} (y = 0.08) single crystal with a T _c = 38 K has been investigated. We have studied the growth dynamics of the three dimensional (3D) ordered phase, called Q2, at different temperatures under X-ray illumination. The critical temperature T ₀∼334 K of the order-disorder transition for the Q2 phase has been determined. We have found that the illumination of the sample with a continuous polarized synchrotron radiation X-ray flux stimulates the oxygen ordering kinetics. The photoinduced ordering process shows a threshold characteristic of cooperative phenomena and its kinetics shows a temperature dependence that is controlled by the activation energy E _A = 100 meV.     

Manipulation of Mesoscopic Phase Separation by X-ray Illumination

High resolution synchrotron X-ray diffraction is used to study the dynamics of oxygen ordering in a La₂CuO_4.08 single crystal as a function of the temperature and of the photon flux. We have observed the occurrence of a phase separation between disordered and 3D long-range ordered domains of interstitial oxygen atoms (the latter being commensurate with the lattice). We are able to study and control the phase separation by irradiating the sample while varying the temperature in a range between 350 K and 250 K. These results shed new light on the oxygen ordering in cuprates; in addition, we show that in such systems it is possible to manipulate and to control structural inhomogeneities at the atomic scale using X-ray diffraction. This work was supported by European project 517039 “Controlling Mesoscopic Phase Separation” (COMEPHS) (2005).     

Temperature Dependence of \sqrt{2} \times \sqrt{2} Phase in Superconducting K0.8Fe1.6Se2 Single Crystal

We report the structural phase transition properties of the newly discovered K_0.8Fe_1.6Se₂ superconductor (T _c =31.8 K) using synchrotron single-crystal X-ray diffraction. The basic structure of the sample at room temperature is found to be tetragonal ThCr₂Si₂-type, modulated by a vacancy ordering induced superlattice structure together with a coexisting minority phase having a $\sqrt{2} \times \sqrt{2}$ ordering. At 520 K, the reflections corresponding to the $\sqrt{2} \times \sqrt{2}$ phase merge with the parent tetragonal phase. The superlattice peaks corresponding to the vacancy ordering disappear at 580 K, indicating an order-disorder phase transition at this temperature.     

Redistribution of Electronic Charge in (TMTTF)2ReO4: 13C NMR Investigation

¹³C NMR measurements were performed for one-dimensional organic conductors, (TMTTF)₂ReO₄. An intermediate charge-ordering (CO) phase has been found firstly for a TMTTF salt with a Td-symmetry counter anion: The NMR parameters indicate two inequivalent molecules with unequal electron densities below 225 K. Moreover, the spin-singlet transition associated with ReO₄ anion ordering was confirmed at around 158 K by ¹³C NMR measurements. A drastic change of NMR parameters below 158 K also indicates a redistribution of the electronic charge at the anion ordering temperature.     

Conductivity of the nanostructured ceramic material Zr<Subscript>0.88</Subscript>Sc<Subscript>0.1</Subscript>Ce<Subscript>0.01</Subscript>Y<Subscript>0.01</Subscript>O<Subscript>1.955</Subscript> prepared from mechanically activated powders

The structure of the Zr_0.88 Sc_0.1Ce_0.01Y_0.01O_1.955 solid solution, a candidate for the use as a solid electrolyte in fuel cells with a low temperature, has been investigated using x-ray powder diffraction and Raman spectroscopy. Single-phase ceramic materials have been produced from powders prepared by the mechanochemical synthesis from ZrO₂ nanoprecursors purified of the impurities introduced during grinding of commercial zirconia. The solid solution has a rhombohedral structure at room temperature owing to the partial ordering of oxygen vacancies. The electrical conductivity of the ceramic materials sintered at temperatures below 1570 K exhibits a hysteresis due to the delay of the martensitic transition from the cubic phase to the rhombohedral phase upon cooling of the sample. The nanostructured ceramic materials are characterized by a high mechanical strength and unusually close values of the activation energies for bulk and grain-boundary electrical conduction.     

Inhomogeneity Induced Conductivity Fluctuation in YBa2Cu3O7−y /Cr2O3 Composite

Dimensional fluctuations of superconducting order parameters in YBa₂Cu₃O_7?y +xCr₂O₃ (x=0.0, 2.3, 4.4, and 6.1 wt.%) has been analyzed. SEM micrographs reveal the reduced grain size with the incorporation of magnetic Cr₂O₃ particles in the YBCO matrix. XRD graphs show the unchanged orthorhombic structure and improved oxygen ordering in the composite samples. With the increase of Cr₂O₃ wt.%, it is found that the superconducting transition temperatures determined from standard four-probe method decreases gradually. Excess conductivity fluctuation analysis using Aslamazov–Larkin model fitting reveals transition of two dominant regions (2D and 3D) above T _c . 2D to 3D crossover temperature, i.e., Lawerence–Doniach temperature that demarcates dimensional nature of fluctuation inside the grains is influenced by Cr₂O₃ incorporation in YBCO matrix. The decrease in Lawerence–Doniach temperature in the mean field region has been observed as a consequent dominance of the 3D region with an increase in wt.% of Cr₂O₃ in the composite.     

Partial isothermal sections of Ti–Al–V ternary diagram

Abstract

The phase equilibria between the titanium aluminides, Ti₃Al and TiAl, and V have been investigated in the temperature range 1473–873 K. Microstructures were characterised using light, scanning electron, and transmission electron microscopy and the nature of the phases present was established via X-ray diffractometry, electron diffraction, and energy dispersive X-ray analysis. A series of isothermal sections have been determined and are compared with those obtained in earlier studies. Ordering of the high temperature bcc β phase to a simple cubic B2 (β₂) structure was observed: the ordering temperature was found to decrease with increasing V content, whereas the opposite effect was observed with Al. The effect of oxygen is to stabilise the α phase and to shift the α₂+β₂+γ field in the V rich direction. The lattice parameters of α₂ and β phases decrease with additions of Al and V: empirical equations have been developed to predict these changes.

MST/1881     

State of oxygen in the structure of YBa2Cu3O6+δ: Reversible changes during annealing and their influence on T c

The valence state of oxygen in YBa₂Cu₃O_6+δ has been studied by x-ray photoelectron spectroscopy. The results indicate that, in the temperature range 298–510 K, the state of some of the cations in this compound changes significantly. These changes are interpreted in terms of ordered and disordered states in the anion sublattice of YBa₂Cu₃O_6+δ, the boundary between which lies within the temperature range studied. Oxygen ordering is assumed to play a central role in determining the complex, stepwise variation of the superconducting transition temperature with composition.     

Nonstoichiometry and phase transition in NdMnO3

The nonstoichiometry of neodymium manganite NdMnO₃ phase has been studied by a gravimetric method at 1273 K. The oxygen partial pressures were controlled by using flowing $\text{O}{}_2\text{Ar}$ or $\text{H}{}_2\text{CO}{}_2$ mixtures. The existence of hyperstoichiometric phase and the homogeneity range of the phase NdMnO_3+x from x = 0 to 0.065 at 1273 K under various oxygen partial pressures has been revealed. And also the effect of oxygen nonstoichiometry on the phase transition at high temperature has been studied by electrical conductivity and DTA measurements. The transition temperature decreases with increasing oxygen content.     

About SnF2 stannous fluoride. VI. Phase transitions

Investigation of SnF₂ from room temperature up to the melting point, by several techniques (D.T.A., D.S.C., microcalorimetry, thermogravimetry, electrical conductivity, X-ray diffraction) has shown the existence of two phase transitions. Upon heating the α → γ transition is observed between 125 and 190°C. This transition is a first order reconstructive one, which is temperature, pressure and grain size dependent. Upon colling the γ phase, a second order phase transition (β ? γ) occurs at 66°C which is a displacive ferro-paraelastic transition. Analogies between the phase transitions observed in SnF₂, TeO₂ and GeF₂ are given.     

Temperature and pressure dependence of orientational disorder and bonding in Li2CsC60

The structural properties of the Li₂CsC₆₀ fulleride, which has been shown before to be face centred cubic with strong Li⁺–C interactions, were studied by synchrotron X-ray powder diffraction at both ambient and elevated pressures. We find no evidence for a monomer→polymer phase transition, analogous to that encountered in sodium intercalated fullerides, either on slow cooling to 150 K at ambient pressure or on the application of pressure to 6.14 GPa at ambient temperature, with the structure always remaining strictly face centred cubic. Analysis of the high-resolution diffraction data at elevated temperatures shows again no evidence for a phase transition with only a small weakening of the Li⁺–C interaction.     

Stripe correlation of tilts and superconductivity in La2CuO4+Σ

In single crystal La₂CuO_4+Σ,Σ ≈ 0.015, c-axis interstitial oxygen ordering (staging) in the oxygen-rich phase of the phase-separated sample was observed through satellite peaks in the neutron scattering which first appear at 250 K. If the sample is cooled through the temperature region around 210 K over a period of several hours, these satellite peaks show splitting in the a*-direction. This first-order phase transition involves a periodic one-dimensional modulation of the in-plane order. Microscopic modeling leads to a picture where the excess oxygen, and therefore also the doped holes, order in stripes along the a-direction in addition to staging along the c-direction. This ordering in stripes can be regarded as a continuation of the phase separation but now on a local scale, leading to a local increase in oxygen and hole density. This increase can account for the increase of the superconductingT _c of 2.5 K observed in our sample and also for many previously reported annealing experiments.     

Low Temperature Magnetic Behavior of Ca3Co4O x Polycrystalline Material

Misfit layered Ca₃Co₄O₉ samples have been fabricated by solid-state reaction method. X-ray diffraction analysis showed that the samples have the c-axis oriented monoclinic structure. The temperature dependence of the DC magnetization of the sample fabricated was investigated in an applied field of 2000 Oe. Magnetization of the sample increased smoothly down to 20 K, then increased rapidly with decreasing temperature, suggesting a transition to the ferrimagnetic phase. M–H analysis indicated a paramagnetic behavior between 20 and 300 K and hysteretic effect below 20 K which corresponds to paramagnetic–ferrimagnetic phase transition. The ferrimagnetisms are most likely caused by interlayer coupling between the Ca₂CoO₃ and CoO₂ subsystems in unit cell.     

Organic metal (EDO-TTF)2PF6 with multi-instability

Abstract

The multi-instability of the electronic structure of (EDO-TTF)₂PF₆, where EDO-TTF means ethylene-dioxytetrathiafulvalene, is reviewed. This complex showed the metal–insulator transition at 280 K associated with distinct molecular deformations. The mechanism is interpreted as the cooperation of Peierls transition, charge ordering, and the order–disorder transition of the countercomponent. The charge ordering pattern in the low-temperature phase is of the novel [0, 0, 1, 1] type. The sensitivity of the electronic state to external perturbations is demonstrated applying not only static but also instantaneous stimuli. In the latter case, the photo-induced phase transition is ultrafast and highly efficient. One photon causes the transition of several hundreds of donor molecules in the low-temperature phase to relax into a highly conducting metastable state within about 1.5 ps. In the early stage of the transient state, the charge ordering of the [1, 0, 1, 0] type occurs. As for the chemical modifications of this material, the partial deuteration of this complex increases the metal–insulator transition temperature. The introduction of a methyl group greatly modulates the electronic structure of the complex, i.e. (methyl-EDO-TTF)₂X (X=BF₄, ClO₄) shows a two-dimensional electronic structure. The working hypotheses for developing the systems with multi-instability are described.     

Anomalous Magnetic Properties of the Bilayered LaSr2Mn2−z Co z O7 (z=0–0.15) Manganite

The Co-doped bilayered LaSr₂Mn₂O₇ manganite at low Co concentrations (0–0.15) was synthesized by the sol–gel process. The X-ray diffraction (XRD) technique confirms phase formation for all the samples under investigation. The results of ac magnetic susceptibility measurements indicate the effect of Co doping on the magnetic ordering phases. The indications of charge ordering (CO) transitions were observed in all the prepared compounds. The CO magnetic phase transition temperature was observed systematically shift to lower temperatures as the Co concentration increases. There was an anomalous oscillating magnetic behavior in all samples with a few peaks before the CO temperature in the paramagnetic (PM) region so that with an increasing Co doping, the number of peaks and amplitude were decreased. Also, the ac susceptibility measurements were performed in the presence of an applied dc magnetic field to further study of this oscillating behavior.     

High-temperature ordering-phase separation transition in the Fe<Subscript>50</Subscript>Co<Subscript>50</Subscript> alloy

High-temperature (above 1200°C) B2 ordering has been detected in the Fe₅₀Co₅₀ alloy by selected area electron diffraction. X-ray photoelectron spectroscopy data indicate that the transition from phase separation to ordering is accompanied by changes in both the d-electron valence band spectrum (ordering increases 3d-electron localization at the Fe atoms) and the 3s core level spectrum (phase separation increases the exchange interaction between the electron spins of the partially filled 3d shell and ionized 3s shell of the Fe and Co atoms). We conclude that an ordering-phase separation transition occurs not only at 730°C but also at a temperature slightly above 1200°C.     

Refinement of the V-O phase diagram in the range 25–50 at % oxygen

The boundaries of the V₁₄O₆ + V _x O _z two-phase region in the V-O system at temperatures from ? 1050 to ? 1650 K have been determined experimentally. The V-O phase diagram has been refined in the range 25–50 at % oxygen using structural and microstructural data for vanadium oxides containing less than 50 at % oxygen in conjunction with earlier results. The possibility of ordering of cubic vanadium monoxide has been examined.     

Effect of Ni doping on the microstructure and high Curie temperature ferromagnetism in sol–gel derived titania powders

Undoped, 0.05 and 0.5 mol% Ni-doped TiO₂ powders were prepared by a modified sol–gel route. The doping effects on the microstructure and magnetism for the powdered samples have been systematically investigated. Doping of Ni in TiO₂ inhibited rutile crystal growth. The probable reason for this is discussed on the basis of band calculation based analysis of electronic structures of 3d transition metal-doped TiO₂ and the energetic, transformation kinetics and phase stability of anatase over rutile as the function of particle size. Room temperature ferromagnetism (RTFM) with the saturation magnetization of 12 m emu g⁻¹ and Curie temperature as high as 820 K is observed only in case of 0.05 mol% Ni:TiO₂ powdered sample, whereas undoped TiO₂ was diamagnetic and 0.5 mol% Ni:TiO₂ was paramagnetic in nature. The role of any magnetic impurity or any Ni metal in the origin of the RTFM has been ruled out by energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and high resolution TEM (HRTEM) analysis, whereas magnetic force microscopy (MFM) established the presence of magnetic domains, supporting the intrinsic diluted magnetic semiconductor behavior. The observed ferromagnetism has been attributed to the spin ordering through exchange interaction between holes trapped in oxygen orbitals adjacent to Ni substitutional sites.