In-situ studies on phase transformations under electron irradiation in a high voltage electron microscope

High voltage electron microscopy (HVEM), using electron energies adequate for causing displacements of atoms from lattice sites, is a very effective technique for studying mechanisms of solid state phase transformations and for charting the path of phase evolution in real time. This has been demonstrated in studies on chemical ordering in nickel-molybdenum alloys and on theβ → Ω displacement ordering in zirconium-niobium alloys. The enhanced diffusivity due to electron irradiation makes it possible to explore a sequence of phase evolution at low enough temperatures where even some first-order transformations are driven by free energy (G) instabilities with respect to the relevant order parameter (η). Specific issues addressed in these studies are reviewed in this paper.     

Electronic spectrum of Bi2Sr2CaCu2O8 near the fermi level from results of numerical calculations and ultraviolet (8.43 eV) photoelectron spectroscopy

The structure of the spectrum of the partial density of filled states of Bi₂Sr₂CaCu₂O₈ in the range E _b<4 eV, obtained by ultraviolet (hv=8.43 eV) photoelectron spectroscopy was compared with that calculated by the strong coupling method. The results of the calculations agree with the experimental results in the range of binding energies 1<E _b <4 eV. Analysis of the evolution of the spectra under thermal and phototreatment in ultrahigh vacuum and in oxygen reveals that oxygen atoms from Cu-O planes possess the highest mobility in the lattice and the surface of the single crystal is formed by Bi-O planes. Pis’ma Zh. Tekh. Fiz. 24, 69–74 (February 12, 1998)     

Kinetics of oxygen defect in YBCO

Monte-Carlo simulations are described using the Kawasaki model of oxygen (dis)ordering in the basal plane of YBa₂Cu₃O_7−δ(YBCO). In this model the presence or absence of an interstitial oxygen defect is mapped into a pseudo Ising spins=+1 or − 1. We study the rate law and the relaxation time of the order parameter and most importantly, the diffusion of oxygen in the basal plane across the structural phase transition in this system. In a separate investigation we have carried out numerical analysis of a mean-field dynamical model (with Kawasaki kinetics) to examine phase ordering dynamics and pattern formation, for δ=0.     

Effect of Oxygen Concentration on Superconducting Properties of Rubidium Tungsten Bronzes Rb<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>WO<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

Superconducting transition temperature (T _c) as a function of oxygen concentration for hexagonal rubidium tungsten bronzes Rb _x WO _y with 2.80 ≤ y ≤ 3.07 and x = 0.19, 0.23, and 0.27 has been systematically investigated. Three regions corresponding to T _c < 2 K (denoted as superconductivity suppressed region), T _c∼ 3 K (superconductivity uniform region) and T _c > 3 K (superconductivity enhanced region) were identified in T _c–y phase diagram for Rb_0.19WO _y and Rb_0.23WO _y . No superconductivity enhanced region was observed for Rb_0.27WO _y . The superconductivity suppressed region shifts toward higher oxygen content as rubidium concentration increases. The local ordering of the intercalated rubidium atoms might be responsible for the intriguing T _c–y phase diagram of Rb _x WO _y .     

Oxygen isotope shifts of the Raman modes of YBa2Cu3O7−x

Raman scattering experiments have been carried out on sintered pellets of YBa₂Cu₃ ¹⁶O_7−x and YBa₂Cu₃ ¹⁸O_7−x obtained both by gas exchange and by growth with substituted oxides. The frequencies of the modes at 340, 435 and 502 cm⁻¹, which involve motion of the oxygen atoms and which shift significantly upon oxygen isotope substitution, have been measured for several sets of samples. The measured frequency shifts indicate that the isotope exchange on the O(2) and O(3) sites is more complete than the exchange on the O(4) site. The 502 cm⁻¹ line of the¹⁸O samples is observed to be broadened and this is attributed to¹⁸O-¹⁶O disorder on the O(4) sites. The results are discussed with reference to previous measurements of site activation energies and models for the exchange mechanism.     

Influence of ion-induced interfacial chemical reactivity on contact resistance

Contact resistance measurements of chromium contacts deposited by partially ionized beam deposition on transparent conducting indium tin oxide (ITO) were performed. These provide a direct experimental evidence of the influence of interfacial chemical interaction on the contact resistance. The interfacial reactivity is controlled by modifying the energy and flux of ionized chromium atoms deposited on ITO employing a specially designed partially ionized deposition system with very high ionization efficiency. The true contact resistivityρ _c is obtained by iteratively correcting the experimentally measured values for the finite sheet resistance of the ITO layer.ρ _c decreases linearly with the energy of the ionized chromium. Auger sputter profiling shows no structural modifications at the interface due to a change in the energy of the chromium atoms, confirming that the observed change in the contact resistivity is directly related to interfacial chemical bonding of the atoms with the oxygen atoms in the ITO leading to a local increase of carrier concentration and lower interfacial resistance.     

Application of a geometric model to the hydrides of FeTi

Criteria of hole size and hydrogen-hydrogen distance have been used to develop rationales for the observed stoichiometries and preferred hydrogen sites in theα-,β ₁-,β ₂- andγ-phases of the FeTi-H system. For these four phases of FeTiH _n , the respective values ofn approximate 0, 1, 1.4 and 2. Structures and position vectors for the metal atoms were obtained from the literature for use in calculating the radii of the various interstices and the intersite distances in each phase. The model does not allow one to predict the nature of the structural transformations that occur in this system with increasing hydrogen concentrations, but knowing the metal atom positions in each phase does allow one to predict the preferred sites for hydrogen atoms. Most of the occupied interstices are octahedral sites coordinated by four titantium atoms and two iron atoms, but the reported occupation of a pseudo-octahedral site in FeTiD is explained within the framework of the model by allowing a deuterium atom to occupy a hexahedral interstice coordinated by three titanium atoms and two iron atoms. Forn −-2, i.e. for theγ phase, geometric considerations allow the correct prediction of some hydrogen atoms situated in octahedral interstices coordinated by four iron atoms and two titanium atoms. Occupation of such a site could not have been predicted by considering the relative affinities of the metals titanium and iron for hydrogen.     

Diffusion of platinum, vanadium and manganese in Ni3Al phase under high pressure

Diffusion of platinum, vanadium, and manganese in the Ni₃Al phase is investigated under high pressure. Platinum atoms occupy cubic face centred sites (α) in the L1₂ ordering structure. Vanadium atoms occupy cubic corner sites (β). Manganese atoms occupy both sites. Activation volumes ΔV for diffusion of these diffusing atoms to the molar volume of the Ni₃Al phase V₀ are as follows:

Thermal strain in zinc pyrovanadate

The α- andc-axis thermal expansion coefficients of α-Zn₂V₂O₇ (monoclinic structure) were found to be negative between 20 and 560‡C. The volume expansion coefficient is negative above 260‡C. As a result, the zinc atoms tend to be in sixfold rather than fivefold coordination. Similar behavior results from substitution of larger sized ions on Zn sites. These effects are associated with the shear strain suffered by the soft Zn-0 polyhedra as a result of significant changes in Β. On cooling from 20‡C to liquid-nitrogen temperature, as well as upon substitution of smaller sized ions on Zn sites, the major structural changes occur along theb axis, without changes in theac plane: the soft polyhedra experience uniform compression and the Zn ions remain in fivefold coordination. Only in the latter case is the extent of substitutional solid solutions governed by the relative difference in size between the host and substituent ion.     

Acoustical parameters of polyethylene glycol/water mixtures

Ultrasonic velocity (v) and density (d) have been measured for polyethylene glycol/water mixtures at 30°C. The adiabatic compressibility (β _ad), molar compressibility (β), specific acoustic impendance (Z), Rao number (R) and van der Waals constant (b) have been computed. The variations ofv, d, β _ad,β, Z, R andb with mole ratio of water/ether group oxygen have been studied. The association between the components and the formation of tetrahydrate have been reported.     

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.     

A structural transition of Fe-doped superconducting cuprates

A series of LaBa₂Cu_3-xFe_xO_y (LBCO) samples with 0.0≤x≤0.75 have been synthesized. As the iron content increases, the ferric ion prefers to occupy Cu(2) sites instead of Cu(l), andT _c of LBCO decreases more rapidly than that of YBCO. This means that the action of Cu(2) on superconductivity is greater than that of Cu(l). Crystal structures of the series show an anomalous structural phase transition depending on Fe contents. As the Fe content increases, the structure changes from orthorhombic to tetragonal, and then the orthorhombic phase is regenerated from the tetragonal phase. By least squares fitting with the X-ray diffraction data, the structural transitions can be determined from the changes of the cell parametersa andb. The relation of the lattice parametersa, b, andc as a function of both the Fe content and the oxygen contenty supports the fact that the excess oxygens may occupy the (0 0 1/2) and (0 1/2 0) sites of the crystal cell.     

Effect of Pb Doping on the Superconducting Properties of?(Cu0.5?xPbxTl0.5)Ba2Ca2Cu3O10?δ

The effect of Pb doping on the superconducting properties of (Cu_0.5−x Pb _x Tl_0.5)Ba₂Ca₂Cu₃O_10−δ (x=0.0, 0.15, 0.25, 0.35) samples has been investigated. Lead is doped in Cu_0.5Tl_0.5Ba₂O_4−δ charge reservoir layer and at the CuO₂ planar sites. A multiphase material is achieved with the doping of Pb at the CuO₂ planar sites; however, a predominant single-phase (Cu_0.5−x Pb _x Tl_0.5)Ba₂Ca₂Cu₃O_10−δ (x=0.0, 0.15, 0.25, 0.35) material is synthesized with the doping of Pb at the charge reservoir layers. Formation of multiphase material with the doping of lead at the planar sites showed that its substitution at the planar site is not possible and the formation of PbO₂ planes is less likely. In the samples doped at the charge reservoir layer, the zero critical temperature [T _c (R=0)] is systematically depressed with the increased concentration of lead. The T _c (R=0) and magnitude of the diamagnetism are enhanced after post-annealing the samples in oxygen atmosphere. An apical oxygen mode is observed around 438 cm⁻¹ in undoped samples, which is shifted to 457–461 cm⁻¹ in the Pb-doped samples. This shift in the peak position is most likely associated with the connectivity of apical oxygen atoms with Pb atoms of (Cu_0.5−x Pb _x Tl_0.5)Ba₂O_4−δ (x=0.0, 0.15, 0.25, 0.35) charge reservoir layers. The presence of Pb in the charge reservoir layer and its increased concentration, somehow, stops the flow of mobile carriers to the conducting CuO₂ planes. The decreased density of mobile carriers diminishes the critical temperature and magnitude of diamagnetism in the final compound. The increased oxygen diffusion in the unit cell achieved by post-annealing in oxygen replenishes the concentration of carriers in conducting CuO₂ planes, which increases the T _c (R=0) and the magnitude of diamagnetism. These experiments have shown that the density of mobile carriers plays a vital role in the mechanism of superconductivity and their depressed density suppresses the superconductivity parameters.     

Synthesis and structural studies of a new potassium uranyl selenate K(H5O2)[(UO2)2(SeO4)3(H2O)] with strongly deformed layers

Single crystals of a new uranyl selenate, K(H₅O₂)[(UO₂)₂(SeO₄)₃(H₂O)] (I), were prepared by isothermal evaporation at room temperature. The crystal structure of I was solved by the direct method (space group P2₁/c; a = 11.456(2), b = 10.231(1), c = 14.809(2) ?; β = 101.901(4)°, V = 1698.4(4) ?³; Z = 4) and refined to R ₁ = 0.0547 (wR ₂ = 0.0825) for 3375 reflections with |F _o| ≥ 4σ _F . The structure of I is based on layers of the composition [(UO₂)₂(SeO₄)₃(H₂O)]²⁻. The charge of the inorganic layer is compensated by potassium and oxonium ions located in the interlayer space. Each potassium ion is coordinated by seven oxygen atoms belonging to uranyl selenate layers, including uranyl oxygen atoms, which leads to bending of uranyl selenate structural elements.     

Crystal Structure of Double Np(V) Malonate with Co(NH3)6 3 + in the Outer Sphere, [Co(NH3)6][NpO2L]2HL (L = OOCCH2COO)

The single crystal X-ray analysis of [Co(NH₃)₆][NpO₂L]₂HL (L = OOCCH₂COO) was carried out [monoclinic system, a = 17.925(6), b = 7.736(4), c = 7.879(3) Å, = 103.65(3)°, space group C2/m]. The structure consists of infinite anionic chains [NpO₂L] _n ^n-, [Co(NH₃)]₆ ^{3 +} cations, and HL^- anions. The Np(V) atoms have hexagonal-bipyramidal oxygen surrounding. The coordination capacity of malonate ions is 6; two oxygen atoms (one of each carboxylate group) are bridging atoms between two Np atoms, owing to which the CPs of Np atoms share common equatorial edges to form infinite chains.     

On the conditions for contraction of a multielectrode corona discharge in He/Ar, Kr, Xe mixtures

The results of an investigation of the contraction of a corona discharge in “needle-grid” and “needle-plane” electrode systems maintained by a constant negative voltage are presented. The discharge was ignited in working media of infrared lasers on p-d transitions of atoms of heavy inert gases (He/Ar, Kr, Xe mixtures). Investigation of the current-voltage characteristics showed that in the mixtures He/Ar/Kr there is no hysteresis on the rising section of the current-voltage characteristic, while hysteresis does appear for a corona discharge in the mixture He/Xe. For voltages ≥5.5 kV on the needles and ballast resistance R_b≥0.5 MΩ dynamical contraction of a corona discharge together with current pulses with f=3–5 kHz and hysteresis loops were found on the descending section of the curve I=f(U), where I and U are the average current and voltage on the discharge gap. The maximum content of atoms of heavy inert gases for [He]=200–300 kPa is [Ar]≤12, [Kr]≤8, and [Xe]≤4 kPa. Pis’ma Zh. Tekh. Fiz. 25, 90–94 (September 12, 1999)     

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).     

Electrical and magnetic studies of iron (III) vanadate

Iron(III) vanadate (FeVO₄) is an n-type semiconductor between 300 and 800 K. Electrical conduction in this phase occurs due to small deviation from oxygen stoichiometric composition. The mechanism of electrical transport is of a thermally activated hopping of charge carriers (electrons) on equivalent iron lattice sites. The FeVO₄ obeys Curie-Weiss law between 80 and 300 K. The measured magnetic moment (μ _eff) of Fe³⁺ ion in FeVO₄ is 5·270 BM at 298 K, which is lower than theμ _spin only value. The predominant exchange interactions are the weak 90° M-O-M superexchange and M-O-O-M super-super-exchange. The negative Weiss constantϑ=− 30 K of the phase indicated the possibility of an antiferromagnetic ordering of the iron(III) vanadate lattice. The IR absorption spectrum of FeVO₄ gave bands at 990, 900, 825 and 725 cm⁻¹ due to the presence of distorted VO₄ polyhedra of the lattice.     

Crystal and molecular structure of a crystal solvate of cesium tris(pivaloyltrifluoroacetonato)dioxouranate, Cs[UO2(ButCOCHCOCF3)3]·1.5C6H6·H2O

The crystal and molecular structure of cesium tris(pivaloyltrifluoroacetonato)uranylate prepared in the form of a crystal solvate Cs[UO₂(Bu ^t COCHCOCF₃)₃]·1.5C₆H₆·H₂O was studied by single crystal X-ray diffraction. The compound crystallizes in the monoclinic system, a = 26.9099(9), b = 13.1549(4), c = 24.4586(8) ?, β = 90.418(2)°, V = 8658.0(5) ?³ (at 100 K), Z = 8, space group C2/c. Of the three β-diketonate ligands, two are coordinated to the uranyl ion in the bidentate fashion, and the third ligand, in the monodentate fashion, so that the equatorial coordination number of the uranyl ion is 5. The crystal structure is built of sandwich-type dimers in which two cesium cations are arranged between two tris(pivaloyltrifluoroacetonato)uranylate anions. The coordination sphere of the cesium cation is built of oxygen atoms of the β-diketone and uranyl groups, fluorine atom of the CF₃ group, and π-coordinated benzene molecule. In addition, 0.5 benzene molecule and one water molecule (per formula unit) occupy structural voids.     

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.