X-ray photoelectron studies of changes in the electronic structure upon phase transitions in Co–Ni and Co–Fe alloys

The changes in the electronic structure of Co–Ni and Co–Fe systems upon phase transitions are studied. X-ray photoelectron study of the valence-band spectra and the parameters of the multiplet splitting of Co, Ni and Fe 3s spectra is carried out at different temperatures. It is established that the ordering–separation phase transition in Co–Ni alloys takes place in the temperature range of 600–700°C. As opposed to Co–Ni alloys, in the Fe–Co alloy, ordering–separation–ordering phase transitions are observed. High-temperature ordering of the Fe₅₀Co₅₀ alloy is observed above 1200°C. The transition from ordering to separation is shown to lead to changes in the d electron spectra of the valence band and in the parameters of the multiplet splitting of the 3s spectra.     

Synchrotron X-ray diffraction studies on the phase transitions in the spinel LixMn3−xO4 intercalation compounds

Detailed investigations have been undertaken of the lithium for manganese substitution effect on the LiMn₂O₄, in the system Li_xMn_3−xO₄, for 0.95≤x≤1.05, that is for the nearly stoichiometric lithium content. Synchrotron X-ray measurements have been performed in the temperature range 10–300 K. The diffraction experiments were carried out at the DESY-HASYLAB high-resolution powder diffractometer (beamline B2), equipped with a closed-cycle He-cryostat. Very small changes in the lithium content influence clearly the low-temperature crystal structure of Li_xMn_3−xO₄, spinels and the nature of phase transitions. It was found that for x=0.95 the sample remains tetragonal in the whole 10–300 K temperature range. The stoichiometric LiMn₂O₄ transforms from cubic to orthorhombic at about 280 K. For x=1.0125 the temperature of phase transition from cubic to orthorhombic decreases down to about 260 K, whereas for x=1.025 the transformation goes from cubic to tetragonal phase, at the temperature 220 K. No phase transition has been observed for the cubic sample with x=1.0375. These results partly explain the divergences in recent reports on the low-temperature structure and phase transformations of lithium manganese oxides.     

Special features of the long-range order – short-range order phase transition in FCC-based alloys

Results of x-ray analysis of the long-range order in FCC-based alloys with L1₂, L1₂(M), L1₂(MM), and D1_a superstructures are generalized. Regularities of the behavior of the long-range order parameter and average size of the thermal and periodic antiphase boundaries (APB) are traced, main mechanisms of temperature order-disorder phase transition are elucidated, and influence of the thermal and periodic APB on the special features of the order-disorder phase transition is revealed. The character of change of the periodic antiphase boundary density determines the special features of the order-disorder phase transition in alloys with longperiod superstructures.     

Calculation of the structure properties and P–T phase diagram of hafnium

Abstract

The full-potential linear muffin-tin orbitals (FP-LMTO) method is used to calculate the total energy and equilibrium lattice properties for the observed phases of Hf. The temperature and pressure dependences of the Gibbs energy are found for these structures within the Debye model. A quantitative agreement with the experimental points of the P-T phase diagram is obtained.     

X-ray studies of the smectic B phase of the 4-bromobenzylidene-4′-alkoxyanilines

The results of X-ray studies of the nine members (4?÷?12) of the 4-bromobenzylidene-4′-alkyloxyaniline homologous series (nBBAA) in smectic B (SmB) and smectic A (SmA) phases are presented. Interestingly, no SmC phase was identified. The thickness of smectic layers in the SmA and SmB phases was estimated showing unexpectedly that in SmA it is bigger than in the SmB. For the SmB phase, orthorhombic unit cell parameters were determined as a function of temperature. The obtained data are discussed in relation to rotational dynamics.     

Investigation on the 773 K isothermal section of Dy–Ni–Si ternary phase diagram by X-ray powder diffraction

The 773 K isothermal section of the Dy–Ni–Si ternary system was investigated and constructed by X-ray powder diffraction in this paper. Eighteen ternary phases (DyNiSi, DyNi₂Si₂, DyNiSi₂, Dy₂Ni₃Si₅, DyNiSi₃, Dy₂NiSi₃, Dy₃Ni₆Si₂, DyNi₁₀Si₂, Dy₄NiSi₇, DyNi₂Si, DyNi₅Si₃, DyNi₄Si, Dy₃NiSi₂, DyNi₆Si₆, Dy₈Ni₃₁Si₁₁, Dy₃Ni₂Si₄, Dy₄Ni₅Si and Dy₉Ni₂Si₁₄) were confirmed to exist in this work. In those ternary phases, Dy₉Ni₂Si₁₄ is a new phase, Dy₂NiSi₃ and Dy₄NiSi₇ have solid solution phenomena and the solid solution ranges are Dy_33.3Ni_14.7–18.7Si_52–48 and Dy₄Ni_0.3–1.2Si_7.7–6.8, respectively. We constructed 14 two phase regions and 52 three phase regions in the Dy–Ni–Si ternary phase diagram at 773 K. Because the phase relation is not very clear between 66.7 and 50 Si at.% in the Dy–Si binary system, we use dot lines to estimate tentative phase regions in this region.     

Fine structure in the α decay of202Rn

The α decay of mass-separated²⁰²Rn is studied at the ISOLDE separator. Time sequential α-singles spectra together with α-X-t and α-e-t coincidence events are collected. Fine structure in the α decay is observed and feeding of a low-lying 0 ₂ ⁺ state at 816 keV in¹⁹⁸Po is evidenced. This 0⁺ state can be interpreted as the bandhead of an intruder-state based deformed band, coexisting with the spherical groundstate band. Mixing between normal and intruder states is discussed. A preliminary α-decay study of²⁰⁰Rn did not yet reveal any fine structure.     

The formation of α at triple junctions of parent β phase in titanium alloys

We have examined the formation of α phase at grain boundary triple junctions of parent β in a metastable β titanium alloy with orientation imaging microscopy based on electron backscattered diffraction (EBSD). As in the case of α formed at grain boundaries of parent β grains, α at a triple junction also forms with the Burgers orientation relationship with one of the three neighbouring β grains. The experimental results are analyzed in terms of the deviation of the 36 possible α variants that can form at a triple junction from the Burgers orientation relationship with neighbouring grains.     

High-pressure X-ray diffraction study of the δ-phase in the uranium-neptunium binary system

Abstract

High pressure X-ray diffraction studies were performed at room temperature on a uranium-neptunium binary alloy (U_{0, 40} Np_0.60) using a diamond anvil cell in an energy dispersive facility. The sample maintained its simple cubic phase up to 62 GPa (highest pressure reached in This experiment). The bulk modulus and its pressure derivative were determined to be B₀ = 82 (2) GPa and B₀′ = 9.4 (1.3), from the experimental data in the pressure range 0–20 GPa. The present results are compared with those obtained by the same techniques used for uranium and neptunium.     

High pressure X-ray diffraction and Raman spectroscopic studies of the phase change of D2O ice VII at approximately 11 GPa

High pressure experiments were performed on D₂O ice VII using a diamond anvil cell in a pressure range of 2.0–60 GPa at room temperature. In situ X-ray diffractometry revealed that the structure changed from cubic to a low symmetry phase at approximately 11 GPa, based on the observed splitting of the cubic structure's diffraction lines. Heating treatments were added for the samples to reduce the effect of non-hydrostatic stress. After heating, splitting diffraction lines became sharp and the splitting was clearly retained. Although symmetry and structure of the transformed phase have not been determined, change in volumes vs. pressure was calculated, assuming that the low-symmetry phase had a tetragonal structure. The bulk modulus calculated for the low-symmetry phase was slightly larger than that for the cubic structure. In Raman spectroscopy, the squared vibrational frequencies of ν₁ (A_1g), as a function of pressure, showed a clear change in the slope at 11–13 GPa. The full width at half maxima of the O-D modes decreased with increasing pressure, reaching a minimum at approximately 11 GPa, and increased again above 11 GPa. These results evidently support the existence of phase change at approximately 11 GPa for D₂O ice VII.     

Effect of local atomic phase separation in the x-ray absorption near edge structure spectroscopy of FeSexTe1−x

Ab initio X-ray absorption near edge structure (XANES) calculations for FeSe_xTe_1−x, using a structural model that combines FeSe and FeTe phases at the nanoscale, are compared with Fe K-edge XANES measurements in the “pre-edge” region. The important aspects of this model are (i) magnetic order in the FeTe phase; (ii) Se and Te atoms placed randomly in both FeSe and FeTe crystallographic positions and; (iii) the two distinct distances for Fe–Se and Fe–Te of the bulk phases. The calculated spectra reproduce the observed increase of spectral weight in the experiments on FeSe_xTe_1−x with Se concentration. This is consistent with an inhomogeneous local electronic structure of FeSe_xTe_1−x. Additionally, we have calculated projected electronic density of d-states for the Fe atom, revealing increased spectral weight in the “pre-edge” region of the XANES spectra, which correlates with the increase in Se concentration. The decrease of calculated Fe d-density of states for the Fermi level, N(ε_F), for high Te content is consistent with the suppression of superconductivity in the title system.     

Reexamining the phase diagram of the Gay—Berne fluid

This work reexamines and updates earlier investigations on the phase behaviour of the Gay-Berne liquid crystal model, concentrating on the effect of varying temperature. Constant volume and constant pressure Monte Carlo simulations are combined for systems consisting of N = 500 molecules along different isotherms over the reduced temperature range 0.60 ≤ T ≤ 1.25. As in previous simulation studies of the model, the study identifies nematic and smectic B phases on compressing the isotropic fluid, the particular phase sequence depending on temperature. The nematic phase is found to be stable with respect to the isotropic phase for reduced temperatures T ≥ 0.75. In the temperature range 0.75 ≤ T ≤ 1.25, the phase boundaries of the isotropic-nematic transition are obtained by computing the Helmholtz free energy of both phases from thermodynamic integration. From the simulation data, the relative volume change at the isotropic-nematic transition is about 2%, and this value appears to be rather insensitive to changes in temperature. On compressing the nematic phase, the Gay-Berne fluid undergoes a strong first-order transition to the smectic B phase. This transition is studied by using constant pressure simulation, and the coexistence properties are estimated from the limits of mechanical stability of the nematic phase. Larger relative volume changes are found at the transition than those suggested by previous studies, with typical values increasing up to 10.5% as the temperature is decreased. The results are consistent with the existence of strong coupling between nematic and smectic order parameters. For temperatures T ≤ 0.70 the nematic phase is no longer stable, and the phase sequence isotropic-smectic B is observed. Therefore, the Gay-Berne model exhibits an isotropic-nematic-smectic B triple point. Extrapolating the present simulation data, this triple point is located approximately at reduced temperature T_INB ? 0.70 and reduced pressure P_INB ? 1.825.     

X-ray diffraction and Mössbauer spectroscopy studies of cementite dissolution in cold-drawn pearlitic steel

Cementite dissolution in cold-drawn pearlitic steel (0.8 wt.% carbon) wires has been studied by quantitative X-ray diffraction (XRD) and Mössbauer spectroscopy up to drawing strain 1.4. Quantification of cementite-phase fraction by Rietveld analysis has confirmed more than 50% dissolution of cementite phase at drawing strain 1.4. It is found that the lattice parameter of the ferrite phase determined by Rietveld refinement procedure remains nearly unchanged even after cementite dissolution. This confirms that the carbon atoms released after cementite dissolution do not dissolve in the ferrite lattice as Fe-C interstitial solid solution. Detailed analysis of broadening of XRD line profiles for the ferrite phase shows high density of dislocations (～10¹⁵/m²) in the ferrite matrix at drawing strain 1.4. The results suggest a dominant role of ?1?1?1? screw dislocations in the cementite dissolution process. Post-deformation heat treatment leads to partial annihilation of dislocations and restoration of cementite phase. Based on these experimental observations, further supplemented by TEM studies, we have suggested an alternative thermodynamic mechanism of the dissolution process.     

Isotopic effect in the solid–liquid phase diagram of quantum clusters

Applying the Fourier path integral formalism to the isothermal-isobaric ensemble, the solid–liquid transition for 13-atom pure Lennard–Jones clusters was characterized. The masses of the clusters were taken as the masses of hydrogen, deuterium and tritium, hence isotopic effects of quantum clusters were considered. The parallel tempering Monte Carlo algorithm was used to solve all multidimensional integral in the FPI method. The volume of the system was defined with respect to the centroids of the quantum particles and a variable constraining potential was used to restrict undesirable thermodynamic events. The maximum value of the constant pressure heat capacity at a given temperature was used to identify the melting temperature. Pressure versus temperature phase diagrams were constructed for these systems with and without the inclusion of quantum effects. A significant difference in the melting temperature was encountered for the different isotopes due to quantum contribution.     

Ab initio study of the composite phase diagram of Ni–Mn–Ga shape memory alloys

The magnetic and structural properties of a series of nonstoichiometric Ni–Mn–Ga Heusler alloys are theoretically investigated in terms of the density functional theory. Nonstoichiometry is formed in the coherent potential approximation. Concentration dependences of the equilibrium lattice parameter, the bulk modulus, and the total magnetic moment are obtained and projected onto the ternary phase diagram of the alloys. The stable crystalline structures and the magnetic configurations of the austenitic phase are determined.     

Random-bond and random-anisotropy effects in the phase diagram of the Blume–Capel model

We report on Monte Carlo studies of the influence of quenched randomness on the phase diagram of the three-dimensional (3D) Blume–Capel model. The randomness is supposed to act either on the exchange coupling constants (bond randomness) or on the anisotropy distribution. With increasing disorder, first-order phase transitions are shown to change into second-order phase transitions. The trajectory of the tricritical point in the phase space as a function of disorder is presented. We have also calculated critical exponents at some points in the second-order phase region which show a change of universality class in agreement with the Harris criterion.     

Calculation of the phase diagram of the TiZr alloy and investigation of the tendency toward separation of the ω phase

The electronic, structural, and thermodynamic properties of the TiZr equiatomic alloy have been calculated in terms of the electron density functional theory and Debye-Grüneisen model. The calculated lattice parameters a and c/a agree well with experimental data for the α, ω, and β phases. It has been shown that the ω phase is stable at atmospheric pressure and low temperatures, and it remains energetically more favorable up to T = 600 K. In the temperature range 600 K < T < 900 K, the α phase becomes stable, and above 900 K, the β phase of the TiZr alloy is stable. The phase diagram constructed in this study agrees qualitatively with the available experimental data. A tendency toward separation of the TiZr equiatomic alloy in the ω phase has been analyzed. It has been demonstrated that, in the ground state, the TiZr equiatomic alloy in the ω phase exhibits a tendency toward ordering rather than toward phase separation.     

In situ X-ray diffraction studies of phase transition in Pb1− x La x Zr0.40Ti0.60O3 ferroelectric ceramics

The temperature dependence of the crystalline structure and the lattice parameters of Pb_{1? x} La _x Zr_0.40Ti_0.60O₃ ferroelectric ceramic system with 0.00 ≤ x ≤ 0.21 was determined. The samples with x ≤ 0.11 show a cubic-to-tetragonal phase transition at the maximum dielectric permittivity, T _max. Above this amount and especially for the x = 0.12 sample, a spontaneous phase transition from a relaxor ferroelectric state (cubic phase) to a ferroelectric state (tetragonal phase) is observed upon cooling below the T _max. Unlike what has been reported in other studies, the x = 0.13, 0.14, and 0.15 samples, which present a more pronounced relaxor behavior, also presents a spontaneous normal-to-relaxor transition, indicated by a cubic to tetragonal symmetry below the T _max. The origin of this anomaly has been associated with an increase in the degree of tetragonality, confirmed by the measurements of the X-ray diffraction patterns. The differential thermal analysis (DSC) measurements also confirm the existence of these phase transitions.     

Effect of copper on the structure–phase transformations and the properties of quasi-binary TiNi–TiCu alloys

The effect of copper alloying up to 25 at % on the structure–phase transformations and the physicomechanical properties of ternary alloys from the quasi-binary TiNi–TiCu section is studied by measuring the physicomechanical properties, transmission electron microscopy, scanning electron microscopy, electron diffraction, and X-ray diffraction (XRD). The data of temperature measurements of the electrical resistivity and the magnetic susceptibility and XRD data are used to plot a general diagram for the thermoelastic B2 ? B19', B2 ? B19 ? B19', and B2 ? B19 martensitic transformations, which occur in the alloys upon cooling as the copper content increases in the ranges 0–8, 8–15, and 15–25 at % Cu, respectively. The experimental results are compared to the well-known data, including differential scanning calorimetry data, obtained for these alloys. The changes in the mechanical properties and the microstructure of the alloys in the state of B19 or B19' martensite are discussed.