Proposed Phase Diagram Research Articles

Structural studies on the phase diagram of cerium

Energy dispersive X-ray diffraction on Ce in the range of pressures up to 14 GPa and temperatures up to 625 K identifies the structures in the previously proposed phase diagram and thereby disproves some of the more speculative phase diagrams from the literature, primarily concerning possible boundaries for the f.c.c. phase at high temperatures.

Head to head domain wall structures in thin magnetic strips

We present calculations of head to head domain wall structures in magnetic strips of Ni/sub 80/Fe/sub 20/ with widths, w, ranging from 75 nm to 500 nm and thicknesses, t, from 1 nm to 64 nm. Neglecting magnetocrystalline and magnetostrictive anisotropy energies, minimization of exchange and magnetostatic energy leads to one of two types of domain wall structures: 'transverse' walls with magnetization at the center of the wall directed transverse to the strip axis and 'vortex' walls where the magnetization forms a vortex at the center of the wall. Calculation of the domain wall energies leads to a proposed phase diagram for head to head domain walls where transverse walls have lower energy when dimensions are less than t/sub crit/w/sub crit//spl ap/130 A//spl mu//sub 0/M/sub s//sup 2/.

Superstructures and domain structures in natural and synthetic kalsilite

Transmission electron microscopy (TEM) investigation of natural kalsilite, (K,77Doo9Na,4b(Si209Alu9Feo12bOg, from San Venanzo, Italy, shows that it contains a pervasive domain structure related to a superstructure. The probable space group for the superstructure is P21. The relationship between the unit -cell parameters of the P6, subcell and those of the P21 supercell is a,upec ~ b,upec ~ v'3a,ub; c,upec = Cmb,and the a,upec axis is rotated 30° about the C axis with respect to the a,ubaxis. Nonperiodic distribution of elongated twin domains causes streaking of superstructure reflections normal to the c* axis. The average structure of this kalsilite sample has P6, symmetry. On the basis of previous structure refinements, this average structure has three 01 positions slightly displaced from the threefold axis, and each 01 position has a site occupancy of V3.High-temperature annealing studies indicated that there is a displacive phase transition from the P21 superstructure to the P63 substructure. The twin domains are related by threefold twin operations. The phase-transition temperature for kalsilite may range from 500 to 600°C and is dependent on the crystal composition (e.g., the proportions of Na, Ca, and vacancies). Kalsilite crystals prepared from nepheline by Na-K exchange are structurally complex. Domain structures in Na-bearing kalsilite crystals with more than 2.5 mol% NaSiAI04 are similar to those in the natural kalsilite sample but exhibit additional twin boundaries parallel to the c axis. Na-poor and Na-free kalsilite crystals are composed of (0001) domains with P63 and P31c symmetries, respectively, and merohedral twins between the neighboring P63 or P31c domains. The merohedral twins in the synthetic crystals formed during transformation of the nepheline structure to the kalsilite structure. The intergrown domains with P31c symmetry formed during the phase transition on cooling of the Na-poor kalsilite. There is no simple symmetry hierarchy among the known structures in the proposed phase diagram.

Excess oxygen diffusion in La2cuo4+? investigated by mechanical spectroscopy

The diffusion of excess oxygen atoms in La2CuO4+δ, (0 < δ < 0.035) was studied by mechanical spectroscopy with a vibrating reed method (220 Hz<f< 1.3kHz; 4K < T < 380K). For samples with δ=0.013(2) we observe a very large internal friction peak (Tmax=275K, f=0.22kHz). It shifts to higher temperatures with increasing frequencies indicating a thermally activated relaxation behavior with an activation enthalpy of 0.31(3) eV. The relaxation peak vanishes after vacuum annealing the sample (δ ≈0) leading to the conclusion that the relaxation peak is due to the diffusion of excess oxygen atoms. A relaxation peak of similar strength as in the sample with δ =0.013 is also detected in an oxygen enriched sample (δ = 0.035). This unexpected finding of the relaxation strength is discussed in terms of the proposed phase diagrams for La2CuO4+δ. In addition, a shoulder at the high temperature wing of the strong relaxation peak is seen the position of which does not vary with the vibration frequencies.

Phase diagram of carbon at high pressures and temperatures.

We have made a systematic study of solid and liquid phases of carbon over a wide range of pressures and temperatures using first-principles molecular dynamics. Our simulations elucidate three aspects of the phase diagram: (1) the melting of the simple cubic phase at 35--40 Mbar; (2) structural changes in the liquid in the range of 4--10 Mbar; and (3) the melting of the BC-8 phase at \ensuremath{\sim}22 Mbar. We combine the results of these three investigations to construct a proposed phase diagram for carbon. In particular, we find that above \ensuremath{\sim}6 Mbar the liquid state has approximately sixfold coordination, and, consequently, that the melting temperatures of the solids in this pressure range tend to decrease with increasing pressure. \textcopyright{} 1996 The American Physical Society.

Phase behaviors of supercooled water: Reconciling a critical point of amorphous ices with spinodal instability

The anomalies of supercooled water in thermodynamic response functions at atmospheric pressure, the phase transition between low and high density amorphous ices (LDA and HDA), and a predicted fragile–strong transition are accounted for in a unified manner by reconciling an idea due to Stanley and co-workers introducing a second critical point separating LDA and HDA ices with a conjecture proposed by Speedy that LDA is a different phase from a normal water, called water II. The reconciliation is made on the basis of results from extensive molecular dynamics simulations at constant pressure and temperature. It is found that there exist large gaps around temperature 213 K in thermodynamic, structural, and dynamic properties at atmospheric pressure, suggesting liquid–liquid phase transition. This transition is identified with an extension of the experimentally observed LDA–HDA transition in high pressure to atmospheric pressure. Thus, we propose a new phase diagram where the locus of the second critical point is moved into negative pressure region. With this simple modification, it becomes possible to account for the divergence of the thermodynamic response functions at atmospheric pressure in terms of the critical point and the spinodal-like instability of HDA. The unstable HDA undergoes a transition to LDA phase in lower temperature. The transition is also observed in high pressure region such as 200 MPa while it disappears at negative pressure, −200 MPa. This reinforces our proposed phase diagram in which there is no continuous path from a supercooled state to LDA at atmospheric pressure. It is argued that the HDA–LDA transition is accompanied by a fragile–strong transition. A possible mechanism of avoiding crystallization of aqueous solutions is also discussed in terms of a difference in hydrogen bond number distribution between LDA and HDA.

Optical birefringence of multilamellar gel phase of cholesterol/phosphatidylcholine mixtures

A new method is proposed to determine optical birefringence of lipid membranes from measurements of dynamic light scattering and polarized light transmitted through a polydisperse system of lipid vesicles. By means of this method, effects of cholesterol on membrane molecular order were studied for the tilted gel phase (the L β′ phase) of dipalmitoylphosphatidylcholine with cholesterol concentrations ranging from 0–20 mol%. The birefringence increased gradually with increasing cholesterol content at concentrations from 6–15 mol%, indicating an increase in longitudinal order of molecular orientation in the multilamellar gel phase. This result provides evidence that the untilted gel phase appears even at low cholesterol concentrations. However, the gradual increase in the birefringence suggests the coexistence of the tilted and untilted phases in this concentration range of cholesterol, which does not accord with a recently proposed phase diagram of the binary mixtures [T.P.W. McMullen and R.N. McElhaney, Biochim. Biophys. Acta, 1234 (1995) 90–98].

Chiral melting of the Si(113) (3 × 1) reconstruction

We report the results of synchrotron X-ray scattering studies of the disordering of the Si(113) (3 × 1) reconstruction. A continuous commensurate—solid to incommensurate—fluid transformation at T c = 950 ± 40 K is observed. At the transformation, the reconstructed layer becomes uniaxially incommensurate along the cubic (11¯0) ( x-direction). It remains commensurate along the (332¯) ( y-direction). Critical scattering shows power-law behavior over nearly two decades of reduced temperature with exponents β¯ for the incommensurability ( ɛ), t x = 0.65 ± 0.07 for the inverse correlation length in the incommensurate direction ( ϰ x ), v y = 1.06 ± 0.07 for the inverse correlation length in the commensurate direction ( ϰ y ), and γ = 1.56 ± 0.13 for the susceptibility ( χ). Below T c the variation of the square of the order parameter, proportional to the peak intensity at the commensurate position ( I 0), varies with an exponent 2 β = 0.22 ± 0.04. It is noteworthy that the correlation lengths in the disordered phase scale anisotropically, i.e. v x ≠ v y , and that the collected exponents do not conform to those of any previously measured universality class. Two universal constants of the transformation have also been measured. The ratio of the incommensurability and the inverse correlation length along the incommensurate direction in the disordered phase is found to be independent of temperature, i.e. β¯, consistent with predictions for a new two-dimensional chiral melting universality class, and to have the value w 0 = 1.6 ± 0.2. Also, the hyperscaling ratio R s = χϰ xϰ y / I 0 V r, where V r is the two-dimensional resolution volume, is independent of the reduced temperature, consistent with the derived hyperscaling relationship v x + v y = γ + 2 β. According to the hypothesis of two-scale-factor universality, R s is a universal constant, which we find takes the value R s = 0.07 ± 0.03. These results are discussed in the context of proposed phase diagrams of two-dimensional threefold-degenerate uniaxial overlayers where the chirality, or difference in free energy of light and heavy domain walls, is varied. A comparison is made to recent LEED measurements of the (3 × 1)-to-disordered transformations of Si and Ge.

Characterization of bismuth lead oxide by vibrational spectroscopy

Bismuth lead oxide is a promising fast-ion conductor, with an oxygen ion conductivity in excess of 1 S cm−1 at 590 °C. The characterization of the system (Bi2O3)1−x (PbO) x has previously been investigated using powder X-ray diffractometry; however, a detailed study using vibrational spectroscopy has not been carried out. This work examines the phases present between x=0 and 1 using both infrared and Raman spectroscopy. Detailed spectra of the phases are included, as well as an analysis of those phases. In general, the phases are reasonably consistent with previously published data; however, certain aspects differ. γ-Bi12PbO19, for example, was found to be a solid solution stable over a wide range of PbO, rather than a single phase as previously published. A summary of the room-temperature phases, after having been quenched from specific temperatures, is included in the form of a proposed phase diagram.

Topology and nematic ordering. II. Observable critical behavior.

This paper is the second in a pair treating a new lattice model for nematic media. In addition to the familiar isotropic (I) and nematically ordered (N) phases, the phase diagram established in the previous paper (Paper I) contains a new, topologically ordered phase (T) occuring at large suppression of topological defects and weak nematic interactions. This paper (Paper II) is concerned with the experimental signatures of the proposed phase diagram. Specific heat, light scattering and magnetic susceptibility near both the N/T and I/T transitions are studied, and critical behavior determined. The singular dependences of the Frank constants ($K_1$, $K_2$, $K_3$) and the dielectric tensor anisotropy ($\Delta \epsilon$) on temperature upon approaching the N/T transition are also found.

Raman Spectroscopy of the Fast Oxide-Ion Conductor Bismuth Lead Oxide

Bismuth lead oxide is a promising fast ion conductor, with an oxygen ion conductivity in excess of 1 S/cm at 590 C. Characterization of the system (Bi{sub 2}O{sub 3}){sub 1{minus}x}(PbO){sub x} has previously been investigated using powder X-ray diffractometry. However, a detailed study using vibrational spectroscopy has not been carried out. This work examines the phases present between x = 0 and 1 using Raman spectroscopy. Detailed spectra of the phases are included as well as an analysis of those phases. In general, the phases are reasonably consistent with previously published data, however certain aspects are contrary. {gamma}-Bi{sub 12}PbO{sub 19}, for example, was found to be a solid solution, rather than a single phase as previously published. A summary of the room temperature phases, after having been quenched from over a wide range of PbO is included in the form of a proposed phase diagram.

Formation and stability of metastable structures and amorphous phases in PU-V, PU-TA, and PU-YB systems with positive heats of mixing

The triode sputtering technique with a “split-target” arrangement was used to obtain metastable crystalline and amorphous phases in the Pu-V, Pu-Ta, and Pu-Yb systems. The proposed phase diagrams for these systems all exhibit liquid immiscibility. The heats of mixing are estimated to be highly positive, and the atomic radii of the component atoms differ by at least 10 pct. Extended amorphous and body-centered cubic (bcc) solid-solution regions were observed in the Pu-V and Pu-Ta systems. The corresponding lattice parameters appear to follow in each case an assumed Vegard’s Law extension. In the Pu-Yb system, no amorphous phase was obtained, but an extended face-centered cubic (fcc) solid-solution region (24 to 78 at. pct Yb) was observed with a large positive deviation of the lattice parameter (∼9 pct at 40 at. pct Yb) from a linear Vegard’s Law between the pure fcc components. The observed ranges of amorphous and metastable solid-solution phases have been interpreted in terms of predicated heats of formation for these phases using Miedema’s thermodynamic approximations that include chemical, elastic, and structural contributions. The effect of the high deposition rates on the formation of amorphous and metastable phases has also been considered. Thermal annealing of Pu-Ta amorphous alloys brings about a rapid diffusion of Pu to the free surface of the amorphous phase without crystallization of the remaining Ta-rich amorphous phase. Microhardness measurements indicate that amorphous Pu-V and Pu-Ta alloys are softer than the crystalline bcc solid-solution alloys in the same composition range. Several similarities in the formation of mixed phase regions (amorphous and solid solutions), microhardness, and resistance to decomposition on heating were noted between the Pu-Ta and Pu-V systems and the Cu-W system studied previously.

Flux creep and fishtail in melt-textured and single-crystalline YBa2Cu3O7

Time-dependent decay of the magnetic moment and magnetization measurements were used to study pinning and flux creep in two samples with and without aj(B) maximum in nonzero field (fishtail effect). From both measurements theE(j) relation was reconstructed forB∥c and the characteristic current exponentμ was obtained. At highj and lowB μ values are between 1 and 4; with increasingB μ passes a maximum and approaches negative values. At lowj μ is below 0.5 and not dependent onB orj. This behavior, which is qualitatively the same in both samples, is compared with proposed phase diagrams of the vortex lattice. Largeμ values are correlated with the plateau of the normalized creep rateS λ0.025; both observations indicate low relaxation and are found in that lowB region for whichj(B) has its minimum. This observation rejects a dominating influence of relaxation on the fishtail effect.

Chiral melting of the Si(113) (3 x 1) reconstruction.

Abstract We report the results of synchrotron X-ray scattering studies of the disordering of the Si(113) (3 × 1) reconstruction. A continuous commensurate—solid to incommensurate—fluid transformation at T c = 950 ± 40 K is observed. At the transformation, the reconstructed layer becomes uniaxially incommensurate along the cubic (11¯0) ( x -direction). It remains commensurate along the (332¯) ( y -direction). Critical scattering shows power-law behavior over nearly two decades of reduced temperature with exponents β¯ for the incommensurability ( ɛ ), t x = 0.65 ± 0.07 for the inverse correlation length in the incommensurate direction ( ϰ x ), v y = 1.06 ± 0.07 for the inverse correlation length in the commensurate direction ( ϰ y ), and γ = 1.56 ± 0.13 for the susceptibility ( χ ). Below T c the variation of the square of the order parameter, proportional to the peak intensity at the commensurate position ( I 0 ), varies with an exponent 2 β = 0.22 ± 0.04. It is noteworthy that the correlation lengths in the disordered phase scale anisotropically, i.e. v x ≠ v y , and that the collected exponents do not conform to those of any previously measured universality class. Two universal constants of the transformation have also been measured. The ratio of the incommensurability and the inverse correlation length along the incommensurate direction in the disordered phase is found to be independent of temperature, i.e. β¯, consistent with predictions for a new two-dimensional chiral melting universality class, and to have the value w 0 = 1.6 ± 0.2. Also, the hyperscaling ratio R s = χϰ x ϰ y / I 0 V r , where V r is the two-dimensional resolution volume, is independent of the reduced temperature, consistent with the derived hyperscaling relationship v x + v y = γ + 2 β . According to the hypothesis of two-scale-factor universality, R s is a universal constant, which we find takes the value R s = 0.07 ± 0.03. These results are discussed in the context of proposed phase diagrams of two-dimensional threefold-degenerate uniaxial overlayers where the chirality, or difference in free energy of light and heavy domain walls, is varied. A comparison is made to recent LEED measurements of the (3 × 1)-to-disordered transformations of Si and Ge.

Phases in rapidly cooled scandium-silicon samples

Small, 0.5 g, samples of high purity ScSi compositions have been melted in an argon arc furnace and cooled at 150–200 °C s −1 while their surface temperatures have been monitored. From the cooling curves and microscopic, hardness, X-ray powder and differential thermal analyses, a phase diagram is suggested which has many similarities to the most recently proposed phase diagram, but does contain evidence for a peritectically formed Sc 5Si 4 phase. The highest silicide is non-stoichiometric within the range ScSi 1.22-ScSi 1.67.

Guidelines for binary phase diagram assessment

The recent publication of Binary Alloy Phase Diagrams,2nd ed, [90Mas] and our extensive screening of phase diagram graphics for this edition has revealed many phase diagram features, which while not explicitly violating phase diagram rules, are to a lesser or greater extent unlikely to represent thermodynamically acceptable conditions. In two previous papers, several thermodynamically improbable features or boundaries in binary phase diagrams were pointed out [91Oka2], and some unlikely thermodynamic models were shown to be unrealistic, or in error [91Okal]. In the present paper, we discuss some of the unlikely features in more detail in order to develop a set of guidelines that may be useful for checking future proposed phase diagram boundaries, or some specific phase diagram features resulting from both experimental and theoretical work, including also phase diagram assessments.

Liquidus temperatures in the Ti-Al system

Liquidus temperatures were determined for the titanium-aluminum system at compositions ranging from 41 to 62 at. pct Al. The measurements were obtained by inducing solidification of slightly undercooled melts under containerless processing conditions using electromagnetic levitation. Absolute temperatures were determined by optical pyrometry in combination with independent measurements of spectral emissivities by laser polarimetry. The present liquidus temperatures are in agreement with two sets of literature values and are consistent with a set of solid-state literature data. These values exceed those selected in one recent proposed phase diagram revision by about 30 K and are as much as 40 to 60 K higher than those in another proposed revision.

Search for low temperature liquid H2 in 2-D films

Thermodynamic, scattering, and nmr measurements on films of H2, HD, D2, and mixtures of these molecules adsorbed on MgO and graphite have found phase diagrams for individual monolayers with solid-liquid-vapor triple points ranging from 7.2K (for the 2nd layer of H2 on MgO) to 5.74K (for a monolayer of H2 on top of a solid layer of D2 on graphite). We review the experimental evidence for the proposed phase diagrams.

PtP251. Phase transitions in highly disordered ferroelectrics

Abstract Critical properties of disordered ferroelectrics are reviewed. The broad spectrum of the relaxation times and the nonergodicity have been observed in these materials. The proposed phase diagrams including polar glassy states are discussed.

Microstructure/conductivity relationship in the scandia-zirconia system

Ageing behaviour has been studied in the scandia-zirconia system (7.8 mol%Sc 2O 3 content) at 1000°C and below with fourprobe dc conductivity measurements and impedance spectroscopy in conjunction with X-ray diffraction and transmission electron microscopy. In the literature, for similar compositions, the decrease in the conductivity with time has been attributed to the precipitation of a low conducting ordered rhombohedral β-phase (Sc 2Zr 7O 17). This study shows that in homogeneously prepared samples of 7.8 mol%Sc 2O 3-ZrO 2, the conductivity deterioration is inexorably tied to the formation and subsequent decomposition of a high temperature tetragonal phase, designated as the t'-phase. This phase is formed by diffusionless transformation from the cubic phase on cooling from the sintering temperature. Some local ordering (formation of γ- or σ-phase) was also detected in the microstructure. The β-phase is not observed at this composition in as-sintered or annealed specimens in agreement with the proposed phase diagrams and is formed only if incomplete reaction has occurred between Sc 2O 3 and ZrO 2 during fabrication.