Sluggish Transition Research Articles

Pressure-tuning, FT-Raman spectra of m-phthalic acid, m-C6H4(COOH)2, and a gadolinium(III) m-phthalate salt

Pressure-tuning, Stokes FT-Raman spectra of m-phthalic acid, m-C6H4(COOH)2 (H2L), and the gadolinium(III) salt, GdL(HL)(H2O)4·4H2O, have been studied at pressures up to ∼50kbar using 1064-nm NIR excitation. Sluggish phase transitions were observed for both compounds in the 26–29 and 28–39kbar regions, respectively. These pressure-induced structural transitions are most likely second-order phase transitions. The pressure sensitivities (dυ/dP) of selected Raman-active modes are reported. The most pressure-sensitive vibrations for m-phthalic acid are those associated with the intermolecular hydrogen bonding modes that occur at low wavenumbers. Some anti-Stokes FT-Raman scattering data are also reported.

In situ Raman study on degradation of edge plane graphite negative-electrodes and effects of film-forming additives

Structural changes in the surface of edge plane highly oriented pyrolytic graphite (HOPG) electrodes were studied in ethylene carbonate (EC)-based electrolytes by in situ Raman spectroscopy. The Raman spectra revealed that the surface crystallinity of graphite was significantly lowered by the initial intercalation and de-intercalation reactions of Li+. This structural degradation resulted in a sluggish stage transition of Li-GIC in the vicinity of the edge plane in the subsequent potential cycle. On the other hand, when the film-forming additive vinylene carbonate was used in the EC-based electrolyte solution, the crystallinity of the edge plane HOPG was maintained even after potential cycling. In addition, the phase transition of Li-GIC during the 2nd potential cycle proceeded in the same manner as in the initial cycle. Based on the present results, we discuss the suppressive role of film-forming additives on the degradation of the surface structure as it relates to the intercalation mechanism of Li+.

Thermal properties and transition studies of multi-wall carbon nanotube/nylon-6 composites

Transition behavior and thermal properties of a multi-wall carbon nanotube (MWCNT)/nylon-6 composite (P-composite) made by in situ polymerization and subsequently structurally modified by high-pressure–high-temperature treatment have been established. The thermal conductivity ( κ) of nylon-6 improved ∼27% by the addition of 2.1 wt.% MWCNT filler simultaneously as the heat capacity per unit volume decreased ∼22% compared with that of nylon-6 at 1 atm and 298 K. Moreover, the MWCNT filler raises the glass transition temperature ( T g) of nylon-6, but the pressure dependence of T g remains unchanged. A model for κ indicates that the interfacial thermal resistance between the MWCNT filler and the nylon-6 matrix decreases 20% up to 1 GPa and most significantly above 0.8 GPa. P-composite was structurally modified by a sluggish cold-crystallization transition at 1.0 GPa, 530 K, which further increased κ by as much as ∼37% as the crystallinity of nylon-6 improved from 31% to 58% with a preferred crystal orientation and increased crystal size.

High-pressure high-temperature synthesis of Cr2O3 and Ga2O3

We report the results of high-pressure high-temperature (HP-HT) synthesis of two sesquioxides, Cr2O3 at 22 GPa and 1950°C and Ga2O3 at 21 GPa and 1600°C, and their X-ray diffraction and Raman studies. After HP-HT synthesis, both oxides presented assemblies of single crystallites that adopted a corundum structure. For HP-HT-synthesized single crystal of Cr2O3, we determined the lattice parameters and the atomic positions in the corundum structure as follows: a=4.9530 Å, c=13.5884 Å, Cr (12 c): 0, 0, 0.3475; O (18 c): 0.3058(5), 0, 0.25. We explained the ‘evidence’ for the existence of a ‘distorted’ corundum lattice above 13–15 GPa by resonant effects in Raman spectra. For Ga2O3, it was demonstrated that HP-HT synthesis can fully turn a sample from a monoclinic C2/m phase (β-Ga2O3) to a corundum phase (α-Ga2O3), although at ambient temperature this sluggish transition is usually completed at ∼35–40 GPa. It was found that α-Ga2O3 exhibits a very strong luminescence doublet that resembles the one in ruby, and hence could have potential for pressure calibration.

Low-spin Fe 2+ in silicate perovskite and a possible layer at the base of the lower mantle

We investigated the spin state of iron in Mg 0.82Fe 0.18SiO 3 silicate perovskite using Mössbauer spectroscopy and nuclear forward scattering (NFS) at pressures up to 130 GPa and temperatures up to 1000 K. Majorite starting material was loaded into diamond anvil cells in three separate experiments, and transformed to silicate perovskite through laser heating. We found, in agreement with previous work, the predominance of a component with high isomer shift (∼1 mm/s relative to α-Fe) and high-quadrupole splitting (QS) (>4 mm/s) in Mössbauer and NFS spectra up to 115 GPa at room temperature, and in accordance with previous work this component was assigned to intermediate-spin Fe 2+. At higher pressures, the intensity of the high QS component in the silicate perovskite spectrum decreased, while the intensity of a new component with low isomer shift (∼0 mm/s relative to α-Fe) and low quadrupole splitting (<0.5 mm/s) increased. This new component was assigned to low-spin Fe 2+, and its intensity increased with both increasing pressure and increasing temperature: at 120 GPa and 1000 K all Fe 2+ was in the low-spin state. X-ray diffraction data showed well crystallised perovskite in all runs, and although the stable phase above ∼110 GPa is expected to be post-perovskite, sluggish transition kinetics likely preserved the perovskite phase in a metastable state. Our results combined with data in the literature and thermodynamic and topological considerations suggest that there may be a region where silicate perovskite containing low-spin Fe 2+ is stable, which coincides with predicted pressure–temperature conditions near the D″ layer.

An anatomy of male labour market earnings inequality in Serbia, 1996–2003

This study uses a regression-based framework to identify the key factors that determine the level and changes in main job earnings inequality for men. A number of different inequality measures are used in our work. The analysis uses data for Serbia drawn from eight annual labour force surveys, which cover both the early episode of sluggish transition and a more recent concerted phase of economic reform. It thus provides some useful insights on the evolution of labour earnings inequality through an uneven transitional process and identifies factors likely to retain an influence on earnings inequality as the market reform processes take greater hold.

In situ, high-pressure differential thermal analysis and ionic conductance of PMMA-based gels with and without TiO 2 nano-particle filler

The transition behaviours of PMMA-based gels with and without nano-particle filler have been investigated at elevated pressures up to 0.9 GPa and in the temperature range 220–310 K. Both gels had molar ratios of 53.9:22:5.2:18.9 of ethylene carbonate (EC), propylene carbonate (PC), lithium perchlorate, PMMA, and one gel contained 8 wt.% TiO 2 nano-particles. The results show a sluggish transition between a low-pressure (high temperature) state and a high-pressure (low temperature) state. The nano-particle filler facilitates the kinetics of the transition, yielding a significantly lower transition pressure as well as a better homogeneity in the sample after a transition back to the low-pressure state. These results can be explained by a filler-induced increase of the nucleation rate, which yields smaller crystallites. The transition is observed also by differential scanning calorimetry and Raman spectroscopy at ambient pressure and is likely associated with crystallization of the EC–PC solvent. The logarithmic pressure derivative of the conductance and, consequently, the activation volume changes a factor of 3 as a result of the transition.

Comparison of rolling texture in low and medium stacking fault energy austenitic stainless steels

Cold rolling texture in AISI 304L and 316L austenitic stainless steels (SS) having low and medium stacking fault energy (SFE), respectively, has been studied. Though at small deformation, both the steels show similar texture, at higher deformation levels, the behaviour on transition from copper to brass-type textures expected in face centered cubic (fcc) structures was different in the two steels. 304L SS showed a sluggish transition compared to 316L SS. This is explained to be due to the effect of the strain-induced α′-martensite formation during the cold rolling of 304L SS, and is confirmed by varying the volume fraction of α′-martensite phase at the same deformation levels by controlled rolling conditions.

High pressure Raman spectroscopic study of spinel MgCr2O4

An in situ Raman spectroscopic study was conducted to investigate the pressure induced phase transformation of MgCr2O4 spinel up to pressures of 76.4GPa. Results indicate that MgCr2O4 spinel undergoes a phase transformation to the CaFe2O4 (or CaTi2O4) structure at 14.2GPa, and this transition is complete at 30.1GPa. The coexistence of two phases over a wide range of pressure implies a sluggish transition mechanism. No evidence was observed to support the pressure-induced dissociation of MgCr2O4 at 5.7–18.8GPa, predicted by the theoretical simulation. This high pressure MgCr2O4 polymorphism remains stable upon release of pressure, but at ambient conditions, it transforms to the spinel phase.

Pressure-Effect on Anatase Titanium Dioxide

Pressure-induced phase transition of anatase titanium dioxide was investigated by Raman, absorption spectroscopy and X-ray diffraction. The change in Raman and absorption spectra with pressure revealed that the transition from anatase to high pressure phase with f -PbO 2 structure (TiO 2 -II) occurred in the pressure range of 4.0-4.6 GPa for a single crystal. The X-ray powder diffraction patterns indicate the presence of superstructural lattice of anatase at pressures more than 3 GPa. The superstructure of anatase disappears on the release of the pressure. A sluggish transition to the high pressure phase is also observed. The anatase coexists with the high pressure phase at 5.2 GPa. The difference in the results between optical spectroscopy (single crystal) and X-ray diffraction (powder) will be due to crystalinity of the sample.

High-Pressure Structures of Ge above 100 GPa

We have investigated structural phase transitions of Ge at pressures up to 190 GPa using monochromatic synchrotron X-ray diffraction. The phase appearing near 100 GPa is found to have an orthorhombic structure (space group Cmca, Z = 16 atoms per unit cell, isotypic to Si–VI). Ge further undergoes a sluggish transition to a hcp structure at around 160–180 GPa. With these new findings, the sequence of structural transitions observed for Ge under pressure up to 190 GPa is similar to that of Si in the range up to 50 GPa. We have performed full potential linear muffin tin orbital calculations of the volume-dependent total energies of Ge in different crystal structures. The calculated phase transitions are compared to the experimental results reported here.

Equation of state of magnetite and its high-pressure modification: Thermodynamics of the Fe-O system at high pressure

Fe 3 O 4 has been studied by high-pressure diffraction to 43 GPa. No major changes in the spinel-type structure of magnetite is observed below 21.8 GPa. At higher pressure a sluggish transition to a highpressure modification, h-Fe 3 O 4 , is observed. The X-ray diffraction pattern of the high-pressure modification is consistent with the orthorhombic unit cell (CaMn 2 O 4 -type structure, space group Pbcm) recently proposed for h-Fe 3 O 4 by Fei et al. (1999), however, it is also consistent with a more symmetric CaTi 2 O 4 - type structure (space group Bbmm). Bulk modulus values for magnetite, K T0 = 217 (2) GPa, and h-Fe 3 O 4 , K T0 = 202 (7) GPa, are calculated from the pressure-volume data using a third-order Birch-Murnaghan equation of state. A thermodynamic analysis of the Fe-O system at high pressure is presented. The proposed equation of state of h-Fe 3 O 4 gives an increased stability of wüstite relatively to a two-phase mixture of iron and h-Fe 3 O 4 compared to earlier equations of state and removes an inconsistency in the thermodynamic description of the Fe-O system at high pressure.

Thermomechanical study of the high temperature phase transition in KH 2PO 4

The method of thermomechanical analysis (TMA) has been employed to examine the high temperature polymorphic transition in potassium dihydrogen phosphate (KH 2PO 4). Both loose powder and pressed pellet samples show an anomalous expansion in the temperature region 200–220°C. The anomaly is explained by the occurrence of a sluggish polymorphic transition. The transition has been found to be reversible.

Crystallization of glassy selenium during its plastic deformation

Crystallization of glassy selenium (Se) during its plastic deformation has been investigated in the temperature interval between 60°C and 95°C. We found that plastic reshaping of small Se pellets into thin discs greatly enhances the notoriously sluggish transition from glassy to crystalline state. However, the content of the crystalline phase in the deformed samples is found to be strongly dependent upon the temperature of the process. While the 30-min long reshaping at 60°C and 95°C gives only partially crystallized samples (6% and 10%, respectively), an equally long process at 80°C gives 100% crystallized samples. Average crystallite size in the samples prepared at 60°C and 80°C is very similar (25 nm and 15 nm, respectively), but in the sample prepared at 95°C the grains are much larger: about 80 nm. The results are discussed taking into account the possibility that the mechanism of plastic deformation enhances both the nucleation frequency and the rate of crystal growth.

Charged defects and ferroelectricity in (1−x)PbZrO3−x(Na1/2Bi1/2)TiO3 solid solution

The crystal structure, dielectric/ferroelectric properties, and domain configuration of (1−x) PbZrO3−x(Na1/2Bi1/2)TiO3 solid solution with 0.14⩽x⩽0.4 were investigated using x-ray powder diffraction, D-E hysteresis, εr(T) measurements, and transmission electron microscopy. A sluggish transition from normal ferroelectric to relaxor state was observed as a function of composition. With an increasing amount of (Na1/2Bi1/2)TiO3, this transition was accompanied by changes in crystal structure (rhombohedral to pseudocubic), domain size and morphology (normal micron-size domain to polar nanodomain), and thermal phase transition behavior (normal ferroelectric transition to diffuse transition behavior with polarization relaxation). For intermediate compositions (i.e., x=0.25) between normal ferroelectric (x=0.16) and relaxor (x=0.4), a spontaneous relaxor-to-normal ferroelectric transition occurred at 80 °C in the εr(T) curve. It is suggested that this transition originates from the introduction of charged defects such as NaPb′ and BiPb•, resulting in the disturbance of long-range interactions among polar ordered BO6-octahedra and short-range structural order of charged defects (i.e., NaPb′−BiPb•).

Comment on abnormality of structural phase transition in the KNO3 crystal

The structural phase transition of KNO3 has been investigated by using Brillouin scattering as a function of temperature and pressure. The following results were obtained: the intermediate phase appears between II- and I-phases on heating, there are large differences between isobaric heating process and isothermal pressurizing process for the transition including the III phase. The KNO3 crystal has been known to have an abnormal structural phase transition such as sluggish transition. Various other experiments have shown such unusual phenomena: particle size effects by heat-flow calorimetry, healing effect by acoustic emission technique, alternative structure by Raman scattering and so on. Abnormality is discussed and the cause of the occurrence of the intermediate phase is considered by taking account of the characteristics of the KNO3 crystal.

Testing hypotheses on plate‐driving mechanisms with global lithosphere models including topography, thermal structure, and faults

A popular concept of plate tectonics is that the most important density anomalies are in oceanic plates, which descend from rise to trench while subducting slabs act as velocity regulators and the drag on the base of the lithosphere is resistive. This hypothesis has been shown to be consistent with velocities of rigid plates and to approximately predict stress directions in a global elastic shell. Here I test whether the hypothesis leads to correct plate velocities and stress directions in models of laterally heterogeneous plates of nonlinear rheology separated by faults with low friction. All models have bottom boundary conditions based on simple shear in an olivine asthenosphere extending to the transition zone, where various patterns of lower mantle flow are assumed. These models show that topography alone is not sufficient to drive plate motion at actual rates over a static or sluggish transition zone. If the forward component of velocity of each subducting slab is also imposed, then velocities are improved, but errors in stress direction become unacceptable. Better models are found by assuming that at least some parts of the transition zone have velocities greater than surface velocities, leading to active or forward basal drag. In the most plausible model, this forward drag acts only on continents, while oceanic lithosphere experiences negligible basal shear tractions. Probably the dense descending slabs of oceanic lithosphere not only pull the oceanic plates, but also stir the more viscous lower mantle, and this in turn helps to drive the slower drift of continents.

Magnetic-Electronic, Conductivity, and Structural Pressure Studies of Sr2FeO4 and Sr3Fe2O7.

Sr2FeO4 and Sr3Fe2O7 have been investigated by resistance-temperature, 57Fe-Mossbauer magnetic and x-ray diffraction studies up to ∼30 GPa. Magnetic moments in Sr2FeO4 remain intact across the conductivity transition to a metallic state at ∼17 GPa. The complex spiral-type magnetic structure at low pressure changes to collinear in the metallic state. In Sr3Fe2O7 the original disproportionated state and associated magnetic-electronic structure first exhibits a change at ∼12-15 GPa, and only a single valence state is discernible at P ≥ 21 GPa. A very narrow gapped state prevalent at ∼10 GPa undergoes a sluggish transition to incipient metallic behavior at ∼28 GPa. X-ray data suggest that the magnetic-electronic and conductivity transitions in both compounds are not driven by symmetry-lowering structural transitions.

Thermophysical Properties of Cholesteryl Oleyl Carbonate Determined with Microscopic Ftir/Dsc System

AbstractA microscopic Fourier transform infrared spectrometer (Micro FTIR) equipped for differential scanning calorimetry (DSC) was used to measure simultaneously the chemical structural variation and the thermal response of phase transition of cholesteryl oleyl carbonate (COC). The differential scanning calorimeter served also to determine the phase transitions of COC during heating or cooling. Two endothermic features due to phase transition were found in the thermogram: 18. 3 °C for a smectic‐cholesteric transition and 37.5 °C for a cholesteric‐isotropic transition during heating; 35.1 °C for an isotropic‐cholesteric transition and 15.8 °C for a cholesteric‐smectic transition during cooling. The breadth of the feature indicated sluggish phase transitions. The three‐dimensional plot indicated that the intensities of lines due to the C‐H stretching and scissoring deformation modes and C=O stretching mode of COC decreased suddenly near the temperature of phase transition during heating but the intensity of the line dues to the C‐O stretching mode of carbonate ester of COC increased. Lattice vibrations or coupling of vibrational modes might be responsible for the result. These intensities in the cooling process varied inversely to those in heating process.

High-pressure polymorphism of the copper(I) halides: A neutron-diffraction study to ~10 GPa.

The structural changes within the copper(I) halides induced by hydrostatic pressure have been investigated using the powder neutron-diffraction technique. The expected transition from the ambient zinc-blende structure with tetrahedral coordination to the octahedrally coordinated rocksalt structure is observed in both CuCl and CuBr. No rocksalt phase is observed in CuI, presumably due to the limited maximum pressure of our apparatus (\ensuremath{\sim}10 GPa). The lower symmetry phases which occur as intermediate structures between the zinc-blende and rocksalt phases have been studied in detail. CuCl and CuI undergo abrupt transitions whilst CuBr has three sluggish transitions, involving extensive coexistence of neighboring phases. There is one intermediate phase in CuCl (CuCl-IV), two in CuBr (CuBr-IV and CuBr-V) and at least two in CuI (CuI-IV and CuI-V). Three different structure types have been identified, in space groups Pa3\ifmmode\bar\else\textasciimacron\fi{} (CuCl-IV and CuBr-V), P4/nmm (CuBr-IV and CuI-V) and R3\ifmmode\bar\else\textasciimacron\fi{}m (CuI-IV). The intermediate structures all retain the tetrahedral cation coordination of the ${\mathrm{Cu}}^{+}$, though the environment is rather more distorted in CuCl and CuBr than in CuI. The evolution of the structures of these I-VII compounds is very different from that observed in the more covalent II-VI and III-V systems.