Evidence For Structural Phase Transition Research Articles

High-Pressure Vibrational and Structural Studies of the Chemically Engineered Ferroelectric Phase of Sodium Niobate

Pure NaNbO3 has an antiferroelectric phase at ambient pressure. The structural behaviour of the chemically engineered ferroelectric phase of sodium niobate, NNBT05: [(0.95) NaNbO3-(0.05) BaTiO3], under high-pressure has been studied using Raman scattering and angle-dispersive synchrotron X-ray diffraction techniques. At pressure > 1 GPa, noticeable changes in the Raman spectra can be seen in the low wavenumber modes (150–300 cm−1). Large changes in the positions and intensities of the Raman bands as a function of pressure provide evidence for structural phase transition. The results indicate significant changes in the bond-lengths and the orientation of the NbO6 octahedra at ~1 GPa, and a transition to the paraelectric phase at ~5 GPa, which are at lower pressures than previously found in pure NaNbO3. The powder X-ray diffraction pattern shows an appreciable change in the peak profile in terms of position and width on increasing pressure. The pressure dependences of the structural parameters show that the response of the lattice parameters to pressure is strongly anisotropic. By fitting the pressure–volume data using the Birch–Murnaghan equation of state, the isothermal bulk modulus was estimated. The experimental results suggest that on doping BaTiO3 in NaNbO3, the bulk modulus increases. The bulk modulus of NNBT05 has been estimated to be 164.5 GPa, which is fairly close to 157.5 GPa, as previously observed in NaNbO3.

Anisotropic thermal expansion and electronic transitions in the Co3BO5 ludwigite.

The investigations of the crystal structure, magnetic and electronic properties of Co3BO5 at high temperatures were carried out using powder X-ray diffraction, magnetic susceptibility, electrical resistivity, and thermopower measurements. The orthorhombic symmetry (Sp.gr. Pbam) was observed at 300 K and no evidence of structural phase transitions was found up to 1000 K. The compound shows a strong anisotropy of the thermal expansion. A large negative thermal expansion along the a-axis is observed over a wide temperature range (T = 300-600 K) with αa = -35 M K-1 at T = 500 K with simultaneous expansion along the b- and c-axes with αb = 70 M K-1 and αc = 110 M K-1, respectively. The mechanisms of thermal expansion are explored by structural analysis. The activation energy of the conductivity decreases significantly above 700 K. Electronic transport was found to be a dominant conduction mechanism in the entire temperature range. The correlations between the thermal expansion, electrical resistivity, and effective magnetic moment were revealed and attributed to the evolution of the spin state of Co3+ ions towards the spin crossover and gradual charge-ordering transition.

Magnetoelectric effect arising from a field-induced pseudo Jahn-Teller distortion in a rare-earth magnet

Magnetoelectric materials are attractive for several applications, including actuators, switches, and magnetic field sensors. Typical mechanisms for achieving a strong magnetoelectric coupling are rooted in transition metal magnetism. In sharp contrast, here we identify $\mathrm{CsEr}{({\mathrm{MoO}}_{4})}_{2}$ as a magnetoelectric material without magnetic transition metal ions, thus ensuring that the Er ions play a key role in achieving this interesting property. Our detailed study includes measurements of the structural, magnetic, and magnetoelectric properties of this material. Bulk characterization and neutron powder diffraction show no evidence for structural phase transitions down to 0.3 K and therefore $\mathrm{CsEr}{({\mathrm{MoO}}_{4})}_{2}$ maintains the room temperature $P2/c$ space group over a wide temperature range without external magnetic field. These same measurements also identify collinear antiferromagnetic ordering of the ${\mathrm{Er}}^{3+}$ moments below ${T}_{N}=0.87\phantom{\rule{4pt}{0ex}}\mathrm{K}$. Complementary dielectric constant and pyroelectric current measurements reveal that a ferroelectric phase with a maximum polarization $P\ensuremath{\sim}0.6\phantom{\rule{4pt}{0ex}}\mathrm{nC}/{\mathrm{cm}}^{2}$ emerges when applying a modest external magnetic field, which indicates that this material has a strong magnetoelectric coupling. We argue that the magnetoelectric coupling in this system arises from a pseudo Jahn-Teller distortion induced by the magnetic field.

Structural stability and vibrational characteristics of CaB6 under high pressure**Project supported by the National Natural Science Foundation of China (Grant Nos. 51572108, 51632002, 11504127, 11674122, 11574112, 11474127, and 11634004), the 111 Project, China (Grant No. B12011), the Program for Changjiang Scholars and Innovative Research Team in University, China (Grant No. IRT_15R23), and the National Found for Fostering Talents of Basic

In situ Raman spectroscopy and x-ray diffraction measurements are used to explore the structural stability of CaB6 at high pressures and room temperature. The results show no evidence of structural phase transitions up to at least 40 GPa. The obtained equation of state with smooth pressure dependencies yields a zero-pressure isothermal bulk modulus B0 = 170 (5) GPa, which agrees well with the previous measurements. The frequency shifts for A1g, Eg, and T2g vibrational modes of polycrystalline CaB6 are obtained with pressure uploading. As the pressure increases, all the vibration modes have smooth monotonic pressure dependence. The Grüneisen parameter of Eg modes is the largest, indicating its largest dependence on the volume of a crystal lattice.

Structural and redox features of La0.7Bi0.3Mn1−xCoxO3 nanoperovskites for ethane combustion and CO oxidation

AbstractIn this work, structural and catalytic properties of La0.7Bi0.3Mn1−xCoxO3 nanocatalysts with x=0.00, 0.25, 0.50, 0.75, and 1.00 prepared by citrate method are investigated. The structural characterization using X'Pert package and Fullprof program is an evidence for structural phase transition. The values of refined unit cell volume obtained from the Rietveld analysis show decreasing and increasing tendencies for values of x≤0.5 and x>0.5, respectively. The catalytic performance tests of the catalysts show that the samples x=0.00 and 0.25 have lower temperature of CO oxidation and C2H6 combustion, respectively.

La1−xCaxCo1−yMgyO3 nano‐perovskites as CO oxidation catalysts: Structural and catalytic properties

AbstractIn this work, structural and catalytic properties of LaCo1−yMgyO3 and La1−xCaxCo0.50Mg0.50O3 nanocatalysts with x=0.00, 0.15, 0.30, 0.45, 0.60, 0.75 and y=0.00, 0.25, 0.50 prepared by citrate method are investigated. The structural characterization using X'Pert package and Fullprof program is an evidence for structural phase transition. The structural results suggest the presence of Co4+ and Co3+ ions that help to overall oxidation activity of these samples and this feature is higher for x=0.60. When substituting Mg and Ca in LaCoO3 the catalytic activity of CO oxidation shifts to lower temperature. La0.5Ca0.50Co0.5Mg0.5O3 nanocatalyst exhibits excellent CO conversion rate of 95% at 267°C. These results suggest that the lower temperature conversion of the sample x=0.60 in series of La1−xCaxCo1−yMgyO3 can be mainly attributed to the presence of (i) higher Co3+–O–Co4+ couples, (ii) a structure having the higher tolerance factor, and (iii) a lower crystallite size.

Calcium and strontium substituted lanthanum manganite–cobaltite [La1−x(Ca,Sr)xMn0.5Co0.5O3] nano-catalysts for low temperature CO oxidation

In this work, structural and catalytic properties of La1−xCaxMn0.5Co0.5O3 and La1−xSrxMn0.5Co0.5O3 nano-catalysts in the concentration range of 0.00≤x≤1.00 are investigated. The structural characterization of the compounds obtained by X-ray powder diffraction and using X′Pert package and Fullprof program is an evidence for structural phase transition of the samples that can be described by variation of tolerance factor. The obtained results also have been confirmed by FTIR measurements. The structural results, and the non-uniformly changes of activation energy, band gap energy and electrical conductivity values suggest the presence of different concentration of various cations Co+2, Co+3, Co+4, Mn+4 and Mn+3 in the compounds studied. The decrease of the crystallite size with increasing Ca and Sr substitution for x up to 0.5 in La1−x(Sr, Ca)xMn0.5Co0.5O3 can be related to the increase and decrease of micro-strain, respectively. Substituting Sr having higher ionic radius with respect to La in LaMn0.5Co0.5O3 shifts the catalytic activity of CO oxidation to lower temperature, whereas, for Ca substitution the catalytic activity shifts to higher temperature. The trend of catalytic activity observed for Ca- and Sr-substituted samples can be related to different morphology observed in SEM micrographs due to different types of micro-strain. La0.5Sr0.5Mn0.5Co0.5O3 nano-catalyst exhibits excellent CO conversion rate of 95% at 175°C.

First-principles high-pressure unreacted equation of state and heat of formation of crystal 2,6-diamino-3, 5-dinitropyrazine-1-oxide (LLM-105).

We report dispersion-corrected density functional theoretical calculations of the unreacted equation of state (EOS) of crystal 2,6-diamino-3, 5-dinitropyrazine-1-oxide (LLM-105) under hydrostatic compression of up to 45 GPa. Convergence tests for k-points sampling in the Brillouin zone show that a 3 × 1 × 2 mesh is required to reproduce the X-ray crystal structure at ambient conditions, and we confirm our finding with a separate supercell calculation. Our high-pressure EOS yields a bulk modulus of 19.2 GPa, and indicates a tendency towards anisotropic compression along the b lattice vector due to molecular orientations within the lattice. We find that the electronic energy band gap decreases from a semiconductor type of 1.3 eV at 0 GPa to quasi-metallic type of 0.6 eV at 45 GPa. The extensive intermolecular hydrogen bonds involving the oxide (-NO) and dioxide (-NO2) interactions with the amine (-NH2) group showed enhanced interactions with increasing pressure that should be discernible in the mid IR spectral region. We do not find evidence for structural phase transitions or chemically induced transformations within the pressure range of our study. The gas phase heat of formation is calculated at the G4 level of theory to be 22.48 kcal/mol, while we obtain 25.92 kcal/mol using the ccCA-PS3 method. Density functional theory calculations of the crystal and the gas phases provided an estimate for the heat of sublimation of 32.4 kcal/mol. We thus determine the room-temperature solid heat of formation of LLM-105 to be -9.9 or -6.5 kcal/mol based on the G4 or ccCA-PS3 methods, respectively.

Combined experimental and computational study of the pressure dependence of the vibrational spectrum of solid picene C22H14

We present high-quality optical data and density functional perturbation theory calculations for the vibrational spectrum of solid picene (C${}_{22}$H${}_{14}$) under pressure up to 8 GPa. First-principles calculations reproduce with a remarkable accuracy the pressure effects on both frequency and intensities of the observed phonon peaks. We use the projection on molecular eigenmodes to unambiguously fit the experimental spectra, resolving complicated spectral structures, in a system with hundreds of phonon modes. With these projections, we can also quantify the loss of molecular character under pressure. Our results indicate that picene, despite a $\ensuremath{\sim}20%$ compression of the unit cell, remains substantially a molecular solid up to 8 GPa, with phonon modes displaying a smooth and uniform hardening with pressure, without any evidence of structural phase transitions. The Gr\"uneisen parameter of the 1380 cm${}^{\ensuremath{-}1}$ ${a}_{1}$ Raman peak (${\ensuremath{\gamma}}_{p}=0.1$) is much lower than the effective value (${\ensuremath{\gamma}}_{d}=0.8$) due to K doping. Therefore, doping and pressure have very different effects and it can be argued that softening of the 1380 cm${}^{\ensuremath{-}1}$ mode is probably due to coupling with electronic states in K-doped solid picene.

Communication: Evidence of structural phase transitions in silicalite-1 by infrared spectroscopy

The adsorption of trichloroethylene, perchloroethylene, and p-xylene on a MFI (Mobile-FIve) zeolite is studied using in situ FTIR spectroscopy at 298 K. Spectra of self-supported zeolites in contact with increasing pressures of pure gas were recorded at equilibrium in the mid-infrared domain. Analysis of the evolution of the shape and location of vibrational bands of the zeolite as a function of the amount adsorbed allowed the observation of structural modifications of the adsorbent for the first time by infrared spectroscopy.

Evidence for structural phase transitions and large effective band gaps in quasi-metallic ultra-clean suspended carbon nanotubes

We report evidence for a structural phase transition in individual suspended metallic carbon nanotubes by examining their Raman spectra and electron transport under electrostatic gate potentials. The current-gate voltage characteristics reveal anomalously large quasi-metallic band gaps as high as 240 meV, the largest reported to date. For nanotubes with band gaps larger than 200 meV, we observe a pronounced M-shape profile in the gate dependence of the 2D band (or G’ band) Raman frequency. The pronounced dip (or softening) of the phonon mode near zero gate voltage can be attributed to a structural phase transition (SPT) that occurs at the charge neutrality point (CNP). The 2D band Raman intensity also changes abruptly near the CNP, providing further evidence for a change in the lattice symmetry and a possible SPT. Pronounced non-adiabatic effects are observed in the gate dependence of the G band Raman mode, however, this behavior deviates from non-adiabatic theory near the CNP. For nanotubes with band gaps larger than 200 meV, non-adiabatic effects should be largely suppressed, which is not observed experimentally. This data suggests that these large effective band gaps are primarily caused by a SPT to an insulating state, which causes the large modulation observed in the conductance around the CNP. Possible mechanisms for this SPT are discussed, including electron-electron (e.g., Mott) and electron-phonon (e.g., Peierls) driven transitions. Open image in new window

Suppression of antiferroelectric state in NaNbO3 at high pressure from in situ neutron diffraction

We report direct experimental evidence of antiferroelectric to paraelectric phase transition under pressure in NaNbO3 using neutron diffraction at room temperature. The paraelectric phase is found to stabilize above 8 GPa and its crystal structure has been determined in orthorhombic symmetry with space group Pbnm. The variation of the structural parameters of the both orthorhombic phases as a function of pressure was determined. We have not found evidence for structural phase transition around 2 GPa as previously suggested in the literature based on Raman scattering experiments; however, significant change in Nb-O-Nb bond angles are found around this pressure. The response of the lattice parameters to pressure is strongly anisotropic with a largest contraction along 〈100〉. The structural phase transition around ∼8 GPa is followed by an anomalous increase in the orthorhombic strain and tilt angle associated with the R point (q = ½ ½ ½). Ab-initio calculation of the enthalpy in the various phases of NaNbO3 is able to predict the phase transition pressure well.

Evidence for Structural Phase Transitions Induced by the Triple Phase Line Shift in Self-Catalyzed GaAs Nanowires

Self-catalyzed growth of GaAs nanowires are widely ascribed to the vapor-liquid-solid (VLS) mechanism due to the presence of Ga particles at the nanowire tips. Here we report synthesis of self-catalyzed GaAs nanowires by molecular-beam epitaxy covering a large growth parameter space. By carefully controlling the Ga flux and its ratio with the As flux, GaAs nanowires without Ga particles and exhibiting a flat growth front are produced. Using scanning electron microscopy and high-resolution transmission electron microscopy, we compare the growth rate and structure, especially near the growth front, of the nanowires with and without Ga droplets. We find that regardless of whether Ga droplets are present on top, the nanowires have a short wurtzite section following the zinc-blende bulk structure. The nanowires without Ga droplets are terminated by a thin zinc-blende cap, while the nanowires with Ga droplets do not have such a cap. The bulk zinc-blende phase is attributed to the Ga droplet wetting the sidewall during growth, pinning the triple phase line on the sidewall. The zinc-blend/wurtzite/(zinc-blende) phase transitions at the end of growth are fully consistent with the triple phase line shifting up to the growth front due to the progressive consumption of the Ga in the droplet by crystallization with As. The results imply an identical VLS growth mechanism for both types of GaAs NWs, and their intricate structures provide detailed comparison with and specific experimental verification of the recently proposed growth mechanism for self-catalyzed III-V semiconductor nanowires ( Phy. Rev. Lett. 2011 , 106 , 125505 ). Using this mechanism as a guideline, we successfully demonstrated controllable fabrication of two distinct types of axial superlattice GaAs NWs consisting of zinc-blende/defect-section and wurtzite/defect-section units.

Superconducting properties and thermal expansion of YbGaxSi2−x

YbGaxSi2−x forms the hexagonal AlB2-type crystal structure in the chemical composition range of 1.12 ≤ x ≤ 1.49 and shows superconductivity with critical temperatures TC ≤ 2.5 K. In this paper, we investigate the electronic and thermal properties of the YbGaxSi2−x compounds by means of electrical resistivity (ρ) and specific heat (C) measurements under magnetic fields, as well as by the temperature dependent x-ray diffraction study. The onset temperature of superconductivity, TonsetC, decreases with the increase of the Ga content x, from 2.5 K for x = 1.15 to 1.9 K for x = 1.31. Estimated upper critical field at zero temperature, Hc2(0), concomitantly decreases from 5.9 kOe for x = 1.15 to 3.1 kOe for x = 1.31. The normal-state electronic specific-heat coefficient is found to be almost independent to x; γ = 8mJ/molK2 for x = 1.18, 1.31, and 1.41. The small value of γ indicates electron-electron correlation is weak in this compound. An irreversible behavior is observed in ρ(T). ρ(T) of YbGa1.2Si0.8 shows a hump-like increase below about 200 K on cooling while it shows a rather monotonic increase on heating. Powder x-ray diffraction of this sample taken at 12 K shows no evidence of structural phase transition. Temperature dependence of the lattice parameters in this sample show normal behavior: both a and c decrease with decreasing temperature with thermal expansion coefficients similar to those in YbGaGe. Possible mechanism for the irreversible behavior in ρ(T) is discussed.

High-pressure structure of half-metallicCrO2

Evidence for a structural phase transition from rutile {alpha}-CrO{sub 2} phase I (P4{sub 2}/mnm) to orthorhombic {beta}-CrO{sub 2} phase II (CaCl{sub 2}-like, Pnnm) is presented using angle-resolved synchrotron x-ray diffraction and high sensitivity confocal Raman spectroscopy. The transition to the CaCl{sub 2} structure, which appears to be second-order, occurs at 12 {+-} 3 GPa without any measurable discontinuity in volume, but is accompanied by an apparent increase in compressibility. Raman data are also presented to show further evidence for a second-order structural phase transition as well to demonstrate soft-mode behavior of the B{sub 1g} phonon mode.

Rheological evidence of structural phase transitions in asphaltene-containing petroleum fluids

Rheological evidence of structural phase transitions in asphaltene-containing petroleum fluids

Rheological evidence of structural phase transitions in asphaltene-containing petroleum fluids

The aim of this paper is to provide data on the rheological/structural properties of “synthetic oils,” composed of light hydrocarbons (toluene) and a heavy fraction, containing asphaltenes (vacuum residue, VR). Samples with asphaltene concentrations 20–85 g/l have been studied at temperatures 0–60 °C and shear rates up to 1500 s −1. The non-Newtonian flow curves were approximated by the Bingham and the Herschel–Bulkley models to determine the apparent yield stress and the shear-rate exponent as functions of the asphaltene concentration and the temperature. Sharp variations of these parameters were attributed to formation/destruction of extended ordered structures in asphaltene colloid suspensions. Structural changes were observed in the temperature range 20–30 °C, particularly important for industrial processes of reservoir development and pipeline transportation. A molecular model of the observed macroscopic effects takes into account possible first-order structural phase transitions in the nanometer-size resin/asphaltene colloid microparticles.

Temperature- and pressure-induced structural transitions in rare-earth-deficient (R = Y, Sm, Gd, Tb) Laves phases

Electrical resistivity, thermal expansion, and temperature-dependent x-ray diffraction measurements on compounds give mutually consistent evidence for structural phase transitions at 740 K, 550 K, 600 K, and 450 K respectively for R = Y, Sm, Gd, Tb; 0 < x < 0.05. Arguments are given as to why most of the rare-earth - nickel compounds with the 1:2 ratio do not crystallize in the simple cubic Laves phase (C15 type) but show a superstructure of the cubic Laves phase at room temperature and at ambient pressure. This superstructure with the space group and a doubled cell parameter is characterized by ordered vacancies on the R sites. It is shown that the observed structural instabilities result in transitions to the cubic Laves phase (space group ), however with disordered vacancies at high temperatures. High-pressure x-ray powder diffraction experiments show that the phase transition in shifts down to room temperature for a pressure of 27 GPa.

Optical and X-ray evidence of structural phase transitions in mixed (Rb1−xKx)CaF3crystals

Abstract Structural phase transitions have been found in the mixed fluoroperovskites crystals (Rb1−xKx)CaF3 (x=0.05, 0.2, 0.35, 0.5, 0.88, 1.0) by optical birefringence and X-ray diffraction. Profile analysis of powder diffraction lines was performed to determine the change of lattice parameters versus temperature in order to establish the crystal symmetry of each structural phase and to measure the temperatures of the transitions. From the splitting of the 400 cubic Bragg line and consideration of Glazer's classification of tilted octahedra in perovskites, the various types of distortions were found. Sequences of CaF6 tilting octahedra occurring during the phase transitions are suggested and discussed for each concentration.

Dielectric properties and phase transition in cesium tetrachlorozincate Cs2ZnCl4

Abstract Dielectric and specific heat measurements of Cs2ZnCl4 single crystals were made in a wide temperature range. The temperature dependence of dielectric constants shows two anomalies at T1=572 and T2=326 K. Specific heat and X-ray intensity of (8 0 0) exhibit also two corresponding anomalies, which are an evidence of structural phase transitions. Ferroelectric hysteresis loop has not been detected yet, but there exist at least two phase transitions.