Intermediate Crystalline Phase Research Articles

Nanomechanics: Small-scale transformations.

Under shear stress, silicon nanocrystals become amorphous through an intermediate hexagonal crystalline phase.

Kinetics of Multi‐Step Redox Processes by Time‐Resolved In Situ X‐ray Diffraction

AbstractThe kinetics of redox reactions of iron oxide in oxygen carrier 50Fe2O3/MgAl2O4 are examined using different time‐resolved techniques. Reduction kinetics are studied by H2 temperature‐programmed reduction (H2‐TPR) monitored by time‐resolved in situ XRD. In contrast to conventional TPR, in situ XRD distinguishes the three‐stage reduction of Fe2O3 → Fe3O4 → FeO → Fe. It also shows that the oxidation of Fe → Fe3O4 by CO2 has no intermediate crystalline phases, explaining why its kinetics can easily be investigated by conventional CO2 temperature‐programmed oxidation (CO2‐TPO). A shrinking core model which takes into account solid state diffusion allows describing the experimental data.

In situ TEM study of deformation-induced crystalline-to-amorphous transition in silicon

The mechanism responsible for deformation-induced crystalline-to-amorphous transition (CAT) in silicon is still under considerable debate, owing to the absence of direct experimental evidence. Here we have devised a novel core/shell configuration to impose confinement on the sample to circumvent early cracking during uniaxial compression of submicron-sized Si pillars. This has enabled large plastic deformation and in situ monitoring of the CAT process inside a transmission electron microscope. We demonstrate that diamond cubic Si transforms into amorphous silicon through slip-mediated generation and storage of stacking faults (SFs), without involving any intermediate crystalline phases. By employing density functional theory simulations, we find that energetically unfavorable single-layer SFs create very strong antibonding interactions, which trigger the subsequent structural rearrangements. Our findings thus resolve the interrelationship between plastic deformation and amorphization in silicon, and shed light on the mechanism underlying deformation-induced CAT in general. Sharper insights into stress-induced, crystalline-to-amorphous transitions in silicon can be realized with crack-resistant micropillars. Mechanical processes such as polishing during the manufacture of photovoltaic cell alter silicon's crystal structure and degrade device performance. Wei Zhang and Zhi-Wei Shan from Xi'an Jiaotong University in China and co-workers have now used high-resolution transmission electron microscopy to capture in situ images of these transitions in progress. The team designed a system of submicrometre pillars consisting of a crystalline silicon core encased in a nanoscale shell of amorphous silicon. The flexible shell helped prevent brittle fracture from occurring while the pillars were gradually compressed into mushroom shapes by an indentation tool. Real-time imaging revealed that mechanical force deformed the silicon framework through plastic motions only – a finding that upsets previous concepts of intermediate crystalline phases taking part in the amorphization process. A novel and effective crystalline-Si/amorphous-Si core/shell sample configuration has been devised. The malleable amorphous Si shell helped inhibit brittle fracture, provide the confinement to significantly raise the stress level, and extend the plastic flow in crystalline Si core. This enabled a real-time observation of the stress-induced crystalline-to-amorphous transition (CAT) process in Si. In situ TEM compression experiment demonstrated a direct amorphization process from the single crystalline diamond cubic Si phase, owing to the accumulation of plastic strain and profuse stacking faults. Deep insights of the CAT process have been achieved from density functional theory simulations.

Reduction of CO2 with KBH4 in solvent-free conditions

Metal hydrides have been commonly used as reducing agents in organic and inorganic chemistry. Until today, the capability of potassium borohydride (KBH4) to reduce aldehydes and ketones to alcohols has been known for its advantage of high stability on air and in alkaline solutions. Conversion of CO2 to formates and boron methoxide compounds by metal borohydrides has been recently studied. In this work we investigated the solid–gas non-catalytic reaction between KBH4 and CO2 under both mechanochemical and thermal-induced conditions with the simultaneous formation of potassium formylhydroborates, K[HxB(OCHO)4 − x] (x = 1–3), as main products. The first crystal structure of a product of solid–gas metal borohydride – CO2 reaction, potassium triformylhydroborate, K[HB(OCHO)3], obtained mechanochemically, was elucidated. The evolution of the reaction between solid KBH4 and CO2 was monitored by a combination of thermogravimetric analysis coupled with mass spectrometry and infrared spectroscopy from room temperature to 500 °C, revealing the generation of hydrogen, methanol and carbon monoxide in a three-step mass increase reaction. Variable temperature in situ synchrotron X-ray powder diffraction under CO2 pressure revealed the formation of a new crystalline intermediate phase with an unidentified composition but crystallizing in a monoclinic space group, and KBO2 during the second and third steps, respectively. Gas chromatography of evolving species under CO2 flow revealed for the first time the formation of methanol and methane in water-free conditions.

Detailed magnetic monitoring of the enhanced magnetism of ferrihydrite along its progressive transformation into hematite

AbstractUnder certain aging conditions, ferrihydrite evolves into hematite through intermediate products of enhanced magnetism. Although the magnetic properties of the end product, hematite, are to date satisfactorily known, those of ferrihydrite (and especially the products of its progressive aging) are not sufficiently explored. To this end, magnetic experiments, conducted mostly by using alternating magnetic fields, have been performed. The results reveal that two‐line ferrihydrite exhibits conspicuous low‐temperature spin glass behavior, a new finding that, on the other hand, could be expected given the previous microstructural observations that described this mineral as a very disordered gel. Upon aging, a progressive increase of the effective magnetic moment per iron ion is detected, in agreement with previous observations that ascribed ferrimagnetic character to an intermediate crystalline phase (sometime called ferriferrihydrite) that disappears just before the full transformation into hematite. Transmission electron microscopy observations suggest that this intermediate crystalline phase nucleates at the expense of the primordial gel, remaining physically attached to it until complete transformation into hematite. This microstructural picture appears well supported as, upon aging, the glassy magnetic dynamics found in ferrihydrite persists for the aged products even when dispersed in a nonmagnetic matrix, very likely because part of the so grown ferrimagnetic nanoparticles form aggregates within remains of the nontransformed gel. We propose a coherent model of ferrihydrite aging, based on integrated microstructural and magnetic observations, which is useful for the identification of aging products in geological and biological environments.

Unraveling structural and magnetic information during growth of nanocrystalline SrFe12O19

Hydrothermal synthesis of SrFe12O19is followedin situusing PXRD, and successfully reproducedex situusing a custom-designed batch-type reactor.

Phase structure and mechanical properties of disentangled ultra-high molecular weight polyethylene/polyhedral oligomeric silsesquioxane nanocomposites in a solid state

Ultra-high molecular weight polyethylene (UHMWPE)/disilanolisobutyl polyhedral oligomeric silsesquioxane (POSS) nanocomposites in a disentangled state were synthesized by in situ ethylene polymerization. This is the first report of a detailed study on the phase structure and mechanical properties of disentangled UHMWPE/ POSS nanocomposites in a solid state. The phase structure and chain dynamics of UHMWPE nanocomposites were characterized using wide-line proton NMR and 13C cross-polarization/magic-angle spinning (CP/MAS) solid-state nuclear magnetic resonance (NMR). The results showed that a strong interaction between the POSS and nascent UHMWPE matrix occurred in a solid state, especially in the intermediate and monoclinic crystalline phases. This strong interaction between POSS particles and UHMWPE chains may result in improved melt recovery properties. Mechanical testing indicated that the lowest friction coefficients and highest breaking strength were achieved when 1 wt% loading of POSS was incorporated. The influence of the microstructure on lubrication and mechanical properties was also studied.

XANES Investigation of Dynamic Phase Transition in Olivine Cathode for Li‐Ion Batteries

Dynamic phase transformation in olivine LiFePO4 involving formation of one or more intermediate or metastable phases is revealed by an in situ time‐resolved X‐ray absorption near edge structure (XANES) technique. The XANES spectra measured during relaxation immediately after the application of relatively high overpotentials, where metastable phases are expected, show a continuous shift of the Fe K‐edge toward higher energy. Surprisingly, the Fe K‐edge relaxes to higher energies after current interrupt regardless of whether the cell is being charged or discharged. This relaxation phenomenon is superimposed upon larger shifts in K‐edge due to changes in Fe2+/Fe3+ ratio due to charging and discharging, and implies an intermediate phase of larger FeO bond length than any of the known crystalline phases. No intermediate crystalline phases are observed by X‐ray diffraction (XRD). A metastable amorphous phase formed during dynamic cycling and which structurally relaxes to the equilibrium crystalline phases over a time scale of about 10 min after cessation of charging/discharging current is consistent with the experimental observations.

Structure and crystallization behavior of La2O3⋅3B2O3 metaborate glasses doped with Nd3+ or Eu3+ ions

The local structure and crystallization behavior of the stoichiometric La2O3⋅3B2O3 (LaMB) metaborate glass doped with Nd3+ or Eu3+ ions are studied using differential thermal analysis (DTA), X-ray diffraction (XRD), Raman scattering and 11B magic-angle spinning nuclear magnetic resonance (MAS NMR), optical absorbance and luminescence techniques. In the crystallized samples, XRD, NMR and Raman spectroscopy have detected the formation of only one crystalline phase (congruent crystallization of LaB3O6). No intermediate metastable crystalline phase has been detected before LaB3O6 crystals formation (single stage crystallization process). The observation of heat treated glass samples by scanning electron microscopy (SEM) and optical microscopy coupled with the study of the effect of varying the glass particle size on the DTA curves have both revealed that LaB3O6 crystallization only occurs by a heterogeneous nucleation mechanism (needle-shape crystals) from glass surface. The activation energy Ec of crystal growth has been determined by performing DTA experiments at different heating rates with the Kissinger (784kJ/mol) and Ozawa (801kJ/mol) equations than can be used for surface crystallization processes. The heterogeneous crystallization behavior and the spectroscopic results obtained in this work by comparing the LaMB glass with the LaB3O6 crystalline phase suggest the existence of significant structural differences between the amorphous and the crystalline phases contrary to what was previously reported in literature.

Investigation into the Palladium‐Europium Acetate Reductive Decomposition with Synchrotron Radiation‐Based X‐ray Diffraction and X‐ray Absorption Spectroscopy

AbstractThe reductive decomposition of the complex Pd2Eu2(μ,η2‐OOCMe)2(μ‐OOCMe)8(H2O)2·(HOOCMe)2upon heating from room temperature to 500 °C in a hydrogen atmosphere was investigated by in situ synchrotron radiation‐based X‐ray absorption fine structure spectroscopy (XAFS) including observations of X‐ray near‐edge structure (XANES) and extended absorption fine structure (EXAFS) at Pd K‐ and Eu L3‐edges and synchrotron X‐ray diffraction (XRD). The Pd‐Eu2O3nanocomposite was found to be the final thermolysis product. The decomposition proceeds via series of intermediate crystalline phases, including europium(II) and (III) anhydrous acetates and EuO(CH3COO) oxidoacetate. Surprisingly, the major fraction of europium occurs in the reduced form Eu2+within a limited temperature range of 200–300 °C. Presumably, the reversible Eu3+↔Eu2+redox‐process is associated with the stability of nanocrystalline palladium hydride, which acts as a stronger reducing agent as compared to molecular hydrogen.

Systematical investigation on characteristics of a photocatalyst: tantalum oxynitrides.

Tantalum oxynitrides with various nitrogen contents were synthesized from Ta2O5 powder by nitridation under a flow of ammonia at 1123 K for various durations. X-ray powder diffraction, transmission electron microscopy, Ultraviolet-visible spectroscopy, energy-dispersive spectroscopy, elemental analysis and photocatalytic reaction were performed to investigate these samples. Selected-area electron diffraction analysis of the mixed crystalline phases of powder samples revealed that each particle had only one crystalline phase. This indicates that entire particles underwent a rapid structural transformation once their nitrogen content reached a critical value. We discovered a new intermediate crystalline phase of tantalum oxynitride, TaO(a > 1)N(b <1), appeared before the generation of the β-TaON phase. The crystal structure of TaO(a > 1)N(b <1) is suggested to be monoclinic, with unit cell parameters of a = 5.1 Å, b = 35.6 Å, c = 5.4 Å and β = 93.5°. The ratio of nitrogen to oxygen in the samples increased with increasing nitridation duration. The increasing rate is different in the different nitridation stage due to the different structure of the samples. Nitrogen entered the samples quickly during the initial 5 h of nitridation, and a monoclinic β-TaON phase was formed. A mesoporous structure emerged in the nitrided particles during the phase transition, greatly increasing the surface area of the samples. The more the nitrogen entered one sample, the darker the color of it due to the narrower the band gap. H2 and O2 evolved by water splitting from the nitrided samples irradiated with visible light. Change in the evolution rate of H2 and O2 had a relation with the structure of the samples.

High yield exfoliation of two-dimensional chalcogenides using sodium naphthalenide

Transition-metal dichalcogenides like molybdenum disulphide have attracted great interest as two-dimensional materials beyond graphene due to their unique electronic and optical properties. Solution-phase processes can be a viable method for producing printable single-layer chalcogenides. Molybdenum disulphide can be exfoliated into monolayer flakes using organolithium reduction chemistry; unfortunately, the method is hampered by low yield, submicron flake size and long lithiation time. Here we report a high-yield exfoliation process using lithium, potassium and sodium naphthalenide where an intermediate ternary Li(x)MX(n) crystalline phase (X=selenium, sulphur, and so on) is produced. Using a two-step expansion and intercalation method, we produce high-quality single-layer molybdenum disulphide sheets with unprecedentedly large flake size, that is up to 400 μm(2). Single-layer dichalcogenide inks prepared by this method may be directly inkjet-printed on a wide range of substrates.

Conversion of Nuclear Waste into Nuclear Waste Glass: Experimental Investigation and Mathematical Modeling

The melter feed, slurry, or calcine charged on the top of a pool of molten glass forms a floating layer of reacting material called the cold cap. Between the cold-cap top, which is covered with boiling slurry, and its bottom, where bubbles separate it from molten glass, the temperature changes by up to 1000K. The processes that occur over this temperature interval within the cold cap include liberation of gases, conduction and consumption of heat, dissolution of quartz particles, formation and dissolution of intermediate crystalline phases, and generation of foam and gas cavities. These processes have been investigated using thermal analyses, optical and electronic microscopies, x-ray diffraction, as well as other techniques. Properties of the reacting feed, such as heat conductivity and density, were measured as functions of temperature. Investigating the structure of quenched cold caps produced in a laboratory-scale melter complemented the crucible studies. The cold cap consists of two main layers. The top layer contains solid particles dissolving in the glass-forming melt and open pores through which gases are escaping. The bottom layer contains bubbly melt or foam where bubbles coalesce into larger cavities that move sideways and release the gas to the atmosphere. The feed-to-glass conversion became sufficiently understood for representing the cold-cap processes via mathematical models. These models, which comprise heat transfer, mass transfer, and reaction kinetics models, have been developed with the final goal to relate feed parameters to the rate of glass melting.

Biomineralization in the Sea Hare Aplysia punctata Initiated by Nano-Dolomite

X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), high resolution transmission electron microscopy (HRTEM), environmental scanning electron microscopy (ESEM) and energy dispersive X-ray analysis (EDS) were used in study of starting biomineralization processes in embryos of the sea hare species Aplysia punctata. 10 days old embryos appeared amorphous according to XRD patterns. TEM of the same sample showed that first grains of nanocrystalline dolomite began to form in the amorphous area. The identification of dolomite was confirmed according to TEM dark field images and SAED, as well as by HRTEM. In further development stages of the embryos very faint aragonite rings became visible by SAED. It was shown that the biomineralization process in A. punctata started by formation of the dolomite nanograins which served as centres of crystallization for further aragonite deposition in the larval shell. The creation of unusual intermediate crystalline phase of nano-dolomite in A. punctata embryos is of equal interest for biologist and ecologist as an evolutionary ancestral trait of molluskan biomineralization, as well as for materials scientists, as a promising template in potential bioengineering application and design of appropriate biomimetic routes that could lead to the development of new implantable biomaterials. The discussion of the present results is based on recent knowledge on general biomineralization in mollusks.

Self-propagating reactions in Al/Zr multilayers: Anomalous dependence of reaction velocity on bilayer thickness

High temperature, self-propagating reactions are observed in vapor-deposited Al/Zr multilayered foils of overall atomic ratios 3 Al:1 Zr and 2 Al:1 Zr and nanoscale layer thicknesses; however, the reaction velocities do not exhibit the inverse dependence on bilayer thickness that is expected based on changes in the average diffusion distance. Instead, for bilayer thicknesses of 20-30 nm, the velocity is essentially constant at ∼7.7 m/s. We explore several possible explanations for this anomalous behavior, including microstructural factors, changes in the phase evolution, and phase transformations in the reactant layers, but find no conclusive explanations. We determine that the phase evolution during self-propagating reactions in foils with a 3 Al:1 Zr stoichiometry is a rapid transformation from Al/Zr multilayers to the equilibrium intermetallic Al3Zr compound with no intermediate crystalline phases. This phase evolution is the same for foils of 90 nm bilayer thicknesses and foils of bilayer thicknesses in the range of 27 nm to 35 nm. Further, for foils with a bilayer thickness of 90 nm and a 3 Al:1 Zr overall chemistry, the propagation front is planar and steady, in contrast to unsteady reaction fronts in foils with 1 Al:1 Zr overall chemistry and similar bilayer thicknesses.

Model for the conversion of nuclear waste melter feed to glass

The rate of batch-to-glass conversion is a primary concern for the vitrification of nuclear waste, as it directly influences the life cycle of the cleanup process. This study describes the development of an advanced model of the cold cap, which augments the previous model by further developments on the structure and the dynamics of the foam layer. The foam layer on the bottom of the cold cap consists of the primary foam, cavities, and the secondary foam, and forms an interface through which the heat is transferred to the cold cap. Other model enhancements include the behavior of intermediate crystalline phases and the dissolution of quartz particles. The model relates the melting rate to feed properties and melter conditions, such as the molten glass temperature, foaminess of the melt, or the heat fraction supplied to the cold cap from the plenum space. The model correctly predicts a 25% increase in melting rate when changing the alumina source in the melter feed from Al(OH)3 to AlO(OH). It is expected that this model will be incorporated in the full glass melter model as its integral component.

Effect of Synthesis Conditions on Formation Pathways of Metal Organic Framework (MOF-5) Crystals

Metal Organic Frameworks (MOFs) represent a class of nanoporous crystalline materials with far reaching potential in gas storage, catalysis, and medical devices. We investigated the effects of synthesis process parameters on production of MOF-5 from terephthalic acid and zinc nitrate in diethylformamide. Under favorable synthesis conditions, we systematically mapped a solid formation diagram in terms of time and temperature for both stirred and unstirred conditions. The synthesis of MOF-5 has been previously reported as a straightforward reaction progressing from precursor compounds in solution directly to the final MOF-5 solid phase product. However, we show that the solid phase formation process is far more complex, invariably transferring through metastable intermediate crystalline phases before the final MOF-5 phase is reached, providing new insights into the formation pathways of MOFs. We also identify process parameters suitable for scale-up and continuous manufacturing of high purity MOF-5.

Crystallization and semiconductor-metal switching behavior of thin VO2 layers grown by atomic layer deposition

Crystalline vanadium dioxide (VO2) thin films were prepared by annealing amorphous VO2 films which were deposited by atomic layer deposition on a SiO2 substrate. A large influence of the oxygen partial pressure in the annealing ambient was observed by means of in-situ X-ray diffraction. In the range between 1 and 10Pa of oxygen the interesting VO2(R) phase crystallized near 450°C. Between 2 and 10Pa of oxygen, metastable VO2(B) was observed as an intermediate crystalline phase before it transformed to VO2(R). Anneals in inert gas did not show any crystallization, while oxygen partial pressures above 10Pa resulted in oxidation into the higher oxide phase V6O13. Film thickness did not have much effect on the crystallization behavior, but thinner films suffered more from agglomeration during the high-temperature crystallization on the SiO2 substrate. Nevertheless, continuous polycrystalline VO2(R) films were obtained with thicknesses down to 11nm. In the case where VO2(R) was formed, the semiconductor–metal transition was observed by three complementary techniques. This transition near 68°C was characterized by X-ray diffraction, showing the transformation of the crystal structure, by spectroscopic ellipsometry, mapping optical changes, and by sheet resistance measurements, showing resistance changes larger than 2 orders of magnitude between the low-temperature semiconducting state and the high-temperature metallic state.

Molecular engineering of microporous crystals: (VII) The molar ratio dependence of the structure-directing ability of piperazine in the crystallization of four aluminophosphates with open-frameworks

The crystallization fields of initial mixtures with compositions of Al2O3:3.0 P2O5:x pip:277 H2O, where x=2.4–12 and pip=piperazine, were investigated. Pure and highly crystalline three-dimensional (3D) AlPO-CJB2, two-dimensional (2D) AP2pip, 3D AlPO-JU88, and 2D AlPO-CJ9 were obtained when x=3, 3.4, 6.5, and 12, respectively. Along with the increase in the ratio of piperazine to Al2O3 in the initial mixture, the density of piperazine in the final structure as well as the non-bonding interaction between the inorganic framework and the protonated piperazine increased. The crystallization processes of AlPO-CJB2, AP2pip, and AlPO-JU88 were investigated using multiple techniques. In the crystallization of AlPO-CJB2, a pure and highly crystalline AP2pip intermediate phase was first formed and then dissolved. A species that may promote the dissolution of AP2pip and the formation of AlPO-CJB2 was identified. Under the synthetic condition of AP2pip, however, this species was not formed; thus, the transformation of AP2pip to AlPO-CJB2 was not observed even for significantly prolonged crystallization times. Additionally, the crystallization rate of AP2pip was significantly accelerated by an increased molar ratio of the reactants with respect to the aluminum source. The results indicate that the type and concentration of the species in the liquid phase were the governing factors in determining the final products and that the formation of particular structures is energy-directed.

Crystallization kinetics of lithium niobate glass: determination of the Johnson–Mehl–Avrami–Kolmogorov parameters

The formation of crystalline LiNbO3 (LN) from LN glass has been studied by means of differential scanning calorimetry and in situ synchrotron X-ray diffraction. The LN glass with no glass former was prepared by the polymerized complex method. The isothermal kinetics of the crystallization process is described using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation and the Avrami exponent n is found to be ~2.0, indicating that the crystallization mechanism is diffusion-controlled growth with a decreasing nucleation rate. The effective activation energy of crystallization calculated from isothermal measurements is 6.51 eV. It is found that the LN glass directly transforms into a rhombohedral LN crystal without any intermediate crystalline phase and most crystal grains are confined within the size of ~40 nm irrespective of different isothermal temperatures. Application of JMAK theory to the non-isothermal thermoanalytical study of crystallization of LN glass is discussed.