Intermediate Metastable Phase Research Articles

Solvothermal Synthesis of LiMn1–xFexPO4Cathode Materials: A Study of Reaction Mechanisms by Time-Resolved in Situ Synchrotron X-ray Diffraction

We applied time-resolved in situ synchrotron X-ray diffraction (XRD) to study the reaction processes and pathways during the solvothermal synthesis of the olivine-structured LiFePO4, LiMnPO4, and LiFe0.4Mn0.6PO4 with ethylene glycol C2H6O2 (EG) as the solvent by following the evolution of the crystal structures of the Fe/Mn-containing phases. We identified a stable intermediate phase in the synthetic reaction process of LiFePO4, viz., a ferrous oxalate EG complex (FeC2O4·C2H6O2), and resolved its structure; thus, we established a two-step reaction mechanism involving dissolution–precipitation followed by interface-coupled dissolution–reprecipitation for the synthesis of LiFePO4. The synthetic reactions in an LiFe0.4Mn0.6PO4 solid solution also followed a two-step process via the formation of a metastable intermediate phase bearing a structural similarity to FeC2O4·C2H6O2 that, however, has a slightly larger unit-cell, indicating that the substitution of Fe by Mn occurred at the intermediate stage. In cont...

On the question of two-step nucleation in protein crystallization.

We report a real-time study on protein crystallization in the presence of multivalent salts using small angle X-ray scattering (SAXS) and optical microscopy, focusing particularly on the nucleation mechanism as well as on the role of the metastable intermediate phase (MIP). Using bovine beta-lactoglobulin as a model system in the presence of the divalent salt CdCl2, we have monitored the early stage of crystallization kinetics which demonstrates a two-step nucleation mechanism: protein aggregates form a MIP, which is followed by the nucleation of crystals within the MIP. Here we focus on characterizing and tuning the structure of the MIP using salt and the related effects on the two-step nucleation kinetics. The results suggest that increasing the salt concentration near the transition zone pseudo-c** enhances the energy barrier for both MIPs and crystal nucleation, leading to slow growth. The structural evolution of the MIP and its effect on subsequent nucleation is discussed based on the growth kinetics. The observed kinetics can be well described, using a rate-equation model based on a clear physical two-step picture. This real-time study not only provides evidence for a two-step nucleation process for protein crystallization, but also elucidates the role and the structural signature of the MIPs in the nonclassical process of protein crystallization.

Metastable Intermediate Phase during Phase Transformation of Calcium Phosphates

Phase transformation of calcium phosphates from amorphous to crystalline phases around neutral pH proceeds via direct structure conversion using non-ionic elementary units. This transformation inevitably forms metastable intermediate-structured phase(s) between the two end phases. In addition to conventional calcium phosphate phases appearing in the transformation, new unknown phases were observed. They did not correspond to simply poor crystalline materials of conventional phases and instead had particular structures. One was pseudo-OCP, which lacked the HPO4-OH layers in the conventional OCP structure.

A study on the thermal conversion of scheelite-type ABO4 into perovskite-type AB(O,N)3.

Phase-pure scheelite AMoO4 and AWO4 (A = Ba, Sr, Ca) were thermally treated under an ammonia atmosphere at 400 to 900 °C. SrMoO4 and SrWO4 were shown to convert into cubic perovskite SrMoO2N and SrWO1.5N1.5, at 700 °C and 900 °C respectively, and to form metastable intermediate phases (scheelite SrMoO4-xNx and SrWO4-xNx), as revealed by X-ray diffraction (XRD), elemental analysis and FTIR spectroscopy. High-temperature oxide melt solution calorimetry reveals that the enthalpy of formation for SrM(O,N)3 (M = Mo, W) perovskites is less negative than that of the corresponding scheelite oxides, though the conversion of the scheelite oxides into perovskite oxynitrides is thermodynamically favorable at moderate temperatures. The reaction of BaMO4 with ammonia leads to the formation of rhombohedral Ba3M2(O,N)8 and the corresponding binary metal nitrides Mo3N2 and W4.6N4; similar behavior was observed for CaMO4, which converted upon ammonolysis into individual oxides and nitrides. Thus, BaMO4 and CaMO4 were shown to not provide access to perovskite oxynitrides. The influence of the starting scheelite oxide precursor, the structure distortion and the degree of covalency of the B-site-N bond are discussed within the context of the formability of perovskite oxynitrides.

The complex polymorphism and thermodynamic behavior of a seemingly simple system: naphthalene on Cu(111).

Naphthalene, C10H8, is a polycyclic aromatic hydrocarbon (PAH) consisting of two fused benzene rings. From previous studies, it is known to form three different commensurate structures in thin epitaxial films on Cu(111), depending on the preparation conditions. One of these structures even exhibits a chiral motif of molecular rotations within the unit cell. In an attempt to elucidate this polymorphism, we performed in situ low-energy electron diffraction (LEED) as a function of temperature and surface coverage, revealing an unexpected and extraordinarily complex structural and thermodynamic behavior. We present experimental evidence for a phase transition from a two-dimensional gas to a highly ordered molecular solid via an intermediate metastable phase with moderate order (extending over a few lattice constants only) which undergoes a reversible orientational shift upon temperature variation. At monolayer coverage and above, we find that two different point-on-line (POL) coincident epitaxial relations constitute the dominant structures. This is remarkable because, so far, POL structures of naphthalene on Cu(111) and other substrates have either not been recognized or not obtained under the respective experimental conditions. Our results are corroborated by the analysis of characteristic moiré patterns observed in scanning tunneling microscopy (STM), indicative of a noncommensurate epitaxial registry.

The phase evolution of CuIn0.7Ga0.3Se2 compound powder prepared using high-energy ball milling

We successfully prepared a homogeneous single-phase CuIn0.7Ga0.3Se2 compound powder using high-energy ball milling method. The phase evolution from initial elemental powders to intermediate metastable phases, to the final CuIn0.7Ga0.3Se2 compound at various milling stages (5 to 50min) was thoroughly investigated. The obtained powders at various stages were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe micro-analysis (EPMA) to evaluate the crystal structure, morphology, and composition of the metastable phases. The intermediate metastable phases including InSe, In2Se3, Ga2Se3, GaSe, CuSe, Cu7Se4, Cu3Se2, and CuInSe2 were formed after 5min milling. After milling for 50min, a single phase of quaternary CuIn0.7Ga0.3Se2 compound formed. The average grain size of the milled powder was about 9.3nm. The results of EPMA also confirmed that a homogeneous CuIn0.7Ga0.3Se2 alloy was formed.

Synthesis of nanostructured tungsten carbide via metal-organic chemical vapor deposition and carburization process

Nanostructured tungsten carbide particles were successfully synthesized by metal-organic chemical vapor deposition in a spouted bed followed by carburization in CH4/H2 atmosphere in the temperature range 700–900°C. The carburization process was a little bit complex, which involved the coating of carbon on the outer surface of the decomposed W(CO)6 precursor particles and then followed by carbon diffusion into the particles, leading to the formation of nanostructured WC via an intermediate metastable phase W2C. The carbon deficient phase W2C was formed initially at lower carburization temperature and then transformed to stable WC phase by increasing the temperature and holding time.

Sol–gel synthesis of Mg2+ stabilized Na-β″/β-Al2O3 solid electrolyte for sodium anode battery

Mg2+ stabilized Na-β″/β-Al2O3 powder was successfully synthesized by a sol–gel method using PVP-10000 as complexing agent and metal nitrates as corresponding metal sources. The thermal decomposition of the xerogel and the evolution of crystallization behavior of Na-β″/β-Al2O3 were studied. The obtained powders have been unambiguously characterized using X-ray diffraction (XRD), N2 adsorption, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The Mg2+ stabilized Na-β″/β-Al2O3 crystallized gradually from amorphous phase to metastable intermediate phases and further formed the major phase at 1000°C. The results showed that the large surface area of the amorphous intermediate phase with mesoporous structure followed by the generation of intermediate phases named as m-Al2O3 and MgAl2O4 with spinel structure to a great extent promoted the formation of Na-β″/β-Al2O3 phase. The Na-β″/β-Al2O3 nano-powders exhibited a plate-like two dimensional structure with clear crystallographic plane for Na+ diffusion. The transformation from m-Al2O3 only required the shift of Na+ from alternate layers of oxygen and the accommodation of every fifth layer since the structure of m-Al2O3 is only slightly different from that of Na-β″/β-Al2O3. The relative density of the sintered ceramic was approximately 98.8% and the ionic conductivity can reach 0.24Scm−1 at 350°C.

Correlation between thermodynamic anomalies and pathways of ice nucleation in supercooled water.

The well-known classical nucleation theory (CNT) for the free energy barrier towards formation of a nucleus of critical size of the new stable phase within the parent metastable phase fails to take into account the influence of other metastable phases having density/order intermediate between the parent metastable phase and the final stable phase. This lacuna can be more serious than capillary approximation or spherical shape assumption made in CNT. This issue is particularly significant in ice nucleation because liquid water shows rich phase diagram consisting of two (high and low density) liquid phases in supercooled state. The explanations of thermodynamic and dynamic anomalies of supercooled water often invoke the possible influence of a liquid-liquid transition between two metastable liquid phases. To investigate both the role of thermodynamic anomalies and presence of distinct metastable liquid phases in supercooled water on ice nucleation, we employ density functional theoretical approach to find nucleation free energy barrier in different regions of phase diagram. The theory makes a number of striking predictions, such as a dramatic lowering of nucleation barrier due to presence of a metastable intermediate phase and crossover in the dependence of free energy barrier on temperature near liquid-liquid critical point. These predictions can be tested by computer simulations as well as by controlled experiments.

Features of High-Alumina Sinter Synthesis in the Presence of Metastable Phases1

Production of refractory cements by sintering is difficult as a result of the narrow temperature range for sintering and high synthesis temperature. In order to expand the temperature range for sintering and reducing firing temperature it is proposed to form an intermediate metastable phase in of high-alumina cement sinters. Results are presented for synthesis of high-alumina cement sinters with formation within them of intermediate metastable sulfatized phases.

Study of size effects in structural transformations of bcc Zr films by molecular-dynamics simulation

The effect of the film thickness and temperature on structural phase transformations in infinite films of bcc zirconium with (001) crystallographic orientation is studied by the molecular dynamics method with a many-body potential of interatomic interaction obtained within the embedded atom model. It is shown that for bcc (001) films there exists a critical thickness (∼6.1nm) below which they are unstable at any temperatures, while at a thickness above 6.1nm the films are stable in a certain temperature range. The thin bcc (001) films (<6.1nm) experience an orientational transition into the bcc (110) phase through an intermediate metastable fcc phase, and then, on cooling, a diffuse bcc (110)→hcp transition is observed. In films 6.1 to 8.2nm thick there forms, on cooling, a twin fcc phase as a result of shear deformation, so that the film surface acquires stepped relief. With further increase of the bcc (001) film thickness there occurs martensitic transformation with the formation of a twin hcp structure, and the film has a wavy surface.The bulk and elastic moduli are calculated for the bcc and fcc lattices. It is shown that for the fcc phase the lattice stability conditions are satisfied in both bulk and film systems.

Microstructure and functional properties of Fe73.5Cu1Nb3Si15.5B7 amorphous alloy

Fe73.5Cu1Nb3Si15.5B7 amorphous alloy undergoes a series of thermally induced structural transformations in temperature region between 25 and 800 °C, including structural relaxation, two Curie temperatures and crystallization. The changes in microstructure caused by these transformations are characterized in detail using XRD, TEM, Mössbauer spectroscopy and EDX elemental analysis. Mössbauer spectroscopy, in particular, reveals that there is a very small degree of crystallinity in the as-prepared alloy, which is lost during structural relaxation after annealing at temperatures below 400 °C. In addition, it suggests that crystallization of the alloy is much more complex process than suggested by XRD data, with several metastable intermediate phases serving as precursors to crystallization of stable crystalline phases. Magnetic, electrical and mechanical properties of the alloys are heavily influenced by the observed structural changes, most notably during crystallization of the alloy, where formation of crystalline phases and creation of new types of interfaces are the main factor to determine the performance of the alloy.

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.

The Effect of Si on the Precipitation Behaviour in Al-Mg-Si Alloys Studied by Thermo-kinetic Simulation and DSC Experiments

The precipitation sequences of T4 (solutionized and quenched) treated Al-Mg-Si alloys that occur during continuous heating—investigated by Differential Scanning Calorimetry (DSC)—are modelled with the help of computational thermo-kinetic methods. Simulations, including coupled nucleation growth and coarsening of fine dispersed precipitates, are presented to study the influence of the Mg/Si ratio on the formation of the precipitates, especially on the metastable s” phase. Excess silicon above the content required to form stoichiometric Mg2Si is not believed to alter the precipitation sequence but to increase the effective amount of the intermediate metastable and stable Mg-Si phases. The applied framework is able to predict number densities and sizes of the main hardening precipitates. DSC thermographs of different Mg/Si ratios in 6xxx alloys are simulated, discussed, and compared to experimental data.

Nucleation of a Stable Solid from Melt in the Presence of Multiple Metastable Intermediate Phases: Wetting, Ostwald’s Step Rule, and Vanishing Polymorphs

In many systems, nucleation of a stable solid may occur in the presence of other (often more than one) metastable phases. These may be polymorphic solids or even liquid phases. Sometimes, the metastable phase might have a lower free energy minimum than the liquid but higher than the stable-solid-phase minimum and have characteristics in between the parent liquid and the globally stable solid phase. In such cases, nucleation of the solid phase from the melt may be facilitated by the metastable phase because the latter can "wet" the interface between the parent and the daughter phases, even though there may be no signature of the existence of metastable phase in the thermodynamic properties of the parent liquid and the stable solid phase. Straightforward application of classical nucleation theory (CNT) is flawed here as it overestimates the nucleation barrier because surface tension is overestimated (by neglecting the metastable phases of intermediate order) while the thermodynamic free energy gap between daughter and parent phases remains unchanged. In this work, we discuss a density functional theory (DFT)-based statistical mechanical approach to explore and quantify such facilitation. We construct a simple order-parameter-dependent free energy surface that we then use in DFT to calculate (i) the order parameter profile, (ii) the overall nucleation free energy barrier, and (iii) the surface tension between the parent liquid and the metastable solid and also parent liquid and stable solid phases. The theory indeed finds that the nucleation free energy barrier can decrease significantly in the presence of wetting. This approach can provide a microscopic explanation of the Ostwald step rule and the well-known phenomenon of "disappearing polymorphs" that depends on temperature and other thermodynamic conditions. Theory reveals a diverse scenario for phase transformation kinetics, some of which may be explored via modern nanoscopic synthetic methods.

Investigating the phase transformations in AuCuPd with computer simulation of ion images

We present the results from investigations of structural formation in gold-copper-palladium jewelry alloys at low-temperature atomic ordering. It is found that Au20.6-Cu55.2-Pd24.2 (in at %) alloy is ordered in the L10 structure through the formation of an intermediate metastable phase with B2 structure. This phase transformation presumably occurs via a martensite mechanism. Orientation relations between B2 and L10 lattices are obtained from the experimental data.

Ab initio study of the bcc-to-hcp transition mechanism in Fe under pressure

We perform ab initio calculations on two different transition mechanisms of the bcc-to-hcp phase transition in Fe under pressure distinguished by the occurrence of the metastable fcc intermediate phase on the transition path, that is, the bcc-hcp and the bcc-fcc-hcp. The calculated results indicate that the occurrence of the fcc intermediate state during the transition is energetically unfavorable, which is consistent with the recent in situ XRD experiments. The enthalpy barrier of the fcc-hcp increases with pressure increasing, which indicates that the pressure tends to impede the transformation from fcc to hcp phase in Fe. The details of the structural and magnetic behaviors of the intermediate states during the transition are investigated, which indicates that there are complex magnetism transitions during the phase transition. The physical origins of the influence of magnetism on the phase transition are discussed. Moreover, the origin of the occurrence and evolution of the fcc metastable structure during the transition in the MD simulations are also discussed.

Nucleation pathway of core-shell composite nucleus in size and composition space and in component space

The kinetics of nucleation of a core-shell composite nucleus that consists of a core of stable final phase surrounded by a wetting layer of intermediate metastable phase is studied using the kinetic theory of binary nucleation not only in the size and composition space but also in the component space. The steady-state solution of the Fokker-Planck equation is considered. Various formulas for the critical nucleus at the saddle point as well as for the postcritical nucleus are derived. The kinetics of nucleation at the saddle point is more appropriately characterized in the size and composition space, while the kinetics of the postcritical nucleus is more appropriately described in the component space. Although both the free-energy landscape and the reaction rates play decisive role to determine the kinetics of nucleation at the saddle point, the details of the free-energy landscape are irrelevant to the kinetics of the postcritical nucleus.

Steady-state nucleation rate and flux of composite nucleus at saddle point

The steady-state nucleation rate and flux of composite nucleus at the saddle point is studied by extending the theory of binary nucleation. The Fokker-Planck equation that describes the nucleation flux is derived using the Master equation for the growth of the composite nucleus, which consists of the core of the final stable phase surrounded by a wetting layer of the intermediate metastable phase nucleated from a metastable parent phase recently evaluated by Iwamatsu [J. Chem. Phys. 134, 164508 (2011)]. The Fokker-Planck equation is similar to that used in the theory of binary nucleation, but the non-diagonal elements exist in the reaction rate matrix. First, the general solution for the steady-state nucleation rate and the direction of nucleation flux is derived. Next, this information is then used to study the nucleation of composite nucleus at the saddle point. The dependence of steady-state nucleation rate as well as the direction of nucleation flux on the reaction rate in addition to the free-energy surface is studied using a model free-energy surface. The direction of nucleation current deviates from the steepest-descent direction of the free-energy surface. The results show the importance of two reaction rate constants: one from the metastable environment to the intermediate metastable phase and the other from the metastable intermediate phase to the stable new phase. On the other hand, the gradient of the potential Φ or the Kramers crossover function (the commitment or splitting probability) is relatively insensitive to reaction rates or free-energy surface.

Framework reconstruction between hR8 and cI16 germaniums: A molecular dynamics study

Using molecular dynamics simulations and a Density Functional based Tight Binding method, the metastable germanium allotropes hR8 and cI16 are shown to interconvert by means of two sets of quasi-1D chains. The first set hosts sequences of SN2 inversions of Ge tetrahedral centers, and represents the activated step. The second set does not imply any reconstruction, but assists the first one in propagating the reconstruction. The overall process is commenced by bond nucleation, followed by chain formation and reconstruction into either structure. A novel intermediate metastable phase is visited in the process. Elementary steps of chemical reactivity are accessible due to the appropriate time and spatial resolution of the methods used. This paves the way for a chemical understanding of structure reconstruction and metastable phase formation in solid materials.