Metastable Crystals Research Articles

Magmatic cristobalite and quartz in the NWA 856 Martian meteorite

Abstract— Silica-rich late-stage crystallization pockets in the Martian meteorite Northwest Africa (NWA) 856 were investigated by transmission electron microscopy (TEM). The pockets occur as wedges between maskelynite laths or between maskelynite and pyroxene. They consist of elongated grains of cristobalite and quartz embedded in a silica-rich glass. Interstitial to the amorphous phase and silica minerals, a number of small accessory minerals have been identified, typical for late-stage crystallization products. They are ilmenite, tranquillityite, fayalite, troilite, baddeleyite, apatite, and chloroapatite. Cristobalite and quartz are shocked, as revealed by the occurrence of numerous amorphous lamellae. This assemblage suggests metastable dendritic crystallization under hydrous conditions. Cristobalite crystallization was probably facilitated by the presence of impurities such as Na or H2O. Our observations show that silica minerals can be formed under magmatic conditions on Mars.

Formation of metastable crystals of [C 4mim][NTf 2] and [C 6mim][NTf 2

Heat capacities of crystalline 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C 4mim][NTf 2] and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C 6mim][NTf 2] in the range 80 K– T fus were measured in an adiabatic calorimeter. Anomalies in the heat-capacity curves for the both compounds occurred near 240 K. Positions of the anomalies depended on thermal history of the samples. More stable crystals had higher heat capacities in the range 220–260 K. Below 200 K heat capacities of all the crystals of the same compound were indistinguishable.

Thermochromatism and Structural Evolution of Metastable Polydiacetylenic Crystals

Topochemically polymerized sodium 10,12-pentacosadiynoate (PCDA-Na) microcrystals show an irreversible red-to-blue chromatic transition accompanied by a distinct structural evolution upon initial thermal treatment, and show a subsequent completely reversible blue-to-red chromatic transition upon further thermal stimuli. Visible absorption spectroscopy, X-ray diffraction (XRD), and differential scanning calorimetry (DSC) are used to investigate the thermochromatic transition behavior of the polydiacetylenic microcrystals. Brief quantum mechanical geometry optimization is employed to explain the lattice dimensional change during the irreversible red-to-blue chromatic transition of the metastable polydiacetylenic crystals.

Hydroxyapatite crystallization from a highly concentrated phosphate solution using powdered converter slag as a seed material

A system for recovering phosphorus from membrane-filtrate from a sludge reduction process containing high phosphorus concentrations was developed. In this system, referred to as the completely mixed phosphorus crystallization reactor, powdered converter slag was used as a seed material. In a preliminary experiment, the optimal pH range for metastable crystallization of phosphorus from membrane-filtrate containing about 100 mg/L PO 4-P was found to be 6.6–7.0. The laboratory scale completely mixed phosphorus crystallization reactor, actually operated in pH range of 6.8–7.6 for influent 72.9 mg/L PO 4-P, achieved an average efficiency of phosphorus removal from the membrane-filtrate of 52.4% during a 30-day experiment. Mixed-liquor suspended solids (MLSS) measurements revealed that, out of 0.24 kg PO 4-P in the original membrane-filtrate fed into the reactor, 0.12 kg PO 4-P was recovered on the seed particles after 30 days. X-ray diffraction (XRD) pattern and Fourier transform infrared (FT-IR) spectra of the crystalline material deposited on the seed particles showed peaks consistent with hydroxyapatite. Scanning electron micrograph (SEM) images exhibited that finely distributed crystalline material was formed on the surfaces of seed particles. Energy dispersive X-ray spectroscopy (EDS) mapping analysis revealed that the molar composition ratio of Ca/P of the crystalline material was 1.84. The Ca/P molar ratio > 1.67 for crystalline substance might result from the presence of CaCO 3 on the crystalline surfaces. A particle size distribution analysis showed that the average particle size increased from 22 μm for the original converter slag seed particles, to 94 μm after 30 days of phosphorus crystallization. Collectively, the present results suggest that the proposed phosphorus crystallization recovery system is an effective tool for recycling phosphorus from phosphate solution.

Phase-field modeling on morphological landscape of isotactic polystyrene single crystals

Spatio-temporal growth of isotactic polystyrene single crystals during isothermal crystallization has been investigated theoretically based on the phase field model by solving temporal evolution of a nonconserved phase order parameter coupled with a heat conduction equation. In the description of the total free energy, an asymmetric double-well local free energy density has been adopted to represent the metastable melt and the stable solid crystal. Unlike the small molecule systems, polymer crystallization rarely reaches thermodynamic equilibrium; most polymer crystals are kinetically stabilized in some metastable states. To capture various metastable polymer crystals, the phase field crystal order parameter at the solidification potential has been treated to be supercooling dependent such that it can assume an intermediate value between zero (melt) and unity (perfect crystal), reflecting imperfect polycrystalline nature of polymer crystals. Two-dimensional simulations exhibit various single crystal morphologies of isotactic polystyrene crystals such as faceted hexagonal patterns transforming to nonfaceted snowflakes with increasing supercooling. Of particular interest is that heat liberation from the crystallizing front influences the curvature of the crystal-melt interface, leading to directional growth of lamellar tips and side branches. The landscape of these morphological textures has been established as a function of anisotropy of surface energy and supercooling. With increasing supercooling and decreasing anisotropy, the hexagonal single crystal transforms to the dense lamellar branching morphology in conformity with the experimental findings.

Phase behaviour of three homologues of the discotic hexa‐n‐alkoxyanthraquinones under pressure

The phase transition behaviour of three homologous discotic mesogens, the hexa‐n‐alkoxyanthraquinones HOAQ(n), n indicating the number of carbon atoms in the alkoxy group, was investigated under hydrostatic pressures up to 500 MPa using a high pressure differential thermal analyser. The T vs. P phase diagrams of HOAQ(6), HOAQ(8) and HOAQ(9) were constructed for solution‐ (Cr0) and melt‐crystallized (Cr1) samples of the compounds. HOAQ(6) shows the reversible Cr0–rectangular columnar phase (Colr)–hexagonal columnar phase (Colh)–isotropic liquid (I) phase sequence at atmospheric pressure. The stable Colr phase of HOAQ(6) has a decreased temperature range with increasing pressure and then the Colr phase disappears under pressures above about 350 MPa; instead the Cr0–Colh–I phase sequence is exhibited. For HOAQ(8), the solution‐grown sample exhibits the stable Cr0–Colh–I phase sequence at atmospheric pressure. Applying pressure to the solution‐grown sample induces the formation of the stable Colr phase in the pressure region between 10 and 350 MPa, leading to the Cr0–Colr–Colh–I phase sequence. The pressure‐induced Colr phase disappears under higher pressures. The melt‐cooled sample of HOAQ(8) shows the formation of the metastable crystal (Cr1), unknown mesophase (X) and Colr phases at lower temperatures under atmospheric pressure, and exhibits the reversible Cr1–X–Colr–Colh–I phase sequence on subsequent thermal cycles. The metastable phase sequence was observed under pressures up to 100 MPa, but the phase transitions were too small to be detected under higher pressures. In HOAQ(9) the stable Cr0–Colh–I phase sequence is observed at all pressures, while the melt‐cooled sample shows the metastable Cr1–Colr–Colh–I phase sequence under pressures up to 300 MPa. The metastable Colr phase disappears under higher pressures.

Nucleation mechanisms: A crystal-chemical investigation of phases forming in highly supercooled aluminosilicate liquids

Crystal nucleation and growth have been investigated approximately 100 K above the glass transition for 12 liquids in the system CaO–Al 2O 3–SiO 2. Nucleation takes place heterogeneously at the gas–liquid interface, yielding various crystal structures, namely, those of yoshiokaite, gehlenite, larnite, anorthite and wollastonite. Characterization by Raman spectroscopy, transmission electron microscopy and electron microprobe indicate that these phases are highly disordered, poorly crystallized and generally non-stoichiometric. Even for incongruent crystallization, crystals have the same Si/Al ratio as their parent melts but variable CaO contents. These features may be rationalized in terms of the contrasting mobility of network-forming cations, which practically vanishes at the glass transition, and that of calcium which remains significant. The fact that the mobility of network-modifying cations controls the structure and composition of nucleating phases implies that liquid viscosity (controlled by the mobility of network formers, i.e. the frequency of Si–O and Al–O bond breaking) is not an appropriate scaling parameter for crystal nucleation. In turn, this may provide an explanation for the discrepancies, reaching many orders of magnitude, between experimentally determined nucleation rates and those predicted from classical and non-classical nucleation theories. An additional factor contributing to such discrepancies is that thermodynamic data obtained on stoichiometric phases stable at liquidus temperatures are not appropriate for estimating the thermodynamic barrier to nucleation of the metastable crystals that actually form at large degrees of supercooling.

Conformational Dimorphism of 1,1,3,3,5,5‐Hexachloro‐1,3,5‐trigermacyclohexane: Solvent‐Induced Crystallization of a Metastable Polymorph Containing Boat‐Shaped Molecules

Two crystalline modifications of 1,1,3,3,5,5-hexachloro-1,3,5-trigermacyclohexane have been experimentally obtained as phase pure products and studied by single-crystal X-ray diffraction. The six-membered heterocycles adopt a chair conformation in the alpha-phase; this polymorph is accessible by crystallisation from solution and from the melt. In contrast, the beta-form is built up from boat-shaped molecules; it can exclusively be crystallised from n-hexane. At the molecular level, formation energies of the 1,1,3,3,5,5-hexachloro-1,3,5-trigermacyclohexane conformers have been compared by using molecular mechanics, semiempirical and ab-initio quantum mechanical calculations. Possible reasons for the selective formation of the alpha- or beta-phase in specific solvents have been considered. Formation of the metastable phase is suggested to occur via a hypothetical intermediate of composition [(GeCl2CH2)3].0.5C6H14. For such an in-silico solvate, a crystal structure of favourable lattice energy, closely related to the experimentally observed beta-modification, has been found through global energy minimisation. Elimination of the n-hexane molecules from this computer-generated solid and subsequent simulated annealing resulted in a crystal structure that corresponds to the experimentally observed beta-phase within the limits of the force field calculations. This scenario implies solvent directed crystallisation of a metastable polymorphic molecular crystal.

Spontaneous and Inductive Thickenings of Lamellar Crystal Monolayers of Low Molecular Weight PEO Fractions on Surface of Solid Substrates

Crystalline macromolecules form folded-chain lamellar and metastable crystals. Annealing at a temperature below melting point results in a thickening of the metastable crystals to construct more stable ones. Such a process was visualized by AFM equipped with a hot stage in monolayers of poly(ethylene oxide) lamellar crystals on a silicon wafer surface. Our observations show that the monolayers can be thickened in a stepwise manner during stepwise heating to temperatures below the melting point. Our analyses show the thickening process and mechanism as follows: At the first step a small portion of the lamellar crystals have the capability to be spontaneously thickened into a thicker one at a certain temperature range, giving rise to a difference in lamellar thickness. In the following step the thinner lamellae are forced to melt, and then the melted molecules enter an amorphous phase in which they are transported toward the thicker lamellae and finally recrystallize in the thicker ones. This inductive mec...

De- and rehydration behavior of α,α-trehalose dihydrate under humidity-controlled atmospheres

Effects of humidity were investigated on de- and rehydration behavior of α,α-trehalose dihydrate ( T h) throughout simultaneous measurements of differential scanning calorimetry and X-ray diffractometry (DSC–XRD) and simultaneous thermogravimetry and differential thermal analysis (TG–DTA). When T h was heated from room temperature under dry nitrogen atmosphere, a metastable anhydrous crystal ( T α) was formed at 105 °C after dehydration of T h. The resulting T α melted at 125 °C and became amorphous, followed by cold crystallization from 150 °C giving rise to a stable anhydrous crystal T β. Under a highly humid atmosphere, on the other hand, T β was formed at 90 °C directly as a result of T h dehydration. T α was readily rehydrated and turned back to T h when nitrogen gas with low water vapor pressure of 2.1 kPa was admitted, whereas high water vapor pressure up to 7.4 kPa was required for rehydration of T β into T h. This study provided a picture of pathways that link various solid forms of trehalose, taking into account the effects of a humid environment.

Salt crystallization as damage mechanism in porous building materials—a nuclear magnetic resonance study

Salts can damage building materials by chemical reactions or crystallization, which is a serious threat to cultural heritage. In order to develop better conservation techniques, more knowledge of the crystallization processes is needed. In a porous material, the size of a salt crystal is limited by the sizes of the pores. It has been predicted that as a consequence, the solubility of a salt increases with decreasing pore size. This increase seems to be related to an increase of the stress generated by a crystal on the pore wall. It has been suggested that the resulting stress could become high enough to induce failure. We have studied the crystallization of salts in porous materials with well-defined pore sizes. Samples were saturated at 40 degrees C with saturated Na2SO4 and Na2CO3 solutions. Next we have cooled the samples to 0 degrees C and waited for nucleation. After nucleation occurred, the solubility in the porous material was measured with nuclear magnetic resonance (NMR) as a function of the temperature. The measurements on Na2CO3 indeed show an increase in solubility with a decrease in pore size. For Na2SO4, we did not observe a pore size-dependent solubility. However, we have to remark that these results show a metastable crystal phase. The results can be used to calculate the actual pressure exerted by the crystals onto the pore wall.

Non-experimentally available thermodynamic properties: the two-component system (CH 3)CCl 3 + CBrCl 3

The orientationally disordered stable and metastable mixed crystals of the two-component system (CH 3)CCl 3 (1,1,1-trichloroethane) + CBrCl 3(bromotrichloro-methane) have been characterised by means of X-ray powder diffraction and thermal analysis techniques. The thermodynamic assessment coherently reproduces the melting equilibria (the stable [R + L] and the partially metastable [FCC + L]) and provides a coherent set of data for the thermodynamic properties of non-experimentally available phase transitions of pure compounds which agree perfectly with those properties obtained from the pressure–temperature phase diagrams.

Thermotropic phase behaviour of long-chain alkylmaltosides

The thermotropic phase behaviour and phase structure of crystalline and non-crystalline n-tetradecyl-beta-D-maltoside (C14G2) and n-hexadecyl-beta-D-maltoside (C16G2) have been investigated by means of differential scanning calorimetry and X-ray techniques. Upon lyophilisation, both compounds form a solid, lamellar phase comprising disordered head groups and hexagonally packed alkyl chains that are suggested to be tilted and interdigitated. This ordered lamellar phase melts into a metastable lamellar liquid crystal, which re-crystallises to a high-temperature crystalline polymorph comprising interdigitated, non-tilted alkyl chains. Remarkably, the high-temperature polymorph of C14G2 has the same melting point as that of C16G2, namely 105 degrees C for both surfactants. A low-temperature polymorph of anhydrous C14G2 crystallises from water at room temperature, whereas the hemihydrate of C14G2 crystallises at 6 degrees C from water, or from chloroform containing trace water. X-ray data suggest both these crystalline modifications to comprise interdigitated and tilted alkyl chains.

Growth and Structural Characterization of Needlelike Metastable Crystals in the Nd−Ba−Cu−(Al)−O System

An unconventional technique was used to grow crystals in the Nd−Ba−Cu−O system. A Nd4Ba2Cu2O10 (Nd-422) bar was supported on two NdBa2Cu3Ox (Nd-123) pellets in an alumina crucible and was heated in a horizontal furnace at a higher temperature than the peritectic decomposition point of Nd-422. Brown Al-doped Nd−Ba−Cu−O needlelike crystals belonging to a new phase were grown directly on the crucible. The structure of the new phase, whose composition is Nd6Ba9.78Sr0.22Cu2Al6O30, has been determined by single-crystal X-ray diffraction. The space group is P63mc; the unit cell parameters are a = 11.526(2) A, c = 6.963(1) A, and Z = 1. A full matrix least-squares refinement yielded R(F) = 0.0405 for 3131 independent reflections. This new phase is a metastable polymorphic modification of Nd-422 solid solution. The preferential directional growth of the needles lies along the c-axis. Many needles are affected by twinning by merohedry, with the twinning plane parallel to the (001) crystallographic plane.

Process Control of Seeded Batch Cooling Crystallization of the Metastable α-Form Glycine Using an In-Situ ATR-FTIR Spectrometer and an In-Situ FBRM Particle Counter

In this paper, we report a process control strategy for the production of metastable α-form glycine crystals of a desired mean mass size by manipulating the alternating temperature profile and the final termination temperature. The seed crystals of the α-form glycine introduced were grown successfully to the size of the product with no fine crystals. Generation of the γ-glycine crystals (stable polymorph) was completely avoided. This crystallization method is flexible and easy to operate, because the alternating temperature profile can be determined on-site according to the transient supersaturation and particle count number data obtained from an in-situ ATR-FTIR spectrometer and an in-situ FBRM particle counter, respectively. The termination time or batch time was also determined on-site to a point that the residual supersaturation became zero. This on-site strategy-determination technique is expected to be applied widely for a variety of polymorphic systems other than the glycine−water system as a pract...

First-principles calculations of ELNES and XANES of selected wide-gap materials: Dependence on crystal structure and orientation

Theoretical calculations of electron energy-loss near-edge structure (ELNES) and x-ray absorption near-edge structure (XANES) of selected wide-gap materials including ${\text{TiO}}_{2}$, AlN, GaN, InN, ZnO, and their polymorphs are performed using a first-principles method. Calculations of 39 $K$ and ${L}_{3}({L}_{2,3})$ edges are made using large supercells containing 72 to 128 atoms. A core hole is included in the final state, and the matrix elements of the electric dipole transition between the ground state and the final state are computed. Structures of some metastable crystals are optimized by a plane-wave basis pseudopotential method. Spectral differences in ELNES and XANES among polymorphs are quantitatively reproduced in this way. The origin of the spectral differences is pursued from the viewpoint of chemical bondings. Crystallographic orientation dependence of ELNES and XANES is also examined both by experiment and theory. The dependence is found to be much larger in $K$ edges than that in ${L}_{3}({L}_{2,3})$ edges.

Impactite from Henbury, Australia

Impactite was formed in several small, closely spaced craters by the impact of iron meteorites on sandstone at Henbury, central Australia. Through application of energy dispersive X-ray spectroscopy, convergent-beam electron diffraction, and selected-area electron diffraction, chemical compositions and space groups were determined for multiple crystalline phases in the impactite. Based on this chemical and structural information, sub-micrometer fayalite (approximately 40% of the total volume of crystalline phases), magnetite (25%), α-quartz (20%), hercynite (10%), diamond (3%), and a Nisulfide phase (2%) were identified, in addition to abundant glass. Compared with the pre-shock rock, the Henbury impactite was subjected to dramatic phase transformation and chemical redistribution and fractionation processes during the impact event. The newly formed dendritic crystalline phases such as fayalite, magnetite, and hercynite, as well as the surrounding glass, may represent metastable eutectic crystallization that occurred between approximately 1600 °C (liquidus temperature of hercynite) and 850 °C (75% of the liquidus temperature of fayalite) on a time scale of seconds. Generally, the composition of glass surrounding magnetite, fayalite, and hercynite is close to SiO 2 , plus small amounts of Fe (about 10%), but the glass around magnetite can also be considerably more enriched in Fe. The distribution of magnetite and fayalite is probably related to the Fe 3+ /Fe 2+ ratio in the original melt as altered by the local oxygen fugacity, and the formation of hercynite is governed by both the Fe 3+ /Fe 2+ ratio and the distribution of Al in the sample. Alpha quartz probably formed from Fe-depleted silica in the autoannealing period, at low pressure and relatively low temperatures. Diamond crystals in the impactite are euhedral and may have formed through chemical vapor deposition (CVD) at very high temperatures. Nickel sulfide probably crystallized directly from sulfide vapor or quenched sulfide melts, under supercooled and reduced conditions. The impactite is chemically very heterogeneous, presumably due to the limited diffusion time available for mixing of Fe and silica during the rapid cooling history. Iron, Ni, and Mg tend to concentrate in fayalite, magnetite, and hercynite, whereas K and Si are enriched in glass. Hercynite contains more Al, and other crystalline phases less Al, than the glass. In general, the Henbury impactite reflects a chaotic mixture of equilibrium, metastable, and non-equilibrium domains produced at high temperatures in a very short time, and most crystallization appears to have occurred in the post-shock period.

Conditions for metastable crystallization from undercooled melts

The limits of validity of Ostwald's rule of stages are investigated theoretically in the case of crystallization of undercooled melts. The treatment is within the limits of capillary theory. Two basic models are compared: (1) According to the first one (model A), the phase with lower energy of formation of critical nucleus is predominantly formed. In an enantiotropic-type phase diagram there is no region of homogeneous preferential formation of the low temperature phase. If the phase diagram is monotropic-type there is a certain temperature below which the metastable crystalline phase is preferentially formed. (2) The second assumption takes into consideration that the nature of extremely small phases is somewhat undefined. One certainly cannot determine whether, say 3-particle complex, is of phase 1 or of phase 2. Moreover, it is known that properties of extremely small clusters could be different from the corresponding volume phase. The main assumption is that there is a certain crucial size ( n-particle complex) at which the nature of the two phases can be distinguished. Complex of the phase, which has lower chemical potential at the crucial size, will be formed first. According to the model, in the case of enantiotropic-type transition there is a critical temperature.

The Two-Component System CCl4 + CBrCl3. Inference of the Lattice Symmetry of Phase II of CBrCl3

The phase diagram of the two-component system between two halomethanes, carbon tetrachloride (CCl4) and bromotrichloromethane (CBrCl3) has been determined by means of X-ray powder diffraction and thermal analysis techniques from 200 K to the liquid state. From the continuous series of mixed crystals, three relations of isomorphism have been evidenced: (i) between the stable low-temperature monoclinic (M) phases, (ii) between the stable orientationally disordered (OD) rhombohedral (R) phases, and (iii) between the stable OD face-centered cubic (FCC) of CBrCl3 and the metastable FCC phase of CCl4. The existence of such a metastable phase gives rise to the appearance, for a large range of composition, of metastable FCC mixed crystals that behave monotropically with respect to the R mixed crystals accordingly with the well-known monotropic character of the R to FCC phase transition of the CCl4. The isomorphism relationships i and ii enable us to validate the previously proposed structure of the monoclinic ph...

Colloidal crystals of core–shell-type spheres in deionized aqueous suspension

The structure, crystal growth kinetics and rigidity of colloidal crystals of core–shell-type latex spheres (diameters 280–330 nm) with differences in shell rigidity have been studied in aqueous suspension, mainly by reflection spectroscopy. The suspensions were deionized exhaustively for more than 2 years using mixed-bed ion-exchange resins. The five kinds of core–shell spheres examined form colloidal crystals, where the critical sphere concentrations, ϕ c, of crystallization (or melting) are high and range from 0.01 to 0.06 in volume fraction. Nearest-neighbor intersphere distances in the crystal lattice agree satisfactorily with values calculated from the sphere diameter and concentration. The crystal growth rates are between 0.1 and 0.3 s−1 and decrease slightly as the sphere concentration increases, indicating that the crystal growth rates are from the secondary process in the colloidal crystallization mechanism, corresponding to reorientation from metastable crystals formed in the primary process and/or Ostwald-ripening process. The rigidities of the crystals range from 2 to 200 Pa, and increase sharply as the sphere concentration increases. The g factor, the parameter for crystal stability, is around 0.02 irrespective of the sphere concentration and/or the kind of core–shell sphere. There are no distinct differences in the structural, kinetic and elastic properties among the colloidal crystals of the different core–shell-type spheres, showing that the internal sphere structure does not affect the properties of the colloidal crystals. The results show that colloidal crystals form in a closed container owing to long-range repulsive forces and the Brownian movement of colloidal spheres surrounded by extended electrical double layers and that their formation is not influenced by the rigidity and internal structure of the spheres.