Four phases of amorphous water: Simulations versus experiment

Multiplicity of the liquid-liquid phase transitions in supercooled water, first obtained in computer simulations [Brovchenko et al., J. Chem. Phys. 118, 9473 (2003)], has got strong support from the recent experimental observation of the two phase transitions between amorphous ices [Loerting et al., Phys. Rev. Lett. 96, 025702 (2006)]. These experimental results allow assignment of the four amorphous water phases (I-IV) obtained in simulations to the three kinds of amorphous ices. Water phase I (rho approximately 0.90 gcm(3)) corresponds to the low-density amorphous ice, phase III (rho approximately 1.10 gcm(3)) to the high-density amorphous ice, and phase IV (rho approximately 1.20 gcm(3)) to the very-high-density amorphous ice. Phase II of model water with density rho approximately 1.00 gcm(3) corresponds to the normal-density water. Such assignment is confirmed by the comparison of the structural functions of the amorphous phases of model water and real water. In phases I and II the first and second coordination shells are clearly divided. Phase I consists mainly of the four coordinated tetrahedrally ordered water molecules. Phase II is enriched with molecules, which have tetrahedrally ordered four nearest neighbors and up six molecules in the first coordination shell. Majority of the molecules in phase III still have tetrahedrally ordered four nearest neighbors. Transition from phase III to phase IV is characterized by a noticeable drop of tetrahedral order, and phase IV consists mainly of molecules with highly isotropic angular distribution of the nearest neighbors. Relation between the structures of amorphous water phases, crystalline ices, and liquid water is discussed.     

Raman and X-ray scattering studies of high-pressure phases of urea

Single-crystal and polycrystalline urea samples were compressed to 12 GPa in a diamond-anvil cell. Raman-scattering measurements indicate a sequence of four structural phases occurring over this pressure range at room temperature. The transitions to the high-pressure phases take place at pressures near 0.5 GPa (phase I --> II), 5.0 GPa (II --> III), and 8.0 GPa (III --> IV). Lattice parameters in phase I (tetragonal, with 2 molecules per unit cell, space group P42(1)m (D3(2d))) and phase II (orthorhombic, 4 molecules per unit cell, space group P2(1)2(1)2(1) (D2(4))) were determined using angle-dispersive X-ray diffraction experiments. For phases III and IV, the combined Raman and diffraction data indicate that the unit cells are likely orthorhombic with four molecules per unit cell. Spatially resolved Raman measurements on single-crystal samples in phases III and IV reveal the coexistence of two domains with distinct spectral features. Physical origins of the spatial domains in phases III and IV are examined and discussed.     

Thermotropic phase behaviour of sodium oleate as studied by FT-Raman spectroscopy and X-ray diffraction

The thermotropic behaviour of sodium oleate (NaOl) has been studied in the temperature range 10–125°C by using Fourier transform-Raman spectroscopy, X-ray diffraction and differential scanning calorimetry (DSC). The temperature dependence of conformationally sensitive bands in the CH₂ stretching (2800–2900 cm⁻¹), C–C stretching (1050–1150 cm⁻¹) and CH₃ rocking region (830–900 cm⁻¹) has been used to characterize the order/disorder behaviour of alkyl chains. It is found that in phase I, NaOl exhibits the crystalline ordered lamellar structure with a repeat period of 4.51 nm. The first broad peak in the DSC trace is due to superposition of two transitions (phase I to phase II and phase II to phase III), therefore, it is not possible to determine the lamellar structure of phase II. This broad transition from phase I to phase III is associated with the melting of methyl-sided chains and increase in gauche conformers in carboxylate-sided chains. Finally, NaOl undergoes a transition from crystalline to a liquid crystalline phase IV, which is associated with the melting of the carboxylate-sided chain.     

Structure and phase transitions in poly[bis-(2,2,3,3-tetrafluoropropoxy) phosphazene]

The structure and phase transitions in poly[bis-(2,2,3,3-tetrafluoropropoxy)phosphazene] have been studied by differential scanning calorimetry (DSC) and x-ray diffraction. Two crystalline phases and one mesomorphic phase are found, denoted I, II, and III, respectively. These phases convert reversibly one into the other on heating and cooling. The Phase I–Phase II transition occurs in a temperature range from 5 to 30°C whereas the Phase II mesophase (Phase III) transition proceeds above 80°C. Heats of transitions are measured to be about 29.0 J/g and 3.6 J/g, respectively. Crystalline Phase I is characterized by a monoclinic unit cell with the parameters: α = 24.4 Å, b = 9.96 Å, c = 4.96 Å, γ = 123°. The axes of both chains, traversing the unit cell, are directed along the “c” axis, the main chains having cis-trans conformation. Phase I is the common crystalline structure for the main chain and side chains. The structure of Phase II is controlled mainly by packing of the side chains. Transition of Phase II into mesomorphic Phase III is accompanied with distortion of packing of the side chains. Only regular packing of the main chains of macromolecules in the plane perpendicular to their axes exists in Phase III. Mesomorphic phase III is stable up to the degradation temperature of the polymer. A significant effect of stress on the Phase II–III transition in oriented samples was found.     

Vibron frequencies of solid H2 and D2 to 200 GPa and implications for the P-T phase diagram

Vibrational spectroscopy of the intramolecular stretching mode (vibron) of the hydrogen isotopes has been used for the past 20 years in different laboratories using various techniques to probe phase diagrams of this system under extreme conditions. Available vibrational spectroscopy data in hydrogen and deuterium to 200 GPa at 10-300 K are analyzed and reassessed to identify the existence of an additional molecular phase (I') to phases I, II, and III previously identified at megabar pressures. The results do not support the existence of phase I' in the pressure-temperature range studied. Previously proposed boundaries between phases I, II, and III are re-examined and updated phase diagrams of hydrogen and deuterium are proposed.     

Phase diagram and phase structure of the sodium di-n-pentyl phosphate-water system.1H pulsed-gradient NMR self-diffusion,31P NMR and x-ray diffraction studies

Sodium di-n-pentylphos phate (DPP) has been synthesized, and the phase diagram of the DPP-water system consisting of five regions (I, II, III, IV, and V) has been determined. The phase structure has been investigated by¹H pulsed-gradient NMR self-diffusion,³¹P NMR and x-ray low angle diffraction methods. The results are summarized as follows. In region I, there exist two critical micelle concentrations (CMC), indicating that this region is in a monomer-micelle equilibrium and that variation in the aggregated state occurs at the second CMC. Region II is a two phase area in which regions I and IV coexist. In region III, hydrated crystals and an aqueous solution of DPP coexist. Region IV is a homogenous, transparent and fluid phase and the results of³¹P NMR spectra and x-ray diffraction patterns reveal the formation of a highly organized structure, similar to a lamellar-like structure. Region V is a homogenous, transparent and fluid phase and the self-diffusion coefficient value and³¹P NMR spectra show that its phase structure is very similar to that for the concentrated sample in region I.     

H-bonding dependent structures of (NH4+)3H+(SO4 2-)2. Mechanisms of phase transitions

The role of different H-bonds in phases II, III, IV, and V of triammonium hydrogen disulfate, (NH(4)(+))(3)H(+)(SO(4)(2)(-))(2), has been studied by X-ray diffraction and (1)H solid-state MAS NMR. The proper space group for phase II is C2/c, for phases III and IV is P2/n, and for phase V is P onemacr;. The structures of phases III and IV seem to be the same. The hydrogen atom participating in the O(-)-H(+).O(-) H-bond in phase II of (NH(4)(+))(3)H(+)(SO(4)(2)(-))(2) at room temperature is split at two positions around the center of the crucial O(-)-H(+).O(-) H-bonding, joining two SO(4)(2)(-) tetrahedra. With decreasing temperature, it becomes localized at one of the oxygen atoms. Further cooling causes additional differentiation of possibly equivalent sulfate dimers. The NH(4)(+) ions participate mainly in bifurcated H-bonds with two oxygen atoms from sulfate anions. On cooling, the major contribution of the bifurcated H-bond becomes stronger, whereas the minor one becomes weaker. This is coupled with rotation of sulfate ions. In all the phases of (NH(4)(+))(3)H(+)(SO(4)(2)(-))(2), some additional, weak but significant, reflections are observed. They are located between the layers of the reciprocal lattice, suggesting possible modulation of the host (NH(4)(+))(3)H(+)(SO(4)(2)(-))(2) structure(s). According to (1)H MAS NMR obtained for phases II and III, the nature of the acidic proton disorder is dynamic, and localization of the proton takes place in a broader range of temperatures, as can be expected from the X-ray diffraction data.     

Polymer single crystals of poly(4-hydroxybenzoate). II. A contribution to crystal structure and polymorphism

Electron diffraction has been used to investigate the structure of a wide range of as-polymerized crystals of poly(4-hydroxybenzoate) [systematic name: poly(1,4-oxybenzoyl)]. The chemical composition and the degree of polymerization (DP) have been varied and some samples have been thermally treated. At room temperature two crystalline modifications with orthorhombic unit cells coexist. The chains adopt a 2₁ helical conformation in both forms, but there are differences for oligomer and polymer crystals. Oligomers of low DP have an extended chain-conformation, whereas in polymers a shortening of the repeat distance along the chain is observed as a function of both the DP and the crystallization conditions. From the most extensive data sets we have derived the lattice parameters a = 7.52, b = 5.70, and c = 12.49 Å for polymer crystals of phase I, and the subcell parameters for oligomer crystals of phase II a = 3.77, b = 11.06, and c = 12.89 Å. Both phases contain two chains per unit cell. In addition to modifications I and II several defect structures exist the unit cells of which contain more than two chains. At temperatures which depend on the degree of polymerization, a phase transition to a third modification takes place. The large difference between the densities of phase III as compared to both phase I and II suggests that torsional degrees of freedom exist in phase III which allow a certain mobility of the phenyl and ester groups. This mobility enables the end groups of adjacent layers in interlamellar regions of oligomer crystals to undergo transesterification reactions and therefore to increase the molecular weight of the samples.     

两种晶型酞菁氧钒纳米颗粒的制备及形成机理

在水溶液中利用激光消融制备了酞菁氧钒(VOPc)相I型纳米颗粒，在加入一种非离子型表面活性剂的情况下通过激光消融制备得到了其相II型纳米颗粒.X射线衍射(XRD)、紫外可见吸收光谱(UV-Vis)和傅立叶变换红外光谱(FT-IR)表征了其纳米颗粒中的晶体结构.扫描电子显微镜(SEM)观察显示相I和相II型酞菁氧钒纳米颗粒的直径分别约为100和60 nm.对相II型酞菁氧钒纳米颗粒的形成机理进行了讨论.     

Charge-transfer phase transition and zero thermal expansion in caesium manganese hexacyanoferrates

A series of caesium manganese hexacyanoferrates is prepared; Cs(I)(1.78)Mn(II)[Fe(II)(CN)6]0.78[Fe(III)(CN)6](0.22) (1), Cs(I)(1.57)Mn(II)[Fe(II)(CN)6]0.57[Fe(III)(CN)6](0.43) (2), Cs(I)(1.51)Mn(II)[Fe(II)(CN)6]0.51[Fe(III)(CN)6](0.49) (3), and Cs(I)(0.94)Mn(II)[Fe(II)(CN)6]0.21[Fe(III)(CN)6](0.70).0.8H2O (4). 1-3 show charge-transfer phase transitions between the high-temperature (HT) and low-temperature (LT) phases with transition temperatures (T(1/2 downward arrow), T(1/2 upward arrow)) of (207 K, 225 K) (1), (190 K, 231 K) (2), and (175 K, 233 K) (3) at a cooling and warming rates of 0.5 K min(-1). Variable temperature IR spectra indicate that the valence states of the LT phases of 1-3 are Cs(I)(1.78)Mn(II)(0.78)Mn(III)(0.22)[Fe(II)(CN)6], Cs(I)(1.57)Mn(II)(0.57)Mn(III)(0.43)[Fe(II)(CN)6], and Cs(I)(1.51)Mn(II)(0.51)Mn(III)(0.49) [Fe(II)(CN)6], respectively. The XRD measurements for 1-3 show that crystal structures of the HT and LT phases are cubic structures (Fm3[combining macron]m), but the lattice constants decrease from the HT phase to the LT phase; a = 10.5446(17) --> 10.4280(7) A (1), 10.5589(17) --> 10.3421(24) A (2), and 10.5627(11) --> 10.3268(23) A (3). The magnetization vs. temperature curves and the magnetization vs. external magnetic field curves show that the LT phases are ferromagnetic with Curie temperatures of 4.3 (1), 5.0 (2), and 5.6 K (3). At a cooling rate of -0.5 K min(-1), 4 does not show the charge-transfer phase transition, but does show a behavior of zero thermal expansion with a thermal expansivity of +0.2 x 10(-6) K(-1) throughout the temperature range 300 and 20 K.     

Electron diffraction based analysis of phase fractions and texture in nanocrystalline thin films, part III: application examples

In this series of articles, a method is presented that performs (semi)quantitative phase analysis for nanocrystalline transmission electron microscope samples from selected area electron diffraction (SAED) patterns. Volume fractions and degree of fiber texture are determined for the nanocrystalline components. The effect of the amorphous component is minimized by empirical background interpolation. First, the two-dimensional SAED pattern is converted into a one-dimensional distribution similar to X-ray diffraction. Volume fractions of the nanocrystalline components are determined by fitting the spectral components, calculated for the previously identified phases with a priori known structures. These Markers are calculated not only for kinematic conditions, but the Blackwell correction is also applied to take into account dynamic effects for medium thicknesses. Peak shapes and experimental parameters (camera length, etc.) are refined during the fitting iterations. Parameter space is explored with the help of the Downhill-SIMPLEX. The method is implemented in a computer program that runs under the Windows operating system. Part I presented the principles, while part II elaborated current implementation. The present part III demonstrates the usage and efficiency of the computer program by numerous examples. The suggested experimental protocol should be of benefit in experiments aimed at phase analysis using electron diffraction methods.     

Hydrostatic pressure effects on a hydrated lipid inverse micellar Fd3m cubic phase

Over a range of hydration, unsaturated diacylglycerol/phosphatidylcholine mixtures adopt an inverse micellar cubic phase, of crystallographic space group Fd3m. In this study hydrated DOPC:DOG mixtures with a molar ratio close to 1 : 2 were examined as a function of hydrostatic pressure, using synchrotron X-ray diffraction. The small-angle diffraction pattern at atmospheric pressure was used to calculate 2-D sections through the electron density map. Pressure initially has very little effect on the structure of the Fd3m cubic phase, in contrast to its effect on hydrated inverse bicontinuous cubic phases. At close to 2 kbar, a sharp transition occurs from the Fd3m phase to a pair of coexisting phases, an inverse hexagonal H(II) phase plus an (ordered) lamellar phase. Upon increasing the pressure to 3 kbar, a further sharp transition occurs from the H(II) phase to a (fluid) lamellar phase, in coexistence with the ordered lamellar phase. These transitions are fully reversible, but show hysteresis. Remarkably, the lattice parameter of the Fd3m phase is practically independent of pressure. These results show that these two lipids are miscible at low pressure, adopting a single lyotropic phase (Fd3m); they then become immiscible above a critical pressure, phase separating into DOPC-rich and DOG-rich phases.     

Low-temperature dynamics of (S)-4-(1-methylheptyloxy)-4ʹ-cyanobiphenyl (8*OCB) and (S)-4-(2-methylbutyl)-4ʹ-cyanobiphenyl (5*CB) in disordered crystalline and glassy phases

The inelastic neutron scattering (INS) spectra were measured for two materials of chiral molecules: (S)-4-(1-methylheptyloxy)-4?-cyanobiphenyl (8*OCB) and (S)-4-(2-methylbutyl)-4?-cyanobiphenyl (5*CB), revealing solid state polymorphism with two partially disordered crystalline phases I and II and glassy state of liquid and of crystalline phase in each substance. The experiments were performed in the energy range up to 30 µeV in the temperature range from 4 to 35 K. For 8*OCB the elastic scans were measured as well up to 300 K illustrating well the phase diagram. For all solid phases of both substances in the µeV range of INS spectra, the existence of the excess density of vibrational states over that typical for fully ordered crystalline phases was evidenced. Contribution of this so-called boson peak occurred to be much larger in glass of isotropic phase than in the phase II and glass of phase I of 8*OCB, while for 5*CB it was larger in the phase I and glass of phase II than in glass of cholesteric phase. The quasi-elastic broadening of elastic peak corresponding to stochastic reorientations in the ns time scale was detected for both substances. Comparison of the results obtained for glassy and crystalline phases of 8*OCB and 5*CB compounds have been given and confronted with those obtained previously in meV energy range.     

Lamellar and whisker single crystals of poly(2,6-oxynaphthoate)

Electron diffraction from single crystal lamellae and whiskers of poly(2,6-oxynaphthoate) reveals the presence of at least 3 unit cells. The equatorial reflections in the patterns from the whiskers correspond to the dominant (phase I) hk0 diffraction pattern from the lamellae; phase I is monoclinic with 2 chemical repeats per physical repeat. The intensity distributions in the hk0 patterns of phase I and II resemble those of the same phases in poly(p-oxybenzoate). The hk0 reflections of phase III suggest a common internapthalene unit spacing, but variable lateral (and possibly axial) shifts; apparently related orthorhombic and monoclinic patterns, with variable γ^*, are observed. At elevated temperature, above the crystalliquid crystal transition (ca. 330°C), quadrant reflections are retained; the change in the hk0 pattern from any given crystal is gradual, extending over some 40°C. Above the liquid crystal-liquid crystal transition (ca. 460°C) the pattern can be interpreted in terms of nematic or possibly smectic A packing. © 1992 John Wiley & Sons, Inc.     

A reexamination of the ice III/IX hydrogen bond ordering phase transition

Ice III is a hydrogen bond disordered crystal which when cooled 1 K / min or faster transforms to an antiferroelectric hydrogen bond ordered structure, ice IX. Throughout its region of stability, experiments indicate that the H bonds in ice III are, in fact, partially ordered, i.e., some proton arrangements are preferred. In addition, there has been evidence that the structure of ice IX retains some residual disorder after the transition. Diffraction experiments and calorimetry apparently conflict with regard to the degree of ordering at the ice III/IX transition. Mean field statistical mechanical theories have been used to link partial occupations from diffraction data with thermodynamics. In this work, we investigate the ice III/IX proton ordering phase transition using electronic density functional theory calculations for small unit cells, extended to simulate the phase transition in a large unit cell using graph invariants. In agreement with experiment, we observe partial ordering over a wide range of temperatures as ice III transforms to partially disordered ice IX, near 126 K, which becomes fully ordered at lower temperatures. We compare our results from full statistical mechanical simulations with mean field models, finding small errors for the low-temperature ice IX phase and much larger errors for the high-temperature ice III phase. The failure of mean field theories may explain the apparent conflict between diffraction experiments and calorimetry.     

Modular construction of oxide structures--compositional control of transition metal coordination environments

The effects of reaction temperature and pO2 were investigated on a series of (Ba,Ca,Nd)FeO3-delta perovskite systems in order to isolate phases containing ordered arrangements of the distinct vacancy and cation ordering patterns identified in less compositionally complex iron oxide systems. Initial synthesis in air at high temperature yields cubic perovskite phases (I) with average iron oxidation states higher than 3; selected area electron diffraction together with diffuse features observed in the synchrotron X-ray diffraction (SXRD) patterns of these materials show evidence of small domains of short-range cation and vacancy order. Annealing these materials in nitrogen or in a sealed tube in the presence of an NiO/Ni buffer yielded the Fe(3+) phase Ca2Ba2Nd2Fe6O16 (II), closely related to Sr2LaFe3O8 but with partial cation order as well as anion order present the larger Ba cations are largely present in the 12-coordinate site between the octahedral iron layers, and Ca is largely present in 10-coordinate sites between octahedral and tetrahedral sites. Further reduction of Ca2Ba2Nd2Fe6O16 using a Zr getter yields the mixed-valence phase Ca2Ba2Nd2Fe6O15.6 (III). The structure of III was solved by maximum entropy analysis of XRD data coupled with analysis of high-temperature neutron diffraction data and refined against combined SXRD and high-Q ambient-temperature neutron data. This material crystallizes in a 20-fold perovskite super cell (Imma, a approximately square root(2 x a(p), b approximately 10 x a(p), c approximately square root(x 2a(p)) and can be visualized as an intergrowth between brownmillerite (Ca2Fe2O5) and the YBa2Fe3O8 structure. There are three distinct iron coordination environments, octahedral (O), square-pyramidal (Sp), and trigonal planar (Tp, formed by distorting the tetrahedral site in brownmillerite), which form a Sp-O-Tp-O-Sp repeat. Bond valence calculations indicate that Tp is an Fe(2+) site, while the O and Sp sites are Fe(3+). The A-site cations are also partially ordered over three distinct sites: 8-coordinate between the Sp layers, 10-coordinate between Tp and O layers, and 12-coordinate between Sp and O layers. Mossbauer spectroscopy, magnetometry, and variable-temperature neutron diffraction show that the material undergoes two magnetic transitions at approximately 700 and 255 K.     

Pressure-induced collapse of guanidinium nitrate N-H···O bonded honeycomb layers into a 3-D pattern with varied H-acceptor capacity

Highly favoured N-H···O bonded honeycomb layers in guanidinium nitrate, C(NH(2))(3)(+)NO(3)(-), have been destabilized by a pressure of 0.6 GPa, and the novel motif of 3-dimensional N-H···O bonded aggregation in high-pressure phase IV determined for in situ grown single-crystal by X-ray diffraction. The mechanism of the transition involves the collapse of voids present in phases I, II and III. In the P/T phase diagram a large hysteresis of the phase IV boundaries is caused by the strongly reconstructive character of the transition and pressure dependent H-accepting capacity of oxygen atoms.     

Confined linear molecules inside an aperiodic supramolecular crystal: the sequence of superspace phases in n-hexadecane/urea

High-resolution studies of the host-guest inclusion compound n-hexadecane/urea are reported at atmospheric pressure, using both cold neutrons and x-ray diffraction. This intergrowth crystal presents a misfit parameter, defined by the ratio c(h)/c(g) (c(host)/c(guest)), which is temperature independent and irrational (γ = 0.486 ± 0.002) from 300 to 30 K. Three different structural phases are reported for this aperiodic crystal over this temperature range. The crystallographic superspaces are of rank 4 in phases I and II, whereas phase III is associated with an increase in rank to 5, with a supplementary misfit parameter (δ = 0.058 ± 0.002) that is constant throughout this phase. The superspace group of phase I is hexagonal P6(1)22(00γ) down to T(c1) = 149.5 ± 0.5 K; phase II, which persists down to T(c2) = 127.8 ± 0.5 K is orthorhombic P2(1)2(1)2(1)(00γ), and phase III is orthorhombic P2(1)2(1)2(1)(00γ)(00δ).     

A model proposed for the interpretation of DTA and X-ray data of the III → I transition in sodium sulfate

The transition between phase I and phase III in sodium sulfate (Na₂SO₄) was studied by high temperature X-ray diffraction and the calculation of diffraction patterns. An intermediate phase (phase II) was detected on cooling in the undoped and 1.7 at.% yttrium-doped Na₂SO₄, but not observed in the 3.8 at.% Y-doped sample. The DTA and X-ray diffraction results of the III ? I transition were explained well by a proposed model which consisted mainly of the following steps: (i) the rotation of the SO₄ tetrahedra, and (ii)the disappearance of interfaces among the transformed domains.     

On the structures of the intermediate phases in the terbium oxide system

A high-resolution transmission electron microscope investigation of the ordered phases in the terbium oxide system in the range $\text{1.714 <}\text{O}\text{Tb}\text{< 2}$ has been performed. Two ordered phases belonging to the R_nO_2n?2 homologous series of defective fluorite phases have been examined, namely, the n = 11 and n = 12 members. Selected area electron diffraction has revealed two polymorphs of the n = 12 phase in the TbO_x system. In addition, a metastable n = 16 phase was identified by diffraction and imaging techniques. High-resolution (～3 Å) images of thin crystals of these phases are presented. Computer-simulated lattice images based on dynamical scattering theory with the incorporation of the appropriate instrumental parameters at through-focus and through-thicknesses for proposed defect structures of the intermediate terbium oxide phases are presented for comparison. Images calculated at optimum defocus and periodic thicknesses correspond to the projected crystal potential. The remarkably good match between experimental and calculated images at optimum defocus and in a through-focus series supports the choice of the structural models for the real defect structure of the intermediate phases.