Comparison of the Dynamics of Water Molecules in Crystals of Proton-Ordered Ice IX and Its Proton-Disordered Analog III

A computer simulation of proton-ordered ice IX and its proton-disordered analog III (768 molecules, 90 K) was carried out by the molecular dynamics method using Poltev–Malenkov's potential. For ice IX, the differences in the dynamic characteristics of molecules with O(1) and O(2) are much wider than those in the case of ice III. The libration spectrum of ice IX has a number of distinct acute peaks, and the spectrum of ice III is strongly smoothed. These peculiarities are explained by the proton ordering of ice IX and disordering of ice III. The latter is responsible for the great differences in the short- and especially long-range environment of water molecules in ice crystals and hence for the presence of many molecules with different dynamic characteristics. Thus averaging over a large number of different vibrational spectra of molecules leads to a smoothed total spectrum in the case of the proton-disordered crystal modification of ice.     

New crystal structures of β-[Ni(NCS)2(4-methylpyridine)4] clathrates with furan,tetrahydrofuran, methylene chloride,benzene + ethanol and methylcellosolve as guest molecules

Single crystal X-ray structures of clathrates of -[Ni(NCS)₂(4-methylpyridine)₄] with furan, tetrahydrofuran, benzene + ethanol, methylene chloride, and methylcellosolve as guests molecules are reported. The location of the guest molecule in the partially decomposed clathrate with methylene chloride was defined by X-ray diffraction and compared with the fully occupied one. The host lattices of all clathrates studied are tetragonal (I4₁/a) and do not differ significantly from typical -phase clathrates of [M(NCS)₂(4-methylpyridine)₄] (M = divalent metal cation). Arrangements of guest molecules represent different types of packing: one type of guest molecule occupies both possible types of positions, two different guest molecules occupy different positions, with only one type of positions occupied by one type of guest molecule. Possible stoichiometries of clathrates with -type lattices are discussed.Presented at the Sixth International Seminar on Inclusion Compounds, Istanbul, Turkey, 27–31 August 1995.     

Electron radiation damage in copper in the high voltage electron microscope

The various factors influencing the nucleation of radiation damage in the HVEM are reviewed. A method for obtaining quantitative information from observations is given. The effect of reirradiation at room temperature on the damage formed at elevated temperatures is shown. This effect may give experimental information concerning the strain fields surrounding large dislocation loops.     

Molecular dynamics studies on the local disordering of Σ3 and Σ11 grain boundaries in aluminium bicrystal

Molecular dynamics (MD) simulations using Morse interaction potential are performed in studies of [110] symmetrical tilt grain boundary (GB) structures with mis-orientation angles 50.5°(Σ11), 129.5°(Σ11), 70.5°(Σ3) and 109.5°(Σ3) at various tempratures. The GB structures are found to start local disordering at about 0.5T _m (T _m is the melting point of aluminium) for 50.5°(Σ11), 0.32T _m for 129.5° (Σ11) and 0.38T _m for 70.5°(Σ3), respectively. These results agree with conclusions deduced from the anelastic measurements. But, for twin-boundary structure 109.5°(Σ3), this disordering has not been found even when temperature increases up to 0.9T _m . The project supported by the National Natural Science Foundation of China and Laboratory for Non-linear Mechanics of Continuous Media, Institute of Mechanics, Academia Sinica.     

Unusual pressure-induced phase behavior in crystalline poly-4-methyl-pentene-1

We report some highly unusual phase behavior, of general implication for condensed matter, on the polymer poly(4-methyl-pentene-1) (P4MP1) induced by changes in pressure and temperature, as observed in situ by x-ray diffraction. Upon increasing pressure beyond a threhold, the polymer, crystalline under ambient conditions, loses its crystalline order isothermally, passing through a continuously varying sequence of mesomorphic states, the process being reversible. This behavior is observed in two widely separated temperature regions, suggesting, for the first time in a single component system, the possibility of reentrant liquid-crystal and amorphous phases. At the upper temperature region (ca. 250°C) there is a consecutive increase and decrease of melting point with pressure. In the lower temperature region (room temperature) the pressure converts the crystal into an amorphous-like glass obviating the need for going through the melt first, and this in a reversible manner. The latter pressure-induced disordered phase converts into crystal on raising the temperature, and reverts to the glassy, disordered phase on lowering the temperature. Some aspects of this behavior have been found quite recently in water-ice and silica but the process of “melting on cooling” has no precedent in any known system. Other unexpected findings include a new pressure-induced modification of P4MP1 with a one-way only entry with temperature, but full reversibility with pressure leading to a triple point in the PT phase diagram. The above highly uncommon results are putting several prevailing preconceptions to test which are being scrutinized. In the course of it some early expectations on general phase behavior, allowing among others for reentrant phases in one component systems, are being invoked as potentially appropriate for certain polymeric systems, if not for condensed matter in general. © 1993 John Wiley & Sons, Inc.     

On the novel double perovskites A2Fe(Mn0.5W0.5)O6 (A= Ca,Sr, Ba). Structural evolution and magnetism from neutron diffraction data

New A₂Fe(Mn_0.5W_0.5)O₆ (A = Ca, Sr, Ba) double perovskite oxides have been prepared by ceramic techniques. X-ray diffraction (XRD) complemented with neutron powder diffraction (NPD) indicate a structural evolution from monoclinic (space group P2₁/n) for A = Ca to cubic (Fm-3m) for A = Sr and finally to hexagonal (P6₃/mmc) for A = Ba as the perovskite tolerance factor increases with the A²⁺ ionic size. The three oxides present different tilting schemes of the FeO₆ and (Mn,W)O₆ octahedra. NPD data also show evidence in all cases of a considerable anti-site disordering, involving the partial occupancy of Fe positions by Mn atoms, and vice-versa. Magnetic susceptibility data show magnetic transitions below 50 K characterized by a strong irreversibility between ZFC and FC susceptibility curves. The A = Ca perovskite shows a G-type magnetic structure, with weak ordered magnetic moments due to the mentioned antisite disordering. Interesting magnetostrictive effects are observed for the Sr perovskite below 10 K.     

High-Temperature Structural Disorder in R3NbO7

R₃NbO₇ (R = La, Nd, Gd, Y) niobates have been studied by Raman spectroscopy at 298-973 K. The temperature and composition effects on the structure of these compounds are considered. The changes observed in the Raman spectra at elevated temperatures are caused by a reversible structural disordering due to oxygen redistribution over anion sites.     

Low-temperature X-ray study of benzoid-quinoid tautomerism in crystals of 2-(N,N-diphenylhydrazinoethylidene)-3(2H)-benzo[b]thiophenone

An X-ray investigation of 2-(N,N-diphenylhydrazinoethylidene)-3(2H)-benzo[b]thiophenone was performed at room and low (–80 °C) temperatures. It was established that the structure of this compound in the crystalline state at R-20 °C is intermediate between the ketoenehydrazine and hydroxyhydrazone forms, while at –80 °C the molecular structure is completely transformed to the hydroxyhydrazone form. This confirms the dynamic nature of hydrogen atom disordering in the molecule at 20 °C.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 104–106, January, 1995.This work was financially supported by the Russian Foundation for Basic Research (Project No. 93-03-04355).     

Concentration Phase Transitions and Structural Disordering in the System of Solid Solutions LixNa1?xTa0.1Nb0.9O3

Concentration phase transitions and structural disordering in the system of solid solutions Li _x Na_1–x Ta_0.1Nb_0.9O₃ (x = 0–0.16) are investigated by the methods of Raman scattering, x–ray structural analysis, and electrophysical measurements. Points of disruption of the translational order in the arrangement of cations at x = 0.05–0.06, 0.09, 0.11, 0.135, and 0.14 that correspond to the boundaries of morphotropic regions and concentration structural anomalies are detected. At points x = 0.11; 0.135, and 0.14, the Raman spectrum in the region of vibrations of the cations located in octahedral and cubooctahedral holes (0–400 cm^–1) consists of two wide maxima and is practically smeared into the wing of the Rayleigh line, and on the concentration dependences of the electrophysical properties one observes sharp anomalies. The oxygen ochahedrons are distorted less significantly as x changes: the corresponding lines broaden, and new lines demonstrating a symmetry change of the oxygen octahedrons appear only for x 0.14. It is shown that the system of solid solutions Li _x Na_1–x Ta_0.1Nb_0.9O₃ for x < 0.015 is characterized by the antiferroelectric type of dipole ordering, and for x 0.015 the structure of a solid solution becomes noncentrosymmetric.     

Crystal structure of the 1:3 thiourea-hexachloroethane inclusion compound at 295 K

The crystal structure of the 1:3 thiourea-hexachloroethane inclusion compound at 295 K has been determined. The parameters of the rhombohedral Bravais cell (space group R3ˉc) are a = 16.097(2) Å, c = 12.450(3) Å, V = 2793.6(8) ų, d _calc = 1.659 g/cm³, d _exp = 1.661±0.005 g/cm³, Z = 6 for C₅H₁₂Cl₆N₆S₃. The disordered guest molecules lie in the channels (parallel to the c axis) of the clathrate framework constructed from thiourea molecules linked by N-H...S hydrogen bonds. The mutual arrangement of the carbon and chlorine atoms is such that they define four orientations of the C-C bond relative to the channel of the host framework with nearly eclipsed conformations of hexachloroethane: one orientation along the triple bond axis (channel axis) and three equiprobable orientations at an angle of 74(2)° to the axis; for the coaxial orientation, there are two different configurations of the guest with slightly different geometries. The relative contributions of each of the five orientations were determined from the site occupancies of the guest atoms: 26.18 (coaxial orientations) and 3×19%. The resulting structure model is compared with the literature data about guest disordering in the structure of an adduct of the same composition determined at 233 K and with other similar structures. Original Russian Text Copyright ? 2007 by S. F. Solodovnikov, G. N. Chekhova, G. V. Romanenko, N. V. Podberezskaya, Z. A. Solodovnikova, D. V. Pinakov, and A. R. Semenov __________ Translated from Zhurnal Strukturnoi Khimii, Vol. 48, No. 2, pp. 348–357, March–April, 2007.