Raman and X-ray investigations of ferroelectric phase transition in NH4HSO4

Temperature-dependent Raman spectroscopy and X-ray diffraction studies have been carried out on NH(4)HSO(4) single crystals in the temperature range 77-298 K. Two structural transitions driven by the molecular ordering and change in crystal symmetries are observed below 263 and 143 K. These phase transitions are marked by the anomalies in the temperature dependence of wavenumber and fwhm of several internal vibrational modes. The Raman spectra and X-ray data enable us to understand the nature of the molecular ordering resulting in the ferroelectric phase below 263 K, sandwiched between two nonferroelectric phases. The crystal structure of the ferroelectric phase is determined correctly as Pc, which has been earlier solved in Ba symmetry. The temperature dependent Raman and X-ray results suggest that the disorder to order transition leading to lower symmetry below 263 K is driven by the change in HSO(4)(-) ions and that below 143 K is driven by the change in both HSO(4)(-) and NH(4)(+) ions.     

High Temperature Phase Transitions in Na_xCoO_2 Materials

1 INTRODUCTION In recent years, much attention has been focused on the NaxCoO2 system due to its notable properties and potential technologic applications as a cathode material in high-energy density, reversible solid state battery systems[1, 2]. Recently, large thermoelectric power[3～6] in NaxCoO2 and superconductivity (～4 K) in hydrated NaxCoO2yH2O (x = 0.35, y = 1.33) ma- terial have been observed[7]. This hydrated layered phase is the first 3d-transition metal oxide with superco…     

A temperature dependence kinetic study of O(1D) + CH4: overall rate coefficient and product yields

Using a recently-developed chemiluminescence technique for monitoring O(1D), the rate coefficient, k1, of the important atmospheric reaction O(1D) + CH4 --> products has been determined over a wide temperature range, 227 to 450 K. The rate coefficient was shown to be independent of temperature, having a value of (1.91 +/- 0.08) x 10(-10) cm3 s(-1); the quoted uncertainties are with 95% confidence. This highly precise value, based on an extended set of determinations with very low scatter, is significantly greater, 26%, than current recommended values. Secondly, the fraction of O(1D) quenched to O(3P) by CH4, k(1q)/k1, was precisely determined from chemiluminescence decays over the temperature range 236 to 340 K. A temperature independent value for k(1q)/k1 of 0.002 +/- 0.003 was found. Finally, LIF detection of OH has been applied to accurately determine the product branching fraction to OH of O(1D) + CH4 at room temperature. Our value, k(1a)/k1 = 0.76 +/- 0.08 (95% confidence), is in line with recent determinations by other groups.     

Calorimetric study on the thermotropic cubic mesogen, 4'-n-hexadecyloxy-3'-nitrobiphenyl-4-carboxylic acid, ANBC(16)

The heat capacity of ANBC(16) has been measured between 15 and 500 K by adiabatic calorimetry. Three (one known and two newly found) crystal-crystal phase transitions and all the known liquid crystalline phases (SmC, cubic D and SmA) were detected. The temperatures, enthalpies and entropies of transition were determined for all the phase transitions observed. The entropy of transition is very small for the transition from/to the cubic D mesophase. The results are compared with the thermal properties of another cubic mesogen, BABH(8). The logical possibility is pointed out that the cubic mesophases of ANBC(16) and BABH(8) are of identical higher order structure, while discussing the fact that they are immiscible.     

Calorimetric studies of binary systems of 1,3,5-trinitrobenzene with naphthalene, anthracene and carbazole. I. Phase transitions and heat capacities of the pure components and charge-transfer complexes

The enthalpies and temperature of fusion and solid—solid transitions and the heat capacity curves for naphthalene (NAP), anthracene, carbazole, 1,3,5-trinitrobenzene (TNB) and charge-transfer complexes of TNB with these donors have been determined by using a Perkin-Elmer DSC-1B scanning calorimeter in the temperature range 180 K to just below the melting point.

Three modifications of TNB, one stable and two metastable, with melting points 398.4, 380.3 and 383.0 K, have been observed. Two phase transitions in the NAP—TNB complex, at 220 and 424.5 K, have been revealed. The probable nature of the phase transitions is discussed.

The quantity ΔC_p, where ΔC_p = C_p (complex) − C_p (donor) − C_p (TNB), is negative at higher temperatures, being equal to zero or positive at 180 K. This result is interpreted as an indication of a decrease in the complex stability in the solid state with decreasing temperature     

Thermal expansion of irradiated polytetrafluoroethylene

The thermal expansion coefficient of gamma-irradiated Polytetrafluoroethylene (PTFE) has been measured in the temperature range 80–340 K by using a three-terminal capacitance technique. The samples are irradiated in air at room temperature with gamma rays from a Co⁶⁰ source at a dose rate of 0.26 Mrad/h. The change in crystallinity is measured by an x-ray technique. The expansion coefficient is found to increase with radiation dose below 140 K owing to the predominant effect of degradation. Above 140 K, the expansion coefficient is found to decrease with radiation dose because of the enhanced crystallinity. The temperatures of the two first-order phase transitions are also found to shift to lower temperatures owing to degradation of PTFE.     

Possibility of isostructural condensed states of matter in the D phase of ANBC and the cubic mesophase of BABH: heat capacity of 4'-n-octadecyloxy-3'-nitrobiphenyl-4-carboxlic acid, ANBC(18)

Heat capacity measurements have been made on ANBC(18) at temperatures from 8 to 490 K by adiabatic calorimetry. All known phases were detected. The temperatures, enthalpies and entropies of transition were determined for the phase transitions observed. On the basis of the entropy of transition to the SmC phase from the D or cubic phases, it is pointed out that the D phase of ANBC and the cubic phase of BABH might be identical in nature. It is shown that the arrangement of 'molecular' cores has a higher degree of order in the isotropic (D and cubic) phases than in the SmC phase, whereas the terminal alkoxy chains are more disordered in the isotropic phases than in the SmC phase. The degrees of disorder in the D and cubic phases relative to the SmC phase are very similar in terms of the entropy of transition per methylene group. The inverted phase sequence in ANBC (SmC D on heating) and BABH (cubic SmC) can be accounted for in terms of the competing roles in the entropy between the molecular core and the chains.     

Phase Transitions in Channel Clathrates of Thiourea with Hexachloroethane and Tetrachloroethane

An explanation is proposed for the nature of phase transitions in channel clathrates of thiourea with hexachloroethane and tetrachloroethane. Analysis of experimental temperature dependences of specific heat, entropy, and NQR spectra indicates the existence of first and secondorder phase transitions. In the thiourea + hexachloroethane compound, the secondorder phase transition (94.60 K, entropy gain Rln2) implies a transformation of two equiprobable states (lowtemperature phase) into a single new state (hightemperature phase). The firstorder phase transition corresponds to clathrate channel distortion. In the thiourea + tetrachloroethane compound, the secondorder phase transition (224K, entropy change Rln 6) occurs with transformation of six equiprobable states (lowtemperature phase) into a single new state (hightemperature phase). The firstorder phase transition corresponds to clathrate channel distortion. The next secondorder phase transition observed in this compound at 248 K leads to disappearance of the four possible orientations of guest molecules in the clathrate channel of hexagonal section and to a transition of guest molecules to chaotic orientation (entropy increment equals Rln 4).     

A molecular "phase ordering" phase transition leading to a modulated aperiodic composite in n-heptane/urea

n-Heptane/urea is an aperiodic inclusion compound in which the ratio of host and guest repeats along the channel axis is very close to unity and is found to have a constant value (0.981) from 280 K to 90 K. Below 280 K, two phase transitions are observed. The first (T(c1) = 145 K) is a ferroelastic phase transition that generates superstructure reflections for the host while leaving the guest with 1D order. The second (T(c2) = 130 K) is a "phase ordering" transition to a four-dimensional structure (P2(1)11(0βγ)) with pronounced host-guest intermodulation and a temperature dependent phase shift between guests in adjacent channels.     

The rotation and the phase transition in solid C60

The interaction between C60's in solid C60 has been calculated by (exp-6-1) potential, and the cause and the controlled factor of the high rapid rotations of C60 's were discussed. In order to describe the disordered degree of C60 rotation, an equivalent M is introduced. The phase transitions at the ~260 K and at the ~90 K are studied from the viewpoint of C60 rotation. The potential barriers of the ordered rotation below the ~260 K and the disordered rotation above the ~260 K have been given, and the effect of the external pressure on the temperature of phase transition has also been given.     

Surface order-disorder phase transitions and percolation

In the present paper, the connection between surface order-disorder phase transitions and the percolating properties of the adsorbed phase has been studied. For this purpose, four lattice-gas models in the presence of repulsive interactions have been considered. Namely, monomers on honeycomb, square, and triangular lattices, and dimers (particles occupying two adjacent adsorption sites) on square substrates. By using Monte Carlo simulation and finite-size scaling analysis, we obtain the percolation threshold theta(c) of the adlayer, which presents an interesting dependence with w/k(B)T (w, k(B), and T being the lateral interaction energy, the Boltzmann constant, and the temperature, respectively). For each geometry and adsorbate size, a phase diagram separating a percolating and a nonpercolating region is determined.     

Structural chemistry of A3CuBO6 (A = Ca, Sr; B = Mn, Ru, Ir) as a function of temperature

Variable temperature X-ray and neutron powder diffraction techniques have been used to identify structural phase transitions in Cu-rich A(3)A'BO(6) phases. A transition from monoclinic to rhombohedral symmetry was observed by X-ray diffraction between 700 and 500 K in Sr(3)Cu(1-x)M(x)IrO(6) (M = Ni, Zn; 0 < or = x < or = 0.5). The temperature of the phase change decreased in a linear manner with Cu-content and was essentially independent of the nature of M. Ca(3.1)Cu(0.9)MnO(6) was shown to pass from a rhombohedral phase to a triclinic phase on cooling below 290 K; the structure of the triclinic phase was refined against neutron diffraction data collected at 2 K. Ca(3.1)Cu(0.9)RuO(6) undergoes a transition between a disordered rhombohedral phase and an ordered monoclinic phase when cooled below 623 K. Neutron diffraction has been used to determine the structure as a function of temperature in the range 523 < or =T/K < or = 723 and hence to determine an order parameter for the low temperature phase; the second-order transition is shown to be incomplete 100 K below the critical temperature.     

The phase diagram of ammonium nitrate

The pressure-temperature (P-T) phase diagram of ammonium nitrate (AN) [NH(4)NO(3)] has been determined using synchrotron x-ray diffraction (XRD) and Raman spectroscopy measurements. Phase boundaries were established by characterizing phase transitions to the high temperature polymorphs during multiple P-T measurements using both XRD and Raman spectroscopy measurements. At room temperature, the ambient pressure orthorhombic (Pmmn) AN-IV phase was stable up to 45 GPa and no phase transitions were observed. AN-IV phase was also observed to be stable in a large P-T phase space. The phase boundaries are steep with a small phase stability regime for high temperature phases. A P-V-T equation of state based on a high temperature Birch-Murnaghan formalism was obtained by simultaneously fitting the P-V isotherms at 298, 325, 446, and 467 K, thermal expansion data at 1 bar, and volumes from P-T ramping experiments. Anomalous thermal expansion behavior of AN was observed at high pressure with a modest negative thermal expansion in the 3-11 GPa range for temperatures up to 467 K. The role of vibrational anharmonicity in this anomalous thermal expansion behavior has been established using high P-T Raman spectroscopy.     

Adiabatic calorimetry of the metallomesogen purple cobalt stearate Co(O2CC17H35)2

The heat capacity of the metallomesogen purple cobalt stearate Co(O₂CC₁₇H₃₅)₂ has been measured by adiabatic calorimetry at temperatures between 16 and 420 K. This compound exhibits two crystalline phases (low temperature Cr2 and high temperature Cr1 phases), mesophase (M phase), and isotropic liquid (I phase). A third crystalline phase Cr3, which is entirely metastable with respect to all the others, is suggested by DSC studies. The Cr2-to-Cr1, Cr1-to-M, and M-to-I phase transitions occurred at 362.1, 380.9, and 400.4 K, respectively. The enthalpy and entropy gains at these phase transitions were determined. The mesophase is either smectic A or nematic.     

四氯合铜酸二烷基铵相变的热分析和红外光谱

用DSC和TG研究了(n-C_nH_(2n+1)NH_3)_2CuCl_4(n=7-12)(记为C_nM)配合物的热稳定性和固-固相变。由红外光谱讨论了C_9Cu三个相的性质。发现C_nM的热稳定性呈奇偶效应; 主相变峰温随链长增长而升高; 相变总ΔH和ΔS也随链增长而加大; 当n≤9时, 高温相为部分无序相; 而n≤10时, 高温相为构象无序相。C_9Cu的主相变主要源自链间堆积结构变化。而在307.7 K的相变主要与烃链有序-无序变化有关。     

Studies on Thermal Decomposition of Pitch-polymer Compositions

The temperature range of phase transitions has been established for coal–tar pitches modified with various types of polymers. Mass loss rate was determined in relation to modifying polymer type. Simultaneous thermoanalytical method (DTA, DTG, TG), appeared to be very useful in studies on pitch-polymer compositions. This revised version was published online in August 2006 with corrections to the Cover Date.     

Thermal desorption of bromine from a graphite-bromine intercalation compound

Gas chromatography and static adsorption have been used to examine the bromine-graphite acceptor system, which has intercalant structures comparable with the adjacent graphite: layers. The bromine adsorption-desorption isobars and isotherms have been determined at 278–393 K, whose shapes indicate reversible phase transitions between intercalation stages 2 and 4. Measurements at 390–550 K give the desorption activation energies for the two forms of intercalated bromine; the heat involved in the conversion of the ionized intercalant form to the molecular one is 10.4 kJ/mole.Translated from Teoreticheskaya i Ékperimental'naya Khimiya, Vol. 25, No. 1, pp. 120–123, January–February, 1989.     

丙酮+环己烷+甲醇组成的最低共沸混合物热力学性质的研究

Molar heat capacities of the pure samples of acetone,methanol and the azeotropic mixture composed of acetone,cyclohexane and methanol were measured by an adiabatic calorimeter from 78 to 320 K.The solid-solid andsolid-liquid phase transitions of the pure samples and the mixture were determined based on the curve of the heatcapacity with respect to temperature.The phase transitions took place at(126.16±0.68)and(178.96±1.47)K forthe sample of acetone,(157.79±0.95)and(175.93±0.95)K for methanol,which were corresponding to thesolid-solid and the solid-liquid phase transitions of the acetone and the methanol,respectively.And the phase tran-sitions occurred in the temperature ranges of 120 to 190 K and 278 to 280 K corresponding to the solid-solid andthe solid-liquid phase transitions of mixture of acetone,cyclohexane and methanol,respectively.The thermody-namic functions and the excess thermodynamic functions of the mixture relative to standard temperature of 298.15K were derived based on the relationships of the thermodynamic functions and the function of the measured heatcapacity with respect to temperature.     

Di-hydroxybenzenes: Catechol, resorcinol, and hydroquinone: Enthalpies of phase transitions revisited

Molar enthalpies of sublimation of 1,2-di-hydroxybenzene, 1,3-di-hydroxybenzene, and 1,4-di-hydroxybenzene were obtained from the temperature dependence of the vapor pressure measured by the transpiration method. The molar enthalpies of fusion of 1,2- and 1,4-isomers were measured by differential scanning calorimetry (DSC). A large number of the primary experimental results on the temperature dependences of vapor pressure and phase transitions have been collected from the literature and have been treated in a uniform manner in order to derive sublimation, vaporization and fusion enthalpies of di-hydroxybenzenes at the reference temperature 298.15 K. The data sets on phase transitions were checked for internal consistency. This collection together with the new experimental results reported here has helped to resolve contradictions in the available thermochemical data and to recommend consistent and reliable sublimation, vaporization and fusion enthalpies for all three isomers under study.     

Far infrared study of the phases KNO3

The literature has indicated that KNO₃, a crystalline ionic solid, can undergo solid - solid transitions between seven different phases (ref.1). Four of these solid phases are found within small pressure (O < P < 4K. bar) and temperature (O < T < 403K) ranges, while the three other structures are found at higher pressures (> 4k. bar) (ref. 2). The phase transitions (I – IV) are investigated here as they offer an opportunity to study the implied negative restoring force, whilst the particular phase allows the study of its particular structure and behaviour.