The Gibbs energies of monomeric and polymeric C60 phases at a 0.1 MPa pressure and temperatures from 0 to 800 K

The Gibbs energies of monomeric (face-centered cubic and simple cubic) and polymeric (orthorhombic, tetragonal, and rhombohedral) C₆₀ phases at normal pressure (0.1 MPa) were calculated over the temperature range 0–800 K. The calculation results were used to determine the relative stabilities of the monomeric and polymeric fullerene C₆₀ phases over the temperature range specified. The data obtained were compared with the experimental data and calculation results reported by other authors.     

Phase transitions of LiAlO2 at high pressure and high temperature

This work presents a comprehensive study on phase transitions in LiAlO₂ system at high pressures and temperatures (0.5-5.0 GPa and 300-1873 K, respectively), as well as the phase stability for polymeric phases of LiAlO₂ in the studied P-T space by X-ray diffraction (XRD). Besides the previously described polymorphic hexagonal α-phase, orthorhombic β-phase and tetragonal δ-phase, a possible new phase of LiAlO₂ was observed after the tetragonal γ-LiAlO₂ sample was treated at 5.0 GPa and 389 K. The stable regimes of these high-pressure phases were defined through the observation of coexistence points of the polymeric phases. Our results revealed that LiAlO₂ could experience structural phase transitions from γ-LiAlO₂ to its polymorphs at lower pressures and temperatures compared to the reported results. Hexagonal α-LiAlO₂ with highly (003) preferential orientation was prepared at 5.0 GPa and 1873 K.     

The Gibbs energies of monomeric and polymeric fullerene C60 phases at pressures up to 2.0 GPa and temperatures up to 800 K

The Gibbs energies of monomeric and polymeric orthorhombic, tetragonal, and rhombohedral fullerene C₆₀ phases were calculated. The results were used to determine the relative stabilities of the phases and construct the phase diagram of C₆₀ over the pressure and temperature ranges of up to 2.0 GPa and 800 K, respectively. The data obtained are compared with experimental data and calculation results reported by other authors.     

Heterogeneous equilibria in the rhenium-oxygen system

The heterogeneous phase equilibria in the rhenium-oxygen system (over a composition range of Re₂O₇-Re) were studied using differential thermal analysis, x-ray diffraction analysis, and tensimetry. The system was found to contain the equilibrium phases of Re₂O₇, cubic ReO₃, and orthorhombic ReO₂. A region of homogeneity of orthorhombic ReO₂ was hypothesized to exist. Monoclinic ReO_2+x was demonstrated to be metastable. The p-T and T-x projections of the phase diagram of the system were constructed.     

Thermodynamic properties of crystalline polymeric C60 phases in the temperature region from O → 0 to 340 E

The temperature dependences of the heat capacity C _p° = f(T) were studied in an adiabatic vacuum calorimeter for the orthorhombic, tetragonal, and rhombohedral polymeric C₆₀ phases in the 7—340 K temperature interval with an error of 0.2%. Comparative analysis of C _p° of these phases formed by stacking of one-dimensional and two types of two-dimensional polyfullerenes C₆₀, was performed, and their fractal dimensionalities D were determined for temperatures below 50 K. The thermodynamic functions of the crystalline polymeric C₆₀ phases were calculated in the temperature region from O 0 to 340 K: C _p°(T), H°(T) — H°(0), S°(T) — S°(0), and G°(T) — H°(0). Assuming that S°(0) = 0, the standard entropies of formation _f S° of these phases from graphite at T = 298.15 K and standard pressure were calculated. In addition, the entropies of transformation of the initial face-centered cubic phase of fullerite C₆₀ in the crystalline polymeric C₆₀ phases and entropies of their interconversions under the same conditions were estimated. The thermodynamic characteristics of the polymeric C₆₀ phases were reviewed.     

Thermodynamics of 2D polymerized tetragonal phase of fullerene C60 in the range from T→0 to 650 K at standard pressure

By dynamic calorimetry the temperature dependence of heat capacity for two-dimensional (2D) polymerized tetragonal phase of C₆₀ has been determined over the 300-650 K range at standard pressure mainly with an uncertainty ±1.5%. In the range 490-550 K, an irreversible endothermic transition of the phase, caused by the depolymerization of the polymer, has been found and characterized. Based on the experimental data obtained and literature information, the thermodynamic functions of 2D polymerized tetragonal phase of C₆₀, namely, the heat capacity C°_p(T), enthalpy H°(T)−H°(0), entropy S°(T), and Gibbs function G°(T)−H°(0), have been calculated over the range from T→0 to 490 K. From 150 to 330 K in an adiabatic vacuum calorimeter and between 330 and 650 K in a dynamic calorimeter the thermodynamic properties of the depolymerization products have been examined and compared with the corresponding data for the monomeric phase C₆₀.     

On α-β phase transition in cristobalite-type Al1−xGaxPO4 (0.00?x?1.00)

The results of variable temperature powder X-ray diffraction and differential thermal analysis (DTA) studies on the orthorhombic (α) low-cristobalite to cubic (β) high-cristobalite phase transition for Al_1−xGa_xPO₄, (0.00?x?1.00) are presented. These studies reveal that all these compositions undergo reversible phase transitions from orthorhombic to cubic form at higher temperature. The high-temperature behavior of GaPO₄ is observed to have a different behavior compared to all other compositions in this series. Orthorhombic low-cristobalite-type GaPO₄ transforms to cubic high-cristobalite form at ∼605 °C. Above ∼700 °C, the cubic high-cristobalite-type GaPO₄ slowly transforms to trigonal quartz type structure. At about 960 °C, the quartz type GaPO₄ transforms back to the cubic high-cristobalite form. During cooling cycles the cubic phase of GaPO₄ reverts to trigonal quartz type phase. However, annealing of GaPO₄ at higher temperatures for longer duration can stabilize the orthorhombic low cristobalite phase. The phase transition temperatures and associated enthalpies are related to the change in unit cell volume and the orthorhombicity of the respective low cristobalite lattice.     

Structural and magnetic properties of high-pressure/high-temperature synthesized (Sr1-xRx)CoO3 (R=Y and Ho) perovskites

Polycrystalline perovskite cobalt oxides Sr_1-xR_xCoO₃ (R=Y and Ho; 0?x?1) were prepared by high-pressure/high-temperature technique. X-ray powder patterns of the Y-system indicated cubic perovskite form for 0?x?0.5, and orthorhombic perovskite form for x=0.8 and 1.0, while coexisting of the two phases for x=0.6. The cubic perovskite samples had metallic electric resistivities while the orthorhombic ones with semiconducting or insulating nature. The parent compound SrCoO₃ showed a ferromagnetic transition at 266 K. With the Y substitution, the transition temperature increased slightly to ∼275 K at x=0.1, then decreased rapidly to ∼60 K for x=0.6. The YCoO₃ (x=1) sample showed non-magnetic behavior. The Ho-substituted system showed quite similar structural, transport and magnetic properties to those of the Y-system.     

Phase equilibrium and electrical conductivity of SrCo0.8Fe0.2O3−δ

The phase diagram of the SrCo_0.8Fe_0.2O_3−δ compound has been determined at high temperatures (823?T?1223 K) and in the oxygen partial pressure range (10⁻⁵?pO₂?1 atm) by thermogravimetric measurements of the equilibrium pO₂, high temperature X-ray diffraction and electrical conductivity measurements. The cubic perovskite phase SrCo_0.8Fe_0.2O_3−δ is stable in a broad range of oxygen content, while the orthorhombic brownmillerite phase SrCo_0.8Fe_0.2O_2.5 stabilizes within a small range around 3−δ=2.5 at temperatures below 1073 K. Equilibrium pO₂ measurements under isothermal conditions show chemical hysteresis at the perovskite to brownmillerite transition. The hysteresis loop decreases its amplitude in pO₂ with decreasing temperature. This behavior is discussed considering the evolution from coherent intergrowth interfaces with elastic strain energy to incoherent interfaces without elastic strain energy as T decreases. The thermodynamic quantities h_O2^oxide and s_O2^oxide for the perovskite phase decrease when increasing the oxygen defects concentration. The electrical conductivity (σ) of the cubic phase exhibits a thermally activated behavior at high temperature. The variation of σ with the oxygen content is non-linear and the activation energy varies from 0.4 to 0.28 eV as the oxygen content increases from 2.4 to 2.6. These results are interpreted in the frame of the small polaron model.     

Liquid–liquid equilibria for the ternary system water+n-dodecane+2-(2-n-hexyloxyethoxy)ethanol

Experiments of the fish-shaped phase diagram for the ternary system water+n-dodecane+2-(2-n-hexyloxyethoxy)ethanol (abbreviated by C₆E₂ hereafter) under atmospheric pressure were performed at constant water/n-dodecane weight ratio (1/1) to locate the critical end points. The upper and lower critical consolute temperatures for the system of interest are 307.80 K and 282.30 K, respectively. Compositions of two- and three-phase liquid–liquid equilibrium for the ternary system water+n-dodecane+C₆E₂ at 293.15 K and 303.15 K under atmospheric pressure are presented in this paper.     

Thermodynamic properties of some polymeric forms of fullerite C<Subscript>60</Subscript> at temperatures ranging from 0 to 320 K

The temperature dependences of the heat capacityC ⁰ _p of fullerites C₆₀ were studied at temperatures ranging from 5 to 320 K in an adiabatic vacuum calorimeter with an accuracy of 0.4–0.2%. The fullerite C₆₀ samples were prepared by treating the starting fullerite C₆₀ under 8 GPa at 920 and 1270 K and “quenched” by a sharp decrease in pressure to −10⁵ Pa and in temperature to ∼300 K. Fullerite C₆₀(8 GPa, 920 K), a crystalline polymer with layered structure formed by polymerized fullerene C₆₀ molecules, was obtained at 920 K and 8 GPa. Fullerite C₆₀(8 GPa, 1270 K), a three-dimensional polymer with a graphite-like structure formed by fragments of decomposed C₆₀ molecules and containing many C(sp³)−C(sp³) bonds, was obtained at 1270 K and 8 GPa. Both polymers are metastable polymeric phases. The anomalous character of the temperature dependence of the heat capacity was revealed in the 49–66 K range for the polymer formed at 1270 K. The thermodynamic functions of the substances under study were calculated for the 0–320 K region along with entropies of their formation from graphite. The entropies of transformation of the starting fullerite C₆₀ into metastable phases and that of intertransformation of phases were estimated. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 277–281, February, 2000.     

The formation of a series of barium tungsten bronzes

The phases occurring in samples of gross composition Ba_xWO₃ (0.01 < x < 0.33) heated at temperatures between 1073 and 1373°K have been determined using X-ray diffraction and electron microscopy. At all temperatures a tetragonal tungsten bronze phase with a narrow homogeneity range of x = 0.20?0.21 was observed to form. In addition, at temperatures up to 1273°K, a series of orthorhombic intergrowth bronzes forms within a restricted composition range around x = 0.04. The latter phases are unstable at higher temperatures and were not found in preparations made at 1323°K. Similarly a new type of bronze phase forms at x = 0.14?0.16 at temperatures up to 1323°K, but not at 1373°K. The structure of this phase is unknown. Aspects of the crystal chemistry of the barium bronzes and the relationships to other bronze phases are discussed.     

A series of lead tungsten bronzes

The phases in samples of gross composition Pb_xWO₃ (0.01 ? x ? 0.28) heated at temperatures between 973 and 1373°K have been investigated. At all temperatures a nonstoichiometric tetragonal tungsten bronze phase forms for compositions x > 0.18. At temperatures up to 1273°K a series of orthorhombic intergrowth bronzes also forms, but these appear to be unstable at higher temperatures and were not found in the preparations made at 1373°K. Aspects of the crystal chemistry of these latter materials are discussed, including structure, crystal habit, valence of the Pb atoms in these phases, and the relation of the phases found here to other related intergrowth bronze phases.     

Formation of scandium carbides and scandium oxycarbide from the elements at high-(P, T) conditions

Synchrotron diffraction experiments with in situ laser heated diamond anvil cells and multi-anvil press synthesis experiments have been performed in order to investigate the reaction of scandium and carbon from the elements at high-(P,T) conditions. It is shown that the reaction is very sensitive to the presence of oxygen. In an oxygen-rich environment the most stable phase is ScO_xC_y, where for these experiments x=0.39 and y=0.50-0.56. If only a small oxygen contamination is present, we have observed the formation of Sc₃C₄, Sc₄C₃ and a new orthorhombic ScC_x phase. All the phases formed at high pressures and temperatures are quenchable. Experimentally determined elastic properties of the scandium carbides are compared to values obtained by density functional theory based calculations.     

Vapor—liquid and vapor—liquid—liquid phase equilibria for binary and ternary systems for nitrogen,ethane and methanol: Experiment and data reduction

Zeck, S. and Knapp, H., 1986. Vapor-liquid and vapor-liquid-liquid phase equilibria for binary and ternary systems of nitrogen, ethane and methanol; experiment and data reduction. Fluid Phase Equilibria, 25: 302–322.VLE and VLLE are investigated for three binary and one ternary system containing N₂, C₂H₆ and CH₃OH in a high-pressure phase equilibrium apparatus with vapor recirculation at temperatures 240 < T < 298 K and pressures 4 < p < 75 bar. Two liquid phases are observed in the systems C₂H₆CH₃OH and N₂CH₃OH. Experimental results are reported and compared with available correlations.     

Phase diagram and crystal phases of trans-1,3 butadiene probed by FTIR and Raman spectroscopy

In the present work the first spectroscopic data concerning solid butadiene are presented. FTIR and Raman spectra of solid trans-1,3 butadiene in the ordered phase I were recorded at atmospheric pressure down to 12 K. The FTIR technique was used to characterize the phase diagram of butadiene in the 0–7 GPa and 150–300 K pressure–temperature range. Besides phase I another solid phase (phase II), orientationally disordered, was found to be stable at pressures above 0.5 GPa between the liquid and the ordered phase I. A monoclinic C_2h⁵ crystal structure with two molecules per cell sitting on C_i sites is proposed for phase I.     

The two-phase region in 2(ZnSe)x(CuInSe2)1−x alloys and structural relation between the tetragonal and cubic phases

The two-phase region in the system 2(ZnSe)_x(CuInSe₂)_1−x covers the chemical composition range 0.10<x?0.36, in which a tetragonal and a cubic phase are coexisting. The structural relation between both phases was determined by selected area diffraction (SAD) and transmission electron microscopy (TEM). Both crystal structures are very similar and the extremely small mismatch of the lattice constants of the tetragonal phase and the embedding cubic matrix phase allows for the grain boundaries to be virtually strain-free and, therefore, without notable dislocations. The tetragonal phase forms grains of flat discus-like shape in the ambient cubic matrix, with the short discus axis parallel to the tetragonal c-axis. TEM experiments proved that the discus-shaped tetragonal particles are collinear with the (100)_cub, (010)_cub and (001)_cub planes of the cubic phase. Cooling and annealing experiments revealed a near-equilibrium state only to be realized for small cooling rates less than 2 K/h and/or for a long-time annealing with subsequent rapid quenching. Only then there will be no cation ordering in both, the tetragonal domains and the parental cubic matrix phase. If, however, the samples are kept in a state far away from the equilibrium condition both phases reveal Stannite-type cation ordering. Within the composition range of 0?x?0.10 only tetragonal 2(ZnSe)_x(CuInSe₂)_1−x-alloys exist. At concentration rates above 36 mol% 2(ZnSe) only cubic structured solid solutions of ZnSe and CuInSe₂ are found to be stable. However, in the range 36 mol% to about 60 mol% 2(ZnSe) tiny precipitates with Stannite-like structure exist, too.     

Preparation, phase transition and thermal expansion studies on low-cristobalite type Al1−xGaxPO4 (x=0.0, 0.20, 0.50, 0.80 and 1.00)

The orthorhombic (α) low-cristobalite type AlPO₄ and GaPO₄ and their solid solutions are prepared by co-precipitation followed by high temperature annealing of the precipitate. The single phasic nature of the products is ascertained by powder XRD at room temperature. The high temperature behavior of these samples is studied by HT-XRD over the temperature range of 25-1000°C. All these compositions undergo an orthorhombic to cubic (β, high-cristobalite) phase transition at elevated temperature. The unit cell parameters at different temperatures are determined by refining the observed powder diffraction profiles. The phase transition is accompanied by a significant increase in the unit cell volume, leading to the formation of a low dense structure. The variation of unit cell volume with temperature for each composition shows that the orthorhombic phase has a significantly larger thermal expansion than the cubic (high temperature) phase. The high temperature behavior of all the compositions except the GaPO₄ is similar. GaPO₄ undergoes a phase separation to a more stable quartz type phase above 800°C. However, the quartz type phase again transforms to the high cristobalite (β) phase at 1000°C. Thermal expansions of all these phases are explained in term of the variation of M-O-P angle as a function of temperature.     

Mössbauer study of the thermal decomposition products of BaFeO4

A pure sample of a hexavalent iron compound, BaFeO₄, was decomposed at temperatures below 1200°C at oxygen pressures from 0.2 to 1500 atm. In addition to the already known BaFeO_x (2.5 ≦ x < 3.0) phases with hexagonal and triclinic symmetry, two new phases were obtained as decomposition products at low temperatures. One of the new phases, with composition BaFeO_{2.61 – 2.71}, has tetragonal symmetry; lattice constants are a₀ = 8.54 Å, c₀ = 7.29 Å. The phase is antiferromagnetic with Néel temperature estimated to be 225 ± 10 K. Two internal fields observed on its Mössbauer spectra correspond to Fe³⁺ and Fe⁴⁺. In the other new phase, with composition BaFeO_2.5, all Fe³⁺ ions had the same hyperfine field; it too is antiferromagnetic with a Néel temperature of 893 ± 10 K. Mössbauer data on the hexagonal phase coincided with earlier results of Gallagher, MacChesney, and Buchanan [J. Chem. Phys.43, 516 (1965)]. In the triclinic-I BaFeO_2.50 phase, internal magnetic fields were observed at room temperature, and it was supposed that there were four kinds of Fe³⁺ sites. The phase diagram of BaFeO_x system was determined as functions of temperature and oxygen pressure.     

Order-Disorder Structural Phase Transitions in Alkali Perchlorates

Order-disorder structural phase transitions in alkali perchlorates MClO₄ (M=Na, K, Rb, Cs) are investigated using molecular dynamics simulation. The potentials in the simulations are based on the Gordon-Kim modified electron gas formalism extended to molecular ions. The simulations yield first-order phase transitions in perchlorates from low temperature orthorhombic structures to high temperature cubic NaCl structures. The perchlorate ions are found to be orientational disordered in the high temperature phases.