Lithium hydroxide phase transition under high pressure: an ab initio molecular dynamics study.

The high-pressure phase transition in the deuterated lithium hydroxide crystalline state has been studied by Car-Parrinello molecular dynamics simulations, in the constant-pressure, constant-temperature ensemble. The recently developed metadynamics approach has been applied to encourage the system to transform into different phases in an affordable simulation time. A previously not completely characterized high-pressure phase has been obtained. The structural and spectroscopic properties have been studied and compared with the neutron scattering, infrared and Raman measurements. It has been found that the calculated structure differs slightly from the experimental hypothesis, and that the presence of strong hydrogen bonds is the source of the red shift and of the characteristic features of the OD-stretching bands in both IR and Raman spectra.     

Direct observation of phase transitions of polyethylene under high pressure by a PSPC x-ray system

A position-sensitive proportional counter (PSPC) x-ray measuring system is employed to observe directly phase transition processes of polyethylene at high temperature and high pressure. X-ray diffraction measurements reveal important new experimental data. First, an irreversible crystal transition from the hexagonal to the orthorhombic structures occurs in the critical region where the hexagonal structure begins to appear at a pressure of 350 MPa. That is, the (100) hexagonal reflection is observed only on cooling at 350 MPa. At pressures above about 400 MPa, however, the hexagonal phase is stable and the phase transitions melt ? hexagonal ? orthorhombic occur reversibly. Second, during cooling at pressures above 400 MPa, the (100) hexagonal reflection can be observed at temperatures below the hexagonal ? orthorhombic transition temperature. This behavior suggests that all the crystal morphologies of polyethylene, from “highly-extended-chain” crystals to crystals with a low melting point, are formed by the transitions melt → hexagonal → orthorhombic. Third, in heating at elevated pressures above 500 MPa, a shoulder in the peak intensity versus temperature plot for the (100) hexagonal reflection is observed at a higher temperature than the large maximum which occurs immediately after the crystal transition. This behavior indicates melting in two stages of hexagonal structures with different thermal stabilities, and the shoulder at higher temperature may be due to the fusion of the hexagonal phase annealed either below or above the transition point.     

Tandem aza-Michael additions under high pressure: a shortcut to the azanorbornyl skeleton

The hyperbaric aza-Michael addition of mono- and diamines on α,β-unsaturated β,β-disubstituted mono- and diesters has been studied. While in the case of monoester, this reaction provides a β-aminoester presenting a quaternary center, a direct and efficient access to diester or lactams featuring an azanorbornyl skeleton was obtained when starting from a diester, following an unprecedented double aza-Michael addition.     

First-principles study of the structure,mechanical properties,and phase stability of crystalline zirconia under high pressure

We presented a detailed study on the structure, mechanical properties, and phase stability for three zirconia crystals under hydrostatic pressure of 0–100 GPa by using density functional theory within the generalized gradient approximation. It is found that m-ZrO₂ presents three phase transitions with increasing pressure, while t-ZrO₂ and c-ZrO₂ do not. As the pressure increases, the band gap of m-ZrO₂ presents three abrupt changes. The band gap of t-ZrO₂ firstly decreases and then increases slowly. The band gap of c-ZrO₂ increases monotonically. An analysis of elastic constants shows that the three oxides are anisotropic under compression with increasing pressure. As the pressure increases, their fracture strength and plastic strength are improved and they are all ductile. The calculated formation enthalpies suggest that the elements (Zr + O₂) are able to form m-ZrO₂, t-ZrO₂, or c-ZrO₂ in the whole pressure range, indicating that zirconium and its alloys are easy to be oxidized.     

Morphological phase behaviour under pressure of polycatenar mesogens with a perfluorinated moiety

Two polycatenar materials composed of a four-aromatic-ring core with a perfluorinated moiety attached in one terminal position through either butylene- or pentylene spacer groups, and three tetradecyloxy chains at the other end (abbreviated as 14PC₄F and 14PC₅F), were investigated to study the effect of pressure on the phase transition behaviour. A polarizing optical microscope equipped with a high pressure optical hot stage, was used for the purpose. The T vs. P phase diagrams of 14PC₄F and 14PC₅F were constructed in the pressure region up to 100 MPa. 14PC₄F showed the stable crystal (Cr₁)-columnar tetragonal (Col_tet)-smectic A (SmA)-columnar hexagonal (Col_h)-isoropic liquid (I) phase transition sequence under all pressures. 14PC₅F exhibited the phase sequence metastable crystal (Cr₂)-cubic (Cub)-Col_tet-SmA-I in a melt-cooled sample on heating under pressure. But when the melt-cooled Cr₂ sample was annealed at 52-54°C for 2-3 h, the stable crystal (Cr₁) was formed slowly, giving a stable Cr₁-Cub-Col_tet-SmA-I phase sequence. The temperature region of the stable cubic phase broadened with increasing pressure. Furthermore a new mesophase of 14PC₅F was pressure-induced between the I and SmA phases on cooling at pressures above about 16 MPa. Since the monotropic mesophase exhibited a texture very similar to that of the high temperature Col_h phase of 14PC₄F with planar orientation, the new phase was assigned at a high temperature columnar hexagonal phase of 14PC₅F.     

Morphological phase behaviour under pressure of polycatenar mesogens with a perfluorinated moiety

Two polycatenar materials composed of a four‐aromatic‐ring core with a perfluorinated moiety attached in one terminal position through either butylene‐ or pentylene spacer groups, and three tetradecyloxy chains at the other end (abbreviated as 14PC₄F and 14PC₅F), were investigated to study the effect of pressure on the phase transition behaviour. A polarizing optical microscope equipped with a high pressure optical hot stage, was used for the purpose. The T vs. P phase diagrams of 14PC₄F and 14PC₅F were constructed in the pressure region up to 100 MPa. 14PC₄F showed the stable crystal (Cr₁)–columnar tetragonal (Col_tet)–smectic A (SmA)–columnar hexagonal (Col_h)–isoropic liquid (I) phase transition sequence under all pressures. 14PC₅F exhibited the phase sequence metastable crystal (Cr₂)–cubic (Cub)–Col_tet–SmA–I in a melt‐cooled sample on heating under pressure. But when the melt‐cooled Cr₂ sample was annealed at 52–54°C for 2–3 h, the stable crystal (Cr₁) was formed slowly, giving a stable Cr₁–Cub–Col_tet–SmA–I phase sequence. The temperature region of the stable cubic phase broadened with increasing pressure. Furthermore a new mesophase of 14PC₅F was pressure‐induced between the I and SmA phases on cooling at pressures above about 16 MPa. Since the monotropic mesophase exhibited a texture very similar to that of the high temperature Col_h phase of 14PC₄F with planar orientation, the new phase was assigned at a high temperature columnar hexagonal phase of 14PC₅F.     

A possible route to oxamic-platinum-blue: Nature and reactivity of a stable intermediate

Summary Oxamic acid is a ligand which offers several potential coordination sites, From its reaction in an aqueous medium with K₂[Pt(NO₂)₄] it is possible to isolate a complex (A) which constitutes a first step in the formation of oxamic-platinum-blue (C), together with green species whose structures are much more difficult to define. Elemental analyses and i.r. spectra of such a series allow one to postulate a formation mechanism for the oxamic platinum-blues.     

Measurement of the rotational viscosity, γ1 of nematic liquid crystals under high pressure

A method for the measurement of the rotational viscosity, γ₁ of nematic liquid crystals under high pressure is described. First measurements for the liquid crystals 4'-methoxybenzylidene-4-n-butylaniline, a broad range nematic mixture of substituted cyclohexyl-phenyls and a re-entrant nematic mixture are presented.     

The behavior of NH3 torsional vibration of l-alanine,l-threonine and taurine crystals under high pressure: A Raman spectroscopic study

Raman spectra of l-alanine, l-threonine and taurine amino acids crystals under high-pressure conditions at room temperature were investigated in a diamond anvil cell in order to observe the behavior of the torsional mode of NH₃ unit. It was observed for l-threonine and taurine crystals that increasing pressure produces an increase in the wave number of the band associated to the torsional vibration of ammonium group, while for l-alanine crystal a decrease in the wave number is observed. The anomalous behavior of l-alanine is explained in terms of the length and geometry of the hydrogen bonds within the crystalline structure: due to a short average length the effect of pressure is to deform the hydrogen bond, while for l-threonine and taurine, pressure approximates the N and O atoms in the N–H⋯O intermolecular bond.     

Equations of states for an ionic liquid under high pressure: A molecular dynamics simulation study

The high-pressure dependence of density given by empirical equation of states (EoS) for the ionic liquid 1-butyl-3-methylimidazolium trifluoromethanesulfonate (or triflate), [C₄C₁im][TfO], is compared with results obtained by molecular dynamics (MD) simulations. Two EoS proposed for [C₄C₁im][TfO] in the pressure range of tens of MPa, which give very different densities when extrapolated to pressures beyond the original experiments, are compared with a group contribution model (GCM). The MD simulations provide support that one of the empirical EoS and the GCM is valid in the pressure range of hundreds of MPa. As an alternative to these EoS that are based on modified Tait equations, it is shown that a perturbed hard-sphere EoS based on the Carnahan–Starling–van der Waals equation also fits the densities calculated by MD simulations of [C₄C₁im][TfO] up to ∼1.0 GPa.     

Thermal conductivity of solid argon at high pressure and high temperature: a molecular dynamics study

The thermal conductivity of solid argon at high-pressure (up to 50 GPa) and high-temperature (up to 2000 K) has been calculated by equilibrium molecular dynamics simulations using the Green-Kubo formalism and an exponential-6 interatomic potential. A simple empirical expression is given for its pressure and temperature dependence. The results are compared with predictions based on kinetic theory. The relative change of the thermal conductivity lambda with density rho is found to be consistent with a partial differential ln lambda/ partial differential ln rho slope of approximately 6 in a wide range of pressures and temperatures, in good agreement with predictions based on kinetic theory.     

Structural, electronic, and optical properties of crystalline iodoform under high pressure: a first-principles study

The high pressure phases, electronic structure, and optical properties of iodoform at zero temperature have been investigated by first-principles pseudopotential plane-wave calculations based on the density-functional theory. A new high pressure polar monoclinic structure with space group Cc, denoted as β phase, has been observed after a series of simulated annealing and geometry optimizations. Our calculated enthalpies showed that the transition from α to β phase occurs at 40.1 GPa. Electronic structure calculated results showed that the insulator-metal transition in α phase due to band overlap is found at about 32 GPa. In addition, the calculated absorption spectra of iodoform are consistent with the experimental results.     

Structural phase stability,mechanical and anharmonic properties of actinide sulfides: a high pressure study with the role of temperature

Vital properties (structural and thermo-elastic) of materials are affected on the application of pressure and temperature. These properties play a key role in understanding the thermodynamic behavior and equation of state. Thus, we have formulated a realistic interaction potential model to study the structural and thermo-elastic of US and ThS including the temperature effects. This model is used for the present compounds in a high-pressure study including temperature effects. We have computed phase transition pressures, volume drop, elastic and thermal properties at different pressures, and elevated temperatures. Our results are in agreement with available experimental and theoretical data.     

Raman and infrared study of the crystals with molecular anions in the region of a solid-liquid phase transition

It is presented the Raman and infrared (IR) spectroscopic investigations of the thiocyanates: LiSCN, NaSCN, KSCN, RbSCN, CsSCN; nitrates: Ca(NO3)2, Sr(NO3)2, Ba(NO3)2, Pb(NO3)2; and perrhenates: LiReO4, NaReO4, KReO4 in the region of a solid-liquid phase transition. The differences in a pre-melting behavior of this salts were found which can be attributed to a different geometry of the SCN-, NO3- and ReO4- anions.     

A fresh look at dense hydrogen under pressure. IV. Two structural models on the road from paired to monatomic hydrogen, via a possible non-crystalline phase

In this paper, we examine the transition from a molecular to monatomic solid in hydrogen over a wide pressure range. This is achieved by setting up two models in which a single parameter δ allows the evolution from a molecular structure to a monatomic one of high coordination. Both models are based on a cubic Bravais lattice with eight atoms in the unit cell; one belongs to space group Pa3, the other to space group R3m. In Pa3 one moves from effective 1-coordination, a molecule, to a simple cubic 6-coordinated structure but through a very special point (the golden mean is involved) of 7-coordination. In R3m, the evolution is from 1 to 4 and then to 3 to 6-coordinate. If one studies the enthalpy as a function of pressure as these two structures evolve (δ increases), one sees the expected stabilization of minima with increased coordination (moving from 1 to 6 to 7 in the Pa3 structure, for instance). Interestingly, at some specific pressures, there are in both structures relatively large regions of phase space where the enthalpy remains roughly the same. Although the structures studied are always higher in enthalpy than the computationally best structures for solid hydrogen - those emerging from the Pickard and Needs or McMahon and Ceperley numerical laboratories - this result is suggestive of the possibility of a microscopically non-crystalline or "soft" phase of hydrogen at elevated pressures, one in which there is a substantial range of roughly equi-enthalpic geometries available to the system. A scaling argument for potential dynamic stabilization of such a phase is presented.     

synthesis,phase transitions and optical properties of novel tetrathiafulvalene derivatives with extremely high molecular polarizabilities

The synthesis of novel unsymmetrically-substituted tetrathiafulvalene derivatives based on lithiation procedures is described. The disubstituted derivatives were synthesized as 1:1 cis/trans-isomeric mixtures, the compositions of which were established by ¹H NMR and ¹³C NMR spectroscopy. The transition temperatures, refractive indices, birefringence, polarizability and order parameter of each of the mixtures were determined and the values are discussed in comparison with analogous compounds with the tetrathiafulvalene unit replaced by a 1,4-phenylene group. In addition, we describe the synthesis of a number of monosubstituted tetrathiafulvalene derivatives and their associated thermal properties. The isomeric compounds presented in this paper have exceptionally large values of polarizability anisotropy, as compared with their phenyl-containing counterparts.     

synthesis, phase transitions and optical properties of novel tetrathiafulvalene derivatives with extremely high molecular polarizabilities

The synthesis of novel unsymmetrically-substituted tetrathiafulvalene derivatives based on lithiation procedures is described. The disubstituted derivatives were synthesized as 1:1 cis/trans-isomeric mixtures, the compositions of which were established by ¹H NMR and ¹³C NMR spectroscopy. The transition temperatures, refractive indices, birefringence, polarizability and order parameter of each of the mixtures were determined and the values are discussed in comparison with analogous compounds with the tetrathiafulvalene unit replaced by a 1,4-phenylene group. In addition, we describe the synthesis of a number of monosubstituted tetrathiafulvalene derivatives and their associated thermal properties. The isomeric compounds presented in this paper have exceptionally large values of polarizability anisotropy, as compared with their phenyl-containing counterparts.     

One-step green synthesis of cuprous oxide crystals with truncated octahedra shapes via a high pressure flux approach

Cuprous oxide (Cu₂O) was synthesized via reactions between cupric oxide (CuO) and copper metal (Cu) at a low temperature of 300 °C. This progress is green, environmentally friendly and energy efficient. Cu₂O crystals with truncated octahedra morphology were grown under high pressure using sodium hydroxide (NaOH) and potassium hydroxide (KOH) with a molar ratio of 1:1 as a flux. The growth mechanism of Cu₂O polyhedral microcrystals are proposed and discussed.