Pressure-induced phase transition in transition metal trifluorides

As a fundamental thermodynamic variable, pressure can alter the bonding patterns and drive phase transitions leading to the creation of new high-pressure phases with exotic properties that are inaccessible at ambient pressure. Using the swarm intelligence structural prediction method, the phase transition of TiF₃, from R—3c to the Pnma phase, was predicted at high pressure, accompanied by the destruction of TiF₆ octahedra and formation of TiF₈ square antiprismatic units. The Pnma phase of TiF₃, formed using the laser-heated diamond-anvil-cell technique was confirmed via high-pressure x-ray diffraction experiments. Furthermore, the in situ electrical measurements indicate that the newly found Pnma phase has a semiconducting character, which is also consistent with the electronic band structure calculations. Finally, it was shown that this pressure-induced phase transition is a general phenomenon in ScF₃, VF₃, CrF₃, and MnF₃, offering valuable insights into the high-pressure phases of transition metal trifluorides.     

Pressure-induced phase transition of wulfenite

The in-situ high-pressure structures of wulfenite have been investigated by means of angular dispersive X-ray diffraction with diamond anvil cell and synchrotron radiation. In the pressure up to 22.9 GPa, a pressure-induced scheelite-to-fergusonite transition is observed at about 10.6 GPa. The pressure dependence for the lattice parameters of wulfenite is reported, and the axial compression coefficients Ka₀=－1.36×10^－3 GPa^－1 and Kc₀= －2.78×10^－3 GPa^－1 are given. The room-temperature isothermal bulk modulus is also obtained by fitting the P-V data using the Murnaghan equation of state.

     

Pressure-induced structural phase transition in RbAu

Using the first principle method based on density functional theory, the structural and elastic properties calculations of RbAu have been performed. The results demonstrate that RbAu is stable in the CsCl structure (B2) at ambient pressure, which is in well agreement with the experimental results. And there exists a structural phase transition from CsCl-type structure (B2) to NaTi-type structure (B32) at the transition pressure of approximate 6 GPa. The pressure effects on the elastic properties are discussed and the elastic property calculation indicates elastic instability maybe provide phase transition driving force according to the variations relation of the elastic constant versus pressure.     

Pressure-induced phase transition of wulfenite

The in-situ high-pressure structures of wulfenite have been investigated by means of angular dispersive X-ray diffraction with diamond anvil cell and synchrotron radiation. In the pressure up to 22.9 GPa, a pressure-induced scheelite-to-fergusonite transition is observed at about 10.6 GPa. The pressure dependence for the lattice parameters of wulfenite is reported, and the axial compression coefficients Ka₀=－1.36×10^－3 GPa^－1 and Kc₀= －2.78×10^－3 GPa^－1 are given. The room-temperature isothermal bulk modulus is also obtained by fitting the P-V data using the Murnaghan equation of state.     

Identification of the polymerized orthorhombic phase of C60 fullerene

It is established by x-ray diffraction and Raman scattering that the polymerization of C₆₀ fullerene at 1.5 GPa and 723 K leads to the formation of an orthorhombic phase that is different from the previously identified high-pressure orthorhombic phase. It is determined by a calculation of the optimal packing of linear C₆₀ polymers by the method of atom-atom potentials that the energetically favorable structure of the orthorhombic phase belongs to the space group P n n m and not the previously proposed group I m m m. The computed value of the rotation angle of the polymer chains that corresponds to the minimum packing energy was equal to 61°. The mechanisms leading to the formation of the polymerized phases are discussed on the basis of the results obtained. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 2, 110–114 (25 July 1997)     

Pressure-induced phase transition in silicon nitride material

The equilibrium crystal structures,lattice parameters,elastic constants,and elastic moduli of the polymorphs α-,β-,and γ-Si3N4,have been calculated by first-principles method.β-Si3N4 is ductile in nature and has an ionic bonding.γSi3N4 is found to be a brittle material and has covalent chemical bonds,especially at high pressures.The phase boundary of the β→γ transition is obtained and a positive slope is found.This indicates that at higher temperatures it requires higher pressures to synthesize γ-Si3N4.On the other hand,the α→γ phase boundary can be described as P = 14.37198+ 3.27 × 10?3T-7.83911 × 10?7T2-3.13552 × 10?10T3.The phase transition from α-to γ-Si3N4 occurs at 16.1 GPa and 1700 K.Then,the dependencies of bulk modulus,heat capacity,and thermal expansion on the pressure P are obtained in the ranges of 0 GPa-30 GPa and 0 K-2000 K.Significant features in these properties are observed at high temperatures.It turns out that the thermal expansion of γ-Si3N4 is larger than that of α-Si3N4 over wide pressure and temperature ranges.The evolutions of the heat capacity with temperature for the Si3N4 polymorphs are close to each other,which are important for possible applications of Si3N4.     

Pressure-induced structural phase transition of iodine

The structural phase transition of iodine was observed at about 210 kbar and at room temperature by the high-pressure x-ray diffraction technique using a diamond-anvil cell and a position-sensitive detector. It was found to occur reversibly in both processes of increasing and decreasing pressure.     

Pressure-induced dimerization of C60 fullerene

The dimerization of C₆₀ fullerene under conditions of quasi-hydrostatic compression at temperatures above 293 K is investigated by IR spectroscopy, Raman scattering (RS) spectroscopy, and x-ray diffraction. The measured dimer (C₆₀)₂ content in the products of the polymerization of fullerite as a function of the pressure, temperature, and treatment time shows that dimerization occurs even at room temperature in the entire pressure range above ∼1.0 GPa. However, at least at temperatures above 400 K dimerization does not result in the formation of a dimer phase as a stable modification of the system, since the dimer is an intermediate product of the transformation. It is shown that increasing the holding time at 423 K decreases the content of the dimer fraction in the samples and results in the formation of linear (at 1.5 GPa) and two-dimensional (at 6.0 GPa) polymers, which are structure-forming elements of the orthorhombic and rhombohedral polymerized phases. Pis'ma Zh. éksp. Teor. Fiz. 68, No. 12, 881–886 (25 December 1998)     

Pressure-induced polycondensation of C60 fullerene

Modifications of carbon which are formed from C₆₀ fullerite at pressures up to 10.0 GPa and temperatures up to 1900 K are studied by x-ray diffraction, Raman spectroscopy, and atomic force microscopy methods. The pressures p and temperatures T at which atomic, molecular, and polymolecular structures form under conditions of quasihydrostatic compression are determined. It is shown that, together with polymerization, another type of chemical interaction of the molecules, called polycondensation, which leads to the formation of polymolecular structures with a shortest intermolecular distance of 0.65 nm, is possible in the system. Three-dimensional polycondensation of C₆₀ fullerene is explained by the special properties of the new carbon states. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 10, 778–783 (25 May 1996) The spelling of the authors names are presented here in English as requested by the Russian Editorial office.     

Spectroscopic study of phase transformations between orthorhombic and tetragonal C60 polymers

We present a detailed study of transformations between the orthorhombic and tetragonal polymeric states of C₆₀. The transformations are characterised by Raman spectroscopy and X-ray diffraction. We show that the transformation from the orthorhombic (O) phase to the tetragonal (T) phase is very fast and our results indicate that the transformation goes via an intermediate dimer (D) state in a two-stage process, O↦D and, D↦T transformations, where the second process is slower than the first. On the other hand, the transformation from the tetragonal to the orthorhombic phase is significantly slower, indicating a high-energy threshold to break the polymer bonds at the temperatures used. The results also support earlier suggestions that the tetragonal phase contains disordered dimers that can be viewed as lattice defects in the formation of higher polymers.     

Pressure-induced phase transition in the cubic ScF<Subscript>3</Subscript> crystal

Pressure-induced phase transitions in the ScF₃ crystal were studied using synchrotron radiation diffraction, polarization microscopy, and Raman spectroscopy. The phase existing in the range 0.6–3.0 GPa is optically anisotropic; its structure is described by space group R ₃ c (Z = 2), and the transition is due to rotation of ScF₆ octahedra around a threefold axis. The pressure dependence of the structural parameters and angle of rotation are determined. The number of Raman spectral lines corresponds to that expected for this structure; above the phase transition point, a recovery of soft modes takes place. At a pressure of 3.0 GPa, a transition occurs to a new phase, which remains metastable as the pressure decreases. The results are interpreted using an ab initio method based on the Gordon-Kim approach.     

Pressure-induced lock-in in an aperiodic nanoporous crystal

This Letter reports on the first observation of a commensurate lock-in inside an aperiodic composite. This result is obtained by neutron diffraction, under hydrostatic pressure, in the prototype self-assembled crystal of hexadecane urea. A selective compressibility of the sublattices is a required condition together with the existence of a lock-in energy term in these supramolecular materials. This measurement, under one-dimensional confinement, opens the way for the use of continuously controlled forces to manipulate molecular properties or functions.     

Pressure-induced dimerization kinetics of fullerene C60

Dimerization kinetics was studied for fullerene C₆₀ by IR spectroscopy at a pressure of 1.5 GPa in the temperature range 373–473 K. The kinetic curves for the formation of a dimer (C₆₀)₂ were obtained using its analytical IR band at 796 cm^?1. Under the assumption that pressure-induced C₆₀ dimerization is a second-order irreversible reaction, the reaction rate constants were determined at different temperatures. The corresponding activation energy and preexponential factor were found to be 134±6 kJ/mol and (1.74± 0.24)×10¹⁴ s^?1, respectively. The specific features of the solid-phase C₆₀ dimerization in simple cubic and face-centered cubic fullerite phases are discussed.     

Pressure-induced phase transition of ice in aqueous LiOH solution

I have examined the changes in in situ Raman spectra of ice in aqueous LiOH solution as a function of pressure at liquid nitrogen temperature (77 K). Here, I have shown the possibility that ice in aqueous LiOH solution transforms to a high-density amorphous like phase at around 0.9 GPa. I have mentioned that the results show differences strongly depending on the salts dissolved in the aqueous solutions.