Structural study of ND4I at high pressures and low temperatures

Abstract

Structure, positional, and thermal parameters of ND₄I were studied at high pressures up to 90 kbar and low temperatures down to 10 K using time-of-flight neutron diffraction. The phase transition from a disordered CsCI-type cubic phase ND₄I(II) into a recently discovered high pressure phase ND₄I(V) was observed at P = 80(5) kbar. Surprisingly, the structure of the high pressure phase V was found to bear a strong resemblance to that of the ambient pressure, low-temperature phase III - tetragonal structure with an antiparallel ordering of ammonium ions, space group P4/nmm. The critical value of the deuterium positional parameter corresponding to the II-V transition is close to the one for the phase transition between the disordered and ordered CsCl-type cubic phases II and IV in other ammonium halides.     

Pressure-induced phase changes of cubic monoolein - Cytochrome C mesophases

Incorporation of the protein cytochrome c (cyt c) into the hydrated bicontinuous Ia3d cubic mesophase of monoolein (MO) was investigated within a wide range of pressures by small-angle X-ray scattering (SAXS). We found that incorporation of cyt c into the cubic phase of MO has a drastic effect on the structure and pressure stability of the system: At high pressures, the lipid systems with less than 0.2 wt.% embedded protein undergo a transition to a fluid lamellar phase with smaller partial molar lipid volume. Incorporation of cyt c at levels above 0.2 wt.% promotes the formation of a new cubic phase, probably a cubic micellar phase of space group P4₃32 (Q²¹²) whose pressure stability rises with increasing protein content.     

Ab initio study of phonons and structural stabilities of the perovskite-type MgSi{O_3}

Using the local-density approximation, calculating the Hellmann-Feynman forces, applying the direct method and deriving the phonon dispersion relations, the stability of the perovskite-like structures of MgSiO₃ at T =0 have been studied. The cubic Pmm phase shows a dispersion-less soft phonon branch spreading from the R to M points of the cubic Brillouin zone. This soft branch persists up to high pressures of 150 GPa. The low-symmetry phases I4/mcm and Imma, P4/mbm can be considered as a result of the soft mode condensation at the M and R points, respectively. These phases prove to be unstable at T =0. The experimentally observed Pmnb phase is a consequence of the intersection of Imma and P4/mbm space groups. Thus, it can be regarded as a simultaneous condensation of two soft modes: one at the M and a second at the R high-symmetry points of the cubic Brillouin zone. The phonon dispersion relations of Pmnb show that this phase is stable and its optical phonons appear above 4.0 THz only. Received 15 October 1999 and Received in final form 14 January 2000     

Structural changes in chlorpropamide at high pressure

The crystalline structure of a molecular crystal of chlorpropamide C₁₀H₁₃ClN₂O₃S is studied by X-ray diffraction at high pressures of up to 4.2 GPa at room temperature. Under normal conditions the structure of chlorpropamide has orthorhombic symmetry with the space group P2₁2₁2₁. At high pressures P > 1.2 GPa, a polymorphic phase transition into the monoclinic phase with the space group P2₁ is observed. The baric dependences of the lattice parameters and unit-cell volumes are obtained for both phases of chlorpropamide.     

Phase transitions in 3,5-dichloropyridine

Abstract

Structural phase transitions in 3,5-dichloropyridine crystal have been studied by calorimetry (150–300 K), X-ray diffraction (300 K), ³⁵Cland ¹⁴N nuclear quadrupole resonance (75–300 K) and vibrational spectroscopy (20–300K). Three phases have been shown to exist: phase I, between 338 and 287.5 K, phase 11, between 287.5 and 167.5 K, and phase 111, below 167.5 K. Phase I is characterised by the space group P 2,/m and Z = 2 whereas phases I1 and 111 remain centrosymmetric but double the unit cell. All the phases are ordered and contain crystallo- graphically equivalent molecules. The I ? I1 transition is of first order and probably of the reconstructive type while the II ? III transition is believed to be displacive. Both transitions are monitored by predominantly R′_x and R′_z librational motions.     

Special points on the phase diagrams of langbeinites. The case of (x,T)-phase diagram of mixed K2xRb2(1−x)Cd2(SO4)3 crystals

We have studied special points that appear on the (x, T)-phase diagram of mixed K_2xRb_2(1?x)Cd₂(SO₄)₃ crystals. This phase diagram has been constructed issuing from the changes in domain structure occurring in the course of phase transitions. We have found that two triple points on the (x, T)-phase diagram of the above solid solutions are very close to each other in the phase space and can change their places or transform into a quadruple point. These special points are associated with disappearance of ferroelectric phases with the symmetries P1 and P2₁ taking place when Rb₂⁺ are substituted with K₂⁺ ions.     

Twisted-grain-boundary (TGB) phases: nanostructured liquid-crystal analogue of Abrikosov vortex lattices

Twisted-grain-boundary (TGB) phases shown by some liquid-crystalline materials have properties common to those of both smectic and cholesteric phases. Following analogy between liquid crystals and super conductors proposed by de Gennes [Solid State Commun., 10, 753 (1972)], Renn and Lubensky [Phys. Rev. A, 38, 2132 (1988)] theoretically predicted a chiral analog of the smectic A (SmA*) phase, which is now known as TGBA phase. The TGBA phase was experimentally observed for the first time by Goodby et al. [Nature, 337, 449 (1989)] in the chiral homologous series of ferroelectric liquid-crystal material R- and S-1-methylheptyl 4′-[(4′′-n-alkoxyphenyl)propionoyloxy]-biphenyl-4-carboxylates (nP₁M₇), with n?=?13,14,15. Since then, more than a hundred pure and mixed systems showing TGBA phase have been found. Later, Renn derived a mean-field phase diagram based on the chiral Chen-Lubensky model, and predicted two more TGB phases, namely TGBC and TGBC?. These two phases have also been experimentally observed in many systems, and in a few, the phase diagram is similar to that predicted by Renn. Unlike the TGBA phase, several theoretical models have been proposed for TGBC and TGBC? phases, and it remains to be ascertained whether all the types of proposed TGBC/TGBC? structures experimentally exist. A review of the theoretically predicted and experimentally observed TGB phases is given in the present article. Some recently observed novel optical textures of the TGB phases are also reported.     

Phase transitions of sulfur at high pressure: Influence of temperature and pressure environment

Abstract

Phase transitions of orthorhombic sulfur were investigated above 10 GPa by Raman spectroscopy using red light excitation. Transitions into several phases that have been reported in previous studies using green light excitation, are confirmed. The phase behaviour is observed to depend strongly on the preparation method. In the presence of a pressure transmitting medium (methanol/ethanol, 4:1), a sequence of phases α-S₈ → [intermediate phase (“ip”) + S₆] → [S₆ + high pressure-low temperature phase (“hplt”)] is described and characterized. Without the use of a pressure transmitting medium, the phase sequence α-S₈ → [“ip” + “hplt”] + “hplt” is observed. In addition, contributions of amorphous sulfur are detected around 10 GPa, i.e. at pressures below the transformation of α-S₈ into the above-mentioned phases. Characteristic Raman spectra of the different phases are extracted and documented over a wide pressure range.     

On the magnetic space group and Raman tensors of antiferromagnetic RbCoF3

The magnetic unit cell of the antiferromagnetic RbCoF₃ is identified theoretically, and the magnetic space group is found to be of type IIIb Shubnikov. The study of the splitting of the fundamental energy level of the magnetic ion Co²⁺(⁴F) at the center of BZ is presented, using the theory of crystalline field adapted for magnetic crystals. The symmetries of the electronic Raman activities are determined by group theory, and the respective Raman tensors are presented. Comparison between the theory and the existing experimental data is discussed.     

Induced ferrielectricity in antiferroelectric liquid crystals

The behaviour of the antiferroelectric SmC_A liquid crystal phase under applied electric field is discussed theoretically. The phase diagram involving the SmA, SmC_A and SmC _A ^* phases is worked out and shown to exhibit a Lifshitz critical point. The deformation of the bilayer structures induced by the field transforms the SmC_A phases into a ferrielectric phase whose specific configuration is described. Received: 23 October 1997 / Revised: 8 April 1998 / Accepted: 14 July 1998     

Microphase separation in multiblock copolymers

The phase diagram of a multiblock copolymer containing domains of coexistence of phases having different superlattice symmetries with a density of monomers of a given type that varies periodically in space. The parameters of such superlattices are calculated in the mean-field approximation, and it is shown that their wave vector varies continuously with the temperature. Zh. éksp. Teor. Fiz. 112, 332–341 (July 1997)     

Structural aspects of the antiferroelectric-paraelectric phase transition in double perovskite Pb2MgWO6 at high pressures and temperatures

The crystal structure of antiferroelectric Pb₂MgWO₆ has been studied using neutron diffraction at high pressures to 5.4 GPa at room temperature and energy-dispersive X-ray diffraction at high pressures to 4 GPa in the temperature range 300–400 K. At normal conditions, in Pb₂MgWO₆, there is an antiferroelectric phase with the crystal structure described by the orthorhombic symmetry with space group Pnma. At temperature T = 313 K and normal pressure or at room temperature and pressure P ～ 0.9 GPa, the crystal under-goes a structural phase transition to the cubic phase with space group $Fm\bar 3m$ (paraelectric phase). The temperature and pressure dependences of the lattice parameters, unit cell volume, and interatomic bond lengths have been obtained, and the thermal expansion coefficients and the bulk moduli have been calculated for the antiferroelectric and paraelectric phases of Pb₂MgWO₆.     

Structural study of ND4Br at high pressure

Abstract

A structure of ND₄Br has been studied at pressures up to 9 GPa by means of time-of-flight neutron diffraction. A phase transition to the high pressure phase V was observed at P=8·2(5)GPa. It was found that the phase V has a tetragonal structure with an antiparallel ordering of ammonium ions, space group P4/nmm which is in strong resemblance with low temperature modification ND₄Br(III). Deuterium positional parameter as a function of pressure was obtained.     

New phases of zinc under pressure from first principles

A procedure for finding the many stable phases of an element under hydrostatic pressure p is discussed and applied to zinc. Five new phases are found with several different Bravais symmetries under the constraint of one atom per cell and their stabilities as functions of p calculated. The procedure seems generalizable to find all the phases in all Bravais symmetries in a given range of pressure. In particular fcc Zn, which is unstable at p = 0, is shown to be very stable above 320 kbar to at least 1000 kbar. In agreement with Müller et al. [Phys. Rev. B 60, 16448 (1999)], a rhombohedral (rh) phase is found to be stable at p = 0, and several more rh phases are found at pressures up to 320 kbar. The Gibbs free energies of all phases are evaluated as functions of p, and the pressures of thermodynamically favored phase transitions are found. The behavior of Zn is compared to similar behavior of vanadium, which also shows stable rh phases and cubic phase instability in a range of pressure; the bcc phase instability is also healed by additional pressure.     

High-pressure disproportionation phases of CO2 and CO

First-principles enthalpy minimization simulations with target pressures suggest that dense-packed disproportionation phases of CO₂ and CO can be transformed from selected molecular precursors under hydrostatic compression. In transformations, electrons are pushed from oxygen to carbon atoms to form electronic disproportionation phases with their chemical formulas of C^?2O${}_2^{+}$ and C^?O⁺. Simulations indicate that the space packing properties of reactive groups in a molecular crystal precursor, including the orientation and the number of bonding partners of each reactive group, mainly control transformation bonding pathways. Phonon dispersion spectra and molecular dynamic simulations confirm the metastability of both electronic disproportionation phases under high pressures.     

A CCD based detector for quick detection of pressure induced phase transitions

Abstract

A high sensitivity CCD based two dimensional angle dispersive X-ray are a detector has been developed for quick detection of pressure induced phase transitions for a laboratory X-ray source such as a rotating anode generator. The performance of this detector was tested by successfully carrying out powder X-ray diffraction measurements on element Pd, intermetallics AuIn², AuGa² and low Z scatterer adamantane (C₁₀H_l6) at ambient conditions. Its utility for quick detection of phase transitions at high pressures with diamond anvil cell (DAC) is demonstrated by reproducing the known pressure induced structural phase transitions in RbI and KI. The importance of this detector system in search of unknown phase transitions has been established by observing new structural phase transitions in In_0.25Sn_0.75 and AuGa₂. Various softwares have also been developed such as interactive location of centre of diffraction rings, radial integration and image enhancement to analyze data from this detector.     

Phase transformations of the amorphous Zn-Sb alloy under high pressures

Abstract

Phase transformations occurring in initially amorphous Zn₄₁ Sb₅₉ semiconductor at pressures to 10 GPa and temperatures to 350C were studied using the measurement of electrical resistance, in situ energy dispersive X-ray diffraction and neutron diffraction on quenched high-pressure phases at ambient pressure. The studied T- P region involves the regions of reversible and irreversible crystallisation and phase transitions between the equilibrium crystalline low-pressure and high-pressure phases.     

New high magnetic field phase of the frustrated <Emphasis Type="Italic">S</Emphasis> = 1/2 chain compound LiCuVO<Subscript>4</Subscript>

Magnetization of the frustrated S = 1/2 chain compound LiCuVO₄, focusing on high magnetic field phases, is reported. Besides a spin-flop transition and the transition from a planar spiral to a spin modulated structure observed recently, an additional transition was observed just below the saturation field. This newly observed magnetic phase is considered as a spin nematic phase, which was predicted theoretically but was not observed experimentally. The critical fields of this phase and its dM/dH curve are in good agreement with calculations performed in a microscopic model (M.E. Zhitomirsky and H. Tsunetsugu, Europhys. Lett. 92, 37001 (2010)).     

Evaluation of the rate constants of dissociative and ternary recombination reactions of argon ions on the basis of the results of ballistic experiments

AbstractExperimental data on the change in the electron density in the wake of a ballistic object traveling at velocities V _∞=3.4–4.9 km/s in argon at pressures p _∞=30–100 Torr are processed and analyzed. A reaction scheme is proposed which takes into account the recombination of charged particles, processes of ionic conversion, and the excited states of the atom. The solution of the equations of a nonequilibrium boundary layer for flow in the wake is used to formulate the inverse problem of determining the rate constants for dissociative recombination Ar ₂ ⁺ +e→Ar+Ar and ternary recombination Ar⁺+e+Ar→Ar+Ar. The “nearest-neighbor” approximation is used to obtain theoretically an expression for the ternary recombination coefficient as a function of temperature and pressure. Numerous solutions of inverse problems and a comparison with experiments demonstrates the validity of the expression obtained for the ternary recombination coefficient. It is shown that this expression is valid for moderate pressures and complements the Pitaevskii result for low pressures and the Langevin result for high pressures. Zh. Tekh. Fiz. 67, 12–18 (May 1997)     

Raman scattering investigation of ferroelastic Sb5O7I crystals

Sb₅O₇I undergoes a displacive phase transition at 481 K where the symmetry is changed fromC _6h ² toC _2h ⁵ . In the low temperature monoclinic phase the crystal is ferroelastic. The polarized Raman spectra of Sb₅O₇I have been measured at various temperatures below and above the phase transition. The frequencies and symmetries of most of the theoretically expected Raman active phonons in the ferroelastic phase have been determined. The observation of a soft mode in the ferroelastic phase which disappears above the phase transition together with the fact that the unit cell of the ferroelastic phase is twice as large as that of the paraelastic structure permits the conclusion that the phase transition results from a phonon instability at the Brillouin zone boundaryM-point of the hexagonal phase. The temperature dependent splittings and intensity changes of several Raman lines are discussed with respect to the ferroelastic property of the crystal and the phase transition.