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.     

Vibrational spectra of the ammonia halides NH4I and NH4F at high pressures

The vibrational spectra of ammonium iodide NH₄I at pressures up to 4.1 GPa and ammonium fluoride NH₄F at pressures up to 4.7 GPa were investigated by inelastic incoherent neutron scattering. The pressure dependences of the transverse optical translational and librational modes were obtained. The behavior of the rotational potential barrier for the ammonium ion as a function of the lattice parameter for disordered and ordered cubic phases of ammonium halides with CsCl type structure were calculated. The results obtained confirm that the transition from an orientationally disordered cubic phase into an ordered cubic phase in ammonium halides occurs at close critical values of the positional parameter of hydrogen (deuterium).     

Neutron diffraction study of high-pressure-induced structural changes in the ammonium halogenides ND4Br and ND4Cl

Structural changes in the deuterated ammonium halogenides ND₄Br and ND₄Cl have been studied by neutron time-of-flight diffraction up to pressures of 45 and 35 kbar, respectively. Data on the equations of state and pressure dependence of the deuterium position parameter have been obtained. A comparison with the hydrogen-containing analogs showed that isotopic substitution of deuterium for hydrogen affects only slightly the compressibility of the systems under study, although the effect is noticeable for ND₄Cl. It has been established that the order-disorder transition from the phase with random deuterium distribution (CsCl cubic structure, space group Pm3m) to the ordered phase (same structure, space group ) occurs in both compounds at the same critical value of the position parameter u=0.153±0.002, which is apparently the same for all ammonium halogenides, and, possibly, for other systems of this structural type as well. Fiz. Tverd. Tela (St. Petersburg) 40, 142–146 (January 1998)     

Phases of silicon at high pressure

X-ray diffraction studies are reported on silicon at pressures up to 250 kbar (25 GPa). A transition to the β-Sn structure (II) initiates at 112 ± 2 kbar and two phases (I + II) coexist to 125 ± 2 kbar. At 132 ± 2 kbar a new phase (V) initiates, and the transition is complete at 164 ± 5 kbar. This phase persists to 250 kbar. Its structure is tentatively assigned as primitive hexagonal with c/a = 0.941 ± 0.002 at 250 kbar. On release of pressure, the sequence is V → (V + II) (145 - 110 kbar) → II → (II + III) (108 - 85 kbar) → III, the last phase persisting to room pressure.     

Neutron diffraction investigation of the structural transition in HgSe1−x Sx ternary mercury chalcogenide systems at high pressures

The structure of HgSe_1?x S_x ternary mercury chalcogenides at high pressures up to 35 kbar is investigated by neutron diffraction. It is found under pressure, that the HgSe_1?x S_x compounds undergo, a phase transition from the cubic sphalerite-type to the hexagonal cinnabar-type structure, which is accompanied by a jump-wise change in the unit cell volume and interatomic distances. The unit cell parameters and the positional parameters of Hg and Se (S) atoms in the high-pressure hexagonal phase are determined. A two-phase state is revealed in the phase transition region.     

The effect of pressure on the Raman spectrum of NH4Cl

The Raman spectra of NH₄C1 are reported over a very wide pressure range at room temperature and some features of the well-known disorder-order transition as well as the spectra of the ordered phase at high pressures are discussed. The mode Grüneisen parameter has been determined to be equal to 2.1 ± 0.03 for ν₅(TO) in this phase showing that the volume-dependent anharmonicity is relatively large. Above 110kbar, significant spectral changes take place, a large number of lattice modes appear and some internal modes also reflect changes. Since these features closely resemble the ones observed in the newly discovered V of NH₄I, it is concluded that phase V also exists in NH₄Cl. The structure of phase V as well as the mechanism of the IV–V transition are still largely unknown but it is shown that the IV–V transition pressures in the ammonium halides vary linearly with the anionic radii.     

Line width transitions in the deuteron magnetic resonance of polycrystalline ND4Cl,ND4Br,and ND4I

The line width of the deuteron magnetic resonance in polycrystalline ND₄Cl, ND₄Br, and ND₄I has been measured from 300°K down to 115°K. Below 200, 172, and 140°K in ND₄Cl, ND₄Br, and ND₄I, respectively, the line rapidly broadens. In addition, the I ? II transition of ND₄I causes a change of line width. The observed line widths agree with those calculated by the present writers from theVan Vleck second moment formula assuming a Gaussian line shape. For this agreement, in the phases II and III of ND₄Cl and in the phase I of ND₄I the deuteron-halogen interactions should be taken into account, whereas in the phases II and III of ND₄Br and ND₄I they should be omitted.     

The study of the vibrational spectra of NH4Cl and NH4Br at high pressures

The vibrational spectra of NH₄Cl at pressures of up to 2.6 GPa and of NH₄Br at pressures of up to 7 GPa are investigated by the method of inelastic incoherent scattering of neutrons. It is found that a linear baric dependence of a librational mode changes its slope above the pressure of transition from a disordered cubic phase into an ordered cubic phase with a CsCl-type structure. The slope of the baric dependence of the transverse optical translational mode remains invariant. Estimates for the Grüneisen parameters are presented and the shape of the potential function is calculated in the one-dimensional approximation for librational vibrations in disordered and ordered cubic phases with a CsCl-type structure. It is shown that the phenomena observed are attributed to the high anharmonicity in the disordered phase.     

Vibrational spectroscopy at very high pressures. part 34.1 Raman spectra of polymorphs of mercuric cyanide

Phase transitions have been observed in mercuric cyanide near 2.5, 8, 19 and 60 kbar. The first four phases have been characterized by Raman spectroscopy. Phase II is closely related to the molecular parent phase I. The coordination at mercury appears to increase to four on entering phase III and phase IV has a spectrum consistent with the cubic structure related to anti-cuprite adopted at s.t.p. by Cd(CN)₂. Above 60 kbar deep brown Hg(CH)₂ V is formed irreversibly: it contains C =N rather than C≡N bonds.     

Energy - dispersive X - ray diffraction of the ammonium - halides under pressure

Abstract

The structure of the ammonium halides NH₄X (X = Cl, Br, I) has been studied under pressure up to 40 GPa by energy dispersive X-ray diffraction using synchrotron radiation. Equations of state and a discussion on the possible structure of phase V will be presented.     

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.     

Theoretical investigation of structures and compositions of double neon-methane clathrate hydrates,depending on gas phase composition and pressure

The region of existence of neon clathrate hydrates is an actual problem of hydrate chemistry. The current work presents theoretical study of the equilibrium formation conditions of pure neon clathrate hydrates and double clathrate hydrates of neon-methane mixture. The structures and properties of double clathrate hydrates were described within the scope of the previously developed molecular clathrate hydrate model that takes into account the influence of guest molecules on the host lattice, interaction of guest molecules between themselves, and the possibility of multiple filling of host lattice cages by guest molecules. The model makes it possible to find an equilibrium state and thermodynamic properties of clathrate hydrates at given values of p and T. In the present work, we considered the properties of double clathrate hydrates in the range of pressures from 0 to 4 kbar at 250 K. The results of modeling have shown that the mass fraction of neon in double clathrate hydrate of Ne and CH₄ mixture of cubic structure I (sI) can reach 26%, and 22.5% in double hydrate of cubic structure II (sII) even at a low methane concentration (1%) in gas phase, at high pressure. It is shown that in double clathrate hydrates of the Ne and CH₄ mixture at high pressures, phase transition sII-sI can occur.     

Phase diagram of GaAs

Abstract

Optical measurements in the diamond anvil cell (DAC) as well as thermodynamics, show cubic GaAs I to be unstable at 300 K, at 13 GPa. The thermodynamic phase line from GaAs I to the high pressure (H.P.) form(s) is at 11 ± 2 GPa. Large hyteresis makes the actual I→II transition observable only at 17.5 ± 1 GPa.     

Neutron diffraction study of the pressure-driven structural transition in the HgTe<Subscript>0.85</Subscript>S<Subscript>0.15</Subscript> ternary compound

The structure of the HgTe_0.85S_0.15 ternary mercury compound was studied by neutron diffraction at high pressures of up to 40 kbar. A phase transition from the cubic (sphalerite-type) to the hexagonal (cinnabar-type) structure was established to occur with increasing pressure and to be accompanied by an abrupt change in the unit-cell volume and interatomic distances. The unit cell parameters, the positions of the Hg and Te/S atoms in the hexagonal cinnabar phase, and their pressure dependences were found.     

Transition energies of ND4Cl and ND4Br in the transition I⇄II

The energy of the transition I?II of ND₄Cl and ND₄Br was measured by using a direct differential calorimetric method. The results 1024 cal/mole for ND₄Cl and 847 cal/mole for ND₄Br were obtained. The accuracy was estimated to be about 2%. The approximate transition temperatures 169°C for ND₄Cl and 118°C for ND₄Br were observed.     

Electronic structure of platinum at ultrahigh pressure

Abstract

Platinum is studied, theoretically, under very high compression. The calculated equation of state is found to agree well with the recent experimental data. At V/V₀ = 0.4, where V₀ is the experimental equilibrium volume, we find a transition from the face centered cubic structure (fcc), found at ambient pressure, to the body centered cubic structure (bcc). The calculated transition pressure is 26 Mbar. The stabilization of the bcc structure is explained by the eigen value sum.     

Raman spectroscopic and optical absorption study of the high pressure behaviour and polymorphism in KO2

Abstract

Raman scattering, visible absorption, and optical observation studies have been made on polycrystalline potassium superoxide (KO₂) in a diamond anvil cell as a function of pressure and temperature. Three new phases are observed. With increasing pressure at 298 K, KO₂ transforms from the well known modified CaC₂ structure (Phase II), to two new phases (VII, and VIII). The transformation from III to VII occurs at about 3.2GPa. Phase VII transforms to phase VIII at about 4.4GPa. However, in some samples phase VII does not occur and phase II transforms directly into phase VIII at about 4.2 GPa. These structural transformations are indicated by marked changes in the Raman spectrum. The transitions out of phase II are also marked by a discontinuous red shift in the optical absorption edge. From optical observations we have also determined the pressure and temperature dependence of the transitions from phase II to the high temperature cubic (B1) phase I as well as from the high pressure phases VII and VIII to a new nonbirefringent phase IX. This new phase IX has the cubic B2 (CsCl) structure as is shown by our recent X-ray synchrotron experiments.     

High-Pressure phase diagram of LiInSe2

Abstract

The phase diagram of LiInSe₂ is investigated by electrical and x-ray measurements. The transition from the cubic high pressure phase to the α-NaFeO₂ structure upon heating is followed by a second transition to the CuFeS₂ structure.     

On the high-pressure phase transition in

X-ray diffraction (XRD) experiments have been carried out on quartz-like GaPO₄ at high pressure and room temperature. A transition to a high pressure disordered crystalline form occurs at 13.5 GPa. Slight heating using a YAG infrared laser was applied at 17 GPa in order to crystallize the phase in its stability field. The structure of this phase is orthorhombic with space group Cmcm. The cell parameters at the pressure of transition are a =7.306?, b =5.887? and c =5.124?. Received: 7 October 1997 / Received in final form: 17 November 1997 / Accepted: 18 November 1997     

The Raman spectra of NH4I at high pressures. The existence of a new phase in the ammonium halides

The Raman spectra of NH₄I and ND₄I show significant changes at higher pressures which can only be interpreted in terms of the existence of a new phase. This is not an intermediate phase occurring between phases II and IV, but is observed upon further compression of the ferro-ordered phase IV. Preliminary measurements show that the same phase transition occurs in NH₄Br and NH₄Cl, but at significantly higher pressures than the one in NH₄I.