Volume changes of castor oil during its transformation to the high pressure phase

Abstract

Time dependencies p(t)_V,T, nd V(t),_P,T, at room temperature as castor oil phase transition indicators were investigated. The time after which the transition takes place, within the pressure range from 0.36 up to 1.05 GPa, strongly depends on pressure. Its minimum, at about 0.6 GPa is equal to 15 hours. Under the same conditions of experiment the largest change in volume (about ?2%) was detected. The relative changes of volume on pressure for the normal state of castor oil and for its new high pressure state have been found to be approximately the same. A large hysteresis of volume changes after the phase transition has also been observed.     

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.     

Ultrahigh pressure equation of state of CaS to 1.5 Mbar

Abstract

Energy Dispersive X-ray Diffraction (EDXD) was performed at room temperature to gather structural data on CaS between approximately 1.7 GPa to nearly 150GPa. In these experiments, CaS retained the B1 structure up to approximately 40 GPa above which it began to transform to the B2 structure. The B2 structure remained stable to the highest pressure reached, 149 GPa, where the relative volume V/V₀ was 0.490. Previous studies on CaS extended only up to 52 GPa, which is barely 10 GPa after the B1 phase changes to the B2 structure. Thus it was not possible to accurately extrapolate the equation of state (EOS) for the B2 phase region to significantly higher pressures. In the present study EOS data for CaS was collected to 150 GPa and no other structural change was observed. EOS parameters for the B1 and B2 phase regions agree well with values reported in the literature.     

Structure and SHG of the high pressure phase IV of HgBr2

Angle dispersive X-ray diffraction experiments on mercuric bromide (HgBr₂) under high pressure up to 11.0 GPa were carried out at room temperature using synchrotron radiation. In addition to the already known four different phases of HgBr₂ in the pressure-temperature range of p<4.5 GPa, 90<T<600 K our observations show the existence of a new phase (V) above 9.0 GPa and, together with published material, support the phase transition sequence: (I) orthorhombic-(II) orthorhombic-(III) monoclinic-(IV) trigonal-(V) trigonal/hexagonal. The structure of phase IV with space group symmetry P3 has been determined from powder diffraction data. The observation of second-harmonic-generation signals confirms the absence of an inversion center. The structure of phase IV is a commensurately modulated variant of the CdI₂ type layer structure, where part of the Hg atoms are displaced from the centers of the HgBr₆ octahedra by a much as 0.76 Å in the direction perpendicular to the layers.     

Anomalies in the velocity of longitudinal ultrasonic waves in cesium under the pressure up to 5 GPa

Abstract

The velocity of longitudinal ultrasonic waves, ν₁,(P), in polycrystalline cesium was measured at 293 ±1K in the pressure interval 0–5 GPa. v ₁(P) alterations at BCC-FCC phase transition at 2.3 GPa and at the electronic-structure transformation at 4.3 GPa were obtained. Decrease of v ₁(P) to 4.3 GPa after a maximum at ～3.8 GPa were found, that gave evidence to the appearance of a corresponding soft mode in the FCC-Cs phonon spectrum. The peculiarities of dependence v ₁(P) correlate with s-conduction electrons promotion to the empty d-band in accordance with the theoretically predicted continuous electronic s-d transformation in Cs.     

The effect of pressure on ftir spectra of ammonium halides

Abstract

FTIR spectra of NH₄Cl, NH₄Br and NH₄I have been measured in the range of 600 - 4800 cm^?1 at pressures up to 30 GPa at 135 K. The spectra show significant changes at the phase transition IV - V, particularly the splitting of v₄ in phase V is clearly demonstrated.     

Electrophysical Properties of ZnAs 2 and CdAs 2 at Hydrostatic Pressure up to 9 GPa

Specific electroresistance and Hall coefficient on oriented ZnAs 2 and CdAs 2 single crystals in the region of room temperatures at hydrostatic pressure up to 9 GPa were measured. In p -ZnAs 2 specific electroresistance falls for one order of magnitude with the increase of pressure, and Hall coefficient falls for two orders in magnitude, and at P =7 GPa specific electroresistance and Hall coefficient come out to a saturation. Under mentioned conditions the phase transition in investigated p -ZnAs 2 samples was not observed, in all probability it occurs under the pressure P >10 GPa. Two groups of n -CdAs 2 samples oriented on [1 0 0] and [0 0 1] directions were investigated. The reversible structural phase transition was observed in investigated n -CdAs 2 samples at P =5.5 GPa from the dependencies of specific electroresistance 𝜌 ( P ) and Hall coefficient R H ( P ). On the basis of the values of concentrations and mobilities before and after phase transition a conclusion was made that semiconductor-semiconductor transition takes place in n -CdAs 2 . Maxima that earlier weren't observed, were detected on dependencies 𝜌 ( P ), R H ( P ) at P =1.8 GPa and at P =3 GPa.     

Equation of state and thermal expansivity of LiF and NaF

The high-pressure and high-temperature behaviors of LiF and NaF have been studied up to 37 GPa and 1000 K. No phase transformations have been observed for LiF up to the maximum pressure reached. The B1 to B2 transition of NaF at room temperature was observed at ～28 GPa, this transition pressure decreases with temperature. Unit-cell volumes of LiF and NaF B1 phase measured at various pressures and temperatures were fitted using a P–V–T Birch–Murnaghan equation of state. For LiF, the determined parameters are: α₀ = 1.05 (3)×10^?4 K^?1, dK/dT = ?0.025 (2) GPa/K, V ₀ = 65.7 (1) Å³, K ₀ = 73 (2) GPa, and K′ = 3.9 (2). For NaF, α₀ = 1.34 (4)×10^?4 K^?1, dK/dT = ?0.020 (1) GPa/K, V ₀ = 100.2 (2) Å³, K ₀ = 46 (1) GPa, and K′ = 4.5 (1).     

High Pressure X-Ray Absorption and Diffraction Study of InAs

We have performed X-ray absorption (XAS) and diffraction (XRD) measurements at high pressure on samples of powdered InAs, up to 50 and 80 GPa, respectively. In the lower pressure range, our data are consistent with the following structural sequence: Zincblende M NaCl M Cmcm . The first order transition from the semiconducting Zincblende phase to the metallic NaCl phase is clearly seen by the shift in the absorption onset at the As K-edge and the strong modifications of the extended X-ray absorption fine structure (EXAFS) due to the changes in the local structure from a 4-fold to a 6-fold coordinated environment. XAS shows the high pressure phase to be locally site-ordered. The diffraction data, analized by Rietveld fitting, gives a volume discontinuity of j V/V 0 ～0.18 for the first order transition. There is no apparent volume discontinuity associated to the NaCl M Cmcm transition.     

High-pressure phases of plutonium monoselenide studied by X-ray diffraction

Abstract

Plutonium monoselenide was studied under high pressure up to 47 GPa, at room temperature, using a diamond anvil cell in an energy dispersive X-ray diffraction facility. At ambient pressure, PuSe has the NaC1-type (B1) structure. The compound has been found to undergo a second-order crystallographic phase transition at around 20 GPa. This phase can be described as a distorted B1 structure, with a rhombohedral symmetry. PuSe transforms to a new phase at around 35 GPa, which can be indexed in the cubic CsCl-type (B2). The volume collapse at this phase transition is 11%. When releasing pressure, we observed a strong hysteresis to the inverse transformation down to 5 GPa. From the pressure-volume relationship, the bulk modulus has been determined to B ₀ = 98 GPa and its pressure derivative as B ₀ = 2.6. These results are compared to those obtained with other actinide monmictides and monochalcogenides.     

High-pressure X-ray diffraction study of the δ-phase in the uranium-neptunium binary system

Abstract

High pressure X-ray diffraction studies were performed at room temperature on a uranium-neptunium binary alloy (U_{0, 40} Np_0.60) using a diamond anvil cell in an energy dispersive facility. The sample maintained its simple cubic phase up to 62 GPa (highest pressure reached in This experiment). The bulk modulus and its pressure derivative were determined to be B₀ = 82 (2) GPa and B₀′ = 9.4 (1.3), from the experimental data in the pressure range 0–20 GPa. The present results are compared with those obtained by the same techniques used for uranium and neptunium.     

X-ray studies of the semiconductors snas,InTe, TlS and TISe UP TO 43 GPa

Abstract

The structures of the semiconductors SnAs, InTe, TlS and TlSe have been investigated using high-pressure (HP) diffraction technique-a gasketed diamond anvil cell (DAC)'. The pressure was measured by ruby fluorescence technique. The first order reversible transition from NaCl to CsCl structure was found in SnAs with volume discontinuity 5% the two-phase area extends from 32 to 43 GPa. The volume change V/v,(P) of the SnAs is shown in Figure 1. In agreement with the gomology rule3, the same pressure effect has been found in Sn_xSb1?x atP=9 GPa2.     

Structure and high pressure behaviour of organic crystals based on aromatically substituted 1,3,4-oxadiazoles

The crystalline structure of a new compound containing the 1,3,4-oxadiazole moiety, 4-(5-methyl-1,3,4-oxadiazole-2yl-)-N,N′-dimethyl-phenylamine (MODPA) was determined. It shows a monoclinic structure with space group P2₁/c and lattice parameters: a=1.02997(6), b=0.64840(4), c=1.58117(10) nm and β=99.4820(10)°. To study the intermolecular interactions in oxadiazole containing organic crystals, X-ray studies on MODPA and 2,5-diphenyl-1,3,4-oxadiazole (DPO) were performed up to 5 GPa at room temperature. The Murnaghan equation of state is used to describe the compression behaviour of both substances. From these results, the bulk modulus and its pressure derivative were determined. The values obtained are: K₀=6.3 GPa and K′₀=6.8 for MODPA and K₀=7.3 GPa and K′₀=6.7 for DPO. Additionally, measurements under increasing temperature at ambient pressure were carried out to evaluate the thermal expansion coefficient: α=1.8×10⁻⁴ K⁻¹ for MODPA and α=1.9×10⁻⁴ K⁻¹ for DPO.     

Investigations of phase transition, elastic and thermodynamic properties of GaP by using the density functional theory

The phase transition of gallium phosphide (GaP) from zinc-blende (ZB) to a rocksalt (RS) structure is investigated by the plane-wave pseudopotential density functional theory (DFT). Lattice constant a₀, elastic constants c_ij, bulk modulus B₀ and the pressure derivative of bulk modulus B₀' are calculated. The results are in good agreement with numerous experimental and theoretical data. From the usual condition of equal enthalpies, the phase transition from the ZB to the RS structure occurs at 21.9 GPa, which is close to the experimental value of 22.0 GPa. The elastic properties of GaP with the ZB structure in a pressure range from 0 GPa to 21.9 GPa and those of the RS structure in a pressure range of pressures from 21.9 GPa to 40 GPa are obtained. According to the quasi-harmonic Debye model, in which the phononic effects are considered, the normalized volume V/V₀, the Debye temperature θ, the heat capacity C_v and the thermal expansion coefficient α are also discussed in a pressure range from 0 GPa to 40 GPa and a temperature range from 0 K to 1500 K.     

Thorium: Phase transformations and equation of state to 300 GPa

Abstract

Equation of state and phase transformations of thorium metal have been investigated to 300 GPa at 300 K in a diamond anvil cell using energy dispersive X-ray diffraction employing synchrotron source. Phase transformations in the 70–100 GPa range indicative of 5f-electron bonding are observed and thorium metal is isostructural with its 4f counterpart cerium at ultra high pressures. The measured static equation of state of thorium to 300GPa (volume fraction V/V _o = 0.40) at 300K is given. At high pressures, the sd to f electronic transfer has significant influence on the measured equation of state of thorium.     

Mechanical and electrochemical characterization of vanadium nitride (VN) thin films

Vanadium nitride (V-N) thin films were grown using a reactive d.c. magnetron sputtering process, from a vanadium target (99.999%) in an Ar/N₂ gas mixture at different deposition bias voltage. Films were deposited onto silicon (1 0 0) and RUS-3 steel substrates at 400 °C. Structural, compositional, mechanical and electrochemical characterizations were performed by X-ray diffraction (XRD), elastic forward analysis (EFA), nanoindentation, electrochemical impedance spectroscopy (EIS), and Tafel polarization curves, respectively. X-ray diffraction patterns show the presence of (1 1 1) and (2 0 0) crystallographic orientations associated to the V-N cubic phase. Nanoindentation measurements revealed that when the bias voltage increases from 0 V to −150 V the hardness and elastic modulus are increased from 11 GPa to 20 GPa and from 187 GPa to 221 GPa, respectively. EIS and Tafel curves showed that the corrosion rate of steel, coated with V-N single layer films deposited without bias voltage, diminishes 90% compared to the steel without this coating. On the other hand, when the V-N coating was deposited at the highest d.c. bias voltage (−150 V), the corrosion rate was greater than in the steel coated with zero-voltage (0 V) V-N films. This last result could be attributed to the formation of porosities produced by the ion bombardment during the deposition process.     

Structure and properties of DLC–MoS2 thin films synthesized by BTIBD method

Diamond-like carbon (DLC)–MoS₂ composite thin films were synthesized using a biased target ion beam deposition (BTIBD) technique in which MoS₂ was produced by sputtering a MoS₂ target using Ar ion beams while DLC was deposited by ion beam deposition with CH₄ gas as carbon source. The structure and properties of the synthesized films were characterized by X-ray diffraction, X-ray absorption near edge structure (XANES), Raman spectroscopy, nanoindentation, ball-on-disk testing, and corrosion testing. The effect of MoS₂ target bias voltage, ranging from −200 to −800 V, on the structure and properties of the DLC–MoS₂ films was further investigated. The results showed that the hardness decreases from 9.1 GPa to 7 GPa, the Young?s modulus decreases from 100 GPa to 78 GPa, the coefficient of friction (COF) increases from 0.02 to 0.17, and the specific wear rate coefficient (k) increases from 5×10⁻⁷ to 5×10⁻⁶ mm³ N⁻¹ m⁻¹, with increasing the biasing voltage from 200 V to 800 V. Also, the corrosion resistance of the DLC–MoS₂ films decreased with the raise of biasing voltage. Comparing with the pure DLC and pure MoS₂ films, the DLC–MoS₂ films deposited at low biasing voltages showed better tribological properties including lower COF and k in ambient air environment.     

High volumetric hydrogen density phases of magnesium borohydride at high-pressure: A first-principles study

The previously proposed theoretical and experimental structures,bond characterization,and compressibility of Mg(BH 4) 2 in a pressure range from 0 to 10 GPa are studied by ab initio density-functional calculations.It is found that the ambient pressure phases of meta-stable I4 1 /amd and unstable P-3m1 proposed recently are extra stable and cannot decompose under high pressure.Enthalpy calculation indicates that the ground state of F 222 structure proposed by Zhou et al.[2009 Phys.Rev.B 79 212102] will transfer to I4 1 /amd at 0.7 GPa,and then to a P-3m1 structure at 6.3 GPa.The experimental P 6 1 22 structure(α-phase) transfers to I4 1 /amd at 1.2 GPa.Furthermore,both I4 1 /amd and P-3m1 can exist as high volumetric hydrogen density phases at low pressure.Their theoretical volumetric hydrogen densities reach 146.351 g H 2 /L and 134.028 g H 2 /L at ambient pressure,respectively.The calculated phonon dispersion curve shows that the I4 1 /amd phase is dynamically stable in a pressure range from 0 to 4 GPa and the P-3m1 phase is stable at pressures higher than 1 GPa.So the I4 1 /amd phase may be synthesized under high pressure and retained to ambient pressure.Energy band structures show that they are both always ionic crystalline and insulating with a band-gap of about 5 eV in this pressure range.In addition,they each have an anisotropic compressibility.The c axis of these structures is easy to compress.Especially,the c axis and volume of P-3m1 phase are extraordinarily compressible,showing that compression along the c axis can increase the volumetric hydrogen content for both I4 1 /amd and P-3m1 structures.     

Pressure induced phase transformations and band structure of different high pressure phases in tellurium

We report here high-pressure x-ray diffraction (XRD) studies on tellurium (Te) at room temperature up to 40 GPa in the diamond anvil cell (DAC). The XRD measurements clearly indicate a sequence of pressure-induced phase transitions with increasing pressure. The data obtained in the pressure range 1 bar to 40 GPa fit five different crystalline phases out of Te: hexagonal Te (I) → monoclinic Te(II) → orthorhombic Te (III) → Β-Po-type Te(IV) → body-centered-cubic Te(V) at 4, 6.2, 11 and 27 GPa, respectively. The volume changes across these transitions are 10%, 1.5%, 0.3% and 0.5%, respectively. Self consistent electronic band structure calculations both for ambient and high pressure phases have been carried out using the tight binding linear muffin tin orbital (TB-LMTO) method within the atomic-sphere approximation (ASA). Reported here apart from the energy band calculations are the density of states (DOS), Fermi energy (E _f) at various high-pressure phases. Our calculations show that the ambient pressure hexagonal phase has a band gap of 0.42 eV whereas high-pressure phases are found to be metallic. We also found that the pressure induced semiconducting to metallic transition occurs at about 4 GPa which corresponds to the hexagonal phase to monoclinic phase transition. Equation of state and bulk modulus of different high-pressure phases have also been discussed.     

Crystal Structure Transformations of Rare-Gas Solids Under Pressure

Pressure-induced structural changes in solid krypton (Kr) and xenon (Xe) have been studied using angle dispersive X-ray diffraction in a diamond-anvil cell (DAC) up to 50 GPa. The analysis of the results shows that in solid Kr (Xe) the phase transition from fcc to hcp starts below 3.2 GPa (1.5 GPa). Albeit the hcp/fcc ratio increases under pressure, both phases coexist up to the highest pressure reached in this study. Room temperature (RT) equations of state (EOS) are determined.