Pressure dependent Raman studies in the K2Mo2O7·H2O crystal

The pressure dependent Raman scattering in the potassium molybdenum oxide hydrate crystal, K₂Mo₂O₇·H₂O, was measured. The high pressure Raman study showed, that the compound remains in the triclinic structure within the 0.0–3.81 GPa range and undergoes a structural phase transition between 3.81 and 4.13 GPa. This particular phase transition is most likely connected with changes in the Raman spectrum, in which the number of modes associated originally with the stretching vibrations in the MoO₅ and MoO₆ units is increased. However, the phase at atmospheric pressure shows bands due to the presence of only one equivalent site, while in the high-pressure phase, two bands are associated with the stretching modes. Continuing the pressure evolution up to 17.04 GPa, two further phase transitions occurred in this crystal in the 6.3–8.1 GPa and the 12.3–14.0 GPa range, respectively. The Raman spectra measured at about 17.04 GPa presented a crystal structure, which experienced a pre-amorphization with a total loss of all lattice modes. This particular result is indicative that this material may have undergone a complete amorphization at pressures larger than 17.04 GPa. Then, the reversible character in the triclinic P-1 (C_i¹) structure was recovered after releasing the pressure.     

High pressure Raman scattering of dl-leucine crystals

We have obtained the Raman spectra of dl-leucine crystal through a diamond anvil cell for pressures between 0 and 5 GPa. The observation of several anomalies in the regions of both the lattice mode and the internal mode suggests that the crystal undergoes a phase transition between 2.4 and 3.2 GPa. This phase transition is preceded by a gradual change of the molecular conformation of leucine molecules in the unit cell. We show that, up to 5 GPa, the dl-leucine crystal is more stable than the chiral l-leucine crystal because while the former presents only one phase transition in the 2.4–3.2 GPa interval, the latter presents three different transitions, the first of which is observed at 0.46 GPa. Additionally, when pressure is released to 0.0 GPa, the original Raman spectrum is recovered, indicating that the modification at high pressure on dl-leucine crystal is reversible.     

High pressure Raman spectroscopic study of phase transformation in TaO2F

High pressure Raman spectroscopic measurements on nearly zero thermal expansion material TaO₂F are carried out up to 19 GPa. Earlier report of high pressure X-ray diffraction studies shows two phase transitions, one at 0.7 and the other at 4 GPa with rhombohedral (R-3c) structure above 4 GPa, but the structure between 0.7 GPa and 4 GPa remained unclear. In high pressure Raman measurements, a reversible, cubic to rhombohedral phase transformation onsets around 0.8 GPa and gets completed at 4.4 GPa with all four predicted normal modes corresponding to R-3c phase and retaining the structure up to 19 GPa. A mixture of cubic and rhombohedral phases is observed between 0.8 and 4.4 GPa. Optically silent modes in the ambient cubic structure exhibit strong, broad Raman bands due to anionic (O/F) disorder in TaO₂F altering the local symmetry and allowing for first order Raman scattering. On compression, these disorder induced first order Raman bands gradually decrease in intensity and disappear around 4.4 GPa due to inhibition of local distortion caused by anions, and the modes corresponding to the rhombohedral phase appear. This is a clear evidence of disorder-free rhombohedral single phase exists above 4.4 GPa in agreement with the reported HPXRD results. Temperature dependent Raman measurements reveal that the intensities of Raman bands remain almost unchanged with rise in temperature indicating static disorder in TaO₂F. Disorder-induced first order Raman modes at 176, 212, 381 and 485 cm⁻¹ soften with increase in pressure whereas the other modes show low positive Gruneisen parameter. The thermal expansion coefficient calculated using these Gruneisen parameters (−2.91 ppm K⁻¹) is in fair agreement with the reported values (−1 to +1 ppm K⁻¹). On the other hand, all four modes of disorder-free rhombohedral phase show the usual hardening behavior with increase in pressure contributing to positive thermal expansion.     

High pressure Raman spectra of DL-lysine hydrochloride

DL-lysine hydrochloride crystals were studied by Raman spectroscopy under hydrostatic pressure using a diamond anvil cell from ambient pressure up to 9.8 GPa in the spectral range from 1150 to 40 cm⁻¹. Changes in the Raman spectrum were observed in all spectral regions analyzed. In particular, modifications in the lattice modes indicate that the crystal undergoes a phase transition. The classification of the vibrational modes, the behavior of their band wavenumber as a function of the pressure and the reversibility of the phase transitions are also discussed.     

High-pressure Raman spectra of racemate dl-alanine crystals

Raman spectroscopy investigations of dl-alanine crystal under high pressures have been carried out up to 18.0 GPa. For instance, around 1.0 GPa and between 1.7 and 2.3 GPa changes in the Raman profile were observed and associated to conformational changes of the molecules in the unit cell or to a phase transition accompanied to slight conformational change of the molecule through CH and CH₃ groups. Moreover, between 6.0 and 7.3 GPa, the appearance of a new low energy lattice modes and to the splitting of a band assigned to the stretching vibration of the CCH₃ moiety were related to a second phase transition. Finally, changes in lattice modes, red shift of the band associated to CCH₃ stretching and increasing of line-width of the band associated to the wagging of CO₂, between 11.6 and 13.2 GPa, are ascribed to a third phase transition. On release of pressure the original phase was obtained again.     

High pressure effects on benzoic acid dimers: Vibrational spectroscopy

To understand pressure effects on dimer structure stability, Raman and FTIR spectroscopies were used to examine changes in H-bonded dimers of benzoic acid (BA). Experiments were performed on single crystals compressed to 33 GPa in a diamond anvil cell (DAC). Several changes in Raman spectra were observed in the range 6–8 GPa indicating modification in the dimer structure suggesting the lowering of molecular symmetry. Pressure increase above 15 GPa induced strong luminescence and a gradual change of the crystal color from white to yellow/brownish. FTIR measurements on the sample released from 33 GPa indicated formation of a new compound. It is proposed that molecules of this compound are composed of the hydroxyl group associated with alcohol, carbonyl group associated with ketone, and the sp³ hydrocarbon groups. This study demonstrates that sufficient high pressure compression and subsequent decompression can lead to significant changes in the H-bonded dimer structure, including the breaking of bonds and formation of new chemical compound.     

Raman and infrared vibrational modes of tricosane paraffin under high pressure

This paper reports an experimental study about the pressure dependence of the vibrational modes of tricosane paraffin (C₂₃H₄₈) investigated by in situ Raman and infrared spectroscopy in a diamond anvil cell (DAC). The main vibrational bands were followed up to 11 GPa and the observed behavior indicated a conformational disorder induced by pressure, corresponding to a transition from ordered to disordered state of the linear chains from 2 to 5 GPa followed by a transition to an amorphous state under pressure above 5 GPa. However, this behavior was reversible after compression–decompression cycle showing that this linear compound was structurally and chemically stable up to 11 GPa at room temperature.     

High pressure Raman spectra of l-glutamic acid hydrochloride crystal

Semiorganic nonlinear optical single crystal l-glutamic acid hydrochloride has been studied by Raman spectroscopy under high pressure conditions. Our results show that this amino acid crystal presents one structural phase transition at about 2.1 GPa and one molecular conformational change around 7.5 GPa. If we compare such behavior with that of the l-glutamic acid crystal in the same range of pressure we note a great stability for the hydrochloride samples. The chloride ion plays an important role increasing the number of the hydrogen bonds that hold the crystal together and thus, contributing to improve the structural stability of the crystal.     

Carbon based Si- and Cr-containing thin films: Chemical and nanomechanical properties

Si- and Cr-containing C films were deposited by magnetron sputtering combined with CVD onto silicon wafers. The composition and chemical structure were characterized by X-ray Photoelectron Spectroscopy (XPS) and nanomechanical properties by depth-sensing hardness and scratch techniques.The incorporated Si and Cr are preferentially bonded to carbon, in accordance with simplified thermodynamic calculations and as manifested by the XPS chemical shifts. At relatively high Cr- and low Si-content silicides (Cr_xSi) may also form as indicated by X-ray induced Auger electron spectroscopy. The chromium content in the C–Si–Cr films varied between 1 and 55 at% while the silicon content in the same films between 25 and 0 at%. For comparison two-component films of Si–C and Cr–C were also deposited with Si-content up to 42 at% and Cr-content up to 55 at% by varying the input power of the magnetrons.The nanohardness (H) and reduced modulus (E) were higher for all the films than that of the silicon substrate being 10 GPa, 127 GPa, respectively. Interestingly, the H and E of the three-component CrSiC films were almost invariant of the changes of the components' concentration within the indicated range and varied between 13–16 GPa and 120–140 GPa. H and E values for the two-component Cr–C films were much higher, reaching about 22 GPa and 170 GPa, respectively.     

High-pressure Raman scattering of MgMoO4

In this paper, we present results of high-pressure Raman scattering studies in β-MgMoO₄ from atmospheric to 8.5 GPa. The experiments were carried out using methanol–ethanol as pressure medium. By analyzing the pressure dependence of the Raman data (change in the number of lattice modes, splitting of bands and wavenumber discontinuities) we were able to observe a phase transition undergone by the β-MgMoO₄ at 1.4 GPa, which is only completed at ∼5 GPa. The transition was observed to be irreversible and the modifications in the Raman spectra were attributed to the changes in coordination of Mo ions from tetrahedral to octahedral. The transition possibly changes the original C2/m symmetry to C2/m or to P2/c. Implication on the phase transition for similar molybdate structures, such as α-MnMoO₄, is also highlighted.     

High pressure polymorphs of LiCoPO4 and LiCoAsO4

Olivine-LiCoXO₄ (X = P, As) compounds might transform to the denser spinel-type and Na₂CrO₄-type structures under pressure. In this work, the relative energetic stability of the three polymorphs and the pressure of the possible polymorphic transformations are investigated combining experiments and first principles calculations. Olivine-LiCoAsO₄ is predicted to transform to the Na₂CrO₄-like structure at 0.4 GPa and to the spinel structure at 5.8 GPa (0 K). Quenching HP/HT experiments show that olivine-LiCoAsO₄ treated at 6 GPa/1173 K transforms to the spinel-like structure. Computational results indicate that olivine-LiCoPO₄ transforms to the Na₂CrO₄-like form at around 4 GPa (0 K), the latter being the stable form till very high pressures (21.6 GPa). In good agreement with this, olivine-LiCoPO₄ when subjected to 6 GPa/1173 K and 15 GPa/1173 K is converted to the Na₂CrO₄-type polymorph. Crystallographic data of the new compound LiCoPO₄ within the Na₂CrO₄ structural type are provided.     

Infrared and Raman spectroscopic characterization of the hydrogen-bonding network in l-serine crystal

The IR spectra (4000–400 cm⁻¹) of neat and isotopically substituted (ND/OD ≤ 10% D and ≅30% D) polycrystalline l-serine (α-amino-β-hydroxypropionic acid; HO–CH₂–CH(NH₃)⁺–COO⁻) were recorded in the temperature range 300–10 K and assigned. The isotopic-doping/low-temperature methodology, which allows for decoupling of individual proton vibrational modes from the crystal bulk vibrations, was used for estimating the lengths and energies of the different H-bonds present in l-serine crystal. To this end, the frequency shifts observed in both the NH/OH stretching and out-of-plane bending spectral regions (relatively to reference values for these vibrations in non-hydrogen-bonded l-serine molecules) were used, together with previously developed empirical correlations between these spectral parameters and the H-bond properties. In addition, the room-temperature Raman spectrum (4000–150 cm⁻¹) of a single crystal of neat l-serine was also recorded and interpreted. A systematic comparison was made between the spectroscopic data obtained currently for l-serine and previously for dl-serine, revealing that the vibrational spectra of the two crystals reflect well the different characteristics of their hydrogen-bond networks, and also correlate accurately with the different susceptibility of the two crystals to pressure-induced strain.     

Phase equilibrium measurements and thermodynamic modeling of methane alcohol systems at ambient temperature

The (vapor + liquid) equilibrium data for binary systems of (methane + methanol), (methane + ethanol), and (methane + 1-propanol) at ambient temperature over a wide range of pressures, (1 to 8) MPa, were measured using a designed pressure–volume–temperature (PVT) apparatus. The phase composition and saturated density of liquid phase were measured for each pressure. The density of pure methanol, ethanol and 1-propanol was also measured at ambient temperature over a wide range of pressure (1 to 10) MPa. The experimental (vapor + liquid) equilibrium data were compared with the modeling results obtained using the Peng–Robinson and Soave–Redlich–Kwong equations of state. To improve the predictions, the binary interaction parameters were adjusted and the volume translation technique was applied. Both equations of state were found to be capable of describing the phase equilibria of these systems over the range of studied conditions. The Soave–Redlich–Kwong equation of state gave better predictions of saturated liquid densities than Peng–Robinson equation of state.     

Mechanoelectrochemical synthesis and properties of porous nano-Ti–6Al–4V alloy with hydroxyapatite layer for biomedical applications

Formation of porous morphology in nanocrystalline mechanically alloyed and electrochemically etched Ti–6Al–4V biomedical alloy was investigated. The alloy was electrochemically etched in a mixture of H₃PO₄ and HF. The electrochemical etching results in broad range from micro(nano)-macropores formation in the surface layer, with diameter in the range of 3 nm–60 µm. On the etched surface hydroxyapatite was electrochemically deposited by using 0.042 M Ca(NO₃)₂ + 0.025 (NH₄)₂HPO₄ + 0.1M HCl electrolyte. In this way bioactive surface was prepared. The pores in the surface acts as anchors for the hydroxyapatite, which grows inside them. Due to the porous morphology, the etched as well as HA deposited surface is promising for hard tissue implant applications. The nanocrystalline alloy has a nanohardness and Young modulus in the range of 993–1275 HV and 137–162 GPa, respectively.     

Vapor–liquid equilibrium of methane and methane + nitrogen and an equimolar hexane + decane mixture under isothermal conditions

Experimental vapor–liquid equilibrium data of the ternary system composed of methane and an equimolar hexane + decane mixture are reported. The experimental measurements were carried out under isothermal conditions at 258, 273, and 298 K in the pressure range 1–19 MPa. Also, experimental vapor–liquid measurements were carried out for the quaternary system methane + nitrogen and an equimolar hexane + decane mixture, at 258 K in the range 3.5–12 MPa. The results for the ternary system show that the solubility of methane in the equimolar mixture of alkanes increases when the pressure is increased at constant temperature and it increases as the temperature decreases in the whole pressure range studied. For the quaternary system with a constant amount of nitrogen, the solubility of methane in the liquid phase increases as the pressure increases at the studied temperature. The experimental results for the ternary system were satisfactorily correlated with the Peng–Robinson equation of state in the ranges of pressure and temperature studied. The equation of state was used to predict the behavior of the quaternary system using binary interaction parameters. The applicability of the principle of congruence was corroborated by comparing the vapor–liquid behavior of methane in the equimolar hexane + decane mixture with that in pure octane, at the three temperatures studied in this work.     

Growth of gamma glycine crystal and its characterisation

Single crystal of γ-glycine, an organic nonlinear optical material, has been grown by solvent evaporation technique from a mixture of aqueous solutions of glycine and potassium nitrate, lithium nitrate at room temperature. Gamma glycine crystals have been grown up to the dimension of 20 mm × 15 mm × 12 mm. Powder X-ray diffraction of the grown crystal was recorded and indexed. Single crystal X-ray diffraction studies were carried out and the unit cell parameters were compared with the literature values. The γ-phase of glycine is confirmed by single crystal XRD and FTIR spectral analysis. The crystals were characterised by UV–vis–NIR transmission spectrum in the range 200–1100 nm. The second harmonic generation conversion efficiency of γ-glycine crystal was twice the efficiency of KDP crystal. Thermal characteristics of γ-glycine crystals were determined by thermogravimetric analysis (TGA) and differential thermal analysis, which shows the thermal stability of the grown crystals. Dielectric constant and dielectric loss measurements were carried out at different temperatures and frequencies. The microhardness of the grown crystals has been studied using Vicker's microhardness tester.     

Experimental density and viscosity measurements of di(2ethylhexyl)sebacate at high pressure

Experimental densities and dynamic viscosities of di(2-ethylhexyl)sebacate (DEHS) are the object of study in this work. DEHS could be a useful industrial reference fluid for moderately high viscosity at high pressures as it is often used as a pressure transmitting fluid. At atmospheric pressure the density and viscosity measurements have been performed in a rotational SVM 3000 Stabinger viscometer from (273.15 to 373.15) K, whereas from (0.1 to 60) MPa and from (298.15 to 398.15) K an automated Anton Paar DMA HPM vibrating-tube densimeter, and a high-pressure rolling-ball viscometer were used. Several Vogel–Fulcher–Tammann type equations were used to fit the experimental values of viscosity to the pressure and temperature. The measured viscosity data have been used together with previous data found in the literature to establish a correlation of the viscosity surface η(T, p) of DEHS, covering a temperature range from (273 to 491) K and pressure up to 1.1 GPa. This correlation could be used in industrial equipment like viscometers and other devices that operate at high pressures. Our viscosity data have also been fitted as a function of temperature and volume to the thermodynamic scaling model of Roland et al. [C.M. Roland, S. Bair, R. Casalini, J. Chem. Phys. 125 (2006) 124508].     

Enthalpies of formation and decomposition of nickel hydride and nickel deuteride derived from (p,c,T) relationships

Measurements of equilibrium hydrogen pressure as a function of hydrogen content and of temperature are a convenient way to determine the thermodynamic properties of metal–hydrogen systems. To date such studies have only been carried out for the systems at relatively low hydrogen pressure. We have developed a high-pressure apparatus capable of pressures up to 1.2 GPa and temperatures up to T =  450 K for the studies of equilibrium conditions in the Ni–H systems and the Ni–D systems in order to derive corresponding enthalpies of formation and decomposition. The results show that although the pressures at given temperatures are always higher for (Ni  +  D₂) than for (Ni  +  H₂), the values of enthalpies are almost identical within the experimental error. The enthalpies of the formation and decomposition of both systems derived from these studies are compared with calorimetric measurements carried out at high pressure. The difference between enthalpies of formation and decomposition for both systems reflect hysteresis, a common phenomenon in transition metal hydrides.     

Phase progression via phonon modes in lanthanide dioxides under pressure

The present paper reports the phase progression in nano-crystalline oxides PrO₂ and CeO₂ up to pressures of 49 GPa and 35 GPa, respectively, investigated via in situ Raman spectroscopy at room temperature. The samples were characterized at ambient conditions using X-ray diffraction (XRD), AFM, and Raman spectroscopy and were found to be cubic with fluorite structure. With an increase in applied pressure the cubic bands were seen to steadily shift to higher wavenumbers for both the samples. However, we observed the appearance of a number of new peaks around a pressure of about 34.7 GPa in CeO₂ and 33 GPa in PrO₂ which were characteristic of an orthorhombic α-PbCl₂ type structure. The mode Gruneisen parameters for both the phases were obtained from the pressure dependence of frequency shifts. On decompression, the high pressure phase existed down to a total release of pressure.     

Vapor–liquid equilibria in the binary system (−)-beta-pinene + (+)-fenchone: Analysis in terms of group contributions models of some binary systems containing terpenoids

Isobaric vapor–liquid equilibrium measurements are reported for the binary system (−)-beta-pinene + (+)-fenchone at the constant pressure of 13.33 kPa in the temperature range from 341.60 K to 393.25 K. The boiling temperatures of the mixtures were also measured at seven constant compositions in the pressure range from 2.56 kPa to 20.80 kPa. The experimental data were found to be thermodynamically consistent. Reduction of the vapor–liquid equilibrium data was carried out by means of the Wilson, NRTL and UNIQUAC equations. Our data on vapor–liquid equilibria for mixtures containing terpenoids are examined in terms of the DISQUAC and modified UNIFAC (Dortmund) group contributions models. Interaction parameters of the DISQUAC model are reported.