Infrared Studies of Ar(H2)2 at Low Temperature and High Pressure

An extensive infrared absorption study of the compound Ar(H ₂)₂ in the range of pressure from 5 to 30 GPa, and between 30 K and room temperature is presented. This study has permitted the identification and assignment of high–frequency and low frequency vibron–phonon combination bands, as well as of the roto–vibrational bands. Spectra changes lowering temperature, with the appearance of the Q ₁(0) vibrational peak, are interpreted as arising from conversion of ortho–hydrogen into para–hydrogen in the solid.     

Pressure-induced polymorphous crystallization in bulk Si20Te80 glass

The effect of pressure on the electrical resistivity of bulk Si₂₀Te₈₀ glass has been studied up to a pressure of 8 GPa. A discontinuous transition occurs at a pressure of 7 GPa. The X-ray diffraction studies on the pressure quenched sample show that the high pressure phase is crystalline with hexagonal structure (c/a = 1.5). On heating, the high pressure hexagonal phase has on exothermic decomposition atT = 586 K into two crystalline phases, which are the stable phases tellurium and SiTe₂ obtained by simple heating of the glass.     

Thermoelectric power of YBa2Cu3O7−δ under pressure up to 9 GPa

Thermoelectric power (TEP) of two YBa₂Cu₃O_7−δ compounds (with δ=0·17 and 0·21) was measured as a function of quasi-hydrostatic pressure up to 9GPa at 300K on samples with low porosity. In both cases TEP decreases with increasing pressure, at a rate ∼ 0·8 μVK⁻¹/GPa. The data obtained under hydrostatic pressure up to 3 GPa are in good agreement with those under quasi-hydrostatic pressure. The TEP of both compositions is found to decrease linearly at a rate 0·8 μVK⁻¹/GPa above 1·5 GPa.     

The pressure-volume relation of ytterbium up to 9 GPa

The pressure-volume relation of ytterbium has been determined up to 9 Gpa using tungsten carbide opposed anvil high pressure x-ray camera. The fcc phase of ytterbium is observed between one atmosphere and 4 GPa and the bcc phase above 3·5 GPa. The bcc phase can be metastably retained down to 1 GPa by gradually decreasing the pressure from a region where only bcc phase alone is observed. The bulk modulus,B ₀, at zero pressure and the pressure derivative of the bulk modulus,B’ ₀, are determined by fitting Murnaghan equation to the pressure-volume data. The following values were obtained:B ₀=16·3 GPa andB’ ₀=3·6 for the fcc phase, andB ₀=14·7 GPa andB’ ₀=1·5 for the bcc phase. Based on the present data it is suggested that the thermodynamic equilibrium pressure for fcc ⇆ bcc transformation in ytterbium is below 3·5 GPa. The valence change under pressure has been discussed.     

Pressure Effects in the Iron Chalcogenides

In FeSe_1?x Te _x , the superconducting and magnetic properties can be tuned not only by the Se-Te ratio x, but also by application of hydrostatic pressure. It was seen that both parent compounds FeSe and FeTe are extremely sensitive on hydrostatic pressure and show quite unexpected effects. The at ambient pressure superconducting FeSe exhibits at pressures above 1 GPa a phase, where superconductivity and antiferromagnetism seem to coexist on an atomic scale. The antiferromagnetic FeTe, on the other hand, transforms from a low pressure antiferromagnet to a ferromagnet above 2 GPa. These pressure studies underline the close relation of the electronic phases to the crystallographic properties, since in these simple compounds already minor changes in the structure for example by application of pressure lead to a drastic change of the superconducting and magnetic properties.     

Evolution of microstructures and phases of Al–Mg alloy under 4 GPa high pressure

Optical microscope (OM), energy dispersive X-ray (EDX) analysis, differential scanning calorimetry (DSC), X-ray diffraction (XRD) and transmission electron microscope (TEM) were applied to investigate the solidification microstructures and phases of Al–9.6 wt.% Mg alloy which solidified under 4 GPa high pressure with the melting temperature 1,153 K. Fine dendritic microstructures were obtained, and the second dendritic arm spacing reduced. Area fraction of the primary α-Al phase increased and that of the second phase decreased. In addition, the solid solubility of Mg in α-Al phase increased. The lattice constant of α-Al phase increased. Specially, the new double phase regions (α-Al′ + Al _x Mg _y ) formed besides a small amount of Al₃Mg₂ phases under high pressure. The Al _x Mg _y phase presented a mean size of about 20 nm, and had the hexagonal structure with the lattice constant of a = 0.288 nm, c = 0.8165 nm probably. Wherein the lattice constant of α-Al′ phase differed from that of α-Al phase greatly. Moreover, evolution mechanism of microstructures and phases under 4 GPa high pressure was discussed.     

Charge Order in (TMTTF)2PF6 Investigated by Infrared Spectroscopy

To explore novel physical phenomena related to strong π-d interaction, we measured the resistivity (ρ) and magnetoresistance of the first organic ferrimagnetic π-d system, (EDT-TTFVO)₂FeBr₄ under high pressures up to 8 GPa, where EDT-TTFVO denotes ethylenedithiotetrathiafulvalenoquinone-1,3-dithiolemethide. At ambient pressure, ρ(T) exhibits resistivity minimum near T_min ∼ 170 K followed by a gradual increase below it. With increasing pressure, T_min abruptly decreases till 4 GPa, beyond which it slightly increases. The increase ofT _min above 4 GPa is discussed in terms of the enhancement of the π-d interaction by applying pressure.     

B3–B1 phase transition and pressure dependence of elastic properties of ZnS

We have performed the ab initio calculations based on density functional theory to investigate the B3–B1 phase transition and mechanical properties of ZnS. The elastic stiffness coefficients, C₁₁, C₁₂, C₄₄, bulk modulus, Kleinman parameter, Shear modulus, Reuss modulus, Voigt modulus and anisotropy factor are calculated for two polymorphs of ZnS: zincblende (B3) and rocksalt (B1). Our results for the structural parameters and elastic constants at equilibrium phase are in good agreement with the available theoretical and experimental values. Using the enthalpy–pressure data, we have observed the B3 to B1 structural phase transition at 18.5 GPa pressure. In addition to the elastic coefficients under normal conditions, we investigate the pressure dependence of mechanical properties of both phases: up to 65 GPa for B1-phase and 20 GPa for B3-phase.     

Hard and superhard carbon phases synthesized from fullerites under pressure

A review has been presented on the structural and mechanical properties of hard carbon phases synthesized from fullerite C₆₀ under pressure. The density and nanostructure have been recognized as the key parameters defining the mechanical properties of hard carbon phases. By suggesting a version of the transitional high-pressure diagram of C₆₀ (developed up to 20 GPa), the three areas of the formation of hard carbon phases have been highlighted. The corresponding phases of superhard carbon are (1) disordered sp₂-type atomic structures at moderate pressures and high temperatures (> 1100 K), (2) three-dimensionally polymerized C₆₀ structures at moderate temperatures and high pressures (> 8 GPa), and (3) sp₃-based amorphous and nanocomposite phases at high pressures and temperatures. First region can be in turn separated into 2 subparts with different peculiarities of sp² structures and properties: low pressure part (0.1–2 GPa) and high-pressure part (2–8 GPa). Temperature can be recognized as a factor responsible for the formation of nanostructures by the partial destruction of molecular phases, whereas pressure is a factor responsible for stimulating the formation of rigid polymerized structures consisting of covalently bonded C₆₀ molecules, whereas the combination of both factors leads to the formation of atomic-based phases with dominating sp³ bonding.     

High-pressure phase transition in Ho2O3

High-pressure X-ray diffraction and Raman studies on holmium sesquioxide (Ho₂O₃) have been carried out up to a pressure of ∼17 GPa in a diamond-anvil cell at room temperature. Holmium oxide, which has a cubic or bixbyite structure under ambient conditions, undergoes an irreversible structural phase transition at around 9.5 GPa. The high-pressure phase has been identified to be low symmetry monoclinic type. The two phases coexist to up to about 16 GPa, above which the parent phase disappears. The high-pressure laser-Raman studies have revealed that the prominent Raman band ∼370 cm⁻¹ disappears around the similar transition pressure. The bulk modulus of the parent phase is reported.     

Raman Studies of Vanadates at Low Temperatures and High Pressures

The spin and orbital ordering have been examined for high-quality SmVO₃ polycrystalline compound using Raman spectroscopy. Measurements were obtained on individual microcrystallites in the approximate y(zz)y and y(xx)y scattering configurations at low temperatures (down to 20 K) and high pressures (up to 2.75 GPa). At room temperature and ambient pressure only the A_g phonons have been observed in both polarizations examined. The decrease of temperature leads to the appearance of extra peaks in the Raman spectra related to the magnetic and structural transitions that occur in the system. We present evidence of a coexistence of the monoclinic and orthorhombic phases accompanied with the coexistence of the G- and C-type OO phases for temperatures below 100 K. However, no sign of any structural transition has been observed in the high pressure Raman spectra (1.87<P<2.75 GPa) at low temperatures T≥80 K, indicating that SmVO₃ remains orthorhombic, at least down to 80 K.     

Magnetic Correlation in BETS2(Cl2TCNQ)

We carried out⁷⁷Se NMR measurements on BETS₂(Cl₂TCNQ) under pressure in order to investigate the magnetic properties of the insulating state which appears above 0.6 GPa. The relaxation rate 1/T₁ at 0.7 GPa shows small peak-like anomaly at 20 K, indicating a spin density wave transition as observed in BETS₂(Br₂TCNQ).     

Phase diagram with a region of liquid carbon-diamond metastable states

Metastable cubic diamond has been found in the structure of solid carbon obtained by quenching of a liquid phase at a pressure (0.012 GPa) much lower than that corresponding to the existence of stable diamond. It is suggested that this metastable diamond is formed as a result of the recalescence of supercooled liquid carbon to the melting point (T _dm) of metastable diamond due to a lower energy barrier for the formation of diamond as compared to that of graphite. A comparison between the calculated Gibbs energies of metastable phases provided an estimate of T _dm = 4160 ± 50 K. For the first time, metastable continuations of the curve of diamond melting at pressures of up to 0.012 GPa are constructed on the phase diagrams of carbon (according to various published data) using analytical curves described by a two-parametric Simon equation.     

Solid-state extrusion of nylons 11 and 12: processing,morphology and properties

Monofilaments of poly(11-amino-undecanoic acid) (nylon 11), and poly(laurylactam) (nylon 12) have been produced using solid-state extrusion methods in an Instron capillary rheometer. The resulting morphology, physical and mechanical properties were investigated. For nylon 11, at an extrusion ratio (ER) of 12, the crystalline melting-point temperature increased by 16° C, over the undrawn material, while the per cent crystallinity,X _c, increased by 23%. Nylon 12, extruded to a maximum ER of 6, realized an increase inT _m of 4° C at ER=5 and anX _c increase of 14%. Young's modulus for nylon 11 increased from 3 GPa at an ER=3 to 5.5 GPa at an ER=7 and levelled off at greater ER. For nylon 12, the Young's modulus climbed from 2.5 GPa at ER=3 to about 3.3 GPa at E R=5.5. Conventionally melt-spun and cold-drawn nylon 11 and nylon 12 fibres exhibited Young's modulus values of 2.7 GPa and 2.9 GPa respectively. Atmospheric moisture loss was found not to affect solid-state extrusion of these higher nylons. Increases in extrusion temperature and/or pressure increased the extrusion rate. The flow activation energy of nylon 11 was 73 kcal mol⁻¹ at 0.24 GPa extrusion pressure, and 124 kcal mol⁻¹ at 0.49 GPa extrusion pressure. Calculated apparent viscosities were about 10¹⁴ poise and 10¹⁵ poise, respectively. The morphologies were shown by electron microscopy to be microfibrillar and the resulting monofilaments were transparent to visible light.     

High-pressure volume magnetostriction in the diluted magnetic semiconductor Cd1 ? xMnxGeAs2 (x = 0.06–0.3)

The specific volume of the diluted magnetic semiconductor Cd_{1 − x} Mn _x GeAs₂ (x = 0.06–0.3) has been determined for the first time by strain measurements at pressures of up to 7 GPa. From the pressure dependences of the relative specific volume, we evaluated the volume magnetostriction (spontaneous magnetization coefficient). A scaling relation was used to estimate the bulk modulus of the magnetically ordered and disordered phases.     

Mechanical strength of ultra-fine Al-AlN composites produced by a combined method of plasma-alloy reaction,spray deposition and hot pressing

Ultra-fine Al-AlN composites with high packing density were produced by the simple sequential process consisting of nitrogen plasma-alloy reaction, spray deposition and hot-pressing. The AlN content,V _f, was controlled in the range below about 40 vol % by changing the nitrogen partial pressure in the plasma-alloy reaction. The density of the Al-AlN composite withV _f=36% after hot-pressing for 7.2 ks at 673 K was 2.96 Mg m^–3 which is nearly the same as the theoretical density. The constituent phases were f c c aluminium and hexagonal AlN and their lattice parameters are nearly the same as those of pure aluminium and AlN phases. The grain size and interparticle spacing of the AlN particles were as small as about 90 and 50 nm, respectively. The Vickers hardness number, Young's modulus and compressive strength of the dense Al-AlN composite were 193, 112 GPa and 628 MPa, and the high hardness above 100 was maintained in the temperature range below 673 K. It was therefore concluded that the sequential process is a useful technique to produce ultra-fine metal-ceramic composites with high mechanical strengths.     

High pressure Raman spectroscopic study of structural phase transition in samarium oxide

High pressure Raman spectroscopic study of Sm₂O₃ poly crystal was performed up to 21.0 GPa and room temperature using a diamond anvil cell. Pressure induced phase transition was observed at 2.6 GPa in the pressure increasing process. This phase transition corresponds to the monoclinic B type phase → the hexagonal A type transformation. The A type phase was stable up to 21.0 GPa. In the pressure release process, the A type phase was stable above 1.8 GPa, and was completely reverted to the B type phase at 1.1 GPa. The phase transition was confirmed to be reversible with a hysteresis of approximately 1.0 GPa.     

Effect of Pressure on Transport Properties of Heavy Fermion CePtSi<Subscript>2</Subscript>

We measured the resistivity of heavy fermion CePtSi₂ under pressure. At ambient pressure, CePtSi₂ shows an antiferromagnetic (AF) transition at 2 K and a Fermi liquid like T ² dependence in resistivity below 1.5 K. With increasing pressure, the AF phase and T ² dependence are suppressed. Above 1.4 GPa, a T linear dependence and pressure-induced superconductivity were found with the maximum T _c=0.14 K at 1.7 GPa. Above 2 GPa, the T ² dependence recovers just above T _c.     

Decompression‐Driven Superconductivity Enhancement in In2Se3

An unexpected superconductivity enhancement is reported in decompressed In₂Se₃. The onset of superconductivity in In₂Se₃ occurs at 41.3 GPa with a critical temperature (T_c) of 3.7 K, peaking at 47.1 GPa. The striking observation shows that this layered chalcogenide remains superconducting in decompression down to 10.7 GPa. More surprisingly, the highest T_c that occurs at lower decompression pressures is 8.2 K, a twofold increase in the same crystal structure as in compression. It is found that the evolution of T_c is driven by the pressure‐induced R‐3m to I‐43d structural transition and significant softening of phonons and gentle variation of carrier concentration combined in the pressure quench. The novel decompression‐induced superconductivity enhancement implies that it is possible to maintain pressure‐induced superconductivity at lower or even ambient pressures with better superconducting performance.     

Effect of synthesis pressure on hydride phases in Mg–M systems (M = Mn,Y)

High-pressure synthesis of the hydrides in Mg–M (M = Mn, Y) systems and the influence of applied pressures during synthesis on present phases and their crystal structures have been studied. In Mg–Mn system, it was found that the crystal structure of Mg₃MnH_y changed from hexagonal structure (a = 0:47107(4) nm and c = 1:0297(1) nm) to monoclinic structure (a = 0:8819(8) nm, b = 0:4658(4) nm, c = 0:4678(5) nm and b= 105:6(1)8) in a pressure range of 3–3.5 GPa. This crystal structural change was reversible with respect to pressure. The Mg₃MnH_y synthesized under 5 GPa was stable up to around 620 K. From thermogravimetric and fusion extraction analyses, the hydrogen content was determined as Mg₃MnH_5.0–5.6. In Mg–Y system, the high-pressure hydride (MgY₂H_y) with yellowish color was synthesized at 1073 K for 2 h under 3 GPa or higher. This phase exhibited an FCC-type structure with a cell parameter of a = 0:516 nm. Its hydrogen content was determined to be about 3.7 mass%, corresponding to a chemical formula of MgY₂H_7.8. The hydride was partially dehydrogenated at around 600 K, and the amount of hydrogen partially desorbed was 1.4 mass%. The FCC-type structure was stable even after the partial dehydrogenation.