Volume and structural study of Fe64Mn36 anti-ferromagnetic Invar alloy under high pressure

We have investigated the pressure variation of the volume and structure of an FCC Fe₆₄Mn₃₆ anti-ferromagnetic Invar alloy. The inclination of the pressure-volume (P-V) curve of the FCC structure becomes discontinuous at a pressure of 4 GPa. According to the bulk modulus at zero pressure estimated by the Birch-Murnaghan equation of state, the pressure between 4 and 10 GPa is 33 GPa larger than that at a pressure below 4 GPa. Considering previous experiments on magnetism at high pressure the Neel temperature at 4 GPa almost decreases to room temperature. These results suggest that the increase in the bulk modulus by 33 GPa can be attributed to the pressure-induced magnetic phase transition from anti-ferromagnetism to paramagnetism. Volume at zero pressure was estimated using the Birch-Murnaghan equation of state. The volume of FCC structure in the anti-ferromagnetic state was 1.17% larger than the volume in the paramagnetic state, namely, the spontaneous magnetostriction was 1.17%. Pressure-induced structural transition from FCC to HCP occurs with an increase in the pressure, especially at up to 5 GPa. The value of c/a is 1.62; this value almost corresponds to that of an ideal HCP structure. The bulk modulus of the HCP structure estimated by the Birch-Murnaghan equation of state is larger than that of the FCC structure, and the volume/atom ratio is smaller than that of the FCC structure.     

Transition from the antiferromagnetic to a nonmagnetic state in FeBO3 under high pressure

A ⁵⁷FeBO₃ single crystal is studied by the nuclear forward scattering (NFS) method. The NFS time spectra from ⁵⁷Fe nuclei are recorded at room temperature under high pressures up to 50 GPa in a diamond anvil cell. In the pressure interval 0<p<44 GPa, the magnetic field H ^Fe at the ⁵⁷Fe nuclei is found to increase nonlinearly, reaching a maximum value of 48.1 T at p=44 GPa. As the pressure increases further and reaches the point p=46 GPa, the field H ^Fe abruptly drops to zero, indicating that a transition from the antiferromagnetic to a non-magnetic state occurs in the crystal. In the pressure interval 0<p<46 GPa, the magnetic moments of the iron ions lie in the (111) basal plane of the crystal. Several possible mechanisms of magnetic collapse are discussed.     

Superconductivity of vanadium and vanadium-titanium alloys at high pressure

The effect of pressure on the superconducting transition temperature T _c of vanadium and V₉₄Ti₆, V₈₅Ti₁₅, V₆₇Ti₃₃, and V₄₈Ti₅₂ (at %) bcc alloys has been studied. It has been found that the T _c(P) dependence of pure vanadium is close to linear in the pressure ranges 0–14 and 23–32 GPa, whereas dT _c/dP decreases to zero with a pressure increase in the 14–23 GPa range. The T _c(P) curves for all alloys are nonmonotonic and have two features in the respective pressure ranges of 3–11 and (a peak-shaped feature) 15–25 GPa.     

First-principles calculations of the structural,elastic, electronic and optical properties of Si2P2O and Ge2P2O at high pressures

The structures of Si₂P₂O and Ge₂P₂O with the space group Cmc21 are derived. The structural, mechanical, elastic anisotropy, electronic and optoelectronic properties are calculated by first principles calculations based on density functional theory (DFT) with generalized gradient approximation (GGA) at high pressure for Si₂P₂O and Ge₂P₂O. By using the elastic stability criteria, it is shown that their structures are all stable. The phonon dispersion spectra are researched throughout the Brillouin zone as implemented in the CASTEP code, which indicates that the optimized structures are stable dynamically. The brittle/ductile behaviors are assessed in the pressures from 0 GPa to 50 GPa. Our calculations present that the performances of them become ductile with pressure rise. Moreover, the anisotropies of them are discussed by the Young's moduli at different pressure, and the results indicate that the anisotropies of them are obvious. The direct band structures of Ge₂P₂O and the indirect band gap of Si₂P₂O show that Si₂P₂O and Ge₂P₂O present semiconducting character at 0 GPa and 50 GPa. The band structures of Si₂P₂O are changed obviously with the increase of pressure. The total DOS originate mainly from O ‘s’ states, O ‘p’ states, P ‘s’ states and P ‘p’ states and M ‘p’ states (M=Si, Ge). The trends of DOS for Si₂P₂O and Ge₂P₂O display many similarities, and the change of DOS is obviously affected by the pressure for Si₂P₂O. The optoelectronic properties of them are researched. The calculated static dielectric constants, ɛ₁(0), are 3.2 at 0 GPa and 6.4 at 50 GPa for Si₂P₂O, and the values of Ge₂P₂O are 10.2 and 9.2 at 0 GPa and 50 GPa.     

Heat capacity of CeIrSi3 under pressure

We measured the heat capacity of CeIrSi₃ (100 mK<T<6 K) under high pressure up to P=1.38 GPa. The measurements have been used a quasiadiabatic method utilizing a CuBe piston-cylinder pressure cell in a dilution refrigerator. At 0 GPa, a sharp anomaly which indicates the antiferromagnetically transition is observed at T_N=5 K. T_N decreases monotonically with increasing pressure up to P=1.38 GPa. The magnetic entropy is released below T_N only 19% of R ln 2 at 0 GPa. And the magnetic entropy decreases with increasing pressure up to 1.38 GPa, 64% compared to that at 0 GPa.     

Pressure dependence of the thermal conductivity of aluminium

The thermal diffusivity, a, of aluminium has been measured at pressures up to 2.5 GPa at room temperature, and from these results the pressure dependence of the thermal conductivity, λ, has been calculated. Both quantities increase with pressure. The increase in a amounts to 4.6% to 1 GPa and 10.4% to 2.5 GPa. The initial pressure coefficient of the electronic thermal conductivity λ_e is found to be [λ_e]^-1?λ_e/?P = 3.7 × 10^-2GPa^-1, which agrees very well with a recent theoretical calculation.     

Pressure-induced changes in the magnetic properties of the single crystal of Mn3 single-molecule magnet

The magnetic measurements of the single crystal of Mn3 single-molecule magnet under high pressure at T=2 K have been performed.We find both the antiferromagnetic intermolecular coupling parameter J and the effective energy barrier to vary compared with the measurements at low pressure.The increase of|J|is estimated to be 12%at 0.7 GPa when compared with that of0 GPa,whereas the effective energy barrier becomes smaller with increasing pressure.Our results demonstrate that the intermolecular interaction of single-molecule magnet can be changed by pressure.Compared with the normal magnetic alloy,the effect of pressure on the magnetic properties of Mn3 is much more prominent,which implies that Mn3 may have great potential in magnetic multifunctional material.     

Experimental constraints on the phase diagram of titanium metal

The phase transformations of titanium metal have been studied at temperatures and pressures up to 973 K and 8.7 GPa using synchrotron X-ray diffraction. The equilibrium phase boundary of the α-ω transition has a dT/dP slope of 345 K/GPa, and the transition pressure at room temperature is located at 5.7 GPa. The volume change across the α-ω transition is ΔV=0.197 cm³/mol, and the associated entropy change is ΔS=0.57 J/mol K. Except for ΔV, our results differ substantially from those of previous studies based on an equilibrium transition pressure of 2.0 GPa at room temperature. The α-ω-β triple point is estimated to be at 7.5 GPa and 913 K, which is comparable with previous results obtained from differential thermal analysis and resistometric measurements. An update, more accurate phase diagram is established for Ti metal based on the present observations and previous constraints on the α-β and ω-β phase boundaries.     

In situ high-pressure X-ray diffraction study of densification of a molecular chalcogenide glass

Structural mechanisms of densification of a molecular chalcogenide glass of composition Ge_2.5As_51.25S_46.25 have been studied in situ at pressures ranging from 1 atm to 11 GPa at ambient temperature as well as ex situ on a sample quenched from 12 GPa and ambient temperature using high-energy X-ray diffraction. The X-ray structure factors display a reduction in height of the first sharp diffraction peak and a growth of the principal diffraction peak with a concomitant shift to higher Q-values with increasing pressure. At low pressures of at least up to 5 GPa the densification of the structure primarily involves an increase in the packing of the As₄S₃ molecules. At higher pressures the As₄S₃ molecules break up and reconnect to form a high-density network with increased extended-range ordering at the highest pressure of 11 GPa indicating a structural transition. This high-density network structure relaxes only slightly on decompression indicating that the pressure-induced structural changes are quenchable.     

Phase diagram of kaolinite from Molecular Dynamics calculations

Structural and thermal behaviors of uncharged 1:1 phyllosilicates kaolinite were investigated from molecular dynamics simulations based on the CLAYFF force field. The focus is on the variation of structural properties including bulk modulus with pressure from 0 to 20 GPa under various range of temperature. The largest bulk modulus between the pressures of 200 and 800 MPa varies from 80 GPa at 298 K to 50 GPa at 1473 K. The obtained value of C_p varies between 7.8 and 13.6 Kcal mol⁻¹ K⁻¹ in the pressure range of 0.1 MPa–20 GPa. Besides, a huge difference was noticed regarding the computed properties at the superheating point. Finally, we show the relationship between superheating point temperature and pressure leading to a phase diagram of kaolinite.     

Fermi surface and metallic properties of graphite at high pressures

A potential of superconductivity of pure graphite has been theoretically examined. At normal pressure, the carrier concentration is too low to exhibit superconductivity. On applying pressure, the band dispersion along the c-axis is significantly enhanced, resulting in an increase in the carrier concentration; 10²⁰ cm^-3 at p=30 GPa. This is favorable to observe superconductivity. Accurate Fermi surfaces are illustrated: a new Fermi surface appears around K point at p=25 GPa.     

High pressure equation of state studies using methanol-ethanol-water and argon as pressure media

We have measured the equation of state of the intermetallic compound AuIn₂ up to 20 GPa and Cd_0.8Hg_0.2 up to 50 GPa using methanol-ethanol-water solution or argon as pressure media. In the experiments performed with argon as pressure medium, we minimized non-hydrostatic conditions by thermally annealing the sample. We present data revealing compressibility anomalies in AuIn₂ at 2.7 GPa and in Cd_0.8Hg_0.2 near 8, 18 and 34 GPa with methanol-ethanol-water and argon. At pressures above 5 GPa the P-V data for AuIn₂ and Cd_0.8Hg_0.2 from experiments preformed with argon as a pressure medium start deviating from those using methanol-ethanol-water, and the equation of state based on experiments in argon is stiffer compared with that in methanol-ethanol-water. This behavior is consistent with the relative merits of the two pressure transmitting media as documented in the literature. We also provide a brief summary of the results of electronic structure calculations that associate these anomalies with electronic topological transitions.     

Resistance anomaly and structural disorder in NiSi2 under high pressure

Results of X-ray diffraction, electrical resistance, thermoelectric power measurements and electronic band structure calculations on NiSi₂ under high pressure are reported. The thermoelectric power (TEP) changes sign near 0.5 GPa (from +30 to −20 μV/K). As the pressure is increased, the value of TEP increases further in magnitude and near 7 GPa it becomes −50 μV/K. The pressure vs. resistance curve measured up to 30 GPa using diamond anvil (DAC)-based technique exhibits a broad hump near 12 GPa and exhibits hysteresis on pressure release. The ADXRD patterns up to 42 GPa show a gradual irreversible loss of long-range order in NiSi₂ with the diffraction lines progressively broadening under pressure. The FWHM of the diffraction lines show a rapid increase in the half-widths close to 0.5 GPa and also near 12 GPa. The computed band structure at a compression (without any disorder) corresponding to 12 GPa, exhibits an electronic topological transition (ETT). The rapid increase in disorder above 12 GPa implies that the ETT may be facilitating the structural disorder. It is suggested that the pressure drives the material through a region of entropic and energetic barriers and induces disorder in the material.     

Origin of the non-linear pressure effects in perovskite manganites: Buckling of Mn-O-Mn bonds and Jahn-Teller distortion of the MnO6 octahedra induced by pressure

High-pressure resistivity and X-ray diffraction measurements were conducted on La_0.85MnO_3−δ at ∼6 and ∼7 GPa, respectively. At low pressures the metal-insulator transition temperature (T_MI) increases linearly up to a critical pressure, P^* ∼3.4 GPa, followed by reduction in T_MI at higher pressure. Analysis of the bond distances and bond angles reveals that a bandwidth increase drives the increase in T_MI below P^*. The reduction in T_MI at higher pressures is found to result from Jahn-Teller distortions of the MnO₆ octahedra. The role of anharmonic interatomic potentials is discussed.     

Pressure evolution of the EFG and magnetic hyperfine field in the layered antiferromagnet FeI2

When FeI₂ is subjected to pressures of up to 20 GPa, a change of approximately 20% occurs in the unit cell volume.⁵⁷Fe Mössbauer spectroscopy (MS) in a diamondanvil cell has been used to monitor the pressure evolution of the hyperfine interaction parameters of this layered antiferromagnetic insulator. The pressure dependence of the quadrupole splittingQS at 296 K exhibits a maximum at 12 GPa and the saturation magnetic hyperfine fieldH ₀ increases from 7.4 T at ambient pressure to 12 T at 18 GPa. A qualitative analysis identifies the pressure evolution ofQS with changes in the trigonal component of the crystal field splitting. The pressure variation ofH ₀ is attributed to an increase in the average value of the 3d charge density distribution.     

Effect of high pressure on the period of the basis lattice and concentration of vacancies in titanium monoxide TiO

The results of the experiment on the thermobaric annealing of titanium monoxide with a B1 cubic structure at a temperature of 2273 K have been analyzed. The analysis of these results has shown that an increase in the initial period of the lattice from 417.6 to 418.0 pm after processing at a pressure of 3 GPa and to 418.5 pm at a pressure of 6 GPa is due to the filling of vacant sites of the crystal structure with atoms. The ab initio quantum calculations indicate that the equilibrium period of the lattice should increase monotonically by 2% at a decrease in the concentration of vacancies from 16.7 to 0%. After aging the samples under normal conditions, the period of the lattice is no more than 421.0 pm and the concentration of vacancies is no lower than 11% because of the instability of the structure at lower concentrations of vacancies.     

Half-metallic behavior of Co2YZ (Y = V,Cr; Z = Al,Ga) under pressure: a DFT study

The structural, electronic and magnetic properties of Co-based Heusler compounds Co₂YZ (Y = V, Cr; Z = Al, Ga) under pressure are studied using first principles density functional theory. The calculations are performed within generalized gradient approximation. The total magnetic moment decreases slightly on compression. Under application of external pressure, the valence band and conduction band are shifted downward which leads to the modification of electronic structure. There exists an indirect band gap along Г–X for all the alloys studied. Co₂CrAl shows half-metallic nature up to 85 GPa. After this pressure transition from true half-metallic behavior to nearly half-metallic behavior is observed and at 90 GPa it shows metallic behavior. Co₂CrGa shows nearly half-metallic behavior at ambient pressure, but true half-metallic behavior is observed as pressure is increased to 100 GPa. For Co₂VGa, true half-metallic to nearly half-metallic transition is observed at 40 GPa and around 100 GPa, Co₂VGa shows metallic behavior. For Co₂VAl, true half-metallic behavior is not observed at ambient as well as higher pressures. The half metal-to-metal transition in Co₂VAl and Co₂CrAl is accompanied by quenching of magnetic moment.     

Experimental constraints on the phase diagram of elemental zirconium

The phase diagram of zirconium metal has been studied using synchrotron X-ray diffraction and time-of-flight neutron scattering at temperatures and pressures up to 1273 K and 17 GPa. The equilibrium phase boundary of the α-ω transition has a dT/dP slope of 473 K/GPa, and the extrapolated transition pressure at ambient temperature is located at 3.4 GPa. For the ω-β transition, the phase boundary has a negative dT/dP slope of 15.5 K/GPa between 6.4 and 15.3 GPa, which is substantially smaller than a previously reported value of −39±5 K/GPa in the pressure range of 32-35 GPa. This difference indicates a significant curvature of the phase boundary between 15.3 and 35 GPa. The α-ω-β triple point was estimated to be at 4.9 GPa and 953 K, which is comparable to previous results obtained from a differential thermal analysis. Except for the three known crystalline forms, the β phase of zirconium metal was found to possess an extraordinary glass forming ability at pressures between 6.4 and 8.6 GPa. This transformation leads to a limited stability field for the β phase in the pressure range of 6-16 GPa and to complications of high-temperature portion of phase diagram for zirconium metal.     

Magnetism and electronic properties of BiFeO3 under lower pressure

Ab initio calculations show an antiferromagnetic-ferromagnetic phase transition around 9-10 GPa and a magnetic anomaly at 12 GPa in BiFeO₃. The magnetic phase transition also involves a structural and insulator-metal transition. The G-type AFM configuration under pressure leads to an increase of the y component and a decrease of the z component of the magnetization, which is caused by the splitting of the d_z² orbital from doubly degenerate e_g states. Our results agree with the recent experimental results.     

Distribution of light alkanes in the reaction of graphite hydrogenation at pressure of 0.1–7.8 GPa and temperatures of 1000–1350°C

ABSTRACT

We studied the effect of pressure and temperature on the hydrocarbon (HC) chain length distribution and total amount of HCs in the reaction of direct graphite hydrogenation at pressures of 0.1–7.8?GPa and temperatures of 1000–1350°C. An increase in pressure was found to lead both to an increase in the absolute yield of HCs due to direct graphite hydrogenation and to chain elongation of HC products. Light alkanes predominate among HCs in the entire studied range of P–T parameters. However, their concentration in quenched fluids increases as pressure is elevated, from less than 10?rel.% at 0.1?GPa to more than 40–50?rel.% at P?≥?3.8?GPa. Methane is actually the only light alkane among reaction products at 0.1?GPa and 1000°C, while it is a minor component at 7.8?GPa and 1350°C. The most stable alkane at pressures above 3.8?GPa is ethane (C₂H₆).