The pressure effect on the photoluminescence and Raman spectra of Nd(DBM)3·Phen

Photoluminescence and Raman spectra of rare earth complex Nd(DBM)₃·Phen (DBM, dibenzoylmethane; Phen, 1,10-phenanthroline) are measured at high pressures. A new Raman band appearing at 1070 cm⁻¹ indicates a second-order phase transition around 5.0 GPa. Although the crystal lattice is destroyed for pressures higher than 7.1 GPa, photoluminescence spectra show that the emission intensity of Nd³⁺ is enhanced dramatically with the pressure increasing up to 9.9 GPa, which is attributed to an efficient intramolecular energy transfer from the ligand to Nd³⁺. By analyzing the energy of the ground and excited states at 9.9 GPa, the ⁴H_11/2 energy level is considered as the main resonance energy level that efficiently accepts the transferred energy from the ligand.     

Pressure dependence of the optical-absorption edge of AlN and graphite-type BN

Effect of pressure on the band gaps on AlN and graphite-type BN (g-BN) has been studied up to 2.7 GPa at room temperature by measuring the optical-absorption edge of single crystals of each substance pressurized in a sapphire-anvil cell. The direct band gap of AlN shifted towards higher energy at a rate of 49±1 meV/GPa, whereas in g-BN the pressure dependence of the band gap was −36±1 meV/GPa. The results are compared with existing first-principles calculations.     

Equation of state and thermal stability of Al3BC

The lattice parameters of Al₃BC have been measured up to 5 GPa at ambient temperature using energy-dispersive X-ray powder diffraction with synchrotron radiation. A fit to the experimental p-V data using Birch-Murnaghan equation of state gives values of the Al₃BC bulk modulus 116(4) GPa and its first pressure derivative 9(2). In the 1.6-4.8 GPa range at temperatures above 1700 K Al₃BC undergoes incongruent melting that results in the formation of Al₃BC₃, AlB₂ and liquid aluminum.     

The band gap and electrical resistivity of FeS2-pyrite at high pressures

The indirect energy gap and electrical resistivity of FeS₂-pyrite have been measured at high pressures and 300 K using optical absorption spectroscopy and electrical conductivity measurements. Absorption spectra extend to ∼28 GPa, while resistivity is determined to ∼34 GPa. The band gap of FeS₂ is indirect throughout this pressure range and decreases linearly with pressure at a rate of −1.13(9)×10⁻² eV/GPa. If this linear trend continues, FeS₂ is expected to metallize at a pressure of 80(±8) GPa. The logarithm of resistivity also linearly decreases with pressure to 14 GPa with a slope of −0.101(±0.001)/GPa. However, between 14 and 34 GPa, the logarithm of resistivity is nearly constant, with a slope of −0.011(±0.003)/GPa. The measured resistivity of pyrite may be generated predominantly by extrinsic effects.     

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.     

Synchrotron X-ray diffraction and absorption studies of CeM2X2 (M=Cu, Ni and X=Si, Ge) at high pressure

The equations of state of CeCu₂Si₂ and CeCu₂Ge₂ to about 60 GPa, as well as that of CeNi₂Ge₂ to 22 GPa and the valence state of Ce in CeCu₂Ge₂ to 20 GPa have been studied at room temperature in a diamond-anvil cell using synchrotron radiation sources. In each compound, the ambient-pressure phase (tetragonal ThCr₂Si₂-type structure) persisted to the highest pressure studied. The unit cell volumes of CeNi₂Ge₂ at ∼5 GPa and CeCu₂Ge₂ at ∼7 GPa, respectively, approached that of CeCu₂Si₂ taken at ambient pressure. From the equation-of-state data, the bulk modulus was derived to be 112.0±5.1 GPa for CeCu₂Si₂, 125.6±4.3 GPa for CeCu₂Ge₂, and 178.4±14.3 GPa for CeNi₂Ge₂. The valence state of Ce in CeCu₂Ge₂ remained trivalent throughout the pressure range investigated.     

Magnetic and structural instabilities in the hexagonal Laves hydride ErMn2H4.6 under high-pressure

The structural and magnetic properties of ErMn₂H_4.6 have been studied by X-ray and neutron diffraction up to the pressures of 15 and 6 GPa, respectively. In the pressure range 0<P<3 GPa we observe a first-order phase transition to new high-pressure (HP) phase. The HP phase has the same hexagonal unit cell as the ambient-pressure phase but smaller lattice parameters (ΔV/V=−5%). The structural transition results in suppression of the long-range antiferromagnetic order. Our results suggest that pressure changes positions of the hydrogen atoms in the metal host. We speculate that the new arrangement of hydrogen atoms induces spin frustration and, therefore, suppresses long-range magnetic order in the HP phase.     

First principles study of pressure induced structural phase transition in hydrogen storage material—MgH2

First principles calculation were performed using Vienna ab-initio simulation package within the frame work of density functional theory (DFT) to understand the electronic properties of magnesium hydride. At normal pressure, the most stable structure of MgH₂ is rutile type with a wide band gap of 3.52 eV, which agrees well with the available data. A pressure induced semi-conductor to metallic transition at a pressure of 92.54 GPa is predicted. Our results indicate a sequence of pressure induced structural phase transition in MgH₂. The obtained sequence of phase transition was α→γ→β→δ→ε at a pressure of 0.37 GPa, 3.89 GPa,7.23 GPa and 11.26 GPa, respectively. Thus our results indicate that MgH₂ is one of the best hydrogen storage material and the maximum storage capacity achieved was 7.7%.     

Spin gap of a pressure-induced superconductor Sr2Ca12Cu24O41 at an optimum pressure of 3.8 GPa

Nuclear magnetic resonance (NMR) on ⁶³Cu nuclei was performed in a pressure-induced superconductor Sr₂Ca₁₂Cu₂₄O₄₁ at an optimum pressure of 3.8 GPa. A pressure of 3.8 GPa was achieved by improving a piston-cylinder-type pressure cell and developing a NMR probe with a steady-load control system. We found that the spin gap still exists even at the optimum pressure. The spin gap was almost the same at pressures below 3.5 GPa on the pressure-temperature phase diagram, whereas it decreased rather drastically above 3.5 GPa.     

Structural property of CsCl-type sodium chloride under pressure

The structure and equation of state of CsCl-type sodium chloride have been determined using high-pressure powder X-ray diffraction from 32 to 134 GPa. The CsCl-type phase remains stable over this entire pressure range. Pressure-volume data can be fitted with a Vinet equation of state with K_30 GPa=135.1 GPa, K′_30 GPa=3.9, and V_30 GPa=27.70 Å³. The nearest-neighbour distance between sodium and chlorine atoms decreased as pressure increased. Significant discrepancies of nearest-neighbour distance between previous theoretical predictions and this study were observed at pressures higher than 70 GPa.     

Ionic to electronic dominant conductivity in Al2(WO4)3 at high pressure and high temperature

Electrical conduction and crystal structure of Al₂(WO₄)₃ at 400 °C have been studied as a function of pressure up to 5.5 GPa using impedance methods and synchrotron radiation X-ray diffraction, respectively. AC impedance spectroscopy and DC polarization measurements reveal an ionic to electronic dominant transition in electrical conductivity at a pressure as low as 0.9 GPa. Conductivity increases with pressure and reaches a maximum at 4.0 GPa, where the conductivity value is 5 orders of magnitude greater than the 1 atm value. Upon decompression, the conductivity retains the maximum value until the sample is cooled at 0.5 GPa. The high pressure-temperature X-ray diffraction results show that the lattice parameters decrease as pressure increases and the crystal structure undergoes an orthorhombic to tetragonal-like transformation at a pressure ∼3.0 GPa. The change of conduction mechanism from ionic to electronic may be explained by means of pressure-induced valence change of W⁶⁺→W⁵⁺, which results in electron transfer between W⁵⁺-W⁶⁺ sites at high pressure.     

Plasticity and ab initio characterizations on Fe4N produced on the surface of nanocrystallized 18Ni-maraging steel plasma nitrided at lower temperature

18Ni-maraging steel has been entirely nanocrystallized by a series of processes including solution treatment, hot-rolling deformation, cold-drawn deformation and direct electric heating. The plasma nitriding of nanocrystallized 18Ni-maraging steel was carried out at 410 °C for 3 h and 6 h in a mixture gas of 20% N₂ + 80% H₂ with a pressure of 400 Pa. The surface phase constructions and nitrogen concentration profile in surface layer were analyzed using an X-ray diffractometer (XRD) and the glow discharge spectrometry (GDS), respectively. The results show that an about 2 μm thick compound layer (mono-phase γ′-Fe₄N) can be produced on the top of the surface layer of nanocrystallized 18Ni-maraging steel plasma nitrided at 410 °C for 6 h. The measured hardness value of the nitrided surface is 11.6 GPa. More importantly, the γ′-Fe₄N phase has better plasticity, i.e., its plastic deformation energy calculated from the load-displacement curve obtained by nano-indentation tester is close to that of nanocrystallized 18Ni-maraging steel. Additionally, the mechanical properties of γ′-Fe₄N phase were also characterized by first-principles calculations. The calculated results indicate that the hardness value and the ratio of bulk to shear modulus (B/G) of the γ′-Fe₄N phase are 10.15 GPa and 3.12 (>1.75), respectively. This demonstrates that the γ′-Fe₄N phase has higher hardness and better ductility.     

The effect of pressure on the spin density wave transition in the layered cobaltites [Ca2CoO3]0.62[CoO2] and [Ca2Co4/3Cu2/3O4]0.62[CoO2]

In order to investigate the pressure effect on the magnetism in the layered cobaltites, positive muon spin rotation and relaxation μ⁺SR experiments have been carried out up to 1.3 GPa using c-aligned polycrystalline samples of [Ca₂CoO₃]_0.62[CoO₂] and [Ca₂Co_4/3Cu_2/3O₄]_0.62[CoO₂]. A transverse field μ⁺SR experiment indicates that the transition temperature to an incommensurate spin density wave IC-SDW state is independent of hydrostatic pressure up to 1.3 GPa for the both compounds. Furthermore, there are no changes in the spontanious muon precession frequency in zero field at 5 K even under 1.3 GPa. These results strongly suggest that the IC-SDW exists not in the rocksalt-type block ([Ca₂CoO₃] and/or [Ca₂Co_4/3Cu_2/3O₄]) but in the CoO₂ plane.     

Structural, electronic and thermodynamic properties of cubic Zn3N2 under high pressure from first-principles calculations

The structural, electronic and thermodynamic properties of cubic Zn₃N₂ under hydrostatic pressure up to 80 GPa are investigated using the local density approximation method with pseudopotentials of the ab initio norm-conserving full separable Troullier-Martin scheme in the frame of density functional theory. The structural parameters obtained at ambient pressure are in agreement with experimental data and other theoretical results. The change of bond lengths of two different types of Zn-N bond with pressure suggests that the tetrahedral Zn-N bond is slightly less compressible than the octahedral bond. By fitting the calculated band gap, the first and second order pressure coefficients for the direct band gap ofthe Zn₃N₂ were determined to be 1.18×10⁻² eV/GPa and −2.4×10⁻⁴ eV/(GPa)², respectively. Based on the Mulliken population analysis, Zn₃N₂ was found to have a higher covalent character with increasing pressure. As temperature increases, heat capacity, enthalpy, product of temperature and entropy increase, whereas the Debye temperature and free energy decrease. The present study leads to a better understanding of how Zn₃N₂ materials respond to compression.     

Infrared study of proton-deuteron mutual diffusion in a CsHSO4/CsDSO4 solid under high pressure

Proton-deuteron mutual diffusion in a CsHSO₄/CsDSO₄ solid at 373 K was examined up to 3 GPa by an infrared mapping measurement. Phases HPHT1 and HPHT2 appeared at 1.5 and 2.3 GPa, respectively, after heating. These phases were found to be stable at room temperature, while phase IV, which appeared on compression at room temperature, was metastable. The pressure dependence of the proton-deuteron mutual diffusion coefficient was determined from the temporal change in the deuteron distribution of the solid. The coefficient decreased from 7×10⁻¹⁶ to 1×10⁻¹⁶ m²/s during the transition from phase II to HPHT1 at 1.5 GPa, and showed no significant change during the transition to phase HPHT2. These results suggested that in addition to the hydrogen bond length, other structural factors might also have had an influence on the rate of diffusion.     

Raman and photoluminescence spectroscopy study of benzoic acid at high pressures

Benzoic acid (C₆H₅COOH, BA) has been studied by high pressure Raman and fluorescence spectroscopy up to about 13.40 GPa using a diamond anvil cell at room temperature. The changes of lattice modes are interpreted as the crystal structure transformation. Three possible phase transitions, with the pressure increasing up to about 0.55, 3.67 and 11.10 GPa, are, respectively, elucidated as crystalline-to-crystalline, crystalline-to-amorphous transitions. A new material formed when the pressure is up to above 11.10 GPa remains stable after the pressure is released.     

Pressure-induced phase transitions in CaB6 by means of XRD and resistance measurement

High pressure behavior of CaB₆ with cubic crystal structure is investigated by means of energy dispersive X-ray diffraction and by employing in situ resistance measurement in a diamond anvil cell. Two newcome high pressure phase transitions are found with pressure ranging from ambient to 26 GPa. The first one at 12 GPa is a structural phase transition from CsCl-type structure to orthogonal structure, which is reflected by both the X-ray diffraction and the resistance variation. The other one at 3.7 GPa is suggested to be an electronic transition, which is observed only in resistance measurement. The diffraction pattern recovered while the pressure is released to 0 GPa with a pressure hysteresis over 11 GPa, which implies the reversibility of the two phase transitions. Bulk moduli of the cubic and orthogonal phases are estimated by fitting the data to a Brich-Murnaghan equation of state equal to 169.9 and 48.2 GPa, respectively.     

Simulated equation of state of CaF2 with fluorite-type structure at high temperature and high pressure

The pressure-volume-temperature (P-V-T) equation of state (EOS), isothermal bulk modulus, and thermal expansivity of CaF₂ with cubic fluorite-type structure are investigated using the constant temperature and pressure shell model molecular dynamics (MD) method with effective pair potentials which consist of the Coulomb, dispersion, and repulsion interaction. It was shown that MD simulation is very successful in accurately reproducing the measured volumes of the CaF₂ over a wide range of pressures. The simulated P-V data matched X-ray diffraction experimental results up to 9.5 GPa at 300 K. In addition, volume thermal-expansion coefficient and isothermal bulk modulus were also calculated and compared with available experimental data and the latest theoretical results at ambient condition. At extended temperature and pressure ranges, The P-V EOS under different isotherms at selected temperatures, T-V EOS under different isobars at selected pressures, thermal expansivity, and isothermal bulk modulus were predicted up to 1500 K and 10 GPa. The detailed knowledge of thermodynamic behavior and EOS at extreme conditions are of fundamental importance to the understanding of the physical properties of CaF₂.     

High pressure effects on the electrical resistivity behavior of the Kondo lattice, YbPd2Si2

We report the influence of external high-pressure (P up to 8 GPa) on the temperature (T) dependence of electrical resistivity (ρ) of a Yb-based Kondo lattice, YbPd₂Si₂, which does not undergo magnetic ordering under ambient pressure condition. There are qualitative changes in the ρ(T) behavior due to the application of external pressure. While ρ is found to vary quadratically below 15 K (down to 45 mK) characteristic of Fermi-liquids, a drop is observed below 0.5 K for P=1 GPa, signaling the onset of magnetic ordering of Yb ions with the application of P. The T at which this fall occurs goes through a peak as a function of P (8 K for P=2 GPa and about 5 K at high pressures), mimicking Doniach's magnetic phase diagram. We infer that this compound is one of the very few Yb-based stoichiometric materials, in which one can traverse from valence fluctuation to magnetic ordering by the application of external pressure.     

High pressure X-ray diffraction and electrical resistance study of the quasi-one-dimensional sulfide InV6S8

The ambient structural details and the results of room temperature high pressure angle dispersive X-ray diffraction and electrical resistance measurements on the quasi-one-dimensional sulfide, InV₆S₈, to a pressure of 25 GPa are reported. The material does not undergo a phase transition in this pressure range, though an anomaly in the c/a ratio has been observed around 10 Gpa. A fit of the Murnaghan equation of state to the V/V₀ versus pressure data, with the value of the derivative of B₀ with respect to pressure, B′₀, fixed at 4 has yielded a value of the bulk modulus, B₀, of 110 GPa. We also present data of the pressure dependence of the lattice constants, a and c, the ratio c/a, and the resistance at room temperature.