Formation and composition of the clathrate phase in the H<Subscript>2</Subscript>O-H<Subscript>2</Subscript> system at pressures to 1.8 kbar

The transition of the hexagonal ice phase I_h to the clathrate phase sII has been found in the H₂O-H₂ system at a pressure of about 1 kbar under conditions of an excess of gaseous hydrogen. The pressures of the I_h → sII and sII → I_h transitions have been determined over a temperature range from ?36 to ?18°C, and the pressure dependence of the synthesis temperature of the clathrate phase from a liquid at pressures from 1.0 to 1.8 kbar has been constructed. The solubility of hydrogen in the I_h and sII phases and in liquid water has been measured. The concentration of hydrogen in the clathrate phase sII is about 1.2 wt % (10 mol %) near the boundary of the sII → I_h transition, and it increases to 2 wt % (16 mol %) at a pressure of 1.8 kbar.     

High pressure tuning of optical transitions in Mg[Pt(CN)4]·7H2O

Mg[Pt(CN)₄]·7H₂O belongs to the class of tetracyanoplatinates(II) which crystallize in columnar structures. In different M_x[Pt(CN)₄]·yH₂O (MCP) single crystals the in-chain Pt-Pt-distance R varies between 3.67 Å (NaCP) and 3.15 Å (MgCP). Two optical transitions can be observed in polarized emission with the electric field vector E either parallel or perpendicular to the columnar (c)-axis. Polarized emission spectra of MgCP are recorded under hydrostatic pressure up to p ≈ 18 kbar (at 295 K). The transition energy v?₆ can be tuned from 17,600 cm^-1 to about 12,000 cm^-1 (2.18-1.48 eV). The pressure induced red shift for the two transitions is: E 6 c: dv?₆/dp = -320±20 cm^-1/kbar, E ⊥ c: dv?_⊥/dp = -270±20 cm^-1/kbar. These values are discussed in the context of the known functional relationship (for ambient conditions) between v? and R.     

Phases of silicon at high pressure

X-ray diffraction studies are reported on silicon at pressures up to 250 kbar (25 GPa). A transition to the β-Sn structure (II) initiates at 112 ± 2 kbar and two phases (I + II) coexist to 125 ± 2 kbar. At 132 ± 2 kbar a new phase (V) initiates, and the transition is complete at 164 ± 5 kbar. This phase persists to 250 kbar. Its structure is tentatively assigned as primitive hexagonal with c/a = 0.941 ± 0.002 at 250 kbar. On release of pressure, the sequence is V → (V + II) (145 - 110 kbar) → II → (II + III) (108 - 85 kbar) → III, the last phase persisting to room pressure.     

LaAg under hydrostatic pressure: Superconductivity and phase transformation

The electrical resistivity of LaAg has been measured from 1–300 K at hydrostatic pressures to 12.4 kbar and the superconducting transition temperature T_c determined inductively to 23 kbar. For P ? 6.2 kbar a lattice transformation is observed at a temperature T_M which increases rapidly with pressure. T_c shows an oscillatory pressure dependence, increasing initially from T_c (0) = 1.062 K. There is no obvious correlation between the pressure dependence of T_M and T_c.     

Exciton condensation in intermediate valent Sm0.90La0.10S

Sm_1−xLa_xS for x more than a few percents are metals at ambient conditions. At low temperature and high pressure they develop a small gap in the order of some meV and become semiconductors or insulators. This has been interpreted as a manifestation of the excitonic insulator. In this Letter we will concentrate on Sm_0.90La_0.10S, which is the only composition showing a first order transition. Measurements of the volume change with pressure at ambient temperature show this first order volume collapse at 5 kbar with hysteresis. The resistivity is measured in function of temperature and pressure and exhibits also at 5 kbar and ambient temperature a first order phase transition to a more metallic state. At low temperatures and in function of pressure the resistivity exhibits a peak. The optical reflectivity at 300 K has been measured at low and high pressure and transforms with pressure above 5 kbar into the golden metallic phase.     

Charge density waves in layer structures: A NMR study on a 2HNbSe2 single crystal

We have studied by pulsed NMR a single crystal of 2HNbSe₂ at ambient pressure and under hydrostatic pressure of 21 kbar in the temperature range 4.2–273 K. Our results are consistent with the onset of incommensurate charge density waves (ICDW) at T_CDW = 33 K atP = 1 bar and 26 K at P = 21 kbar. Below T_CDW, the lineshapes of the (m → m ? 1) transitions agree with a local distribution of Knight shift and electric field gradients respecting the symmetry of a triple ICDW, while above T_CDW, pre-transitional broadening is observed. The product T₁T = 500 ± 100 msk was found constant in the temperature range 4.2–77 K and pressure independant.     

Compression and polymorphism of potassium to 400 kbar

Both the compression and polymorphism of K were investigated to about 400 kbar at room temperature in a diamond-anvil pressure cell by optical and X-ray diffraction techniques. The ambient b.c.c.-K(I) is stable to about 120 kbar. The compression data for K(I), fitted to the Birch equation of state, yielded the zero-pressure bulk modulus B₀ = 29.9 ± 0.2 kbar and its first pressure derivative B'₀ = 4.15 ± 0.10. These values agree very well with those of recent data derived from the direct measurement of length changes using piston-displacement (to 20 kbar) and laser interferometer (to 7 kbar) techniques. On the basis of the compression data for K(I) alone, Bridgman's pressure scale may be overestimated by about 10 kbar at 100 kbar and that of Kennedy and LaMori may be underestimated by about 5 kbar at 50 kbar. Two high-pressure polymorphs of K were revealed in the pressure range 100–200 kbar. The b.c.c. → f.c.c. transition in K was observed to occur near 120 kbar, accompanied by a −2.5% discontinuous change in volume. The sample changes gradually from silver to gold in the range 130–150 kbar. The f.c.c.-K.(II) transforms further into an as yet undetermined structure between 170 and 190 kbar. No change in colour was observed in the latter transition. K(III) is stable up to at least about 400 kbar. The equation of state for the f.c.c. phase of K cannot be established. The volume of K was compressed more than 60% in the vicinity of 200 kbar, however.     

A resistance minimum and the electronic structures of some one-dimensional Pt-complexes at high pressures

The effect of pressure has been measured up to 650 kbar on the electrical resistance in six one-dimensional Pt-complexes, including two mixed valence ones. SrPt(CN)₄·2H₂O, MgPt(CN)₄·7H₂O and BaPt(CN)₄·4H₂O do not show a resistance minimum, contrary to other d⁸-metal complexes, suggesting a considerable contribution of π¹ orbitals of CN^- ligands to the conduction.     

Low-energy impact of a heavy Higgs boson sector

The effective theory which characterizes the low-energy sensitivity of the minimal Weinberg-Salam model to a heavy Higgs boson sector is shown to be the gauged SU(2)_L × U(1) non-linear θ model. This theory is the limit of the Weinberg-Salam model as the Higgs boson mass, M_H, is removed (M_H → ∞). Using the symmetry properties of the non-linear theory, along with a power-counting analysis, we are able to classify low-energy observables according to their sensitivity to the regulator (M_H). At one loop, the greatest sensitivity is a logarithmic dependence on the Higgs boson mass. The M_H dependent corrections to some specific, experimentally accessible observables are calculated, and other possible applications of this technique are discussed.     

Dielectric properties and phase transitions in alkali cyanides and cyanospinels under the influence of hydrostatic pressure

The effect of hydrostatic pressure p on the low-frequency dielectric constant ? has been investigated for selected cyanides (NaCN, KCN) and cyanospinels [K₂M(CN)₄ with M = Zn, Cd, Hg and Rb₂Zn(CN)₄] for pressures up to 7 kbar In the low-pressure region (decreases monotomcally resulting in negative first-order pressure denvatives of the dielectric constant The second- and third-order pressure derivatives, however, proved to be positive in most cases Using the dielectric constant as a very sensitive probe we observed phase transitions from the cubic low-pressure phase to an orthorhombic (NaCN) resp trigonal (cyanospinels) high-pressure phase at the following transition pressures (for 293.2 K) 2 260 kbar for NaCN, 1 438 kbar for K₂Hg(CN)₄, 2 660 kbar for K₂Cd(CN)₄, 3 318 kbar for K₂Zn(CN)₄ and 0.690 kbar for Rb₂Zn(CN)₄ The transition temperature T_c, was found to increase strictly linear with pressure between 290 and 340 K at a rate of dT_c/dp = 120 2 and 105 3 Kkbar^?1 for K₂Zn(CN)₄ and K₂Cd(CN)₄, respectively.     

Formation and diffusion of self-interstitial atoms in silicon crystals under hydrostatic pressure: Quantum-chemical simulation

A theoretical modeling of the formation of Frenkel pairs and the diffusion of a self-interstitial atom in silicon crystals at normal and high (hydrostatic) pressures has been performed using molecular dynamics, semiempirical quantum-chemical (NDDO-PM5, PM6), and ab initio (SIESTA) methods. It is shown that, in a silicon crystal, the most stable configuration of a self-interstitial atom in the neutral charge state (I ⁰) is the split configuration 〈110〉. The shifted tetrahedral configuration (T ₁) is stable in the singlet and triplet excited states, as well as in the charge state Z = +2. The split 〈110〉 interstitial configuration remains stable under hydrostatic pressure (P ≤ 80 kbar). The activation barriers for diffusion of self-interstitial atoms in silicon crystals are determined to be as follows: ΔE _a (Si)(〈110〉 → T ₁) = 0.59 eV, ΔE _a (Si)(T ₁ → T′₁) = 0.1 eV, and ΔE _a (Si)(T ₁ → 〈110〉) = 0.23 eV. The hydrostatic pressure (P ≤ 80 kbar) increases the activation barrier for diffusion of self-interstitial atoms in silicon crystals. The energies of the formation of a separate Frenkel pair, a self-interstitial atom, and a vacancy are determined. It is demonstrated that the hydrostatic pressure decreases the energy of the formation of Frenkel pairs.     

Thermal decomposition and electrical conductivity of M(OH)3 and MOOH (M=Y, lanthanide)

The thermal decomposition of M(OH)₃ (M=Y, La, Nd, Sm, Gd) with the Y(OH)₃ structure was examined by the TG and DTA methods. Y(OH)₃, Nd(OH)₃, and Sm(OH)₃ decomposed to MOOH and then to M₂O₃. The decomposition of La(OH)₃ and Gd(OH)₃ occurred via the following schemes: La(OH)₃→LaOOH→La₂O₃·1/2H₂O→La₂O₃, and Gd(OH)₃→Gd₂O₃·3/2H₂O→GdOOH→Gd₂O₃. The highest conductivity of 5.9×1o^?9Scm^?1 at 250°C was found in Gd(OH)₃ and that of 8.9×10^?7 S cm^?1 400°C in GdOOH. The continuous-wave ¹H NMR absorption spectrum of LaOOH at room temperature exhibited no doublet line shape. This shows that protons are magnetically isolated from each other, and very little H₂O and H₃O⁺ can exist.     

Dependence of the fundamental absorption edge of CdInGaS4 on hydrostatic pressure

The optical absorption of CdInGaS₄ single crystals has been measured over a hydrostatic pressure range up to 40 kbar in the 2.0–3.0 eV photon energy range at room temperature. The interband gap for the indirect allowed transition was found to have a pressure coefficient, dEg/dP, of +6.4 × 10^-6ev/bar.     

Phase transformations in equiatomic alloy TiZr at pressure up to 70 kbar

The effect of pressure on the α ? β and ω ? β transformations in the equiatomic alloy TiZr is studied by the differential thermal analysis (DTA) and calorimetric technique. The α-β equilibrium at atmospheric pressure occurs at a temperature of 579°C, and the heat of transition ΔH is 40.9±2.0 J/g. As the pressure increases up to 28 kbar, the temperature of the α-β equilibrium linearly decreases, dT/dP=?2.2±0.3 K/kbar. In the pressure range 28–48 kbar, the β-phase undergoes a transition to the two-phase (α + ω) state upon cooling to room temperature. At pressures above the triple point with the coordinates P=49±3 kbar and T=460±30°C, the cooling of the β-phase gives rise to only the hexagonal ω-phase with the unit cell parameters a=4.843 Å, c=2.988 Å, and c/a=0.617 under normal conditions. The slope of the ω-β equilibrium boundary is positive at pressures up to 70 kbar, dT/dP≈0.46 K/kbar. The ω → α transformation at atmospheric pressure proceeds in the temperature range T=425–470°C with the enthalpy of transition ΔH=2.8 J/g.     

Electrical measurements of the electron irradiation induced E- and H-traps in GaAs under hydrostatic pressure

The change of resistivity of the 2.3 MeV-electron-irradiated bulk n- and p-GaAs have been measured at hydrostatic pressure up to 5 kbar at RT. Corrections for the changes in free electron and hole mobilities with pressure have been neglected. The resistivity changes are explained by a dependence on pressure of the ionisation energy of the radiation-induced E- and H-traps. The results indicate that most from these radiation- induced levels moves away from the conduction-band edge (γ_c-point) at a rate approximately (0.8?1.0)γ_G, here γ_G=11.6×10^?6 eV bar^?1 is the energy gap pressure coefficient for GaAs at RT. The high changes in ionization energies of E2 to E5-traps upon pressure are to be compared with the lower changes in ionization energies found for the deep-lying impurity levels. In accordance with the theoretical investigation it was suggested that most of the investigated radiation-induced levels in GaAs are t₂-states of Ga- and As-vacancies.     

Pressure-induced structural phase transition in ReO3

We have investigated the pressure-induced structural phase transition in ReO₃ by neutron diffraction on a single crystal. We collected neutron diffraction intensities from the ambient and high pressure phases at P=7 kbar and refined the crystal structures. We have determined the stability of the high pressure phase as a function temperature down to T=2 K and have constructed the (P-T) phase diagram. The critical pressure is P_c=5.2 kbar at T=300 K and decreases almost linearly with decreasing temperature to become P_c=2.5 kbar at T=50 K. The phase transition is driven by the softening of the M₃ phonon mode. The high pressure phase is formed by the rigid rotation of almost undistorted ReO₆ octahedra and the Re-O-Re angle deviates from 180°. We do not see any evidence for the existence of the tetragonal (P4/mbm) intermediate pressure phase reported earlier.     

High-field ENDOR and the sign of the hyperfine coupling

Two different approaches for assigning electron nuclear double resonance (ENDOR) signals to their respectiveM _s manifolds by a controlled generation of asymmetric ENDOR spectra, are described and applied to a number of systems. This assignment then allows a straightforward determination of the sign of the hyperfine coupling. Both approaches rely on a high thermal polarization that is easily achieved at high fields and low temperatures. For high-spin systems, such asS = 5/2 the assignment is afforded by the selective inversion of the | ?3/2〉 → | ?1/2〉 electron paramagnetic resonance (EPR) transition which is highly populated as compared to its symmetric counterpart, the |1/2〉 → |3/2〉 EPR transition, and therefore is easily identified. ForS = 1/2 the determination of the sign of the hyperfine coupling becomes possible when the cross- and nuclear-spin relaxation rates are much slower than the electron-spin relaxation rate and variable mixing time pulse ENDOR is used to measure the spectrum. Under these conditions the signals of theM _s = 1/2 (α) manifold become negative when the mixing time is on order of the electron-spin relaxation time, whereas those of theM _s =?1/2 (β) manifold remain positive. Under partial saturation of the nuclear transitions and short mixing time the opposite behavior is observed. Pulse W-band¹H ENDOR experiments demonstrating these approaches were applied and the signs of the hyperfine couplings of the water ligands in Mn(H₂O) ₆ ²⁺ , the H_α and H_β histidine protons in the Cu(histidine)₂ complex, the imidazole protons in Cu(imidazole) ₄ ²⁺ and the cysteine β-protons in nitrous oxide reductase were determined.     

Absorption measurements of H2O at high temperatures using a CO laser

Absorption of CO laser radiation (v = 8→7, J = 14→15 transition at 1901.762 cm^-1) by H₂O has been studied in shock-heated H₂/O₂/Ar mixtures over the temperature range 1300–2300 K. This laser transition is nearly coincident with the v₂-band 12_3,10←11_2,9 transition of H₂O at 1901.760 cm^-1, thereby providing a convenient and sensitive absorption-based H₂O diagnostic useful for studies of combustion. The collision-broadening parameter for this H₂O line, due to broadening by Ar, was determined to be 2γ (cm^-1atm^-1) = 0.027 (T/1300)^-0.9 in the temperature range 1300–2300 K. Calculations of the H₂O absorption coefficient (at 1901.762 cm^-1) based on this expression for 2γ are presented for the temperature range 300–2500 K and pressure range 0.3–1 atm.     

电子回旋共振等离子体增强沉积氟化非晶碳薄膜的光学性质

用紫外可见光透射光谱(UV-VIS)并结合键结构的X射线光电子能谱(XPS)和红外谱(FTIR)分析,研究了电子回旋共振等离子体增强化学气相沉积法制备的氟化非晶碳薄膜的光吸收和光学带隙性质.在微波功率为140—700W、源气体CHF₃∶C₆H₆比例为1∶1—10∶1条件下沉积的薄膜,光学带隙在1.76—2.85eV之间.薄膜中氟的引入对吸收边和光学带隙产生较大的影响,吸收边随氟含量的提高而增大,光学带隙则主要取决于CF键的含量,是由于强电负 关键词： 氟化非晶碳薄膜 光吸收与光学带隙 电子回旋共振等离子体     

Pressure-induced phase transformation in In2Bi

We have made measurements of the pressure dependence of the superconducting transition temperature, T_c, for In₂Bi and related alloys. For In₂Bi- phase alloys, a large discontinuity in T_c is seen at 15–20 kbar, which we associate with a phase transformation first seen by Bridgman [1]. Our measurements suggest that this transformation is produced by the decomposition of In₂Bi into In₅Bi₃ and an In-rich phase. In the low pressure phase, T_c shows a minimum at 9–15 kbar whereas it depends linearly on pressure in the high pressure phase with ?T_c/?P equal to -4.9 × 10^-5 K bar^-1.