M. Tkacz

High pressure study of changes in energy and intensity of excitations in crystalline metal glyoximes

The effect of high pressure has been measured on the energy and integrated intensity of electronic excitations of several layered crystals of glyoximes containing Ni, Pd, or Pt. Large changes in both energy and intensity were observed, both of which were completely reversible. The shifts in energy with pressure, are explained in terms of the relative spatial extent of the outer d and p orbitals of Ni, Pd, and Pt. The effects of back donation from the ligands and intensity borrowing from the higher energy charge transfer excitations are considered as possible causes of the observed intensity changes. It was concluded that intensity borrowing was the major cause of the observed changes.

Useful equations of state of hydrogen and deuterium

Abstract The equations of state for hydrogen and deuterium obtained by special fitting procedure to the large number of experimental data available in literature are presented. The form of equation is very simple and allows for easy calculation of fugacities and chemical potentials of corresponding gases. Although the experimental points employed in the fitting procedure concern only the gaseous phases, the equation can be extrapolated to regions of solid phases when volume change due to fluid–solid phase transition is subtracted. Comparison of experimental data available with data obtained by extrapolation or fitting procedure is given in Tables.

High pressure studies of the rhodium–hydrogen system in diamond anvil cell

High pressure x-ray studies on the rhodium–hydrogen system have been performed up to 8 GPa at ambient temperature. Formation of the hydride phase has been observed at 3.8 GPa of hydrogen pressure, accompanied by a rapid increase of the lattice volume of rhodium metal. Decomposition pressure has been determined as 2.5 GPa. This value has been used for the calculation of the standard free energy of formation of the rhodium hydride, giving a value of 11 500 cal/mol H2. The thermodynamic properties of the RhH have been discussed. The pressure-concentration isotherm has been calculated, using the x-ray data and the “universal” volume increase during the hydride phase formation.

Double clathrate hydrates with helium and hydrogen

Abstract Double cubic structure II clathrate hydrates of tetrahydrofuran (THF) with helium (space group Fd3m, a = 17.155(5) A, 120 K) or hydrogen and those of hexafluorophosphoric acid with hydrogen have been obtained. These hydrates are solid solutions due to the variable degree of the filling of the small cavities by helium and hydrogen molecules. The degree of the filling of the small cavities of the hydrate of THF by helium at 3.5 kbar is 24.5%; hydrogen can occupy 100% of the cavities, and at 7.0 kbar it is possible for two molecules to be accommodated in the small cavity.

Phase transitions in the water-hydrogen system at pressures up to 4.7 kbar.

Using a volumetric technique, a T-P diagram of phase transformations between the hydrogen-rich clathrate hydrate (sII phase), hydrogen-filled ice II (C(1) phase), and the liquid (L) is studied in the H(2)O-H(2) system at pressures up to 4.7 kbar and temperatures from -22 to +15 degrees C. The volume and entropy effects of these transformations are established in the vicinity of the triple point of the L + sII + C(1) equilibrium located at P = 3.6(1) kbar and T = +1(1) degrees C. The estimated molar ratios H(2)/H(2)O of phases at the triple point are X(L) = 0.04(2), X(sII) = 0.32(2), and X(C1) = 0.10(2).

High pressure X-ray diffraction study of copper hydride at room temperature

Abstract High pressure X-ray studies on CuH up to 23 GPa have been performed at room temperature using a gasketed diamond anvil cell. The experimental data on the molar volume of CuH as a function of pressure have been fitted to Murnaghans equation of state giving a bulk modulus: B0 = 72.5±2 GPa and B0 = 2.7 ± 0.3. By comparison with the equation of state for pure copper the effective additive volume of hydrogen has been evaluated as a function of pressure. It decreases from 3.2 cm3/mol H, at ambient pressure reaching a flattening value of 1.7cm3hol H at about 60 GPa. This suggests a continuous transition of CuH from ionic or covalent character at normal pressure to metallic hydride behavior at high pressure

Magnetic properties of cubic and hexagonal chromium hydrides: a comparison of the magnetic susceptibility with the 53Cr NMR Knight shift

Abstract We have measured the 53 Cr Knight shift in cubic and hexagonal chromium hydrides with atomic hydrogen-to-chromium ratio H/Cr≈1 in the temperature range 3–300 K. The shifts (0.30% in cubic phase, 0.53% in hexagonal phase) are temperature independent and imply that the intrinsic magnetic susceptibilities of the respective hydrides are also temperature independent. The values of intrinsic magnetic susceptibilities have been determined from the analysis of magnetisation isotherms recorded in the temperature range 1.7–300 K, and in magnetic field strengths up to 55 kOe. The isotherms indicate the presence of paramagnetic and ferromagnetic impurities in the samples, the magnetisation of which was subtracted from the measured magnetisation data. The cubic and hexagonal chromium hydrides are Pauli paramagnets with intrinsic susceptibilities of 5.9·10 −8 and 5.0·10 −8 m 3 /kg, respectively.

Thermodynamic properties of iron hydride

Abstract X-Ray diffraction studies of the iron sample subjected to high hydrogen pressure inside a diamond anvil cell have been carried out in order to determine the equilibrium conditions of formation and decomposition of the iron hydride. Due to hysteresis, commonly observed in transition metal hydrides, the pressure of decomposition of the corresponding hydride describes the equilibrium as decomposition is considered to be a stress-free process. X-Ray diffraction studies have revealed the structural phase transition from the b.c.c. structure of iron metal to the d.h.c.p. of hydride at 3.5 GPa, while the decomposition process has been observed at a hydrogen pressure of about 2.2 GPa. Both processes are accompanied by a volume change of the host lattice of 2.8 A 3 as compared to the volume of hexagonal iron, extrapolated from high pressure. Free enthalpy of formation of iron hydride was calculated as equal to 23.5 kJ/mol of hydride. Assuming the entropy of formation of 52.25 J/mol/K, enthalpy of iron hydride is positive and equals to 7.9 kJ/mol of FeH.

The effect of pressure on electronic excitations in TCNQ and its complexes

The electronic spectra of TCNQ and TCNQ complexes has been investigated at high pressure up to 150 kbar. The observed pressure shifts are discussed in terms of intermolecular interactions. The results are also compared with available theoretical calculations.

The influence of high pressure (up to 10 kbar) on the limiting currents of the Cd(II)/Cd(Hg) system

Abstract The electroreduction of cadmium(II) at a mercury electrode and the oxidation of cadmium amalgam in 4.0 M NaCl at pressures ranging from 0.001 to 10 kbar, using cyclic voltammetry and chronoamperometry, revealed a moderate decrease in the limiting currents and diffusion coefficients of CdII in solution and Cd in mercury. The partial molar volume of the diffusion activation calculated from these dependences is equal to 1.1–1.2 and 0.93 cm3 mol−1 for diffusion in 4.0 M NaCl and in mercury, respectively. The formal potentials of the CdII/Cd(Hg) system expressed versus the ferrocene-ferricinium ion electrode as a function of pressure led to a value of 10.7 cm3 mol−1 for the volume of the electroreduction reaction of CdII.