Magnetic and structural characteristics of Ho6Fe23 deuterides

The pressure-composition isotherm of the Ho₆Fe₂₃D₂ system at 0°C was established, revealing the presence of two plateau regions. Using the isotherm, five compositions of the Ho₆Fe₂₃D_x system (x = 0, 1.5, 8.2, 12.1, 15.7) were selected for investigation. These were examined by X-ray diffraction and magnetization measurements. The cubic lattice of the host metal is maintained to x = 8.2. A tetragonal structure develops at x = 12.1, but this reverts to cubic symmetry at the highest deuterium concentration studied. The observed structural changes are ascribed to preferential site occupancies by deuterium. Absorption of 15.7 deuterium atoms per formula unit in Ho₆Fe₂₃ caused a 10.2% increase of the unit cell volume. Increase in the deuterium content resulted in a striking increase in the Curie temperature T_c and in a linear decrease in compensation temperature T_comp. The saturation magnetization changed smoothly as the system changed from one structure type to another. There appears to be a significant increase in Fe magnetic moment upon deuterium absorption, which is ascribed to the removal of electrons from the 3d iron band by deuterium.     

Amperometric titration of boron with fructose

Aqueous solutions of boratei can be titrated amperometrically with standard fi uctose solutions This procedure may be used to determine the boron content of various types of samples after conversion of the boron to the borate form The methode is based upon the observation that the polarographic wave of fructose. in a o I M LiCl, 001 N'LiOH solution is deprcssed by the presence of borate ions Under the prescribed conditions the methode is sensitive to a few hundredths of a milligram of boron The few comnonly occurring intcrferenes can be conveniently removed prior to the titration     

Oxygen adsorption on hydrated gold cluster anions: experiment and theory

The discovery that supported gold clusters act as highly efficient catalysts for low-temperature oxidation reactions has led to a great deal of work aimed at understanding the origins of the catalytic activity. Several studies have shown that the presence of trace moisture is required for the catalysts to function. Using near-atmospheric pressure flow reactor techniques, we have studied humidity and temperature effects on the reactivity of gas-phase gold cluster anions with O2. Near room temperature, the humid source produces abundant gold-hydroxy cluster anions, Au(N)OH(-), and these have a reversed O2 adsorption activity: Nonreactive bare gold clusters become active when in the form Au(N)OH(-), while active bare clusters are inactive when -OH is bound. The binding energies for the stable structures obtained from density functional calculations confirm fully these findings. Moreover, the theory provides evidence that electron-transfer induced by the binding of a OH group enhances the reactivity toward molecular oxygen for odd anionic gold clusters and suppresses the reactivity for the even ones. The temperature dependence of O2 addition to Au(3)OH(-) and Au(4)(-) indicates deviations from equilibrium control at temperatures below room temperature. The effects of humidity on gold cluster adsorption activity support the conclusion drawn for the mechanism of O2 adsorption on "dry" gold cluster anions and provides insight into the possible role of water in the enhanced activity of supported gold cluster catalysts.     

Covalent-ionic nature of the potential energy surface of the Li-CO2 complex

Unrestricted Hartree-Fock calculations with large basis sets, including d-functions, and the estimation of the correlation energy, show that the potential energy surface for the Li-CO₂ complex is built from the crossing of two states, each of them corresponding to a different electron arrangement. One has a strong ionic character and the other is of van der Waals type. Each portion of the energy surface presents a minimum, which is stable in respect to the dissociation limit.     

Dissolved oxygen: the electroanalytical chemists dilemma

Electroanalytical chemistry has been plagued for decades with the problems associated with dissolved oxygen. Dissolved oxygen is electrochemically reduced at potentials less negative than ?0.05 V vs. standard calomel electrode (SCE) and, therefore, interferes with detection of many species when using reductive voltammetry. Historically, electroanalytical chemists have taken the view that dissolved oxygen should be removed from the sample solution in order to overcome the interference problems. However, there are drawbacks associated with this approach, particularly when dealing with flowing solutions. More recently an alternative approach, the development of voltammetric methods which are not so prone to dissolved oxygen interference, has been taken by various workers.