Stereoselection in the AlCl3-catalysed ene additions of chloral to 1,2-dialkyl ethylenes

The observed stereoselection in the title reactions is predicted by a simple steric model, fully supported by $\text{X}$-ray data.     

Hall effect in granular materials

We show that in granular materials the Hall coefficient cannot, in general, be used to estimate directly the density of carriers, since it will depend on the ratio of the mobility within the grains to the conductivity mobility. The Hall voltage is predominantly determined within the individual grains, where the mobility may be quite large. On the other hand, the conductivity mobility may be considerably smaller due to a strong potential barrier between grains. The Hall coefficient depends on the ratio of these mobilities as well as on the carrier density.     

Influence of absorbed hydrogen on the magnetic behavior of RCo3 (R = Gd,Dy and Ho) compounds

The pressure-composition isotherm for the RCo₃-H₂ system (R = heavy rare earth) shows two plateau pressures. For GdCo₃ the first plateau extends to the approximate composition GdCo₃H_2.2 (designated as β-hydride) and the second to the composition GdCo₃H_4.6 (γ-hydride). Magnetization measurements have been carried out on GdCo₃, GdCo₃H₂, GdCo₃H_4.6, DyCo₃, DyCo₃H_4.3, HoCo₃ and HoCo₃H_4.2 in the temperature range 4.2 K to 300 K. The results show that hydrogen absorption produces a reduction both in the magnetic moment per cobalt and the Curie temperature; the reduction is slight in going from GdCo₃ to GdCo₃H_2.2, but is drastic in going to GdCo₃H_4.6. In contrast, in isostructural GdFe₃ compound, the magnetic moment per iron increases on hydrogen absorption. Measurements on DyCo₃H_4.3 and HoCo₃H_4.2 suggest the possibility that the rare earth moments are not magnetically ordered even at 4.2 K.     

Low temperature heat capacities and thermal properties of DyAl2, ErAl2 and LuAl2

The heat capacities of the compounds DyAl₂, ErAl₂ and LuAl₂ were measured in an adiabatic calorimeter from approximately 5 to 300 K. The compounds DyAl₂ and ErAl₂ show C_P anomalies at 58.0 and 10.2 K, respectively, which are attributed to the destruction of magnetic order. In order to separate the crystal field and magnetic contributions from the measured heat capacities, it was necessary to evaluate the lattice heat capacity. The lattice term, C_L was obtained from the C_P data of LuAl₂ by a method of interpolation which gave values of C_L for an arbitrary R Al₂ compound. Using this “interpolated lattice blank”, excess entropies associated with the crystal field and magnetic terms were computed throughout the series. These values are quite close to R In (2J + 1). The results also indicate that, for the compounds studied, the degeneracy of the lowest ground state is completely lifted. In addition, the magnetic contribution to the heat capacity of the magnetically ordered R A1₂ phases was found to exhibit an exponential dependence below the temperature corresponding to the spin wave energy gap and a T^{$\text{3}\text{2}$} dependence above this temperature.Detailed calculations were performed to characterize the influence of cubic crystal field in ErAl₂ on the ⁴I_{$\text{15}\text{2}$} ground state multiplet of the Er³⁺ ion. It is concluded that the magnetic ordering in ErAl₂ takes place within the Γ₈³ quartet state. Smoothed values of heat capacity, entropy and related thermodynamic functions are tabulated.