The parameters in the crystal structure of α-GeTe were determined by least-squares analysis of intensity data obtained with an automatic x-ray powder diffractometer. The rhombohedral cell containing 4 Ge and 4 Te atoms has a=5.996±0.001 Å, α=88.18°, and the single atom-position parameter x=0.237±0.002. The displacement of one sublattice with respect to the other from the relative position that would correspond to the NaCl structure (i.e., x=0.250) corroborates the predictions, based on band calculations, of Cohen, Falicov, and Golin.
Diffusion in the weakly paramagnetic aluminum (rich) – copper system has been investigated in a magnetic field. Aluminum – aluminum/3% copper solid diffusion couples were diffusion annealed in a magnetic field of 30 and 20 kilo-oersteds, and then analyzed for diffusion using a microhardness technique. A statistically significant 25% decrease in the diffusivity was found for the couples annealed in a 30-kilo-oersted field applied perpendicular to the diffusion direction. However, from the accuracy of the results it is not possible to determine whether the field affects only the frequency factor D0, the activation energy E, or both factors. A theory based on the plasma–agnetohydrodynamic properties of the alloy system is developed to explain the results. According to the theory, the magnetic field decreases the diffusivity by the factor 1/(1 + ωce2/νe2), where ωce and νc are the cyclotron and collision frequencies respectively of the diffusion-transported electrons. The plasma-oscillation screening effect is discussed, and it is shown that the field does not significantly affect charge screening. It is thus proposed that the diffusivity is decreased through the frequency factor D0, and not through the activation energy of the atom-vacancy jump process.
The degree of normal segregation, occurring during directional (normal) freezing in a transverse magnetic field of 34 000 oersteds, has been determined for aluminium–copper alloys solidified for rates of [Formula: see text] to 4 inches per hour. For ingots in the composition range 0.5% < %Cu < 4.5%, solidified at rates exceeding [Formula: see text] inch per hour, the magnetic field increased normal segregation, corresponding to a maximum decrease in the effective distribution coefficient ke of approximately 13%. It is shown that the decrease in ke can occur only through a rise in the liquidus of the alloy system, which is in agreement with the theory developed to account for the inverse segregation results obtained for aluminium–copper ingots chill-cast in a magnetic field (previously reported). An increase in ke due to the field was found for 0.5% Cu ingots and also for the 4.5% Cu and 7% Cu ingots. The concentration dependence of ke is related to the concentration gradients and diffusion zone lengths, and is shown to be compatible with the thermodynamic constraints on the system undergoing the irreversible process of solidification in a magnetic field.
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