T. B. Massalski

Experimental Evidence for the Fermi Surface Overlap Effect in

Measurements of the electronic specific heat coefficient and of the limiting Debye temperature are reported for ten Ag-Zn alloys in the range of the h.e.p. ε-phase. After a correction for the electron-phonon enhancement, the trend of the electronic specific heat coefficient is consistent with a nearly rigid band behaviour, showing a general decrease of the density of states at the Fermi level when the corners of the Brillouin zone are filled. A slight deviation from this trend occurs at electron concentration values exceeding approximately 1.855, in agreement with other measured properties and confirming a theoretical model involving overlaps of electrons across the {00.2} planes of the Brillouin zone. The estimated band gaps are of the order of 2 eV. I t appears that whereas in the dilute rj-phase alloys of zinc with silver the rigid band condition is not valid the opposite is true in the concentrated ε-phase alloys.

\epsilon

Measurements of the electronic specific heat coefficient and of the limiting Debye temperature are reported for several close-packed hexagonal intermediate phases in the systems Cu-Ge, Ag-Sn and Ag-Cd. The obtained results, together with previous similar work, conform to a general pattern which suggests that the electronic structure of all h. c. p. phases based on the noble metals can be described in terms of a common nearly rigid band for the conduction electrons. The details of the trend in the density of states are similar to the calculated density of states trends for the h. c. p. metals Zn and Be. The main feature is a large peak in the range of electron concentration between 1.4 and 1.8 which is most likely the result of Fermi surface contacts with the {10.1} Brillouin zone planes, and overlaps across the {10.0} zone planes. A secondary feature is the onset of overlaps across the {00.2} zone planes. A model of an inscribed Fermi sphere into the Brillouin zone is sufficient to indicate the expected behaviour of the trend in the electronic heat capacities in individual phases. Electron-phonon enhancement in the h. c. p. phases appears to be a nearly constant factor, amounting to about 30% of the measured electronic heat capacity values.

-phase Ag-Zn Alloys, Obtained from Low Temperature Specific Heats

Measurements of the electronic specific heat coefficient and of the limiting Debye temperature are reported for pure zinc and for two n-phase alloys containing 2 at. % and 4 at. % silver in zinc, respectively. After a correction for electron-phonon enhancement the electronic specific heat coefficient for pure zinc differs by only a small percentage from the calculated value reported in the literature on the basis of a band calculation. The results for the alloys show a decreasing trend of the density of states at the Fermi level when silver is added to zinc. This is contrary to a prediction based on a rigid band approach. Hence, the results indicate a complete breakdown of the rigid band condition on alloying. The reasons for this are most likely associated with the influence of the d band electrons or with charge distribution effects between solute and solvent atoms.

Electronic Band Structure of h.c.p. Electron Phases Based on the Noble Metals

Design of alloys with useful properties often involves a detailed knowledge of the phases that are likely to be present in a given alloy after a given heat treatment. This in turn can be assisted by the knowledge of relative phase stability as controlled by thermodynamic and also kinetic considerations. In this paper, we discuss computer modeling of relative phase stabilities in an increasing order of demand on computational facilities: i) theoretical calculation of the Pu-V phase diagram, ii) modeling of phases present in Pd-Si alloys following rapid cooling from the melt, and iii) first principles calculation of relative phase stability in ordered intermetallic compounds Co 3 Ti and Ni 3 V.