Hall-effect measurements and the electronic structure of amorphous Pd-Si-(Cu) alloys

Abstract

Measurements of the Hall-effect coefficient (${R}_{H}$) have been performed at room temperature on the same set of amorphous alloy ribbons in the systems Pd-Si and Pd-Si-Cu for which electronic specific-heat measurements have been reported earlier. The behavior of the Hall coefficient shows a marked change in the range of 18-20 at.% Si, coinciding with the previously observed maximum in the electronic density of states in the same range. The diameter of the Fermi sphere, $2{k}_{F}$, calculated from the Hall data, appears to be smaller for all alloys investigated than the wave number ${q}_{p}$ corresponding to the occurrence of the first peak in the structure factor. Hence the condition $2{k}_{F}={q}_{p}$ is never reached in the amorphous alloys. The corresponding effective electron concentration ${(\frac{e}{a})}_{\mathrm{eff}}$ in the conduction band, derived from the Hall coefficient and appropriate densities, varies between 0.8 and 1.3 electrons per atom. The increasing trend of ${(\frac{e}{a})}_{\mathrm{eff}}$ with silicon content is approximately linear above 19 at.% Si, and in that range the effective valences of Pd and Si are approximately 0.38 and 4, respectively. Below 19 at.% Si, the effective valence of Si decreases markedly, suggesting that the silicon atoms are no longer fully ionized in the dilute Pd-Si alloys. Under these conditions, the $3s$ and $3p$ electrons are likely to exist in the form of bound states near the Fermi level, and their presence is most likely the reason for the occurrence of a maximum in the corresponding density of states observed previously.