Electron transport properties of glassy and crystalline Pd-Ge ribbons

Abstract

The electrical resistivity and the absolute thermoelectric power (TEP) of Pd₈₀Ge₂₀ and Pd_77.5Ge_22.5 alloys, in both glassy and crystalline states, have been measured over a wide temperature range. In the glassy alloys, the room-temperature resistivity is high (about 100 mu Omega cm) and has a positive temperature coefficient. The room-temperature TEP for the alloys in both states is negative. The magnitude is, however, a factor of four larger in the corresponding crystalline state. In Pd_77.5Ge_22.5 glassy alloys, the TEP also changes sign at about 230K as the temperature is lowered. The overall transport behaviour of the glassy alloys can be understood in terms of the Evans-Greenwood-Lloyd diffraction model (1971) for electron concentration in the alloys. This involves electron localisation at the Ge sites and filling of the transition-metal d band. Since there is a delicate interplay between the structure factor and the d phase shift terms of the TEP, its sign is decided by the relative magnitudes of these two terms.