Complex ɛ-phases in the Al–Pd-transition–metal systems: Towards a combination of an electrical conductor with a thermal insulator

ɛ-Phases in the Al–Pd–(Mn,Fe,Co,Rh,…) alloy systems form in wide compositional ranges and belong to the interesting class of complex intermetallics that are characterized by giant unit cells with quasicrystals-like cluster substructure. In order to see how the exceptional structural complexity and the coexistence of two competing physical length scales affect the physical properties of the material, we performed investigation of the magnetic, electrical, thermal transport and thermoelectric properties of the ɛ-phases in the Al–Pd–Fe, Al–Pd–Co and Al–Pd–Rh systems. Magnetic measurements reveal that the materials are diamagnetic, containing tiny fractions of magnetic transition–metal atoms (of the order 10–100 ppm). Electrical resistivity is moderate, of the order 102 μΩ cm, and shows weak temperature dependence (in most cases of a few %) in the investigated temperature range 4–300 K. An interesting feature of the ɛ-phases is their low thermal conductivity, which is at room temperature comparable to that of thermal insulators amorphous SiO2 and Zr/YO2 ceramics. While SiO2 and Zr/YO2 are also electrical insulators, ɛ-phases exhibit electrical conductivity typical of metallic alloys, so that they offer an interesting combination of an electrical conductor with a thermal insulator. The reason for the weak thermal conductivity of the ɛ-phases appears to be structural: large and heavy atomic clusters of icosahedral symmetry in the giant unit cell prevent the propagation of extended phonons, so that the lattice can no more efficiently participate in the heat transport. The thermoelectric power of the investigated ɛ-phase families is small, so that these materials do not appear promising candidates for the thermoelectric application.