Magnetic, electrical and thermal transport properties of Al–Cr–Fe approximant phases

Two stable approximant phases in the Al–Cr–Fe system – a gamma-brass phase (γ-AlCrFe) and a mixture of two orthorhombic approximants of the decagonal phase (O1/O2-AlCrFe) – were investigated using magnetic susceptibility, electrical resistivity and thermal conductivity measurements, combined with structural investigations using X-ray diffraction, light microscopy (LM) and scanning electron microscopy (SEM). The investigated approximants exhibit physical properties that are in many respects between those of regular metals and quasicrystals (QCs); their electrical resistivities show very weak temperature dependences and the resistivity values are higher than for regular metals and lower than for Al-based QCs. The magnetic susceptibility results show the existence of a small fraction (of about 1% for the γ-AlCrFe and about 10 times less for the O1/O2-AlCrFe) of localized magnetic moments with Curie-like temperature dependence. Thermal conductivity measurements show that the electronic and lattice contributions are of comparable size at room temperature. While the electronic contribution can be described by the Wiedemann–Franz law, the lattice contribution can be reproduced by the sum of the Debye term (long-wavelength phonons) and the term due to hopping of localized vibrations. At the lowest measured temperature (8 K), scattering of phonons on stacking-fault-like defects limits the heat transport, and this type of defect has also been observed in the LM and SEM structural investigations.