Synthesis and superstructure of Al-Ir, Al-Ir-Pd crystalline approximants and their electrical resistivities

Chemically ordered Al2.7Ir and Al64.5Ir22Pd13.(5) 1/0 approximants are prepared by two different methods and characterized by X-ray diffraction and transmission electron microscopy. Al2.7Ir possesses a primitive cubic lattice with a = 15.345(6), whereas Al64.5Ir22Pd13.(5) has a face-centered cubic lattice with a = 15.497(9). Examination of the samples prepared by two different methods reveals that the texture of the samples prepared by spark plasma sintering is more homogeneous than those prepared by arc melting. Electrical resistivities of both the compounds exhibit metallic characters, i.e., a positive temperature coefficient of the resistivity (TCR) with a high resistivity value. The results suggest that the TCR may not always be negative for 1/0 approximants with 15-16 periodicity but the high residual resistivities are common to all the approximants, which is presumably attributed to the common structural unit, an icosahedral cluster of similar to 7.7.