Ordered Al-Pd-Ru icosahedral quasicrystal and its crystalline approximants and their electrical resistivities

In the Al-Pd-Ru system, a structurally ordered icosahedral phase has been successfully produced near the composition Al73Pd15RU12 by heat treatment at 1273 K for several hours. The x-ray diffraction patterns of the Al-Pd-Ru i-phase are very sharp, indicating that the i-phase is quite stable and that samples of high structural quality are attainable by appropriate heat treatment in this system. Prolonged annealing at 1273 K, however, results in phase transformation into the 1/0 cubic approximant (a = 1.5 nm). Also, we found the formation of a 2/1 cubic approximant phase (a = 2.0 nm) near the composition Al71Pd19Ru10 when samples were annealed at 1173 K for one hour. The temperature dependence of the resistivity of the i-phase is significantly large and the resistivity ratio R (R = rho(12K)/rho(300)) reaches as high as 2.5, comparing well to that seen in the ordered Al-Cu-Ru i-phase. The resistivity and its temperature dependence for the 2/1 approximant phase are quite similar to those for the i-phase, exhibiting a large resistivity ratio R similar to 1.6, in contrast with the metallic behaviour of the 1/0 cubic approximant phase. These results indicate that the characteristic length which determines the temperature dependence of the resistivity is somewhere between 1.5 and 2.0 nm and suggest that the long-range quasiperiodicity beyond the length scale is of less significance as far as the electrical properties are concerned.