ICOSAHEDRAL AND APPROXIMANT PHASES IN A MG-GA-AL-ZN ALLOY AND THEIR ELECTRICAL RESISTIVITIES

It has been found in a Mg-Ga-Al-Zn alloy that an icosahedral phase produced by melt-spinning transforms to the 1/1 cubic approximant of the icosahedral phase at 653 K and successively to the 2/1 cubic approximant at around 668 K upon heating. Structural relations of the two approximant phases to the icosahedral phase are discussed, in the framework of the projection scheme, on the basis of X-ray diffraction data. Electrical resistivities have been measured from 4-2 K to 280 K for single-phase samples of the icosahedral and approximant phases. The icosahedral phase and the 2/1 approximant one show similar behaviour, in which the resisitivity is approximately 120-mu-OMEGA cm at 4.2 K and the temperature coefficient is negative. In contrast, the 1/1 approximant phase shows a relatively small resistivity value of approximately 60-mu-OMEGA cm at 4.2 K and a positive temperature coefficient. Changes in the resistivity accompanied by the phase transformations have been confirmed by measuring the resistivity upon heating the melt-spun sample up to above the transformation temperatures.