ALUMINUM-COBALT PAIR POTENTIALS AND THE STRUCTURE OF DECAGONAL AL-CU-CO

摘要

Recently, pair potentials have been devised for aluminum-rich transition metal com-pounds. We apply these potentials to the AI65CU17.5C0175 alloy. We begin with a C-centered orthorhombic periodic approximant to the AlCuCo decagonal phase (a= 60.4, b= 83.2, c= 4.15), chosen because it is the unit cell of the observed mi-crocrystalline phase. We produce a set of possible atomic sites within the periodic approximant, based on the atomic surfaces determined by Steurer and Kuo (Ada Cryst, B, 1990, 46, 703) and geometrical constraints. We then run a Monte Carlo simulation to determine the total occupancies of the various sites and the fractional occupancy by Al and Co atoms. The results are used to map out the atomic sur-faces. We find that the vast majority of cobalt atoms belong to one atomic surface and are concentrated toward the center of this surface. The features of the atomic surfaces are explained in terms of the favorable local environments for Al and Co atoms in physical space. We minimize wR with respect to the phonon Debye-Waller factors, using Steurer and Kuo's data, and find a value that compares well with that for other models.