EXAFS measurements and reverse Monte Carlo modeling of atomic structure in amorphous Ni80P20 alloys

This paper presents a full account of the EXAFS measurements and reverse Monte Carlo (RMC) modeling of the atomic arrangements and short-to-medium range structure in an amorphous Ni–P alloy, expanding on the description included in our recent publication [H.W. Sheng, W.K. Luo, F.M. Alamgir, J.M. Bai, E. Ma, Nature 439 (2006) 419]. The atomic packing is analyzed from the standpoint of solute atoms. The short-to-medium range structure is discussed based on single-solute-centered quasi-equivalent clusters that form due to strong chemical short-range ordering, and the topological order is described in terms of both intra-cluster and inter-cluster dense packing for efficient filling of space. This analysis is also conducted for amorphous Ni80P20 prepared via different processing routes, to observe if the polyamorphism suggested in literature for amorphous Ni–P can be confirmed from the local structure perspective. The structural differences between the proposed polymorphs are apparently subtle and a full resolution of this issue is found to be beyond the capabilities of our EXAFS/RMC modeling approach. The amorphous structural features uncovered are also compared briefly with those observed before in amorphous alloy systems with positive heat of mixing.