Atomic-level characterization of free volume in the structure of Cu₆₇Zr₃₃amorphous alloy

The structure of Cu67Zr33 amorphous alloy was investigated in terms of packing density and free volume by using neutron, x-ray diffraction and reverse Monte Carlo (RMC) modelling. The RMC model was analysed by a method of decomposing the three-dimensional atomic configuration into fundamental polyhedral units (termed as 'holes' referencing the Bernal's works) of which faces are all triangles consisting of chemical bonds. Not only tetrahedral and octahedral holes but also other larger holes were identified. Moreover, the atomic packing fractions and free volumes in the respective polyhedral holes were evaluated with reference to those for the corresponding crystal structures. The results show that the distribution of free volumes for the larger holes can be described by the exponential function assuming that there are no energetic interactions between each other. On the other hand, the local structural fluctuations due to densely and loosely packed tetrahedral holes were observed, leading to the negative free volume spaces.