Dense Random Packed Models for Amorphous Monatomic and Binary Systems.
In the last few years, computer simulated structure models have become very useful in predicting the density distribution functions of amorphous materials. However, no one model has every adequately described a physical amorphous system, with the result that many models have been proposed. These models may differ from each other in detail but the basic requirement that atoms are represented by spheres still remains. These proposed models can be characterized by either hard or soft spheres. Hard sphere refers to the situation where atoms are in contact with each other, while the latter to one where an energy function or many functions are applied resulting in the broadening of peaks. Hard sphere models are generally used with some kind of Gaussian broadenings in the distribution peaks in order for comparison with the experiments. Without broadening or "relaxation", these structural models possess anisotropy which reflects the inhomogeneity of the system. Periodicity is also observed in the interference function which suggests the existence of residual atomic lattices. Both of these effects are not observed in experiments therefore, the models cannot be used to represent physical systems. Relaxation using Morse and Lennard Jones potentials by Heimendahl on a single element computer simulated system makes the peak positions in the pair correlation function agree with the experiment, but the magnitudes do not compare well. Relaxation of binary systems has not yet been fully investigated. A new simplified model was developed here which uses less computer time than previously published models (approximately from half to an hour) in the structural simulations. The anisotropy and periodicity were investigated in this new unrelaxed model and, based on the correlation functions and the interference functions, were not observed. The average densities and distribution functions are comparable to previous published models and also to the experimental results on Co and TbFe2. Relaxation of the model was investigated with Keating's energy expression for single size atoms. The distribution function compares very well with the experimental result of amorphous Co from Leung and Wright. The investigation was extended to the binary system TbFe2 using the same energy function with some modifications. Comparison made with the results of a neutron scattering data of Rhyne et al shows good agreement. The model was then used to study the binary system Tb(,x)Fe(,1-x) as a function of compositional variation of x (having x = .12, .33, .45 and .75). It was observed by Cargill and Chi et al using x-rays that the peaks in the intensity pattern did not shift with composition in Ni(,1-x)P(,x) and Co(,1-x)P(,x); however, it was also observed by Pickart et al using neutron scattering on Tb(,x)Fe(,1 -x) that the peaks in the interference function do shift with variation of x. A reason for such differences might be that the variation of x for the two previous cases was not large enough to see any significant changes in their distribution functions. Popplewell et al in their work on Ag(,x)Gd(,1-x) showed that the peaks in their radial distribution functions shifted as a function of x between 0.05 to 0.16; however, the physically hand built model they presented did not match these results. Structures of Tb(,x)Fe(,1-x) with x equals 0.12 to 0.75 were simulated to study these properties. These G(r)'s generated showed distinct prominent peak positions corresponding to the geometrical arrangements of the atoms within 2 sphere diameters away from an origin, and with magnitudes changing as a function of the compositional variations. The interference functions compare very well with the experiment both with respect to peak shifts as x changes and order of magnitude.
- Pub Date:
- Physics: Condensed Matter