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MULTI-PHASE STABILITY IN THE EAM INTER-ATOMIC POTENTIAL


  Nitzan R. Edelman [1]  ,   Uri Argaman [1]   ,  Eugene Pechenik [2]  ,  Eyal Yahel [3]  ,   Guy Makov [1]  
 [1] Department of Materials Engineering, Ben Gurion University of the Negev, Beer Sheva 84105, Israel
[2] School of Physics and Astronomy, Tel Aviv University, Tel Aviv 669978, Israel
[3] Department of Physics, NCRN, Beer Sheva 84190, Israel

Inter-atomic potentials that are applicable to a wide range of atomic environments are generally obtained by fitting functional forms to experimental and ab-initio data. The validity of the potential outside the fitted range is uncertain. Development of a wide ranging functional form of the inter-atomic potential may improve their transferability.

Baskes [Phys. Rev. Lett. 83, 2592 (1999)] proposed such a wide ranging functional form, the second nearest neighbors Lennard Jones Embedded Atom potential (2nn-LJEAM). With two model parameters, this model can represent a large variety of solid phases (fcc, bcc, etc.) and has been applied to study thermodynamic properties of metals, clusters and binary systems.

The relative thermodynamic stability of crystalline phases was determined within the 2nn-LJEAM model. The original results, considered phases with one geometrical parameter, (e.g. sc, dc). In earlier work, we have shown that the bcc phase in this model is mechanically unstable, casting doubt over the wide range of applicability.

We now, have generalized this model to include the bct phase with two geometrical parameters, of which fcc and bcc are special cases. The bct phase was found to be stable in the parameter space previously occupied by the bcc phase. The mechanical stability of the bct phase was examined by mapping the potential energy surface as a function of the lattice parameters.

Furthermore, we have calculated the unrelaxed vacancy formation energy within the model parameter space and generated from this data a mapping of chemical stability. Negative values of the formation energies imply spontaneous formation of vacancies that render the phase unstable. The bcc phase could not be stabilized while other thermodynamically stable phases could.   

Finally, we have derived the elastic constants of fcc phase within the model and observed a fair fit to real materials. Therefore, the 2nn-LJEAM in its current form is able to represent a limited range of atomic environments and is mainly suited to fcc structures.