The SCF convergence criteria of the ab initio calculations of elastic properties of single-crystal B2 ZrPd phase

Abstract

The mechanical and dynamical stability of the geometry optimised structural phases of Zr50Pd50-xRux have been calculated using the ab initio modelling technique. The accuracy of the elastic constants largely depends on the accuracy of the self consistent field (SCF) setting and also on the level of convergence of geometry optimisation for each distorted structure. Parameters such as energy cutoff, SCF convergence criteria and k-points set form part of these calculations. Often, the importance of setting accuracy of these parameters is neglected and may result in erroneous reporting of elastic constants. In the present study, B2 ZrPd is used to demonstrate the disadvantage of inaccurate selection of energy cutoff and k-points set on the calculation of elastic constants. The computed results of phonon dispersion curves were in excellent agreement with existing experimental data, and further aided in resolving discrepancy among theoretical results. While current results showed that the B2 phase of ZrPd is neither mechanically or vibrationally stable at 0 K, contrary to some of the previous calculations, the phonon dispersion curves of all considered ZrPd phases reveal presence of imaginary frequencies. Introduction of Ru on Pd site of the B2 ZrPd has negligible effect on the thermodynamic stability (Hf), but increased both mechanical and lattice dynamic stability significantly. In addition, it is predicted for the first time from lattice dynamic calculations that addition of 18.75 at.% Ru alters the martenstic transformation path from that of pristine ZrPd towards that of TiNi. Lattice vibration results for other chosen reference alloys are also in excellent agreement with existing theoretical data.