Structural stabilities of layered materials: Pt-Ta

We suggest that metal multilayer systems can be fruitfully modeled by crystalline-layer-type compounds. As an example we have performed first-principles local-density calculations on the platinum-tantalum system for two compounds, PtTa and ${\mathrm{Pt}}_2$Ta. The crystal structures considered were CsCl, CuAuI, ${\mathrm{MoSi}}_2$, and ${\mathrm{MoPt}}_2$. These structures may be viewed as stackings of low-index planes of fcc or bcc crystals and may be transformed from one to another by a tetragonal distortion. In contrast, epitaxial growth often involves stackings of close-packed layers and, in the case of the bcc structure, the low-index (100) and close-packed (110) stackings are connected by an orthorhombic distortion. Total energies have been calculated and from these interface energies defined. Tracing the total energies over the above-mentioned distortions has yielded results consistent with known phase-diagram behavior and has suggested that Pt adsorbed on Ta(110) is locally stable although globally unstable.