First-principles determination of the bulk phase diagram for body-centered-tetragonal copper: Application to epitaxial growth of Cu on Fe{100}

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Abstract
From the results of our ab initio pseudopotential total-energy calculation on Cu in the body-centered-tetragonal (bct) crystal system, a first-principles phase diagram for bulk bct Cu is derived as a function of externally applied stresses. An interesting fcc-to-bcc structural transition is predicted, induced by the application of a biaxial tension and a hydrostatic pressure. The biaxial tension needed to initiate the transformation is found, however, to be too large to attain macroscopically without inducing plastic flow (i.e., dislocation motion) in single crystal copper. An explanation for the existence of a metastable (albeit rather disordered) phase of bcc Cu on an iron substrate, recently observed by Wang et al., is put forth by considering the energy of tetragonal distortions on the cubic phases of Cu. The results of our bulk bct calculations suggest that the disorder observed on Cu/Fe{100} is due to a nonoptimal match of lattice constants. The growth of a stable bct phase of Cu on a suitable cubic substrate with lattice constant 2.76 Å may be possible.