Strain-induced change in the elastically soft direction of epitaxially grown face-centered-cubic metals

The theory of epitaxial strain energy is extended beyond the harmonic approximation to account for large film/substrate lattice mismatch. We find that for fcc noble metals (i) directions 〈001〉 and 〈111〉 soften under tensile biaxial strain (unlike zincblende semiconductors) while (ii) 〈110〉 and 〈201〉 soften under compressive biaxial strain. Consequently, (iii) upon sufficient compression 〈201〉 becomes the softest direction (lowest elastic energy), but (iv) 〈110〉 is the hardest direction for large tensile strain. (v) The dramatic softening of 〈001〉 in fcc noble metals upon biaxial tensile strain is caused by small fcc/bcc energy differences for these materials. These results can be used in selecting the substrate orientation for effective epitaxial growth of pure elements and ApBq superlattices, as well as to explain the shapes of coherent precipitates in phase separating alloys.