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 A/sub p/B/sub q/ superlattices, as well as to explain the shapes of coherent precipitates in phase separating alloys.
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