Generating derivative structures from multilattices: Algorithm and application to hcp alloys

We present an algorithm for generating all derivative superstructures of a nonprimitive parent lattice. The algorithm has immediate application in important materials design problems such as modeling hexagonal-close-packed (hcp) alloys. Extending the work of Hart and Forcade [Phys. Rev. B 77, 224115 (2008)] (which applies only to Bravais lattices), this approach applies to arbitrary multilattices. The algorithm enumerates superlattices and atomic configurations using permutation groups rather than direct geometric comparisons. The key concept is to use the quotient group associated with each superlattice to determine all unique atomic configurations. The algorithm is very efficient; the run time scales linearly with the number of unique structures found. We demonstrate the algorithm in the important case of hcp-derived superstructures. In the list of enumerated hexagonal-close-packed derivative superstructures, we predict several as-yet-unobserved structures as likely candidates for new intermetallic prototypes.