Competing structural instabilities in Ti-based layered-perovskite-oxide superlattices*

Utilizing first-principles computational techniques, we have mapped out structural instabilities in the Ruddlesden-Popper homologous oxide superlattice families with a general chemical formula A$_{n-1}$A$^{\prime}_{2}$Ti$_{n}$O$_{3n+1}$, A = Sr, Ba, Pb (perovskite-type block) and A$^{\prime}$ = Sr (rocksalt-type block), for $n$ = 1--5. Our calculations show that each superlattice family has a unique set of ``instability footprints'' --- including the ferroelectric, antiferroelectric and antiferrodistortive types --- which may or may not have a strong coupling to epitaxial strain. Furthermore, the existence and strength of structural instabilities within each particular family change dramatically with an increasing number of perovskite-type layers $n$, granting us wide flexibility to fine-tune the properties of these materials for various device applications or, e.g., for integration into composites with magnetic Ru- or Mn-based layered-perovskite-oxide compounds.