Competition of defect ordering and site disproportionation in strained LaCoO3 on SrTiO3 (001)

The origin of the $3\ifmmode\times\else\texttimes\fi{}1$ reconstruction observed in epitaxial ${\mathrm{LaCoO}}₃$ films on ${\mathrm{SrTiO}}₃(001)$ is assessed by first-principles calculations including a Coulomb repulsion term. We compile a phase diagram as a function of the oxygen pressure, which shows that ($3\ifmmode\times\else\texttimes\fi{}1$)-ordered oxygen vacancies (${\mathrm{LaCoO}}_2.67$) are favored under commonly used growth conditions, while stoichiometric films emerge under oxygen-rich conditions. Growth of further reduced ${\mathrm{LaCoO}}_2.5$ brownmillerite films is impeded by phase separation. We report two competing ground-state candidates for stoichiometric films: a semimetallic phase with $3\ifmmode\times\else\texttimes\fi{}1$ low-spin/intermediate-spin/intermediate-spin (LS/IS/IS) magnetic order and a semiconducting phase with IS/IS/IS magnetic order. This demonstrates that tensile strain induces ferromagnetism even in the absence of oxygen vacancies. Both phases exhibit an intriguing ($3\ifmmode\times\else\texttimes\fi{}1$)-reconstructed octahedral rotation pattern and accordingly modulated La-La distances. In particular, charge and bond disproportionation and concomitant orbital order of the ${t}_2g$ hole emerge at the Co sites that are also observed for unstrained bulk ${\mathrm{LaCoO}}₃$ in the IS state and explain structural data obtained by x-ray diffraction at elevated temperature. Site disproportionation drives a metal-to-semiconductor transition that reconciles the IS state with the experimentally observed low conductivity during spin-state crossover without the presence of Jahn-Teller distortions.