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

2020/03/04 by Benjamin Geisler, Rossitza Pentcheva
Chemistry · Materials Science · Physics and Astronomy · #Advanced Condensed Matter Physics #Chemistry #Condensed matter physics #Crystal structure #Crystallography #Disproportionation #Electronic and Structural Properties of Oxides #Magnetic and transport properties of perovskites and related materials #Materials science #Octahedron #Order (exchange) #Oxygen #Phase (matter) #Phase diagram #Physical chemistry #Physics #Spin states #Stoichiometry #cond-mat.mtrl-sci

paper · pdf · doi:10.1103/physrevb.101.165108

published as Phys. Rev. B 101, 165108 (2020) · 11 pages, 9 figures, Supplement

arxiv created 2020/03/04 · openalex publication_date 2020/04/09 · openalex created_date 2020/04/17 · arxiv updated 2020/07/15 · openalex updated_date 2026/08/05

Abstract

The origin of the 3\ifmmode×\else\texttimes\fi1 reconstruction observed in epitaxial LaCoO₃ films on SrTiO₃(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×\else\texttimes\fi1)-ordered oxygen vacancies (LaCoO_2.67) are favored under commonly used growth conditions, while stoichiometric films emerge under oxygen-rich conditions. Growth of further reduced 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×\else\texttimes\fi1 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×\else\texttimes\fi1)-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 LaCoO₃ 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.

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