Electron‐beam‐induced structural phase transition related to oxygen vacancy ordering in epitaxial L a2/3 S r1/3 M nO3 films

Abstract

Functional oxides with a perovskite crystal lattice of type ABO 3 may possess corresponding oxygen‐deficient modulation structures, which can be used to tailor material properties including magnetism, ferroelectricity, and superconductivity. One prototypical example is the brownmillerite crystal structure of type ABO 2.5 [1‐5], which due to its high ionic conductivity could find applications in solid oxide fuel cells, oxygen‐separation membranes, gas sensors and other devices requiring anion diffusion. Brownmillerites have been derived from perovskite materials using topotactic reduction [1], optimized film growth [2,3], and oxygen getters [4]. Here, we demonstrate that the evolution of the perovskite‐brownmillerite phase transition can be fully controlled and monitored in epitaxial La 2/3 Sr 1/3 MnO 3 (LSMO) films using electron‐beam irradiation in a transmission electron microscope (TEM) [5]. Atomic‐scale real‐time TEM imaging reveals that the structural transition is driven by an incessant ordering of electron‐beam induced oxygen vacancies in every second MnO x plane. This local depletion of oxygen reduces the coordination of Mn cations, causing a vertical displacement of the La/Sr ions. A map of the out‐of‐plane lattice spacing corroborates this point (Figure 1(b)). Over‐irradiation of the brownmillerite phase induces a second transition to a perovskite‐like structure with disordered oxygen vacancies and a significantly enhanced out‐of‐plane lattice compared to the original LSMO film (Figure 1(c)). Additional information on the distribution of oxygen vacancies in the three structural phases of LSMO is obtained by HRTEM under negative Cs imaging (NCSI) conditions [6]. Compared to the original perovskite LSMO (see Figure 2(a) and inset), the NCSI contrast from brownmillerite LSMO (Figure 2(b) and inset) manifests a depletion of oxygen and predominant tetrahedral coordination of Mn in every other MnO x layer. The modulation structure disappears when the LSMO crystal transforms into the oxygen‐deficient perovskite‐like structure with enhanced out‐of‐plane lattice parameter (Figure 2(c) and inset). In this case, the oxygen is randomly distributed, which is facilitated by oxygen diffusion from MnO 6 octrahedra to MnO 4 tetrahedra during the second structural phase transition. Electron energy loss spectroscopy and energy‐dispersive x‐ray spectroscopy further confirm our findings [5]. This work was supported by the Academy of Finland (Grant Nos. 260361 and 252301) and by the European Research Council (ERC‐2012‐StG 307502).