Topotactic Phase Transformation of the Brownmillerite SrCoO_2.5 to the Perovskite SrCoO_3–δ

Abstract

Oxygen stoichiometry is one of the most important elements in determining the physical properties of transition metal oxides (TMOs). A small change in the oxygen content results in the variation of valence state of the transition metal, drastically modifying the materials functionalities. The latter includes, for instance, (super-)conductivity, magnetism, ferroelectricity, bulk ionic conduction, and catalytic surface reactions. [1][2][3][4][5] In particular, among those applications, TMOs with mixed valence states have attracted attention for many environmental and renewable energy applications, including catalysts, hydrogen generation from water splitting, cathodes in rechargeable batteries and solid oxide fuel cells, and oxygen separation membranes. [6][7][8] For example, previous studies have shown that the ability to control the number of d-band electron population and detailed spin configuration in TMOs is critical for improved catalytic performance of TMOs. [9,10] In this context, SrCoO x (2.5 ≤ x ≤ 3.0) is an ideal class of materials to study the evolution of the physical properties by modifying the valence state in TMOs, due to the existence of two structurally distinct topotactic phases, i.e. the brownmillerite SrCoO 2.5 (BM-2 SCO) (see Figure 1a) and the perovskite SrCoO 3 . [11,12] Especially, BM-SCO has atomicallyordered one-dimensional vacancy channels (see Figure 1a), which can accommodate additional oxygen when the valence state of Co is changed. Moreover, SrCoO x exhibits a wide spectrum of physical properties from antiferromagnetic insulator to ferromagnetic metal depending on the oxygen stoichiometry. [11][12][13] Since SrCoO x has only a single control knob, i.e. the oxygen contentx, to modify the Co valence state without cation doping, it is straightforward to study the valence state (i.e. oxygen content) dependent physical properties. However, so far, the growth of high quality single crystalline materials has not been as successful due to difficulty in controlling the right oxidation state.In this work, we report on the epitaxial growth of high quality BM-SCO single crystalline films on SrTiO 3 (STO) substrates by pulsed laser epitaxy (PLE). In order to examine the topotactic phase transformation to the perovskite SrCoO 3- (P-SCO), some of the samples were subsequently in-situ annealed at various oxygen pressure (P(O 2 )) to fill the oxygen vacancies.While the direct growth of P-SCO films with x = 3.0 was an arduous task, we found that postannealing in high P(O 2 ) (> several hundreds of Torr) could fill sufficient amount of oxygen vacancies, yielding systematic evolution in electronic, magnetic, and thermoelectric properties. (Figure 1b) demonstrate that the films are of high quality. X-ray rocking curve ω-scans revealed a full width half maximum (FWHM) of < 0.04º, demonstrating the excellent crystallinity (cf., FWHM of the 002 STO peak was ~0.02º) of our films (data not shown).While we have shown the XRD data from a well-optimized, high quality thin film, it is worthwhile to mention tha...

Strain Control of Oxygen Vacancies in Epitaxial Strontium Cobaltite Films

“…This again suggests that while the films are increasingly oxygen deficient, no tensile strain was sufficient to topotactically transform the P-SCO film back to BM-SCO under these conditions. This electronic trend has also been observed in our prior study; [24] however, the origin for the less conducting behavior under tensile strain has not been completely understood. Indeed, while the systematic trend is obvious, the more insulating nature from the P-SCO films compared to P-SCO ozone indicates that their carrier transport is strongly influenced by the change in strain-induced carrier concentration owing to the loss of oxygen.…”

“…[23][24][25] Due to both the easy intercalation of O 2-within BM-SCO offered by ordered vacancy channels (OVCs) and the metastability of Co 4+ in P-SCO, the cobaltite has exceptionally low oxygen activation energies (< 1 eV), amplifying the energetic effects of strain. [26] In addition, deviations in oxygen content from the near-stoichiometric P-SCO result in significant property changes from a ferromagnetic metal to an antiferromagnetic insulator.…”

Symmetry‐Driven Atomic Rearrangement at a Brownmillerite–Perovskite Interface

“…Specific details of the growth conditions are reported elsewhere. [18] Characterization: The excellent phase purity and high-structural quality of the films were …”

Comparison of topotactic fluorination methods for complex oxide films

“…For example, post-growth anneals in oxygen, dilute O 3 /O 2 mixtures, vacuum, or forming gas mixtures have been used to oxidize or reduce, depending on the annealing environment, many ABO 3−δ perovskites to achieve a targeted anion stoichiometry, typically δ = 0 or 0.5. [2][3][4][5][6][7][8][9][10][11] The use of aggressive reducing agents, such as Ca 2 H, 12 has enabled the realization of ABO 2 compounds such as SrFeO 2 and LaNiO 2 via topotactic transformations from as-grown ABO 3 and ABO 2.5 films. 13,14 Given the sensitive coupling between δ and physical properties, [15][16][17] topotactic oxidation and reduction reactions have been used to induce large changes to the electronic, optical, and magnetic properties of epitaxial films [18][19][20] and may prove applicable in ionically controlled solid state devices.…”