Topotactic Metal-Insulator Transition in Epitaxial SrFeOx Thin Films

Amit Khare; Dongwon Shin; Tae Sup Yoo; Minu Kim; Tae Dong Kang; Jaekwang Lee; Seulki Roh; In-Ho Jung; Jungseek Hwang; Sung Wng Kim; Tae Won Noh; Hiromichi Ohta; Woo Seok Choi

doi:10.1002/adma.201606566

Topotactic Metal-Insulator Transition in Epitaxial SrFeO_x Thin Films

Adv Mater. 2017 Oct;29(37). doi: 10.1002/adma.201606566. Epub 2017 Jul 31.

Authors

Amit Khare^{1

2}, Dongwon Shin³, Tae Sup Yoo¹, Minu Kim^{4

5}, Tae Dong Kang^{4

5}, Jaekwang Lee⁶, Seulki Roh¹, In-Ho Jung⁷, Jungseek Hwang¹, Sung Wng Kim⁸, Tae Won Noh^{4

5}, Hiromichi Ohta⁹, Woo Seok Choi¹

Affiliations

¹ Department of Physics, Sungkyunkwan University, Suwon, 16419, South Korea.
² Center for Integrated Nanostructure Physics, Institute for Basic Science, Sungkyunkwan University, Suwon, 16419, South Korea.
³ Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN, 37831, USA.
⁴ Center for Correlated Electron Systems, Institute for Basic Science, Seoul National University, Seoul, 08826, South Korea.
⁵ Department of Physics and Astronomy, Seoul National University, Seoul, 08826, South Korea.
⁶ Department of Physics, Pusan National University, Busan, 46241, South Korea.
⁷ Department of Mining and Materials Engineering, McGill University, Montreal, QC, H3A 0C5, Canada.
⁸ Department of Energy Sciences, Sungkyunkwan University, Suwon, 16419, South Korea.
⁹ Research Institute for Electronic Science, Hokkaido University, Sapporo, 001-0020, Japan.

PMID: 28758333
DOI: 10.1002/adma.201606566

Abstract

Topotactic phase transformation enables structural transition without losing the crystalline symmetry of the parental phase and provides an effective platform for elucidating the redox reaction and oxygen diffusion within transition metal oxides. In addition, it enables tuning of the emergent physical properties of complex oxides, through strong interaction between the lattice and electronic degrees of freedom. In this communication, the electronic structure evolution of SrFeO_x epitaxial thin films is identified in real-time, during the progress of reversible topotactic phase transformation. Using real-time optical spectroscopy, the phase transition between the two structurally distinct phases (i.e., brownmillerite and perovskite) is quantitatively monitored, and a pressure-temperature phase diagram of the topotactic transformation is constructed for the first time. The transformation at relatively low temperatures is attributed to a markedly small difference in Gibbs free energy compared to the known similar class of materials to date. This study highlights the phase stability and reversibility of SrFeO_x thin films, which is highly relevant for energy and environmental applications exploiting the redox reactions.

Keywords: electronic structures; optical spectroscopy; perovskite oxides; thin films; topotactic phase transformation.