Formation mechanism of superconducting phase and its three-dimensional architecture in pseudo-single-crystal KxFe2â‹TMySe2

We report how the superconducting phase forms in pseudo-single-crystal KxFe2−ySe2. In situ scanning electron microscopy (SEM) observation reveals that, as an order-disorder transition occurs, on cooling, most of the high-temperature iron-vacancy-disordered phase gradually changes into the iron-vacancy-ordered phase, whereas a small quantity of the high-temperature phase retains its structure and aggregates to the stripes with more iron concentration but less potassium concentration compared to the iron-vacancy-ordered phase. The stripes that are generally recognized as the superconducting phase are actually formed as a remnant of the high-temperature phase with a compositional change after an “imperfect” order-disorder transition. It should be emphasized that the phase separation in pseudo-single-crystal KxFe2−ySe2 is caused by the ironvacancy order-disorder transition. The shrinkage of the high-temperature phase and the expansion of the newly created iron-vacancy-ordered phase during the phase separation rule out the mechanism of spinodal decomposition proposed in an early report [Z. Wang et al., Phys. Rev. B 91, 064513 (2015)]. Since the formation of the superconducting phase relies on the occurrence of the iron-vacancy order-disorder transition, it is impossible to synthesize a pure superconducting phase by a conventional solid state reaction or melt growth. By focused ion beam scanning electron microscopy, we further demonstrate that the superconducting phase forms a contiguous three-dimensional architecture composed of parallelepipeds that have a coherent orientation relationship with the iron-vacancy-ordered phase. Disciplines Condensed Matter Physics | Materials Science and Engineering Comments This article is from Phys. Rev. B 93 (2016): 064509, doi:10.1103/PhysRevB.93.064509. Posted with permission. Authors Yong Liu, Qingfeng Xing, Warren E. Straszheim, Jeff Marshman, Pai Pedersen, Richard McLaughlin, and Thomas A. Lograsso This article is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/ameslab_pubs/386 PHYSICAL REVIEW B 93, 064509 (2016) Formation mechanism of superconducting phase and its three-dimensional architecture in pseudo-single-crystal KxFe2− ySe2 Yong Liu,1,* Qingfeng Xing,1 Warren E. Straszheim,1,2 Jeff Marshman,3 Pal Pedersen,4 Richard McLaughlin,5 and Thomas A. Lograsso1,6 1Division of Materials Sciences and Engineering, Ames Laboratory, U.S. DOE, Ames, Iowa 50011, USA 2Materials Analysis and Research Laboratory, Iowa State University, Ames, Iowa 50011, USA 3Carl Zeiss Microscopy, LLC, Ion Microscopy Innovation Center (IMIC), Peabody, Massachusetts 01960, USA 4Carl Zeiss Microscopy, LLC, Thornwood, New York 10594, USA 5Oxford Instruments America, Incorporated, Concord, Massachusetts 07142, USA 6Department of Materials Science and Engineering, Iowa State University, Ames, Iowa 50011, USA (Received 7 October 2015; revised manuscript received 30 December 2015; published 11 February 2016) We report how the superconducting phase forms in pseudo-single-crystal KxFe2−ySe2. In situ scanning electron microscopy (SEM) observation reveals that, as an order-disorder transition occurs, on cooling, most of the high-temperature iron-vacancy-disordered phase gradually changes into the iron-vacancy-ordered phase, whereas a small quantity of the high-temperature phase retains its structure and aggregates to the stripes with more iron concentration but less potassium concentration compared to the iron-vacancy-ordered phase. The stripes that are generally recognized as the superconducting phase are actually formed as a remnant of the high-temperature phase with a compositional change after an “imperfect” order-disorder transition. It should be emphasized that the phase separation in pseudo-single-crystal KxFe2−ySe2 is caused by the iron-vacancy order-disorder transition. The shrinkage of the high-temperature phase and the expansion of the newly created iron-vacancy-ordered phase during the phase separation rule out the mechanism of spinodal decomposition proposed in an early report [Z. Wang et al., Phys. Rev. B 91, 064513 (2015)]. Since the formation of the superconducting phase relies on the occurrence of the iron-vacancy order-disorder transition, it is impossible to synthesize a pure superconducting phase by a conventional solid state reaction or melt growth. By focused ion beam scanning electron microscopy, we further demonstrate that the superconducting phase forms a contiguous three-dimensional architecture composed of parallelepipeds that have a coherent orientation relationship with the iron-vacancy-ordered phase. DOI: 10.1103/PhysRevB.93.064509