Pressure-induced isostructural phase transition and charge transfer in superconducting FeSe

• Published in: Journal of alloys and compounds Vol. 767; no. C; pp. 811 - 819
• Main Authors: Yu, Zhenhai, Xu, Ming, Yan, Zhipeng, Yan, Hao, Zhao, Jinggeng, Patel, Umeshkumar, Brewe, Dale L., Heald, Steve M., Ma, Jingyuan, Guo, Yanfeng, Yang, Ke, Xiao, Zhili, Wang, Lin
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 30.10.2018; Elsevier BV; Elsevier
• Subjects: Anisotropy; Charge transfer; Crystallography; Diffraction; Electronic properties; Electronic structure; Fe-based superconductor; High pressure; Isostructural phase transition; MATERIALS SCIENCE; Phase diagrams; Phase transitions; Spin transition; Superconductivity; Superconductors; Synchrotron radiation; X ray absorption; X ray spectra; X-ray diffraction; X-ray spectroscopy; Fe-based superconductor; Charge transfer; High pressure; Isostructural phase transition
• ISSN: 0925-8388; 1873-4669
• DOI: 10.1016/j.jallcom.2018.07.161

We present extensive investigations of the crystallographic phase diagram and electronic properties of the Fe-based superconductor FeSe under extreme conditions (high pressure (HP) and low temperature (LT)) by synchrotron X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS). An isostructural phase transition (Tetragonal (T) →high-pressure Tetragonal (Tʹ)) is discovered in FeSe at ∼2.8 GPa based on the axial ratio c/a with finer pressure step as observed in Fe-As-based superconductor such as EuFe2As2. We also find a pressure-induced Tʹ → MnP-type phase transition at 7.6 GPa in FeSe, which is consistent with the documented pressure-induced high-spin → low-spin transition (∼6–7 GPa). These results reveal the pressure-induced structural phase transition sequence in FeSe at room temperature to be T → Tʹ → Tʹ+MnP-type at pressures of 0–10.6 GPa, enriching the crystallographic phase diagram. The HPLT XRD data also indicate that a sluggish structural phase transition (Cmma → Pnma) begins at 7.5 GPa, and these two phases coexist up to 26.5 GPa. The HP X-ray absorption near-edge spectroscopy (XANES) measurement shows that Eo of Se experiences a pressure-induced shift to high energy, evidencing strongly charge transfer between Fe and Se under high pressure. Our results shed lights on the correlation between crystallographic/electronic structure and superconductivity in this material. •An isostructural phase transition (T → Tʹ) is discovered in FeSe at ∼2.8 GPa.•Another pressure-induced Tʹ → MnP-type phase transition at 7.6 GPa in FeSe.•A sluggish structural transformation (Cmma → Pnma) begins at 7.5 GPa (10 K).•XANES results show pressure-induced charge transfer between Fe and Se.