title: Pressure effect on iron-based superconductor LaFeAsO1-xHx: Peculiar response of 1111-type structure.
abstract: A systematic study of the crystal structure of a layered iron oxypnictide LaFeAsO1-xHxas a function of pressure was performed using synchrotron X-ray diffraction. This compound exhibits a unique phase diagram of two superconducting phases and two parent phases. We established that the As-Fe-As angle of the FeAs4tetrahedron widens on the application of pressure due to the interspace between the layers being nearly infilled by the large La and As atoms. Such rarely observed behaviour in iron pnictides implies that the FeAs4coordination deviates from the regular tetrahedron in the present systems. This breaks a widely accepted structural guide that the superconductivity favours the regular tetrahedron, albeit the superconducting transition temperature (Tc) increases from 18 K at ambient pressure to 52 K at 6 GPa for x = 0.2. In the phase diagram, the second parent phase at x ~ 0.5 is suppressed by pressure as low as ~1.5 GPa in contrast to the first parent phase at x ~ 0, which is robust against pressure. We suggest that certain spin-fluctuation from the second parent phase is strongly related to high-Tcunder pressure.
authors.affiliation: Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan.
authors.forename: Kensuke
authors.initials: K
authors.lastname: Kobayashi
authors.affiliation: Materials Research Centre for Element Strategy, Tokyo Institute of Technology, Yokohama, Kanagawa 226-8503, Japan.
authors.forename: Jun-Ichi
authors.initials: JI
authors.lastname: Yamaura
authors.affiliation: Laboratory for Materials and Structures, Tokyo Institute of Technology, Yokohama, Kanagawa 226-8503, Japan.
authors.forename: Soshi
authors.initials: S
authors.lastname: Iimura
authors.affiliation: Materials Research Centre for Element Strategy, Tokyo Institute of Technology, Yokohama, Kanagawa 226-8503, Japan.
authors.forename: Sachiko
authors.initials: S
authors.lastname: Maki
authors.affiliation: Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan.
authors.forename: Hajime
authors.initials: H
authors.lastname: Sagayama
authors.affiliation: Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan.|Department of Materials Structure Science, The Graduate University for Advanced Studies, Tsukuba, Ibaraki 305-0801, Japan.
authors.forename: Reiji
authors.initials: R
authors.lastname: Kumai
authors.affiliation: Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan.|Department of Materials Structure Science, The Graduate University for Advanced Studies, Tsukuba, Ibaraki 305-0801, Japan.
authors.forename: Youichi
authors.initials: Y
authors.lastname: Murakami
authors.affiliation: College of Humanities and Science, Nihon University, Setagaya, Tokyo 156-8550, Japan.
authors.forename: Hiroki
authors.initials: H
authors.lastname: Takahashi
authors.affiliation: Materials Research Centre for Element Strategy, Tokyo Institute of Technology, Yokohama, Kanagawa 226-8503, Japan.
authors.forename: Satoru
authors.initials: S
authors.lastname: Matsuishi
authors.affiliation: Materials Research Centre for Element Strategy, Tokyo Institute of Technology, Yokohama, Kanagawa 226-8503, Japan.|Laboratory for Materials and Structures, Tokyo Institute of Technology, Yokohama, Kanagawa 226-8503, Japan.
authors.forename: Hideo
authors.initials: H
authors.lastname: Hosono