Structural Chemistry & Crystallography Communication

Structural response to pressure in 1111-type iron-based superconductor LaFeAsO1−xHx

International Conference on Applied Crystallography
October 16-17, 2017 | Chicago, USA

Kensuke Kobayashi

High Energy Accelerator Research Organization, Japan

ScientificTracks Abstracts: Struct Chem Crystallogr Commun

DOI: 10.21767/2470-9905-C1-002

Abstract

Iron-based superconductor (iron pnictides) and cuprates are most well-known types of superconductor with critical temperature (Tc) higher than 50 K. In iron-based superconductors, the relation between the maximum Tc and structural parameters of FePn4 (Pn = pnictide) has been proposed as follows: the highest Tc is achieved when the Pn-Fe-Pn bond angle (�?±Pn-Fe-Pn) approaches 109.5�?° as in a regular tetrahedron of FePn4 or when the Pn height from Fe plane (hPn) ~ 1.38 �?�?. The application of pressure is a direct and clean way to modify the local geometry of FePn4 without the degradation of the crystal in comparison to the chemical substitution; hence, the detailed crystal structure under pressure warrants further investigation. A systematic study of the crystal structure of a layered iron oxypnictide LaFeAsO1-xHx, with a unique phase diagram of two superconducting phases and two parent phases, as a function of pressure was performed using synchrotron X-ray diffraction. We established that the �?±As-Fe-As widens on application of pressure due to the interspace between the layers being nearly infilled by the large La and As atoms. This behavior implies that the FeAs4 coordination deviates from the regular tetrahedron in our systems, which breaks a widely accepted structural guide albeit the increase of Tc 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 lowpressure at ~1.5 GPa in contrast to the first parent phase at x ~ 0, which remains robust to pressure. We suggest that the spin/orbital fluctuation from the second parent phase gives rise to the high-Tc under pressure. The pressure responses of the FeAs4 modification, the parent phases, and their correlation are previously unexplained peculiarities in 1111-type iron-based superconductors.

Biography

Kensuke Kobayashi has received his doctor’s degree in science from Osaka City University in 2009. Since April 2010, he has been a researcher at Condensed Matter Research Center (CMRC), Institute of Material Structure Science, KEK. At present, he is a Project Assistant Professor (MEXT Element Strategy Initiative) and worked on experimental studies of the structural and electrical properties of materials by means of synchrotron X-ray diffraction under external fields, such as pressure, electric field and low temperature