29 / 33
Page 62
Notes:
conferenceseries
.com
Volume 3, Issue 2
ISSN: 2470-9905
Crystallography 2017
October 16-17, 2017
2
nd
International Conference on
October 16-17, 2017 | Chicago, USA
Applied Crystallography
Kensuke Kobayashi, Struct Chem Crystallogr Commun, 3:2
DOI: 10.21767/2470-9905-C1-002
Structural response to pressure in 1111-type iron-based superconductor LaFeAsO
1
−
x
H
x
Kensuke Kobayashi
High Energy Accelerator Research Organization, Japan
I
ron-based superconductor (iron pnictides) and cuprates are most well-known types of superconductor with critical temperature
(T
c
) higher than 50 K. In iron-based superconductors, the relation between the maximumTc and structural parameters of FePn4 (Pn
= pnictide) has been proposed as follows: the highest Tc is achieved when the Pn-Fe-Pn bond angle (αPn-Fe-Pn) approaches 109.5° as
in a regular tetrahedron of FePn4 or when the Pn height from Fe plane (h
Pn
) ~ 1.38 Å. The application of pressure is a direct and clean
way to modify the local geometry of FePn
4
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 LaFeAsO
1-x
H
x
, 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 α
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 FeAs
4
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 low-
pressure 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-T
c
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.
Figure1: Crystal structure of LaFeAsO
1-x
H
x
and the contour plots of T
c
as a function of As-Fe-As bond angle (
α
As-Fe-As) and the
Fe-As bond length (dFe-As).
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
kensuke.kobayashi@kek.jp