1. Crystal Structure Refinement of　Fluorine-Free LnFeAsO1-y by Neutron Diffraction
AIST, Japan
C. H. Lee, A. Iyo, H. Eisaki, H. Kito, T. Ito, K. Kihou, H. Matsuhata
IMR Tohoku Univ., Japan
K. Yamada
ILL, France
M. T. Fernandez-Diaz
Köln Univ., Germany
M. Braden

2. Tc of LnFeAsO1-y Rare-earth dependences
Nominal composition:
P.M. Shirage et al.
Tc approaches maximum value around Nd and Sm
Why is Tc saturated around Nd and Sm?

3. Experimental details *Samples *Neutron diffraction
LaFeAsO1-y : y = 0.12
NdFeAsO1-y : y = 0.05, 0.08, 0.14, 0.17
*Neutron diffraction
ILL（Grenoble、France）
・ High-resolution two-axis
diffractometer (D2B)
ILL high-resolution diffractometer

4. Rietveld refinement NdFeAsO0.83 (Tc=51K) T=R.T. x y z B
Space group:P4/nmm
Rwp=2.84
x y z B
Nd (3) 0.42(4)
Fe (3)
As (3) 0.41(5)
O 0.83(1) (8)
The occupation of the oxygen site
was also varied.
The obtained R-factor was over the range
of 2.7%≤Rwp≤4.1%.

5. As - Fe bond length As-Fe Independent of oxygen content.
T. Nomura et al.,
cond-mat/ v1
Independent of oxygen content.
The short As-Fe bond length suggests a covalent bonding.

6. As - Fe(plane) Fe-plane As-Fe(plane)
As atoms deviate from Fe-planes with increasing oxygen deficiency.
The variation rate is larger in Nd compounds.

7. As - Fe(plane) vs Tc
Tc increases with increasing As-Fe(plane) distance.

8. As - Fe - As bond angles α α Regular tetrahedron 109.47º β β As α Fe β
The FeAs4-tetrahedron transforms toward a regular tetrahedron
with increasing oxygen deficiency
FeAs4-tetrahedron
FeAs4-tetrahedrons of Nd compounds approach a regular tetrahedron closer.

9. Tc vs distortion of FeAs4-tetrahedron
Regular tetrahedron
Tc increases as the FeAs4-lattices become a regular tetrahedron.

10. Tc vs distortion of FeAs4-tetrahedron in pnictides
Regular tetrahedron
Tc seems to attain maximum values when FeAs4-lattices form a regular tetrahedron.
This indicates the clear relationship between crystal structure and superconductivity

11. Conclusion
We studied the crystal structure of LnFeAsO1-y by the powder neutron diffraction technique.
As results
Tc seems to attain maximum values for regular FeAs4-tetrahedrons.
These results suggest the clear relationship between crystal structure and superconductivity.

12. Ln-O bond length
T. Nomura et al.,
cond-mat/ v1
Ln-O
The Ln-O bond length increases with increasing oxygen deficiency.
This is probably due to the decrease of the number of electrons in the O-planes.

13. Ln - As bond length (LnO)1+ (FeAs)1- (LnO)1+ Ln-As
Attracting
force
increases
(FeAs)1-
Attracting
force
increases
(LnO)1+
Ln-As
The Ln-As bond length decreases drastically.
Electrons can be transferred from LnO-planes to FeAs-planes
with increasing the oxygen deficiency.
Attracting force between LnO-planes and FeAs-planes can become stronger.

14. Pressure effect pressure (Nd) Regular tetrahedron
Tc decreases by pressurizing since the FeAs4-lattices deviate from a regular tetrahedron.

15. Tc of LnFeAsO1-y and LnFeAsO1-xFx
Rare-earth dependences
Pressure dependences
Nominal composition: La
H. Takahashi et al.,
Nature 453 (2008) 376
At first, Tc increases with increasing pressure Nd
P.M. Shirage et al.
Tc approaches maximum value around Nd and Sm
N. Takeshita et al.
Why is Tc saturated around Nd and Sm?
Tc only decreases