Iron-based layered superconductor: LaOFeP

Article abstract of DOI:10.1021/ja063355c

We report superconductivity in an iron-based layered oxy-pnictide LaOFeP. LaOFeP is composed of an alternate stack of lanthanum oxide (La³⁺O^2--) and iron pnictide (Fe²⁺P^3-) layers. Magnetic and electrical resist

Full text of DOI:10.1021/ja063355c

Published on Web 07/15/2006
Iron-Based Layered Superconductor: LaOFeP
†
†
†,‡
§
§
Yoichi Kamihara, Hidenori Hiramatsu, Masahiro Hirano, Ryuto Kawamura, Hiroshi Yanagi,
†,§
,†,‡
Toshio Kamiya, and Hideo Hosono*
ERATO-SORST, JST, Frontier CollaboratiVe Research Center, Tokyo Institute of Technology, Mail Box S2-13,
259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan, Frontier CollaboratiVe Research Center, Tokyo Institute of
4
Technology, Mail Box S2-13, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan, and Materials and Structures
Laboratory, Tokyo Institute of Technology, Mail Box R3-4, 4259 Nagatsuta, Yokohama 226-8503, Japan
Received May 15, 2006; E-mail: hosono@msl.titech.ac.jp
c
Since the discovery of high transition temperature (T ) super-
conductivity in layered copper-based oxides,1 extensive efforts
have been devoted to the exploration of new material systems
containing transition metal ions other than copper in a hope to
realize higher transition temperatures because it is widely believed
,2
c
that the high T values of the copper oxides are related to the strong
electron correlation associated with the transition metal ions.
Further, researchers have focused mostly on layered structures due
to a freedom to control the carrier density in the transition metal
oxide layer. These efforts have led to the discoveries of several
3
4
novel superconductors, such as Sr
2
RuO
4
,
KOs
2 6
O ,
Na
x
CoO
2
/
5
12 (Ln ) Y, La).6,7 Although their T
1.3H
2
O, and LnFe
4
P
c
’s are
much lower than those of the copper oxides, the discoveries of
superconductivity in the new material systems provide valuable
knowledge for understanding physics underlying the oxide super-
conductors as well as for finding an approach to a novel high T
c
superconductor. Here we report a new class of superconductor, an
iron-based layered oxy-pnictide LaOFeP. LaOFeP is composed of
3
+
2-
an alternate stack of lanthanum oxide (La
O
) and iron pnictide
2+
3-
8
Figure 1. (a) Crystal structure of LaOFeP. The green polyhedra indicate
the FeP4 tetrahedra. (b) Observed XRD pattern for LaOFeP. The vertical
bars at the bottom show the calculated positions of Bragg diffractions. Inset
(
Fe
P ) layers (Figure 1a). It is thought that carriers flow in the
2
+ 3-
two-dimensional (Fe
P
) layers, while impurity doping to the
) layer transfers the carriers generated in the (La
3+
2-
3+ 2-
(La
O
O
)
shows the structure of a FeP tetrahedron with P-Fe-P bonding angles
4
2+ 3-
layer to the (Fe
P
) layers, which works as “natural modulation
analyzed by the Rietveld method.
9
c
doping”, similar to the high T superconductors and transparent
P-type semiconductor copper-based oxychalcogenides.1
0,11
power was measured with a Cu probing to determine the charge
type for the majority carrier.
The magnetic measurements were made by a vibrating sample
magnetometer (VSM; Quantum Design). The magnetic susceptibil-
ity (ø) was obtained from the magnetization in a magnetic field at
The first
synthesis and the crystal structure analysis of LaOFeP were
performed by Zimmer et al., although electronic and magnetic
properties have not been reported.
We employed a different synthesis process from the one reported
by Zimmer et al. in this study. First, LaP, FeP, and Fe P were
2
prepared by heating stoichiometric mixtures of La (Shinetsu
Chemical, purity 99.5%), Fe (Kojundo Chemical, 99.9%+), and P
8
<
10 Oe after zero-field cooling.
Synthesized powders show dark brown color with a metallic
luster, and they are stable in air. Figure 1b shows the powder XRD
pattern of the obtained sample with a calculated diffraction pattern.
The diffraction angles shown were calculated from the lattice
parameters obtained by the least-squares fitting method (a ) b )
(
Rare Metallic Chemical, 99.999%) at 700 °C for 10 h in evacuated
silica tubes. On the other hand, La was prepared by dehydrating
commercial La powder (Kojundo Chemical, 99.99%) at 600 °C
for 10 h in air. Then, nominally pure and F-doped LaOFeP
0.06)FeP] were synthesized by heating stoichiometric
mixtures of LaP, FeP, Fe P, La, LaF (Morita Chemical 99%), and
La powders in silica tubes at 1200 °C for 40 h. The silica tubes
2 3
O
2 3
O
0
.3964(1), c ) 0.8512(3) nm). All the diffractions were indexed to
this crystal structure, showing the sample was single phase. The
refined structure shows that the FeP tetrahedron is distorted with
two different P-Fe-P bonding angles of 104.39(4) and 120.18(8)°
Figure 1b). Figure 2 shows the F-T curves for pure and F-doped
[La(O0.94F
4
2
3
2
O
3
(
were filled with atmospheric pressure of high purity Ar gas at room
temperature to prevent implosion of the silica tubes. The crystal
structure of the resulting powder was examined by powder X-ray
LaOFeP at a magnetic field (H) of 0 Oe. It is evident that both the
data exhibit metallic behavior in the electrical conductivity at
temperatures >10 K. At temperatures below 5 K, a sharp drop in
F is observed, and the resistivity vanishes at 3.2 K (Figure 3a),
implying transition to a superconductivity phase. The transition
temperature shifts to lower temperatures with increasing H,
indicating that the magnetic field breaks the superconductivity state.
The superconducting transition temperature was significantly raised
by F-doping as seen in Figure 3b. The magnetic susceptibilities
diffraction (XRD; Rigaku RINT) using Cu KR radiation and refined
by the Rietveld method using the code RIETAN2000.12 DC
electrical resistivity measurement was performed by a dc four-probe
technique employing silver paste as electrodes. Thermoelectric
†
ERATO-SORST, JST.
‡
Frontier Collaborative Research Center, Tokyo Institute of Technology.
§
Materials and Structures Laboratory, Tokyo Institute of Technology.
10012
9
J. AM. CHEM. SOC. 2006, 128, 10012-10013
10.1021/ja063355c CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
responsible for the conduction. Fe-based superconductors so far
12 (Ln ) La, Y)6
,7,13
in the Skutterudite family
discovered, LnFe
4
P
2+
and the present LaOFeP, commonly contain Fe ions coordinated
by P anions. However, there are two major differences in the crystal
4
structure between them: (1) LnFe P12 has a three-dimensional
structure, while LaOFeP has a two-dimensional layered structure;
2) the coordination around Fe2 is octahedral for LnFe
+
(
4
P12 but
tetrahedral for LaOFeP. Similar to the layered copper-based oxide
high T superconductors, the aliovalent ion doping to LaOFeP
Figure 2. Electrical resistivity (F) versus temperature (T) for pure and
F-doped LaOFeP.
c
caused the significant difference in the superconducting transition
temperature. LaOFeP has a layered structure and contains a
transition metal ion different from Cu and, therefore, constitutes a
new alternative material system to the layered copper-based oxides.
However, there is a marked difference in the coordination
structure in LaOFeP compared with the layered copper-based
2+
superconductors reported so far; that is, a Cu occupies a planar
2
+
4
-fold square site while Fe in LaOFeP occupies a tetrahedral
3-
site coordinated with four P ions, and the tetrahedrons are linked
with each other by edge-sharing to form the (Fe2
consequence, the electrons at the Fermi level occupy Cu 3dx2-y
orbitals in the layered copper-based superconductors, while either
+ 3-
P
) layer. As a
2
+
2
2+
Fe 3dxy or 3dyz/3dzx orbitals form the Fermi level for LaOFeP.
Therefore, we like to note that LaOFeP has an importance for
superconductivity physics and material exploration: the finding of
the iron-based new superconductor with a different type of a layered
structure will provide an opportunity for studying the mechanism
c
of high T superconductivity in layered crystals. It would be
informative that electronic transport at 2 K in LaOMP (M: +2
charge state transition metal cation) is drastically changed by
5
7
varying M from Mn (3d ) to Co (3d ): semiconductor for Mn,
14
superconductor for Fe, and metal for Co were observed. A variety
of derivatives can be synthesized by replacing the La, M, and/or P
sites, which allows for the systematic study on superconductivity.
Acknowledgment. Y.K. is indebted to Dr. H. Okabe (Aoyama
Gakuin University) and Dr. H. Kito (AIST) for their helpful advice.
Supporting Information Available: Crystal structure data of
LaOFeP at room temperature. This material is available free of charge
via the Internet at http://pubs.acs.org.
Figure 3. Superconducting properties of pure and F-doped LaOFeP. (a)
Electrical resistivity (F) versus temperature (T) for pure LaOFeP at various
magnetic fields (H). (b) F versus T for F-doped LaOFeP in comparison
with pure LaOFeP. (c) Magnetic susceptibility (ø) versus T for pure and
F-doped LaOFeP. The dashed line indicates the ø value of the perfect
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J. AM. CHEM. SOC.
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VOL. 128, NO. 31, 2006 10013