Nickel-based oxyphosphide superconductor with a layered crystal structure, LaNiOP

Article abstract of DOI:10.1021/ic701200e

A layered oxyphosphide, LaNiOP, was synthesized by solid-state reactions. This crystal was confirmed to have a layered structure composed of an alternating stack of (La³⁺O^2-)⁺ and (Ni ²⁺P^3-)^-. We found that the resulting LaNiOP shows a superconducting transition at ～3 K. This material exhibited metallic conduction and Pauli paramagnetism in the temperature range of 4-300 K. The resistivity sharply dropped to zero and the magnetic susceptibility became negative at <4 K, indicating that a superconducting transition occurs. The volume fraction of the superconducting phase estimated from the diamagnetic susceptibility reached ～40 vol % at 1.8 K, substantiating that LaNiOP is a bulk superconductor.

Full text of DOI:10.1021/ic701200e

Inorg. Chem. 2007, 46, 7719−7721
Nickel-Based Oxyphosphide Superconductor with a Layered Crystal
Structure, LaNiOP
Takumi Watanabe,† Hiroshi Yanagi,† Toshio Kamiya,†,‡ Yoichi Kamihara,‡ Hidenori Hiramatsu,‡
Masahiro Hirano,‡,§ and Hideo Hosono*,†,‡,§
Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta,
Yokohama, Japan, ERATO-SORST, JST, in the Frontier CollaboratiVe Research Center, Tokyo
Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, Japan, and Frontier CollaboratiVe
Research Center, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, Japan
Received June 19, 2007
A layered oxyphosphide, LaNiOP, was synthesized by solid-state
chemistry of superconductors; it has provided complementary
information to better understand the mechanism of super-
conductivity and clues for exploring new material systems
for higher Tc superconductors.
reactions. This crystal was confirmed to have a layered structure
- +
composed of an alternating stack of (La3+O2
)
and (Ni2+P3-)-.
We found that the resulting LaNiOP shows a superconducting
transition at 3 K. This material exhibited metallic conduction and
Pauli paramagnetism in the temperature range of 4 300 K. The
We have studied a series of quaternary oxyphosphides,
LaMOP (M ) divalent transition-metal cations such as Mn2+,
Fe2+, and Co2+), with the expectation of a new correlated
electron system. These studies have led to the discovery of
a new Fe-based superconductor, LaFeOP.6 In this crystal
structure, the LaO and MP layers are alternately stacked
along the c axis, as shown in Figure 1. Systematic research
on isostructural oxychalcogenides, LaCuOCh (Ch ) S, Se,
and Te), has indicated that the CuCh layer, which is
sandwiched by larger-energy-gap LaO layers, works as a hole
transport path.7 As an analogue, the MP layer in LaMOP is
also expected to be a carrier conduction layer sandwiched
by the LaO insulating layers. This two-dimensional crystal
structure, which contains transition-metal cations, has led to
the expectation that interesting electronic and/or magnetic
properties will be discovered based on the electron correla-
tion. In this work, a new member in the LaMOP system,
LaNiOP, which is another superconductor, is synthesized.
Samples were prepared by solid-state reactions of the
starting materials, La (Shin-etsu Chemical, purity 99.5%),
P (Rare Metallic, 99.9999%), and NiO (Kojundo Chemical
Laboratory, 99.97%). First, a stoichiometric mixture of La
metal powder and P was heated in an evacuated silica tube
at 400 °C for 12 h, and subsequent heating at 700 °C for 6
h resulted in single-phase LaP. Then a stoichiometric mixture
of LaP and NiO was pressed into a pellet, which was heated
at 1000 °C for 1 day in an evacuated silica tube. The resulting
sample was characterized by high-power X-ray diffraction
∼
−
resistivity sharply dropped to zero and the magnetic susceptibility
became negative at <4 K, indicating that a superconducting
transition occurs. The volume fraction of the superconducting phase
estimated from the diamagnetic susceptibility reached
at 1.8 K, substantiating that LaNiOP is a bulk superconductor.
∼
40 vol %
Since the discovery of high-transition-temperature (high-
Tc) Cu-based superconducting oxides, numerous efforts have
been devoted to exploring higher Tc superconductors in a
variety of other materials. One of the attractive systems is
Cu-based and other transition-metal-based oxides because
the high Tc reflects the strong electron correlation among
3d electrons. These efforts on cuprate oxides have led to
discoveries of many superconductors with different crystal
structures, which have raised Tc to 133 K.1 In contrast,
research on non-Cu-based compounds (oxide, pnictide, etc.)
is less active because materials with a Tc higher than those
of the Cu-based ones have yet to be discovered. However,
the discovery of Sr2RuO4,2 NaxCoO2‚yH2O,3 electron-doped
HfNCl,4 and LixNbO25 has largely impacted the physics and
* To whom correspondence should be addressed. E-mail: hosono@
msl.titech.ac.jp.
† Materials and Structures Laboratory, Tokyo Institute of Technology.
‡ ERATO-SORST, JST.
§ Frontier Collaborative Research Center, Tokyo Institute of Technology.
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10.1021/ic701200e CCC: $37.00
Published on Web 08/17/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 19, 2007 7719

COMMUNICATION
Figure 1. Crystal structure of LaMOP (M ) divalent transition-metal
cation). The yellow polyhedron indicates the MP4 tetrahedron. LaO and
MP layers are alternately stacked along the c axis.
Figure 3. Temperature (T) dependence of the electrical resistivity (F) at
0 Oe. The inset shows the F-T curves as a function of the magnetic field
magnified in the temperature range of 1.9-10 K.
Figure 2. XRD pattern of LaNiOP measured (blue dots) and refined by
the Rietveld method (red lines). Vertical bars at the bottom show the
calculated positions of the Bragg diffractions of LaNiOP (upper), LaNi2P2
(middle), and La3PO7 (bottom, which only shows strong peaks).
Figure 4. Temperature (T) dependence of the magnetic susceptibility (ø)
in the temperature range of 1.8-10 K under conditions of ZFC and FC at
10 Oe. The inset shows the field (H) dependence of magnetization (M) at
1.8 K.
(XRD; D8 ADVANCE-TXS, Bruker AXS) with Cu KR
radiation. It was found that the highest phase purity sample
obtained to date contains LaNi2P2 and La3PO7 as impurity
phases. Thus, three-phase Rietveld analyses were carried out
to extract the volume fractions of the impurities using the
code TOPAS3.8 The crystal structure of LaNiOP was refined
with the space group of P4/nmm, and the initial structure
parameters were taken from that of isostructural compound
LaCoOP.9
The electrical resistivity of the sintered pellets (apparent
density ∼50%) was measured in the temperature range of
1.9-300 K by a four-probe technique (using PPMS, Quan-
tum Design). Ohmic contacts were formed with a silver paste.
The magnetic properties were measured with a supercon-
ducting quantum interference device magnetometer (SQUID,
using MPMS, Quantum Design). The temperature depen-
dence of the magnetic susceptibility was measured from 1.8
to 300 K under conditions of zero-field cooling (ZFC) and
field cooling (FC) at HFC ) 10 Oe.
tion, which shows that NiP4 has a distorted tetrahedral
coordination having two different P-Ni-P angles of 101.7°
and 126.4°.
Figure 3 shows the temperature dependence of the electri-
cal resistivity (F) at Hext ) 0 Oe. F at 300 K is very low, 1.2
mΩ‚cm, and exhibits a metallic behavior at temperatures
down to 4 K. The inset shows a magnified view in the
temperature range of 1.9-6 K. The resistivity begins to drop
around 4 K and becomes zero around 2 K. The onset
temperature shifts to a lower temperature as Hext increases,
and the drop in F vanishes at Hext ) 10 000 Oe. These results
suggest that the obtained product exhibits a superconducting
transition at ∼3 K.
Figure 4 shows the temperature dependence of the
magnetic susceptibility (ø) measured under ZFC and FC at
10 Oe. ø is very small and is nearly independent of the
temperature between 4 and 300 K, implying Pauli paramag-
netism. However, ø begins to drop and becomes negative at
∼3 K. It reaches a large negative value of -5.3 × 10-3
emu/g at 1.8 K. These results, together with the zero
resistance, clearly indicate that LaNiOP exhibits supercon-
ductivity at temperatures below 4 K. The volume fraction
of the superconducting phase estimated from the ø value at
1.8 K is ∼40 vol %. The M-Hext curve (field dependence
of magnetization) in the inset shows that the increase in the
negative magnetization is proportional to Hext at Hext ) e20
Oe and then decreases, indicating that ø is a constant value
of ∼40 vol % of the perfect diamagnetic phase when Hext is
e20 Oe and decreases to zero up to Hext ) 800 Oe. These
The obtained LaNiOP is chemically stable in air and dark
gray. Figure 2 shows the XRD patterns measured (blue dots)
and simulated by the Rietveld analysis using the refined result
(red line). The three-phase Rietveld analyses reveal that the
obtained sample is mainly LaNiOP with ∼8 vol % of
LaNi2P2 and <1 vol % of La3PO7. The refined crystal
structure of LaNiOP is summarized in supplemental informa-
(8) Bruker AXS. TOPAS, version 3; Bruker AXS: Karlsruhe, Germany,
2005.
(9) Zimmer, B. I.; Jeitschko, W.; Albering, J. H.; Glaum, R.; Reehuis,
M. J. Alloys Compd. 1995, 229, 238-242.
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COMMUNICATION
observations indicate that LaNiOP is a type II superconductor
with lower and upper critical magnetic fields of Hc1 ) ∼20
Oe and Hc2 ) ∼800 Oe, respectively.
superconductors have layered structures similar to that of
LaNiOP: Ni is tetrahedrally coordinated by not P but B,
and the LaO layers with the PbO-type structure in LaNiOP
are replaced by rocksalt-type layers.
Herein, the effects of the impurity phases should be
assessed. Among the impurity phases found in the XRD
pattern, only LaNi2P2 shows an electronic conduction.
Jeitschko and Reehuis have reported that LaNi2P2 does not
exhibit a superconducting transition down at least to 1.8 K.10
We also measured the M-H curves of single-phase LaNi2P2
samples at 2 K and confirmed that they do not show
diamagnetism. In addition, the volume fractions of the
LaNi2P2 (∼8 vol %) and La3PO7 (<1 vol %) phases are
safely smaller than that of the superconducting phase (∼40
vol %). Accordingly, we conclude that LaNiOP becomes a
superconducting phase at <4 K. Ni-based superconductors,
LnNi2B2C (Ln ) Y, Tm, Er, Ho, and Lu) and La3Ni2B2N,11,12
have been reported by Cava et al. as high-Tc intermetallic
superconductors; Tc of LuNi2B2C and La3Ni2B2N are 16.6
and 13.2 K, respectively. The crystal structures of these
In summary, a layered compound of LaNiOP was first
synthesized by solid-state reactions. It exhibits a type II
superconducting transition of ∼3 K with a lower critical
magnetic field of ∼20 Oe and an upper field of ∼800 Oe.
Systematic study on the derivatives of LaMOP by replacing
La, M, and P with other lanthanides, transition metals, and
N group elements, respectively, would help to understand
the superconductivity mechanism and lead to the discovery
of a new superconductor.
Supporting Information Available: X-ray crystallographic data
for LaNiOP in CIF format. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC701200E
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