Superconductivity at 38 K in the Iron Arsenide (Ba1-xKx)Fe2As2.

Superconductivity at 38 K in the iron arsenide (Ba1−xKx)Fe2As2
Marianne Rotter, Marcus Tegel and Dirk Johrendt∗
Department Chemie und Biochemie, Ludwig-Maximilians-Universita¨t Mu¨nchen,
Butenandtstrasse 5-13 (Haus D), 81377 Mu¨nchen, Germany
(Dated: July 17, 2008)
The ternary iron arsenide BaFe2As2 becomes superconducting by hole doping, which was achieved
by partial substitution of the barium site with potassium. We have discovered bulk superconduc-
tivity at Tc = 38 K in (Ba1−xKx)Fe2As2 with x ≈ 0.4. The parent compound BaFe2As2 as well as
KFe2As2 both crystallize in the tetragonal ThCr2Si2-type structure, which consists of (FeAs)δ− iron
arsenide layers separated by barium or potassium ions. BaFe2As2 is a poor metal and exhibits a
spin density wave (SDW) anomaly at 140 K. By substituting Ba2+ for K+ ions we have introduced
holes in the (FeAs)− layers, which suppress the SDW anomaly and induce superconductivity. This
scenario is very similar to the recently discovered arsenide-oxide superconductors. The Tc of 38 K in
(Ba0.6K0.4)Fe2As2 is the highest critical temperature in hole doped iron arsenide superconductors
so far. Therefore, we were able to expand this class of superconductors by oxygen-free compounds
with the ThCr2Si2-type structure. Our results suggest, that superconductivity in these systems
evolves essentially from the (FeAs)δ− layers and may occur in other related compounds.
PACS numbers: 74.10.+v, 74.70.Dd, 74.20.Mn
The recent discovery of superconductivity in pnictide-
oxides with critical temperatures (Tc) up to 55 K has
generated tremendous interest in the scientific commu-
nity. After first reports on superconductivity in LaFePO
[1] and LaNiPO [2, 3] below 5 K, the breakthrough came
with the fluoride doped arsenide LaFeAs(O1−xFx) [4]
that exhibits Tc = 26 K which increases to 43 K
under pressure.[5] Reports on even higher Tc’s of up
to 55 K, achieved by replacing lanthanum by rare
earth ions with smaller ionic radii, followed quickly.[6]
These compounds represent the second class of high-Tc
materials,[7] 22 years after the discovery of the copper
oxide superconductors.[8]
The parent compound of the new materials, LaFeAsO,
has a quasi two-dimensional tetragonal structure, which
consists of charged (LaO)δ+ layers alternating with
(FeAs)δ− layers (ZrCuSiAs-type).[9] Several recent stud-
ies suggest that superconductivity in doped iron ar-
senides is unconventional and therefore non-BCS-like,
[10, 11, 12] but this issue is not clear at all. In con-
trast to the non-conducting parent compound of the cop-
per oxides, LaFeAsO is a poor metal and exhibits Pauli-
paramagnetism. The existence of a spin density wave
(SDW) anomaly evolving in LaFeAsO at 135-140 K as-
sumes a key role. [13, 14] The SDW is accompanied
by a structural phase transition [15] and anomalies in
the specific heat, electrical resistance and magnetic sus-
ceptibility. Antiferromagnetic ordering of the magnetic
moments occurs just below the structural transition tem-
perature (TN = 134 K, 0.36 µB/Fe).[16] By changing the
electron count within the (FeAs)δ− layers, the structural
phase transition and antiferromagnetic ordering are sup-
pressed and superconductivity emerges.[17, 18] Electron
∗Electronic address: johrendt@lmu.de
doping has been a highly successful approach in the case
of LaFeAsO, either by substituting oxide for fluoride [4]
or by introducing oxide deficiencies in the LaO layer.[19]
In contrast to this, the only case of superconductivity by
hole doping is (La1−xSrx)FeAsO (Tc = 25 K) so far.[20]
The pairing mechanism in iron arsenides is currently in
dispute. But even in these early days it becomes evident,
that superconductivity in LaFeAsO emerges from spe-
cific structural and electronic conditions in the (FeAs)δ−
layer. However, if only the iron arsenide layer is essen-
tial, also other structure types could serve as parent com-
pounds. We reported recently, that BaFe2As2 with the
well-known ThCr2Si2-type structure is an excellent can-
didate. [21] The crystal structure of BaFe2As2 is shown
in Figure 1. This ternary arsenide contains FeAs layers
identical to LaFeAsO, moreover with the same charge
[22], and exhibits a SDW anomaly likewise (vide infra).
In this letter we report on superconductivity in BaFe2As2
induced by hole doping, which was achieved by partial
substitution of the barium by potassium ions.
FIG. 1: (Color online) Crystal structure of BaFe2As2
(ThCr2Si2-type structure, space group I4/mmm).
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2In the very large family of ThCr2Si2-type compounds,
superconductivity occurred at temperatures below 5
K, as e.g. in LaIr2Ge2, LaRu2P2, YIr2−xSi2+x and
BaNi2P2, [23, 24, 25, 26, 27] although closely related
rare earth borocarbides are known for higher Tc’s up
to 26 K in YPd2B2C.[28, 29] We have suggested ear-
lier, that superconductivity in ThCr2Si2-type compounds
may arise under certain electronic conditions. [30] Re-
cently we found that BaFe2As2 and LaFeAsO exhibit
strikingly similar properties. [21] BaFe2As2 is a poor
Pauli-paramagnetic metal that undergoes a structural
and magnetic phase transition at 140 K, accompanied
by strong anomalies in the specific heat, electrical resis-
tance and magnetic susceptibility. In the course of this
phase transition, the space group symmetry changes from
tetragonal (I4/mmm) to orthorhombic (Fmmm).
Based on these findings, we expected superconductiv-
ity in doped BaFe2As2. First attempts to realize electron
doping by lanthanum substitution were unsuccessful, be-
cause the required doping level could not be achieved.
We then decided to try hole doping by substituting the
Ba2+ cations for K+ with a similar ionic radius. In
order to achieve doping levels of 0.15-0.2 electrons per
(FeAs) unit, we had to substitute 30-40% of the barium
ions for potassium ions. This seemed possible, because
isostructural KFe2As2 had been known to exist.[31] We
succeeded in preparing (Ba1−xKx)Fe2As2 (x = 0.3 and
0.4) by heating stoichiometric mixtures of the elements
(all purities > 99.9%) in alumina crucibles, welded in sil-
ica tubes under an atmosphere of purified argon. [31, 32]
The samples were heated slowly (50 K/h) to 873 K, kept
at this temperature for 15 h and cooled to room temper-
ature by switching off the furnace. After homogenization
in an argon-filled glove-box, the products were annealed
at 925 K for 15 h, again homogenized, cold pressed into
pellets and sintered at 1023 K for 12 h. The material is
black and stable in air for weeks. BaFe2As2 and KFe2As2
were synthesized by the same method.
Phase purity was checked by X-ray powder diffrac-
tion with Cu-Kα1 or Mo-Kα1 radiation. Rietveld refine-
ments of the data were performed with the GSAS package
[33, 34]. The pattern of BaFe2As2 could be completely
fitted with a single phase. In the samples of KFe2As2
and (Ba1−xKx)Fe2As2, we detected FeAs (Westerveldite
[35]) as impurity phase, which was included in the re-
finement and quantified to 6 ± 1%. The substitution of
40% barium for potassium is clearly proved by the refine-
ment of the site occupation parameters in the Rietveld
fit of (Ba0.6K0.4)Fe2As2 (Figure 2). A summary of the
crystallographic data is compiled in Table I.
As mentioned above, a crucial aspect of the LaFeAsO
superconductors is the suppression of the SDW anomaly
by doping. Therefore, we have also measured the X-
ray powder pattern of (Ba0.6K0.4)Fe2As2 at 20 K. No
broadening or splitting of the diffraction peaks as found
in BaFe2As2 below 140 K were detected. The in-
sert in Figure 2 shows the temperature dependency of
the (110)-reflections of BaFe2As2 and (Ba0.6K0.4)Fe2As2
for comparison. We successfully refined the pattern of
(Ba0.6K0.4)Fe2As2 measured at 20 K by using the param-
eters of the undistorted tetragonal structure (space group
I4/mmm). Table I shows the almost identical crystallo-
graphic data of (Ba0.6K0.4)Fe2As2 at 297 K and 20 K,
respectively. Thus it is evident that the K-doping has
suppressed the structural transition of BaFe2As2.
31.5 32.5 31.8 33.0 2θ (deg.)
180 K
140 K
100 K
60 K
40 K
20 K
180 K
140 K
100 K
60 K
40 K
20 K

2θ (degrees)


inten
sity (
10
coun
ts)






10.0 20.0 30.0 40.0 50.0 60.0
0.0

1.0

2.0

inten
sity (
arb.
units
)
BaFe2As2 Ba0.6K0.4Fe2As2
4
FIG. 2: (Color online) X-ray powder pattern (+) and Ri-
etveld fit (−) of (Ba0.6K0.4)Fe2As2 at 297 K. Reflection
markers are blue for FeAs and black for (Ba0.6K0.4)Fe2As2.
The insert shows the temperature dependency of the (110)-
reflections of BaFe2As2 and (Ba0.6K0.4)Fe2As2.
TABLE I: Crystallographic data of (Ba0.6K0.4)Fe2As2
Temperature (K) 297 20
Space group I4/mmm I4/mmm
a (pm) 391.70(1) 390.90(1)
c (pm) 1329.68(1) 1321.22(4)
V (nm3) 0.20401(1) 0.20189(1)
Z 2 2
data points 5499 8790
reflections (total) 405 127
d range 0.639 − 6.648 0.971 − 6.606
RP, wRP 0.0202, 0.0258 0.0214, 0.0283
R(F2), χ2 0.026, 1.347 0.093, 1.816
Atomic parameters:
K,Ba 2a (0,0,0) 2a (0,0,0)
Uiso = 130(8) Uiso = 89(8)
Fe 4d ( 12 , 0,
1
4 ) 4d (
1
2 , 0,
1
4 )
Uiso = 47(4) Uiso = 84(7)
As 4e (0,0,z) 4e (0,0,z)
z = 0.3538(1) z = 0.3538(1)
Uiso = 70(3) Uiso = 76(7)
K : Ba ratio 42(1) : 58(1) 38(1) : 62(1)
Bond lengths (pm):
Ba–As 338.4(1)×8 337.2(1)×8
Fe–As 239.6(1)×4 238.8(1)×4
Fe–Fe 277.0(1)×4 276.4(1)×4
Bond angles (deg):
As–Fe–As 109.7(1)×2 109.9(1)×2
109.4(1)×4 109.3(1)×4

3That followed we have measured the electrical resis-
tance of (Ba1−xKx)Fe2As2 (x = 0, 0.4 and 1.0) through
a four-probe method. As depicted in Figure 3, BaFe2As2
has the highest resistance and shows a decrease at 140
K, which is linked to the SDW anomaly.[21] In con-
trast to this, the resistance of KFe2As2 is considerably
smaller and decreases smoothly with temperature, as it
is typical for a normal metal. The resistance of K-doped
(Ba0.6K0.4)Fe2As2 is similar to KFe2As2 and does not
show any sign of an anomaly at about 140 K in agree-
ment with our structural data. But the resistance drops
abruptly to zero at ≈ 38 K, which clearly indicates su-
perconductivity. Figure 4 shows details of the transition.
By using the 90/10 criterion, we find the midpoint of
the resistive transition at 38.1 K and a transition width
of 1.5 K. The first deviation from the extrapolated re-
sistance is at ≈ 39 K and zero resistance is achieved
at 37.2 K. Consequently, we have discovered supercon-
ductivity analogue to the LaFeAsO materials, but in a
oxygen-free compound with ThCr2Si2 structure. The Tc
of 38 K is the highest critical temperature observed in
hole doped iron arsenide superconductors so far (25 K in
(La1−xSrx)FeAsO).[20]
0
50
100
150
200
250
300
0.00.20.40.60.81.01.2
BaF
e 2As
2
(Ba 0
.6K 0.4)
Fe 2A
s 2
KFe
2As 2


Resistance (mΩcm)
Temp
eratu
re (K
)
FIG. 3: (Color online) Electrical resistance of BaFe2As2
(blue), KFe2As2 (green) and (Ba0.6K0.4)Fe2As2 (red).
In order to confirm superconductivity, we have mea-
sured the magnetic susceptibility of finely ground pow-
der of (Ba0.6K0.4)Fe2As2 using a SQUID magnetometer
(MPMS-XL5, Quantum Design Inc.) Zero-field cooled
(shielding) and field cooled (Meissner) cycles measured
at 1 mT and 0.5 mT are shown in Figure 5. The sample
becomes diamagnetic at 38.3 K and shows 10% of the
maximum shielding at 37.2 K. The zero field cooled (zfc)
branches of the susceptibilities measured at 1 and 0.5 mT
are almost identical and amount to −0.94 at 1.8 K, which
is close to ideal diamagnetism (4piχ = −1). The Meissner
effect (fc) depends on the applied field and the measured
susceptibilities at 1.8 K are −0.64 at 0.5 mT and −0.3
at 1 mT. These values of the shielding- and Meissner
fractions should be considered as estimates due to uncer-
20
25
30
35
40
45
50
55
60
65
70
0.00.51.01.5
343
638
404
2
0.00.40.8
(Ba 0.
6K 0.4)
Fe 2A
s 2


T 90 =
38.7
K
T 50 =
38.1
K
T 10 =
37.2
K
ρ/ρ
40 K
Temp
eratu
re (K
)


dρ/dT
Temp
eratu
re (K
)
FIG. 4: Resistivity transition of (Ba0.6K0.4)Fe2As2.
0
10
20
30
40
50
60
-1.0-0.8-0.6-0.4-0.20.0
zfc fc fc
(Ba 0.
6K 0.4)
Fe 2A
s 2
0.5 m
T
1.0 m
T


4πχ
Temp
eratu
re (K
)
FIG. 5: (Color online) Magnetic susceptibility of
(Ba0.6K0.4)Fe2As2 at 0.5 mT (red) and 1 mT (blue).
tainties regarding the density of the compacted powder
and demagnetization effects. However, the susceptibility
data unambiguously prove bulk superconductivity of the
(Ba0.6K0.4)Fe2As2 sample.
In summary, we have discovered the first mem-
ber of a new family of iron arsenide superconductors.
(Ba0.6K0.4)Fe2As2 with the ThCr2Si2-type structure is
a bulk superconductor with Tc = 38 K. The struc-
tural and electronic properties of the parent compound
BaFe2As2 are closely related to LaFeAsO. We have in-
duced superconductivity by hole doping and have found
a significantly higher Tc in comparison with hole doped
LaFeAsO. In contrast to previously stated opinions, our
results suggest that hole doping is definitely a possible
pathway to induce high-Tc superconductivity, at least in
the oxygen-free compounds. Further optimization may
lead to even higher Tc’s in ThCr2Si2-type compounds.

4Acknowledgments
We thank Prof. Thomas Fa¨ssler for support with
magnetic measurements. This work was financially sup-
ported by the German Research Foundation [Deutsche
Forschungsgemeinschaft (DFG)].
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