Superconductivity and Crystal Structures of (Ba1-xKx)Fe2As2 (x = 0 - 1)

Sup DOI: 10.1002/anie.200((will be filled in by the editorial staff))
Sup ctures of (Ba1−xKx)Fe2As2 (x = 0 - 1)
Mar el and Dirk Johrendt*
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erconductivity
erconductivity and Crystal Stru
ianne Rotter, Michael Pangerl, Marcus Teg
iscovery of iron arsenide superconductors has opened n
es for superconductivity research. After first reports
s(O1−xFx) with critical temperatures (Tc) of 26 - 43 K,[1
igher transition temperatures up to 55 K in SmFeAs(O1−x
[3]ed quickly. It is meanwhile accepted, that iron arsenides
ent a second class of high-Tc superconductors[4] since the
ery of the cuprates in 1986.[5]
milar to the cuprates, superconductivity in iron arsenides
es from two-dimensional, magnetically ordered layers in the
compound. LaFeAsO crystallizes in the ZrCuSiAs-type
re,[6] composed by alternating (LaO)+ and (FeAs)− layers as
on the left hand side of Fig. 1. Superconductivity is induced
tial oxidation (hole doping)[7] or reduction (electron doping)
(FeAs)δ− layers. Electron doping has been highly successful
stitution of oxide for fluoride or by oxide vacancies, whereas
ne case of hole doped LaFeAsO has been reported so far.[8]
cently, we proposed the ternary iron arsenide BaFe2As2[9]
he well known ThCr2Si2-type structure as a potential new
compound.[10] Our idea based on the very similar structural
ectronic conditions of this long known ternary arsenide in
rison to LaFeAsO. BaFe2As2 and LaFeAsO contain identical
layers, which also have the same charge according to
FeAs)−]2. Fig. 1 shows both structures in comparison.
dingly, we were able to induce superconductivity at 38 K in
0.4)Fe2As2 by hole doping,[11] thus we have discovered a new
of superconducting iron arsenides. Our discovery was
ed by reports on isotypic compounds with strontium (Tc ≈ 37
13] calcium (Tc ≈ 20 K),[14] and europium (Tc = 32 K)[15] within
. Meanwhile, a large part of the research on superconducting
rsenides is focused on the ternary compounds rather than
de-oxides, because single-phase samples and also large single
ls are much easier to obtain.
en though several findings suggest unconventional (non-
superconductivity in iron arsenides,[16-18] the pairing
nism is far from being clear and disputed in the physical
unity.[19] A generally accepted key aspect for both LaFeAsO
aFe2As2 families is a magnetic and structural phase transition
undoped phases at temperatures between 140 and 203 K.[10, 20-
e tetragonal symmetry (space group I4/mmm) turns into
hombic (space group Fmmm) and antiferromagnetic spin
ng emerges immediately below these temperatures. Recently,
ve proved the antiferromagnetic spin structure of BaFe2As2 by
crystal neutron diffraction.[23]
Figure 1 . Crystal structures of LaFeAsO (left) and BaFe2As2 (right)
It has been believed that the magnetic and structural phase
transitions of LaFeAsO and BaFe2As2 must be suppressed by
doping, so that superconductivity can emerge. But recently it was
reported, that superconductivity up to 29 K occurs even in undoped
AFe2As2 (A = Ca, Sr, Ba) under pressure.[24, 25] Thus all the signs are
that destabilization of the antiferromagnetic state by doping or
pressure is a main issue for superconductivity in iron arsenides.
However, the crucial question, whether both states may coexist, is
still open.
The doping dependency of the structure and superconductivity
has been intensively studied on LaFeAsO-type compounds. In
electron-doped REFeAsO1−x (RE = La - Sm),[26] Tc increases with
higher doping levels and with decreasing lattice parameters. On the
other hand, the hole doped system (La1−xSrx)FeAsO[27] shows also
increasing Tc with higher doping levels, but with increasing lattice
parameters. This indicates that the doping level is the determining
parameter for Tc in LaFeAsO compounds. However, these results
are limited by the facts, that the exact doping levels are unknown in
most cases and confined to x ≈ 0.2 anyway. Furthermore, the
changes in the lattice parameters are very small and their
significance often doubtful.
In contrast to this, the BaFe2As2 system opens a golden
opportunity for doping studies. This is because also KFe2As2 had
been known to exist [28] and consequently K-doping of BaFe2As2
should be easy due to similar ionic radii of Ba2+ (1.42 Å) and K+
(1.51 Å).[29] So far, we have only reported on the occurrence of
superconductivity in (Ba0.6K0.4)Fe2As2. But the dependencies of
superconductivity and crystal structures on the potassium content in
the whole solid solution (Ba1−xKx)Fe2As2 are still unknown. In this
communication, we report on the synthesis, crystal structures and
superconducting transition temperatures of the complete series
(Ba1−xKx)Fe2As2 (x = 0 – 1).
The ternary compounds BaFe2As2 and KFe2As2 both belong to
the more two-dimensional branch of the ThCr2Si2-type structure
without As−As bonds between the layers. The unit cells almost have
the same volumes despite the slightly bigger radius of K+. On the
other hand, the c/a ratios differ considerably because the c lattice
parameter of KFe2As2 is almost 1 Å longer than the c of BaFe2As2.
In other words, the unit cell of KFe2As2 is stretched along c.
Prof. Dr. Dirk Johrendt, M. Sc. Marianne Rotter,
B.Sc. Michael Pangerl, Dipl.-Chem. Marcus Tegel
Department Chemie und Biochemie
Ludwig-Maximilians-Universität München
Butenandtstr. 5-13 (Haus D), 81377 München, Germany
Fax: (+)89 2180 77431
E-mail: Johrendt@lmu.de
Web: www.cup.uni-muenchen.de/ac/johrendt/index.html
We thank Dr. Joachim Deisenhofer for susceptibility
measurements. This work was financially supported by the
DFG.

1

The crystal structures of the compounds (Ba1−xKx)Fe2As2 were
determined by Rietveld refinements of X-ray powder patterns as
shown exemplary in Figure 2. Figure 3 shows the changes of the
structure with doping. The lattice parameters a and c vary linearly
with the potassium content over the whole range. We find a constant
unit cell volume within the experimental error, because the strong
elongation of c is almost compensated by the decrease of a.[30] Also
the Fe−As and Ba(K)−As bond lengths remain unchanged. Both
parameters vary by less than 0.4% and are therefore not shown.

Figure 2. Measured (+) and calculated (⎯) X-ray powder pattern of
(Ba0.9K0.1)Fe2As2.
Figure 3. Variation of structural parameters in (Ba1−xKx)Fe2As2.
Apart from the lattice parameters, only the Fe−Fe bond length
and the As-Fe-As bond angle ε changes significantly (by 5-7%) on
doping. Both decrease linearly with increasing potassium content,
which means that the FeAs4 - tetrahedra get more elongated along c
and the iron atoms within the layers move together. Interestingly, ε
becomes the ideal tetrahedral angle of 109.5° at x ≈ 0.4. The insert
in Figure 3 depicts the ε angle in the FeAs layer. Thus, the main
implication of doping on the crystal structure of (Ba1−xKx)Fe2As2 is a
decreasing As-Fe-As bond angle and a shortening of the distances
between the iron atoms at the same time.

Chemical bonding in ThCr2Si2-type compounds has been
intensively studied.[31] We have shown that the properties of these
compounds depend on a subtle balance between different bonding
interactions, especially on the interplay between metal-ligand
(Fe−As) and metal-metal (Fe−Fe) bonding within the layers.[32] In
the case of BaFe2As2, it is accepted that the Fe-3dx2-y2-orbitals close
the Fermi level play a key role for magnetism and
superconductivity. The angle ε determines the overlap between Fe-
3dx2-y2 and As-3sp orbitals, thus our results suggest a strong coupling
of structural and electronic degrees of freedom by doping.
It is disputed if the structural phase transition in the iron
arsenides has to be completely suppressed before superconductivity
emerges. Recent results suggest that the structural distortion of
LaFeAsO disappears by doping exactly at the border to the
superconducting state.[33] In the case of BaFe2As2, we have already
shown that the tetragonal to orthorhombic phase transition is
suppressed in (Ba0.6K0.4)Fe2As2.[11] In order to delimit the
composition range of the transition, we have measured X-ray
powder diffraction patterns of (Ba1-xKx)Fe2As2 (x = 0 – 0.3) between
300 and 10 K. Figure 4 shows the temperature dependencies of the
(110)-reflections. The reduction of the lattice symmetry is visible by
peak splitting or broadening up to x = 0.2, but absent at x = 0.3. The
transition temperatures (Ttr) decrease strongly with higher potassium
content from 140 K to ≈ 90 K at x = 0.2, where the transition
proceeds over a wide temperature range. From this we conclude
that the orthorhombic phase (space group Fmmm) exists at low
temperatures up to x = 0.2 and becomes tetragonal between x = 0.2
and 0.3.

Figure 4. Temperature dependencies of the (110)-reflections in
(Ba1−xKx)Fe2As2 with x = 0 - 0.3.
We have also investigated the doping effect on the
superconducting transition temperatures. Therefore we have
measured the electrical resistances of (Ba1−xKx)Fe2As2 samples (x =
0 - 1) between 2 and 300 K by a four probe method. The relative
changes of the resistance with temperature (R/R300K) of all samples
are shown in Figure 5. Superconductivity was detected in all cases
except for the undoped parent compound BaFe2As2, but the
transition temperatures vary strongly. BaFe2As2 is a poor metal with
a specific resistivity around 1 mΩcm at room temperature and
exhibits the structural and magnetic phase transition at 140 K,[10]
which is clearly visible in the resistance plot.




2

Figure 5. Relative electrical resistances of (Ba1−xKx)Fe2As2 samples.
At the smallest doping level x ≈ 0.1, the resistance anomaly is
less pronounced, but not completely suppressed. We find an abrupt
drop in the resistance at ≈ 3 K, which is the onset of
superconductivity. However, zero resistance could not be achieved
at 1.8 K, but superconductivity was proved by magnetic
measurements. The anomaly in the resistance appears to be
suppressed when the doping level is at 0.2. At this point, we find the
behavior of a normal metal and superconductivity at Tc ≈ 25 K,
which increases strongly to 36 K and 38 K at x = 0.3 and x = 0.4,
respectively. Doping levels of x > 0.5 lead to a continuous decrease
of Tc down to 3.8 K for KFe2As2.
The phase diagram in Figure 6 shows the superconducting
critical temperatures (Tc), as well as the phase transition
temperatures (Ttr) of (Ba1−xKx)Fe2As2. We find superconductivity in
the range between x = 0.1 - 1.0 with Tc > 30 K between x = 0.3 - 0.6
and a maximum of 38 K close to x = 0.4. The orthorhombically
distorted crystal structure exists up to x = 0.2, that is the point where
Tc is already 25 K. Thus, superconductivity apparently coexists with
the orthorhombic structure and potentially with the
antiferromagnetic state below Ttr known for BaFe2As2.


Figure 6. Phase diagram of (Ba1−xKx)Fe2As2 with the superconducting
(Tc) and phase transition (Ttr) temperatures. Tc is defined as the
temperature where the resistance is dropped to 90% of the
extrapolated value. The dashed lines are guides for the eye.
In summary, we have shown experimental doping dependencies
of the crystal structure and superconductivity in the solid solution
(Ba1−xKx)Fe2As2. As the main effect of doping on the crystal
structure at room temperature, we find linear decreasing As-Fe-As
bond angles (ε) and Fe−Fe distances, equivalent to an elongation of
the FeAs4 tetrahedra along [001]. This contradicts results of recently
reported DFT calculations, where the opposing effect was
proposed.[34] The structural changes are intimately coupled to the
electronic states at the Fermi level, because the most relevant Fe-
3dx2−y2 orbitals are strongly affected by the bond angle ε. We have
observed the structural phase transition (I4/mmm → Fmmm) of
BaFe2As2 with decreasing transition temperatures up to a doping
level of x = 0.2 and the complete suppression at x = 0.3.
Superconductivity occurs over the whole doping range in
(Ba1−xKx)Fe2As2 with a maximum Tc of 38 K at x ≈ 0.4. Solely the
parent compound BaFe2As2 is non-superconducting above 1.8 K.
The superconducting transitions in the orthorhombic compounds
(Ba0.9K0.1)Fe2As2 (Tc ≈ 3 K) and (Ba0.8K0.2)Fe2As2 (Tc ≈ 25 K) give
strong evidence for the coexistence of superconductivity with the
structurally distorted and potentially magnetically ordered state in
the BaFe2As2 family of iron arsenide superconductors.
Experimental Section
Polycrystalline samples of (Ba1-xKx)Fe2As2 with x = 0 - 1 in steps of
0.1 were synthesized by heating stoichiometric mixtures of the
elements (purity > 99.9%) at 823 - 1223 K in alumina crucibles
enclosed in silica ampoules under argon atmosphere. In order to
reduce the loss of potassium by evaporation, the gas volume was
reduced by alumina inlays in the crucibles. The products were black
metallic powders and stable in air for weeks. Sampling EDX
measurements showed homogenous distributions of barium and
potassium (±5%) and confirmed the compositions obtained from the
Rietveld fits. X-ray powder diffraction patterns were recorded between
10 and 300 K using a Huber G670 imaging plate detector (CuK α1-
radiation, Ge(111)-monochromator), equipped with a closed-cycle He-
cryostat). Patterns at room temperature were indexed with tetragonal
body-centered unit cells according to the ThCr2Si2 type (I4/mmm) or
with orthorhombic face-centered unit cells (Fmmm, aortho ≈ √2 atetra−δ,
bortho ≈ √2 btetra+δ, cortho ≈ ctetra) at low temperatures. Small amounts of
FeAs were detected as impurity phase in some samples. The crystal
structures were refined by the Rietveld method using the GSAS[35]
software package using Thompson-Cox-Hastings functions with
asymmetry corrections as reflection profiles.[36] Electrical resistances
were measured by the four probe method on cold pressed and
sintered pellets (1123 K) using a He-closed-cycle refrigerator. Gold
wires were fixed to the sample by silver conduction paint.

Received: ((will be filled in by the editorial staff))
Published online on ((will be filled in by the editorial staff))
Keywords: Superconductivity · Phase transitions · Iron arsenides ·
Barium · Potassium · Crystal structures
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4

Entry for the Table of Contents

Superconductivity
Marianne Rotter, Michael Pangerl,
Marcus Tegel and Dirk Johrendt*
__________ Page – Page
Superconductivity and Crystal
Structures of (Ba1−xKx)Fe2As2 (x = 0 - 1)

The iron arsenides (Ba1−xKx)Fe2As2
with the ThCr2Si2-type structure
exhibit superconductivity between 3
K and 38 K depending on the doping
level. Superconductivity appears
before the structural distortion of the
parent compound BaFe2As2 is
completely suppressed by doping.
The bond angles in the iron arsenide
layers decrease by doping,
suggestive of a coupling of structural
and electronic degrees of freedom.


5