Structural and magnetic phase transitions in the ternary iron arsenides SrFe2As2 and EuFe2As2

RAPID COMMUNICATION
Structural and magnetic phase transitions in the
ternary iron arsenides SrFe2As2 and EuFe2As2
Marcus Tegel1, Marianne Rotter1, Veronika Weiss1, Falko M.
Schappacher2, Rainer Po¨ttgen2 and Dirk Johrendt1
1Department Chemie und Biochemie der Ludwig-Maximilians-Universita¨t Mu¨nchen,
Butenandtstr- 5-13 (Haus D), 81377 Mu¨nchen, Germany
2Institut fu¨r Anorganische und Analytische Chemie, Universita¨t Mu¨nster,
Corrensstrasse 30, D-48149 Mu¨nster, Germany
E-mail: johrendt@lmu.de
Abstract. The structural and magnetic phase transitions of the ternary iron
arsenides SrFe2As2 and EuFe2As2 were studied by temperature-dependent x-ray
powder diffraction and 57Fe Mo¨ssbauer spectroscopy. Both compounds crystallize in
the tetragonal ThCr2Si2-type structure at room temperature and exhibit displacive
structural transitions at 203 K (SrFe2As2) or 190 K (EuFe2As2) to orthorhombic lattice
symmetry in agreement with the group-subgroup relationship between I4/mmm and
Fmmm. 57Fe Mo¨ssbauer spectroscopy experiments with SrFe2As2 show full hyperfine
field splitting below the phase transition temperature (8.91(1) T at 4.2 K). Order
parameters were extracted from detailed measurements of the lattice parameters and
fitted to a simple power law. We find a relation between the critical exponents and
the transition temperatures for AFe2As2 compounds, which shows that the transition
of BaFe2As2 is indeed more continuous than the transition of SrFe2As2 but it remains
second order even in the latter case.
PACS numbers: 61.50.Ks,74.10.+v,33.45.+x
Keywords: Iron arsenides, Phase transition, Structure, 57Fe Mo¨ssbauer spectroscopy
ar
X
iv
:0
80
6.
47
82
v2
[
co
nd
-m
at.
su
pr
-co
n]
1
Se
p 2
00
8

Phase transitions in SrFe2As2 and EuFe2As2 2
1. Introduction
The discovery of the iron arsenide superconductors has provided fresh impetus in the
field of high-TC superconductivity [1, 2]. LaFeAsO with the tetragonal ZrCuSiAs-
type structure [3] becomes superconducting by doping the (FeAs)δ− layers either with
electrons or holes [4, 5]. The initially reported transition temperature of 26 K rose
quickly up to 55 K by replacing La3+ ions for smaller Sm3+ ions [6]. The crystal
structure of LaFeAsO contains alternating layers of edge-sharing La4/4O and FeAs4/4
tetrahedra and superconductivity emerges in the FeAs layers by adding or removing
about 0.2 electrons per formula unit.
Very recently, we reported on the oxygen-free iron arsenide BaFe2As2 with the
ThCr2Si2-type structure as another possible parent compound for superconductivity
[7]. Soon after that we were able to induce superconductivity by hole doping in the
compound (Ba0.6K0.4)Fe2As2 with TC = 38 K and we have therefore established a further
family of iron arsenide superconductors [8]. Another report on superconductivity at 37
K in isostructural K- and Cs- doped SrFe2As2 followed quickly [9]. The crystal structures
of both LaFeAsO and BaFe2As2 are depicted in Figure 1. Both compounds are built up
by almost identical (FeAs)δ− layers, but they are separated by lanthanum oxide sheets
in LaFeAsO and by barium atoms in BaFe2As2 respectively.
Figure 1. (Color online) Crystal structures of LaFeAsO and BaFe2As2.
The non-superconducting parent materials LaFeAsO and BaFe2As2 show
remarkably similar properties. Both compounds are poor metals and only weakly
magnetic. One key finding is the existence of a spin-density-wave (SDW) anomaly,
which occurs at Ttr = 150 K in LaFeAsO and at Ttr = 140 K in BaFe2As2, respectively
[10, 7]. This SDW is linked to abrupt changes in the electrical resistivity and magnetic
susceptibility and also to structural phase transitions. Antiferromagnetic ordering was
found in LaFeAsO 18 K below the structural transition [10], but directly at or at least
very close to the lattice distortion temperature in BaFe2As2 according to recent neutron

Phase transitions in SrFe2As2 and EuFe2As2 3
diffration experiments [11]. It is currently believed that superconductivity in the iron
arsenides is intimately connected with the suppression of this SDW anomaly by doping.
This suggests that spin-fluctuations may play an important role for the mechanism of
superconductivity as it was also assumed for the high-TC cuprates. Thus, the nature
of the phase transitions is important for a deeper understanding of superconductivity
in the iron arsenides. However, precise structural data close to the phase transition are
only available for BaFe2As2 and LaFeAsO [7, 12, 13]. SrFe2As2 has been studied by
single crystal data with relatively low resolution [14], which allow no evaluation of the
order parameter close to the transition temperature. Furthermore, the connection of
the structural transition in SrFe2As2 with magnetic ordering, as well as the structure
of EuFe2As2 at low temperatures has not been investigated yet. We have therefore
studied the structural phase transitions of polycrystalline SrFe2As2 and EuFe2As2 in
detail by temperature-dependent x-ray powder diffraction. We could also confirm the
association of the structural transition in SrFe2As2 with magnetic ordering by 57Fe
Mo¨ssbauer spectroscopy.
2. Experimental
SrFe2As2 and EuFe2As2 were synthesized by heating mixtures of distilled Sr(Eu)-metal,
iron-powder and sublimed arsenic at ratios of 1:2:2 in alumina crucibles, which were
sealed in silica tubes under an atmosphere of purified argon. The mixtures were heated
to 850 K at a rate of 50 K/h, kept at this temperature for 15 h and cooled down to room
temperature. The reaction product was homogenized in an agate mortar and annealed
at 900 K for 15 h. The obtained black crystalline powders of SrFe2As2 and EuFe2As2
are sensitive to air and moisture.
Temperature dependent x-ray powder diffraction data were collected using a Huber
G670 Guinier imaging plate diffractometer (Cu-Kα1 radiation, Ge-111 monochromator),
equipped with a closed-cycle He-cryostat. Rietveld refinements were performed with
the GSAS package [15] using Thompson-Cox-Hastings functions [16] with asymmetry
corrections as reflection profiles.[17]
A 57Co/Rh source was available for the 57Fe Mo¨ssbauer spectroscopy investigations.
The velocity was calibrated relative to the signal of α-Fe. A SrFe2As2 sample was placed
in a thin-walled PVC container at a thickness of about 4 mg Fe/cm2. The measurements
were performed in the usual transmission geometry at 298, 77 and 4.2 K. The source
was kept at room temperature.
3. Results and Discussion
In order to clarify the connection of the structural phase transition in SrFe2As2 with
magnetic ordering, we first present 57Fe Mo¨ssbauer spectra of SrFe2As2 measured at 298,
77 and 4.2 K in Figure 2 together with transmission integral fits. In agreement with the
ThCr2Si2-type crystal structure we observe a single absorption line for SrFe2As2. At 77

Phase transitions in SrFe2As2 and EuFe2As2 4
K, well below the structural transition temperature, we detect full magnetic hyperfine
splitting of the signal. Excellent fits of the data are obtained with the parameters listed
in Table 1. The isomer shifts are similar to those found in BaFe2As2 (δ= 0.31 - 0.44
mm/s). Due to different ionic radii, we observed a smaller c/a ratio of 3.15 for SrFe2As2
in comparison to c/a = 3.29 for BaFe2As2 [18]. The stronger compression of the FeAs4/4
tetrahedra in the strontium compound is also reflected by the larger quadrupole splitting
parameter. Good agreement is observed with the recently published 57Fe data for
LaFePO [19] and LaFeAsO [20, 21], which contain electronically very similar tetrahedral
FeP4/4 and FeAs4/4 layers. The hyperfine field detected at the iron nuclei in SrFe2As2
(Bhf = 8.91(1) T) at 4.2 K is considerably higher than in BaFe2As2 (5.47 T) [7]. The
magnetic behavior of the iron arsenide layers strongly depends on the occupation of the
Fe 3dx2−y2 orbitals, and the latter depends on the position of the arsenic atoms [22].
Thus, with smaller strontium and europium atoms, a stronger magnetic character of the
iron arsenide layers and consequently a higher ordering temperature can be observed,
i.e. 140 K in BaFe2As2, 205 K in SrFe2As2 [22], and 200 K in EuFe2As2 [23]. The
hyperfine fields show the same trend: 5.47 T in BaFe2As2 [7], 8.91 T K in SrFe2As2,
and 8.5 T in EuFe2As2 [23].
Table 1. Fitting parameters of 57Fe Mo¨ssbauer spectroscopy measurements with
SrFe2As2. δ: isomer shift; ∆EQ: quadrupole splitting parameter Γ: experimental line
width; Bhf : magnetic hyperfine field.
T (K) δ (mm·s−1) Γ (mm·s−1) ∆EQ (mm·s−1) Bhf (T)
298 0.31(1) 0.28(1) −0.13(1) –
77 0.44(1) 0.31(1) −0.09(1) 8.70(1)
4.2 0.47(1) 0.37(6) −0.09(1) 8.91(1)
According to recently published single crystal data [14], SrFe2As2 exhibits a
structural transition from the tetragonal space group I4/mmm to the orthorhombic
subgroup Fmmm as first described for BaFe2As2 [7]. Anomalies of the physical
properties have been reported for polycrystalline SrFe2As2 at 205 K [22] and at 198
K for single crystals [14]. We confirm a structural transition at 203 K by x-ray powder
patterns recorded at low temperatures. Figure 3 shows the experimental and fitted
powder pattern of SrFe2As2 with a clear splitting of the (2 1 3)-reflection depicted in the
insert. We have refined the structures above and well below the transition temperature
and obtained the crystallographic data summarized in Tables 2 and 3.

Phase transitions in SrFe2As2 and EuFe2As2 5
-4 -2 0 2 4
velocity (mm s
-1
)
1
%
0
.
5
%
0
.
2
%
r
e
l
a
t
i
v
e
t
r
a
n
s
m
i
s
s
i
o
n
(
%
)
77 K
298 K
4.2 K
Figure 2. (Color online) 57Fe Mo¨ssbauer spectra of SrFe2As2 with transmission
integral fits.
An intensively discussed question is whether the transition is of first or second
order. For second order transitions, the space groups of the distorted and undistorted
structures have to comply with a group-subgroup relationship according to Hermann’s
theorem [24, 25]. The space group Fmmm is a translation equivalent subgroup of
I4/mmm of index 2. Thus, from a group theoretical standpoint one could expect a
second order transition with continuous variation of the order parameter.
Figure 4 shows the unit cell parameters of SrFe2As2 determined by Rietveld
refinements. The a lattice parameter in the tetragonal structure has been multiplied by√
2 for comparison. We have found a rather abrupt splitting of the lattice parameters on
cooling below 203 K. The tetragonal axis at = 555.11 pm splits by +2.0 pm (+0.365%)
and−1.9 pm (−0.343%) within 1 K, leading to ao = 557.13(3) pm and bo = 553.20(3) pm
at 202 K, respectively. Below this temperature, we observe a continuous increase of ao
and decrease of bo. The ao parameter saturates already at 165 K towards a total change
of +0.67%, whereas (bo) decreases further down to a total change of−0.94% at 90 K. The
different behavior of the orthorhombic axis is most likely a consequence of the magnetic
ordering involved in the structural transition. We can understand this anisotropy, if we