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Z. Kristallogr. 226 (2011) 435–446 / DOI 10.1524/zkri.2011.1363
# by Oldenbourg Wissenschaftsverlag, München
Structural chemistry of superconducting pnictides
and pnictide oxides with layered structures
Dirk JohrendtI , Hideo HosonoII, Rolf-Dieter HoffmannIII and Rainer Pöttgen*, III
I
II
III
Department Chemie und Biochemie, Ludwig-Maximilians-Universität München, Butenandtstraße 5–13 (Haus D), 81377 München, Germany
Frontier Research Center, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
Institut für Anorganische und Analytische Chemie, Universität Münster, Corrensstraße 30, 48149 Münster, Germany
Received October 29, 2010; accepted February 6, 2011
Pnictide / Pnictide oxide / Superconductivity /
Intermetallics / Group-subgroup relation
Abstract. The basic structural chemistry of superconducting pnictides and pnictide oxides is reviewed. Crystal
chemical details of selected compounds and group subgroup schemes are discussed with respect to phase transitions upon charge-density formation, the ordering of vacancies, or the ordered displacements of oxygen atoms.
Furthermore, the influences of doping and solid solutions
on the valence electron concentration are discussed in order to highlight the structural and electronic flexibility of
these materials.
Introduction
Among the huge number of ternary AxTyPnz, AEx TyPnz,
and REx TyPnz pnictides (A ¼ alkali metal, AE ¼ alkaline
earth metal, RE ¼ rare earth metal, T ¼ late transition metal, Pn ¼ P, As, Sb, Bi) those with the compositions
1 : 1 : 1 and 1 : 2 : 2 have most intensively been studied in
the past 30 years. Especially the rare earth containing
compounds exhibit interesting magnetic and electrical
properties. These classes of compounds have repeatedly
been reviewed in book chapters of the Handbook on the
Physics and Chemistry of Rare Earths.
Most of the 1 : 2 : 2 compounds crystallize with
the tetragonal ThCr2Si2 type structure [1], space
group I4/mmm, a ternary ordered version of the BaAl4
type [2]. A broader diversity of structure types is observed for the 1 : 1 : 1 compounds. Only few of them
adopt the tetragonal PbFCl type [3]. The basic crystallographic data of all these intermetallics are summarized in
the Pearson Handbook [4].
The PbFCl structure type leaves some empty tetrahedral voids that might be filled, leading either to a ternary
compound of composition 1 : 1 : 2 (HfCuSi2) or to a quaternary one of composition 1 : 1 : 1 : 1, first determined for
ZrCuSiAs [5]. So far, there are only few examples, where
a reversible filling of such voids occurs. This is the case
* Correspondence author (e-mail: [email protected])
for hydride formation of CeRuSi ! CeRuSiH [6] and
CeRuGe ! CeRuGeH [7]. The crystal chemical data of
the huge number of ZrCuSiAs materials have recently
been reviewed [8].
Although the basic crystallographic data of the many
ThCr2Si2 and ZrCuSiAs type compounds are known for
several years, especially for the ZrCuSiAs family, systematic property studies have been performed only recently.
These investigations mainly focused on p-type transparent
semiconductors like LaCuSO (for a review see [9]) or the
colored phosphide and arsenide oxides REZnPO [10] and
REZnAsO [11].
The class of ZrCuSiAs type compounds gained a true
renaissance in 2006, when superconductivity was reported
for LaFePO [12–14] and LaNiPO [15, 16], however, at
comparatively low transition temperatures of 3.2 and
4.3 K. Shortly later, a much higher transition temperature
of 26 K was observed for LaFeAsO1x Fx [17], and even
55 K was determined for SmFeAsO1x Fx [18]. Fluorine
doping on the oxygen sites suppresses the spin-densitywave formation, favoring superconductivity.
In the ThCr2Si2 family only few superconducting compounds, i.e. LaIr2Ge2, LaRu2P2, YIr2x Si2þx , and BaNi2P2
with low transition temperatures have been reported [19–
22]. Also this field grew rapidly, when superconductivity
with a maximum transition temperature of 38 K has been
observed for the solid solution Ba1x Kx Fe2As2 [23, 24].
Again, potassium doping destroys the antiferromagnetic
ordering of the iron substructure of BaFe2As2 [25] and
leads to superconductivity.
These two discoveries led to a tremendous output since
2008 in various fields: (i) searching for new compounds,
(ii) systematic doping experiments for property variations,
(iii) systematic physical property studies, and (iv) theoretical investigations for a deeper understanding of the structure-property relationships and the mechanisms of superconductivity.
Although this period is still quite short, already diverse
reviews on this highly exciting field have been published
[26–34] and special issues in New Journal of Physics [35]
and Physica C: Superconductivity [36] have been edited.
The vast numbers of physical property measurements have
competently been reviewed by Johnston [37].
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Fig. 1. The basic tetrahedral building units in the ZrCuSiAs and
NdZnPO structure types. For each structure one layer of edge-sharing
tetrahedra is emphasized. For details see text.
Herein we focus on diverse structure chemical aspects
of such superconducting pnictides and pnictide oxides
with a stronger emphasis on stacking variants (intergrowth
structures) and group-subgroup relations.
Basic crystal structures
The basic building unit of the various ternary pnictide and
pnictide oxide crystal structures are layers of TPn4/4 tetrahedra. As emphasized in Fig. 1, these TPn4/4 tetrahedra
share four common edges and the tetrahedra keep 4m2
site symmetry. These tetrahedral layers are separated and
charge balanced either by Aþ, AE2þ, or RE3þ cations, by
[REO]þ layers, or by perovskite-related oxydic slabs.
In Fig. 2 we present the structures of LiFeAs [38, 39],
LaFeAsO [40, 41], and BaFe2As2 [25, 42]. The basic crystallographic data of these arsenides and related pnictides
are listed in Table 1. The three tetragonal structures have
similar negatively charged layers of FeAs4/4 tetrahedra. In
LiFeAs and LaFeAsO these layers are well separated by
double layers of lithium atoms, respectively the positively
charged [LaO]þ layers. This is different in BaFe2As2. Here
we observe large cages of coordination number 16 which
are filled by the barium atoms. The double layers of Liþ
also lead to a different packing of the layers as compared
to BaFe2As2.
The Fe––As distances in the three structures vary between 240 and 241 pm, close to the sum of the covalent
D. Johrendt, H. Hosono, R.-D. Hoffmann et al.
radii of 237 pm [43], indicating substantial covalent
Fe––As bonding, in agreement with diverse electronic
structure calculations carried out on these materials.
Furthermore one observes weaker Fe––Fe bonding within
the tetrahedral layers. The Fe––Fe distances of 267
(LiFeAs), 280 (BaFe2As2), and 285 (LaFeAsO) pm are
longer than those of 248 pm in a-Fe [44]. The weak
Fe––Fe bonding was substantiated by electronic structure
calculations.
The structure of NdZnPO [45] shows a different tetrahedral layer. As compared to the many tetragonal
ZrCuSiAs type phases, the ZnP4/4 tetrahedra in rhombohedral NdZnPO only share three common edges (Fig. 1) and
they exhibit site symmetry 3m. These tetrahedral double
layers are then alternately stacked with ONd4/4 tetrahedra
in a rhombohedral fashion (Fig. 3). So far, this peculiar
structure type has only been observed in quaternary zinc
phosphide oxides. CeZnPO and PrZnPO are dimorphic
[46]. The high-temperature (b) NdZnPO type modifications
were obtained at 1170 K while the low-temperature (a)
ZrCuSiAs type modifications crystallizes at 970 K.
The four structure types discussed above have many
representatives. The basic crystallographic data are summarized in Refs. [8] and [37]. Besides these more or less
simple structures, where the [FeAs]d layers are separated
just by the cations or the [REO]dþ layers, a variety of
stacking variants have recently been reported, where the
[FeAs]d layers show larger separation through insulating
oxide slabs. The crystal chemical features of these compounds are discussed in the following paragraph.
The first reported compound with Fe2As2 layers separated by thick perovskite-like oxide blocks was
Sr3Sc2O5Fe2As2 [47]. This was derived from the known
sulfide-oxide Sr3Sc2O5Cu2S2 [48], which crystallizes with
the Sr3Fe2O5Cu2S2-type structure in the space group I4/mmm
[49]. At that time, the latter compounds were considered
as candidates for new oxide superconductors. Figure 4a
shows the crystal structure, where Fe2As2 layers and perovskite-like Sr3Sc2O5 (SrjScO2jSrOjScO2jSr) layers are
stacked along the c axis. Scandium has five oxygen neighbors forming square ScO4/2O1/2 pyramids, which share the
apical oxygen atom. The distance to the arsenic atom is
339 pm. This is not a significant bond, since typical ScAs distances are around 270 pm and the sum of the van-
Fig. 2. The crystal structures of LiFeAs,
LaFeAsO, and BaFe2As2. Lithium (lanthanum, barium), iron, and arsenic atoms are
drawn as medium grey, black filled, and open
circles, respectively. The layers of edge-sharing
FeAs4/4 tetrahedra and the OLa4/4 tetrahedra
in LaFeAsO are emphasized.
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Superconducting pnictides and pnictide oxides with layered structures
Table 1. Basic crystallographic data of selected ternary pnictides and pnictide oxides with layered structures.
Compound
space group
a (pm)
c (pm)
V (nm3)
Ref.
LiFeAs
P4/nmm
377.360(4)
635.679(1)
0.0907
[39]
LaFeAsO
P4/nmm
432.268(1)
874.111(4)
0.1422
[41]
BaFe2As2
Sr3Sc2O5Fe2As2
I4/mmm
I4/mmm
396.25(1)
406.9
1301.68(3)
2687.6
0.2044
0.4450
[25]
[47]
Sr2ScO3FeP
P4/nmm
401.6
1554.3
0.2507
[58]
Sr2VO3FeAs
Sr2CrO3FeAs
P4/nmm
P4/nmm
392.96
391.12(1)
1567.32
1579.05(3)
0.2420
0.2416
[61]
[51]
Na2Ti2As2O
I4/mmm
407.0(2)
1528.8(4)
0.2532
[70]
BaTi2As2O
(SrF)2Ti2As2O
P4/nmm
I4/mmm
404.7(3)
404.865(5)
727.5(4)
1942.04(2)
0.1192
0.3183
[72]
[73]
Sr2Mn2CuAs2O2
I4/mmm
407.913(6)
1858.26(3)
0.3092
[76]
Nd2Fe2Se2O3
La5Cu4As4O4Cl2
I4/mmm
I4/mmm
402.63(1)
413.46(7)
1843.06(2)
4144(1)
0.2988
0.7084
[79]
[80]
der-Waals radii is just 340 pm. One strontium atom has
the 12-fold oxygen coordination as in perovskites. The
second strontium atom encloses the Fe2As2 slabs as in the
ThCr2Si2-type structure, surrounded by four oxygen and
four arsenic atoms. The Fe––As bond length is 243.5 pm
and the As––Fe––As angles are 113.3 (2) and 107.6
(4), thus the FeAs4 tetrahedra are slightly compressed
along the c axis. The structural features of the FeAs layers
in Sr3Sc2O5Fe2As2 are very similar to those of the other
parent compounds of iron arsenide superconductors, and
furthermore the formal charge (1) is identical. Surprisingly, this compound shows none of the crucial properties.
Neither magnetic ordering nor any structural anomaly or
superconductivity could be detected in Sr3Sc2O5Fe2As2
and Ba3Sc2O5Fe2As2 [50, 51]. Only traces of superconductivity up to 20 K were observed in the Ti-doped samples
Sr3(Sc0.8Ti1.2)O5Fe2As2 and Sr3(Sc0.6Ti1.4)O5Fe2As2 [52].
Fig. 3. The rhombohedral crystal structure of NdZnPO. The layers of
edge-sharing ZnP4/4 and ONd4/4 tetrahedra are emphasized. For details see text.
Insertion of further AMO3 perovskite layers lead to the
homologous series Can+1(Mg,Ti)nOyFe2As2 (n ¼ 3, 4, 5)
[53–56], which can also be written as A2MO2(AMO3)n
(n ¼ 1 4) in order to include the first member
Sr3Sc2O5Fe2As2 and to emphasize the stacking of the perovskite blocks. Bulk superconductivity up to 43 K has
been reported in Ca4(Mg,Ti)3O8Fe2As2 [55].
The closely related compounds A2MO3FePn (A ¼ Ca,
Sr, Ba; M ¼ Sc, Cr, V; Pn ¼ As, P) [57–59] crystallize
with the Sr2GaO3CuS-type structure [60] in the space
group P4/nmm, shown in Fig. 4b. The distance between the
Fe2Pn2 layers is about 1500 pm separated by perovskite-like
blocks made of two MO4/2O sheets without connection via
the apical oxygen atoms, in contrast to Sr3Sc2O5Fe2As2.
Sr2ScO3FeP is superconducting below 17 K [58], which is
the highest Tc in iron phosphides so far. Interestingly,
among the series Sr2MO3FeAs (M ¼ V, Sc, Cr), only the
undoped vanadium compound Sr2VO3FeAs exhibits bulk
superconductivity at Tc ¼ 37 K [61], which increases to
45 K under pressure [62]. None of them shows magnetic
ordering of the iron substructure or any kind of structural
distortion at low temperatures. Neutron diffraction experiments with Sr2CrO3FeAs revealed antiferromagnetic ordering of the chromium atoms and an intrinsic doping of
about 7% chromium into the iron layer [63]. This may be
a reason for the absence of superconductivity, because Crdoping has proven to be poisonous to superconductivity in
122-type iron arsenides [64]. On the other hand, it is still
unclear why superconductivity emerges in stoichiometric
Sr2VO3FeAs, but not in Sr2ScO3FeAs. A self-doping effect through V3þ/V4þ mixed valence has been suggested
from X-ray absorption spectroscopy [65]. Combined X-ray
and neutron diffraction experiments verified the ideal stoichiometry, but showed also sensitivity to intrinsic V-doping of the iron site, which suppresses superconductivity in
Sr2VO3[Fe0.91(1)V0.09(1)]As [66]. The role of the V atoms
with respect to the electronic structure of Sr2VO3FeAs is
still under discussion [67, 68]. Recent neutron [66] and
photoemission [69] experiments point to highly correlated
vanadium in the oxygen environment. The V-3d orbitals are
removed from the Fermi level by the magnetic exchange
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D. Johrendt, H. Hosono, R.-D. Hoffmann et al.
b
a
Sr2þ, and filling the tetrahedral voids with oxygen atoms
as depicted in Fig. 4c. Anomalies appear at 200 K in the
heat capacity and resistivity. A subtle structural transition
was also detected, but without changes in the space group
symmetry. Compounds with the A2M3Pn2O2 structure [74]
have also been suggested as parent compounds for superconductors [75]. Sr2Mn3As2O2 contains alternating
Mn2As2 and SrMnO2 layers, where manganese atoms are
tetrahedrally coordinated by phosphorus as well as in
square planar oxygen coordination. Neutron diffraction experiments revealed G-type magnetic ordering in the
Mn2As2 layer (3.50(4) mB/Mn at 4 K) and non-magnetic
manganese in the MnO2 layer, which changes to an Atype ferrimagnetic structure in Sr2Mn2CuAs2O2. Herein,
the Mn atoms in the Mn2As2 layers are ferromagnetically
aligned (3.9(1) mB/Mn), but antiferromagnetically between
the layers [76, 77]. The structure of Nd2Fe2O3Se2 is similar [78], build up by Nd2O2 and Fe2OSe2 layers, where
iron is six-fold coordinated by two oxygen and four selenium atoms. Antiferromagnetic ordering of the Fe spins
occurs at 88 K with the moments oriented in the Fe––O
bond direction [79]. An interesting large stacking variant
represents La5Cu4As4O4Cl2, which was obtained from a
NaCl salt flux [80]. ThCr2Si2-like La[Cu2As2]2 layers alternate with LaOCl blocks as shown in Fig. 4d.
Group-subgroup schemes – superstructures
c
d
Fig. 4. Crystal Structures of (a) Sr3Sc2O5Fe2As2, (b) Sr2VO3FeAs, (c)
(SrF)2Ti2As2O and (d) La5Cu4As4O4Cl2. For details see text.
splitting. However, the progress is somewhat hampered by
the still relatively low quality of the Sr2VO3FeAs samples.
Further related compounds with stacked layers extend
the pnictide oxide chemistry. Na2Ti2(As,Sb)2O [70] crystallizes with a modified anti-K2NiF4 type structure (I4/
mmm) and exhibits anomalies in the magnetic susceptibility and resistivity [71]. BaTi2As2O contains analogue, but
ecliptically stacked Ti2OAs2 layers [72]. A resistance
anomaly around 200 K is suppressed by lithium doping in
Ba1xLix Ti2As2O, but no superconductivity was found.
(SrF)2Ti2Pn2O (Pn ¼ As, Sb) [73] is also derived from the
Na2Ti2(As)2O-type structure through replacing Naþ by
Several of the iron pnictide and related structures (the parent structures of the superconducting phases) show small
structural distortions due to the occurrence of a spin-density-wave, ordered vacancies, or ordered oxygen displacements. Systemization of such superstructures is most effective through group-subgroup schemes. In the present
chapter we exemplarily present such schemes in the concise
and compact Bärnighausen formalism [81–83].
At room temperature BaFe2As2 [42] adopts the tetragonal ThCr2Si2 type structure, space group I4/mmm. A structural phase transition occurs at 140 K [25]. The BaFe2As2
structure becomes orthorhombic, space group Fmmm,
similar to the b-SrRh2As2-type [84]. This corresponds to
a translationengleiche symmetry p
reduction
of index 2.
ffiffiffi
The tetragonal mesh (we list a 2 for better comparison, i.e. F4/mmm) of 560.38 560.38 pm distorts to
561.46 557.42 pm. In parallel, the c lattice parameter
slightly contracts from the tetragonal (1301.68 pm) to the
orthorhombic (1294.53 pm) phase [25]. The decoupling of
the tetragonal a lattice parameter is the only gain of free
parameters for this phase transition. As is evident from
Fig. 5, we do not gain further free x, y, or z parameters,
but lower site symmetries for all atoms. Thus, the phase
transition leads to a slight flattening and distortion of the
FeAs4/4 tetrahedra (Fig. 6).
A structural distortion at low temperature has also been
reported for BaNi2As2 [85]. These authors reported the
low-temperature structure in the triclinic space group P1.
This is astonishing, since LT-BaNi2As2 would have the
lowest symmetry of all BaAl4 superstructures; see the Bärnighausen tree in [86]. In most cases, the lowering of the
symmetry proceeds via few, often only one step.
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Superconducting pnictides and pnictide oxides with layered structures
Fig. 8. (color online) Ordering of the corner-sharing CdSe4/2 tetrahedra in the layers around z ¼ 0 and z ¼ 1=2 in the La2CdSe2O2 structure. Cadmium vacancies are marked with asterisks. For details see
text.
The nature of the BaFe2As2 structural phase transition is
completely analogous to that in LaFeAsO [41] which contains similar layers of condensed FeAs4/4 tetrahedra (vide ultra). Staring from space group P4/nmm we observe a translationengleiche symmetry reduction of index 2 (t2) to space
group Cmme. The corresponding group-subgroup scheme is
shown in Fig. 7. Again, the FeAs4/4 tetrahedra are slightly
flattened in LT-LaFeAsO with lower site symmetry.
The structure of La2CdSe2O2 [87, 88] also derives from
ZrCuSiAs type HT-LaFeAsO, however, with a distinct difference in the cadmium and selenium containing tetrahedral layer. The formal ionic formula splitting for this compound is [La2O2]2þ[CdSe2]2. In order to charge-balance
the insulating [La2O2]2þ layers, only half of the transition
metal positions need to be filled with divalent cadmium.
The ordering pattern for the tetrahedral [CdSe2]2 layers is
shown in Fig. 8 and the corresponding group-subgroup
scheme in Fig. 9. The symmetry reduction proceeds via a
klassengleiche transition of index 2 from P4/nmm to P42/
nmc, leading to superstructure reflections. Since every
other cadmium atom is removed from the subcell layers in
a checkered motif, we only observe corner-sharing CdSe4/2
tetrahedra in the superstructure.
A further interesting example is the selenide oxide
Ba2ZnO2Ag2Se2 [89], which crystallizes with a distortion
variant of the Sr2Cu2CoO2S2 type structure [90–92]. This
type is an intergrowth variant of [Ag2Se2] tetrahedral
layers (similar to LaFeAsO and BaFe2As2) and perovskiterelated [Ba2ZnO2] slabs. In the Sr2Cu2CoO2S2 type subcell
structure, space group I4/mmm, the zinc atoms show an
unusual ZnO2 square plane (Fig. 10). However, in contrast
to the Sr2Cu2CoO2S2 type compounds, if one places the
ZnO2 slab under tension (this is actually the case for the
Fig. 7. Group-subgroup scheme in the Bärnighausen formalism for
the structures of HT-LaFeAsO and LT-LaFeAsO. The index for the
translationengleiche symmetry reduction (t) and the evolution of the
atomic parameters is given.
Fig. 9. Group-subgroup scheme in the Bärnighausen formalism for
the structures of HT-LaFeAsO and La2CdSe2O2. The index for the
klassengleiche symmetry reduction (k2) and the evolution of the
atomic parameters is given.
Fig. 5. Group-subgroup scheme in the Bärnighausen formalism for
the structures of HT-BaFe2As2 and LT-BaFe2As2. The index for the
translationengleiche symmetry reduction (t) and the evolution of the
atomic parameters is given.
Fig. 6. The FeAs4/4 tetrahedra in LT- and HT-BaFe2As2. Relevant interatomic distances, angles, and the site symmetries are given.
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D. Johrendt, H. Hosono, R.-D. Hoffmann et al.
Fig. 10. The zinc (filled circles)-oxygen (open circles) substructures
in the subcell and the superstructure of Ba2ZnO2Ag2Se2. The unit
cells are marked by light grey shading. Relevant Zn––O distances are
indicated. For details see text.
larger ions Ba2þ, Agþ, and Se2), the square planar arrangement distorts and the structure forms discrete, linear
[ZnO2]2 units.
The average structure in space group I4/mmm and the
well ordered superstructure in space group Cmce are related by a group-subgroup scheme (Fig. 11). The corresponding symmetry reduction proceeds in two steps. First
there is a translationengleiche transition from I4/mmm to
Fmmm, followed by a klassengleiche transition (k2) to
Bbem, a non-standard setting of Cmce. The superstructure
has twice the cell volume of the subcell. Besides the standardized data, we also list the transformation to the refined
data in Fig. 11, in order to facilitate comparison with the
published data.
The symmetry reduction has a drastic effect on the
Zn––O distances. In the subcell the average Zn––O distances within the ZnO2 squares are 214.2 pm. In the ordered superstructure always two oxygen atoms move towards the zinc atoms while the other two move away,
leading to shorter (2 189.6 pm) and longer (2 238.9 pm)
Zn––O distances. Thus, Ba2ZnO2Ag2Se2 is a rare example
for the Zn2þ d10 ion in linear coordination. As is evident
from the refined positional parameters, the maximum
shifts from the subcell to the superstructure occur for the
oxygen atoms, while the metal atoms almost remain at
their original positions. The structural distortion is nicely
underlined by bond valence sum (BVS) calculations,
which show BVS ¼ 1.35 for the subcell and 1.63 for the
superstructure [89].
Fig. 11. Group-subgroup scheme in the Bärnighausen formalism for
the structures of Sr2Cu2CoO2S2 and Ba2ZnO2Ag2Se2. The indices for
the translationengleiche and klassengleiche symmetry reductions and
the evolution of the atomic parameters are given. In the last two columns the standardized and refined data are listed for better comparison with the published data. For details see text.
A last example concerns the structure of
Sr2MnO2Cu1.5S2 [32, 93]. At room temperature this sulfide oxide is tetragonal, space group I4/mmm and shows a
75% random copper occupancy within the tetrahedral
layer, similar to several CeCu1x SO samples [94, 95].
Upon cooling the sample below 240 K, long-range ordering of filled (75%) and vacant (25%) copper sites occurs
(Fig. 12). The symmetry reduction proceeds via three
steps (Fig. 13). The first two reductions, t2 to Fmmm and
Fig. 12. The copper (filled circles)-sulfur
(open circles) substructures in the subcell
(75% occupancy of the copper site) and the
superstructure of Sr2MnO2Cu1.5S2. The unit
cells are marked by light grey shading. For
details see text.
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Superconducting pnictides and pnictide oxides with layered structures
Fig. 13. Group-subgroup scheme in the Bärnighausen formalism for the subcell (75%
random copper occupation) and the superstructure of Sr2MnCu1.5O2S2. The indices for
the translationengleiche and klassengleiche
symmetry reductions and the evolution of the
atomic parameters are given. In the last two
columns the standardized and refined data are
listed for better comparison with the published data. For details see text.
k2 to Bmem, lead to an intermediate model, where still all
(or half) of the copper sites would be filled. The third step
(k2) from Bmem to Ibam allows for a splitting of the 4a
Cu site into two fourfold Cu sites 4a and 4b, of which the
latter remains unoccupied in the ordered state. The vacancy formation has only tiny influence on the filled tetrahedral sites. One observes almost regular CuS4 tetrahedra in the ordered low-temperature structure. The copper
deficiency is driven by the manganese valence and an
electron precise formulation (2Sr2þ)4þMn2.5þ(2O2)4–
(1.5Cuþ)1.5þ(2S2)4– is adequate. The manganese valence
has been determined on the basis of XANES measurements [93].
Structural flexibility
Superconductivity in iron based pnictides was first discovered in the stoichiometric phosphide-oxide LaOFeP
[12], but the low Tc of 4 K caused not much attention at
that time. The breakthrough came in 2008 with the isotypic arsenide La(O1x Fx )FeAs, where up to 20% of the
oxide ions are substituted by fluoride ions [17]. Stoichiometric LaOFeAs is non-superconducting, but a poor metal which exhibits a magneto-structural phase transition at
150 K [96]. Fluorine substitution increases the negative
charge in the FeAs layers and is therefore called “elec-
tron doping”, which suppresses the phase transition and
superconductivity emerges at 26 K. Replacing lanthanum
by smaller rare earth ions increases the Tc up to 55 K in
Sm(O0.85F0.15)FeAs [18], which is the highest in iron
based superconductors so far. Subsequently, the iron arsenides with ZrCuSiAs- and ThCr2Si2-type structures
turned out to allow substitutions of all atom sites. Substitution of iron by small amounts of cobalt, e.g. in
Ba(Fe1x Cox )2As2 [97] or LaO(Fe1x Cox )As [98] induced
superconductivity at 22 and 14 K, respectively. This kind
of doping represents a remarkable difference to the cuprate compounds, where any, even small doping of the copper site is detrimental to superconductivity.
Also reducing the negative charge of the (FeAs)d
layer induces superconductivity in iron arsenides. Examples of such “hole doping” are the replacement of La3þ by
Sr2þ in (La1x Srx )OFeAs (Tc up to 25 K) [99] and substituting Ba2þ by Kþ in Ba1x Kx Fe2As2 (Tc up to 38 K) [23]
or by Naþ in Ba1x Nax Fe2As2 (Tc up to 34 K) [103]. Analogous substitutions of the alkaline earth by alkali ions in
SrFe2As2 [101] and CaFe2As2 [102] have also been reported. Finally, it was found that even substitutions without changing the total electron count can induce superconductivity. This “isovalent doping” is possible by replacing
iron with ruthenium in Ba(Fe1x Rux )2As2 [103] or arsenic
with phosphorus in BaFe2(As1x Px )2 [104] and
LaOFe(As1x Px ) [105]. Typically, much higher degrees of
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442
Fig. 14. Phase diagram of Ba1x Kx Fe2As2 [24].
substitution (30–40%) are required at isovalent substitution in comparison with electron- or hole-doping, where
5–20% are sufficient in most cases. Despite these diversified options of inducing superconductivity in parent compounds with FeAs layers, the resulting phase diagrams are
remarkably similar and reminiscent to the cuprates. Superconductivity emerges as the magneto-structural phase transition gets suppressed, and the superconducting domes
generally involve a large composition range as shown for
Ba1x Kx Fe2As2 in Fig. 14 [24]. To what extend superconductivity and magnetic ordering co-exist in the underdoped
region (x 0.15–0.25 in Fig. 14) is still controversial.
Findings from 57Fe-Mössbauer spectroscopy gave evidence
of homogeneous co-existence, while 75As-NMR [106] and
mSR [107] data indicated mesoscopic phase separation in
magnetic and superconducting fractions. On the other
hand, homogeneous co-existence is generally accepted in
the Co-doped system Ba(Fe1x Cox )2As2 [108] (see below).
While the effect of substitution on the electronic and
magnetic properties has been intensively investigated with
respect to the mechanism of superconductivity, not so
much studies were published about the effects on the crystal structure. Potassium doping in Ba1x Kx Fe2As2 leads to
a significant linear elongation of the c-axis. The Fe–As
Fig. 15. Structure parameters of Ba1x Kx Fe2As2 [24].
D. Johrendt, H. Hosono, R.-D. Hoffmann et al.
bond lengths remain remarkably constant close to 240 pm,
while the As–Fe–As angles become smaller and close to
the ideal tetrahedral angle of 109.47 in Ba0.6K0.4Fe2As2
with the highest Tc [24]. The structural changes are shown
in Fig. 15. Interestingly, sodium substitution has an almost
identical effect on the crystal structure, except for the unit
cell volume, which is almost constant in the case of
Ba1x Kx Fe2As2, but decreases significantly by substitution
with smaller Naþ ions in Ba1x Nax Fe2As2 [100]. It is
quite remarkable that the critical temperature is not significantly changed by this rather large volume effect. The latter finding also challenges the report about the coincidence of the structural changes in Ba1x Kx Fe2As2 with
those in undoped BaFe2As2 under pressure, where superconductivity emerges between 2.5 and 6 GPa [109]. A
special role of the As–Fe–As angle has been presumed
after Lee et al. [110] collected structural data of many
pnictide superconductors and showed that the highest Tc’s
occur if the tetrahedral angle is close to the ideal one.
However, the true connection between the bond angle and
superconducting pairing mechanism is not yet clear.
A notable argument for competing structural and
superconducting order parameters in underdoped
Ba(Fe1x Cox )2As2 arose from a remarkable accurate structural study by Nandi et al. [108]. According to this, the
orthorhombic distortion of the antiferromagnetic phase decreases below the onset temperature of superconductivity
as shown in Fig. 16. However, the detailed crystal structure of the low-temperature phase is still unknown. Also
the origin of the phase transition is not yet completely
clear, in particular because the structural distortion occurs
at temperatures well above the magnetic ordering in the
1111-compounds, while both appear simultaneously in the
undoped 122-materials. Lv et al. suggested ordering of the
iron dxz,dyz orbitals [111], while nematic order of the magnetic moments was proposed by other authors [112].
Detailed studies of the effects of isovalent substitution
on the structures of CeOFe(As1x Px ), Ba1x Srx Fe2As2 and
BaFe2(As1x Px )2 have been reported [113, 114]. In the latter case, it has been shown that arsenic and phosphorus
are not exactly at the same coordinates in the unit cell,
and that phosphorus doping is not a simple chemical pres-
Fig. 16. (color online) Orthorhombic distortion in underdoped
Ba(Fe1x Cox )2As2 [111].
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443
Superconducting pnictides and pnictide oxides with layered structures
Fig. 17. (color online) Correlation between
T C and structural parameters in iron pnicides(chalcogenides) superconductors. (a) T C
vs. tetrahedral angle around Fe, (b) T C vs.
pnictgen(chalcogen) height from the iron plane.
a
sure effect, but a subtle reorganization of the crystal structure.
Outlook
Approximately 30 months have passed since the first paper of a La 1111 superconductor with T C ¼ 26 K was reported. More than 2000 papers have been already published to date. Although many iron-based superconductors
have been found, each of them contains a square net of
Fe2þ ions in tetrahedral coordination with pnictogen ions
or chalcogenide ions. The electronic state near the Fermi
level is almost governed by five Fe 3d orbitals and the
contribution of the anion orbitals remains 10%. We may
thus regard these materials as iron-based superconductors.
It has revealed that iron-based superconductors have several unique superconducting properties compared with
high temperature cuprates and MgB2, i.e., high robustness
to magnetic field, insensitiveness to magnetic impurities
and small anisotropy of the physical properties. These
features are favorable for applications in superconducting
wires.
Perhaps, the most important but difficult subject in this
area is to raise the T C above 77 K. The maximum T C
attained to date is 56 K, which was obtained for the Sm
1111 compound reported in June, 2008. What is the primary factor controlling the T C in these material systems?
The crystal chemistry provided a clue to this issue. Lee
et al. [110] examined a correlation between the T C and the
bond angle of Pn(Ch)-Fe-Pn(Ch), finding that the T C is
raised as the bond angle approaches to that (109.47 ) of
the regular tetrahedron. Kuroki et al. [115] calculated the
T C on the assumption that these materials are spin-fluctuation mediated superconductors and proposed that the T C is
pushed up when the height (hPn) of pnictogen(chalcogen)
from the iron plane is increased. This idea was derived
from a consideration of the topology change of the Fermi
surface by the appearance/disappearance of a pocket reflecting the fact that the Fe d x2–y2 level is sensitive to hPn.
Figures 17a and b summarize the data of T C and the tetrahedral angle or hPn for 5 types of parent compounds.
Although both plots well explain the striking difference in T C
between LaFePO and LaFeAsO or among the REFeAsO
(RE ¼ La, Ce, Nd, and Sm) series, a reverse correlation
was seen for the 122 systems or a part of the 11 systems.
These two plots suggest the importance of the local struc-
b
ture around the iron. Ogino et al. [56] examined the effect
of the thickness of the blocking layers between FeAs(P)
layers on T C in iron pnictides with perovskite-like layers,
finding that the T C is increased slightly but saturates
around a thickness of 1.5 nm. The tetrahedral angle
around iron or the pnictogen height from the iron plane is
only one structural parameter to predict the T C, although
some exceptions are seen, at the present stage. The researchers are concentrating on finding materials with higher T C following this tentative guide, expecting totally new
high T C materials which are far from this rule. Strike
while the iron is hot. The authors believe this old saying
is also true in the present case.
Acknowledgments. This work was supported by the Deutsche Forschungsgemeinschaft through SPP 1458 Hochtemperatursupraleitung
in Eisenpnictiden and the JSPS program.
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