Z. Kristallogr. 226 (2011) 435–446 / DOI 10.1524/zkri.2011.1363 435 # by Oldenbourg Wissenschaftsverlag, Mu¨nchen

Structural chemistry of superconducting pnictides and pnictide oxides with layered structures

Dirk JohrendtI, Hideo HosonoII, Rolf-Dieter HoffmannIII and Rainer Po¨ttgen*, III

I Department Chemie und Biochemie, Ludwig-Maximilians-Universita¨t Mu¨nchen, Butenandtstraße 5–13 (Haus D), 81377 Mu¨nchen, Germany II Frontier Research Center, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan III Institut fu¨r Anorganische und Analytische Chemie, Universita¨t Mu¨nster, Corrensstraße 30, 48149 Mu¨nster, Germany

Received October 29, 2010; accepted February 6, 2011

Pnictide / Pnictide oxide / Superconductivity / for hydride formation of CeRuSi ! CeRuSiH [6] and Intermetallics / Group-subgroup relation CeRuGe ! CeRuGeH [7]. The crystal chemical data of the huge number of ZrCuSiAs materials have recently Abstract. The basic structural chemistry of supercon- been reviewed [8]. ducting pnictides and pnictide oxides is reviewed. Crystal Although the basic crystallographic data of the many chemical details of selected compounds and group sub- ThCr2Si2 and ZrCuSiAs type compounds are known for group schemes are discussed with respect to phase transi- several years, especially for the ZrCuSiAs family, systema- tions upon charge-density formation, the ordering of va- tic property studies have been performed only recently. cancies, or the ordered displacements of oxygen atoms. These investigations mainly focused on p-type transparent Furthermore, the influences of doping and solid solutions semiconductors like LaCuSO (for a review see [9]) or the on the valence electron concentration are discussed in or- colored phosphide and arsenide oxides REZnPO [10] and der to highlight the structural and electronic flexibility of REZnAsO [11]. these materials. 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 Introduction comparatively low transition temperatures of 3.2 and 4.3 K. Shortly later, a much higher transition temperature Among the huge number of ternary AxTyPnz, AExTyPnz, of 26 K was observed for LaFeAsO1xFx [17], and even and RExTyPnz pnictides (A ¼ alkali metal, AE ¼ alkaline 55 K was determined for SmFeAsO1xFx [18]. Fluorine earth metal, RE ¼ rare earth metal, T ¼ late transition me- doping on the oxygen sites suppresses the spin-density- tal, Pn ¼ P, As, Sb, Bi) those with the compositions wave formation, favoring superconductivity. 1 : 1 : 1 and 1 : 2 : 2 have most intensively been studied in In the ThCr2Si2 family only few superconducting com- the past 30 years. Especially the rare earth containing pounds, i.e. LaIr2Ge2, LaRu2P2, YIr2xSi2þx, and BaNi2P2 compounds exhibit interesting magnetic and electrical with low transition temperatures have been reported [19– properties. These classes of compounds have repeatedly 22]. Also this field grew rapidly, when superconductivity been reviewed in book chapters of the Handbook on the with a maximum transition temperature of 38 K has been Physics and Chemistry of Rare Earths. observed for the solid solution Ba1xKxFe2As2 [23, 24]. Most of the 1 : 2 : 2 compounds crystallize with Again, potassium doping destroys the antiferromagnetic the tetragonal ThCr2Si2 type structure [1], space ordering of the iron substructure of BaFe2As2 [25] and group I4/mmm, a ternary ordered version of the BaAl4 leads to superconductivity. type [2]. A broader diversity of structure types is ob- These two discoveries led to a tremendous output since served for the 1 : 1 : 1 compounds. Only few of them 2008 in various fields: (i) searching for new compounds, adopt the tetragonal PbFCl type [3]. The basic crystallo- (ii) systematic doping experiments for property variations, graphic data of all these intermetallics are summarized in (iii) systematic physical property studies, and (iv) theoreti- the Pearson Handbook [4]. cal investigations for a deeper understanding of the struc- The PbFCl structure type leaves some empty tetrahe- ture-property relationships and the mechanisms of super- dral voids that might be filled, leading either to a ternary conductivity. compound of composition 1 : 1 : 2 (HfCuSi2) or to a qua- Although this period is still quite short, already diverse ternary one of composition 1 : 1 : 1 : 1, first determined for reviews on this highly exciting field have been published ZrCuSiAs [5]. So far, there are only few examples, where [26–34] and special issues in New Journal of Physics [35] a reversible filling of such voids occurs. This is the case and Physica C: Superconductivity [36] have been edited. The vast numbers of physical property measurements have * Correspondence author (e-mail: [email protected]) competently been reviewed by Johnston [37]. 436 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 tet- rahedral layer. As compared to the many tetragonal Fig. 1. The basic tetrahedral building units in the ZrCuSiAs and ZrCuSiAs type phases, the ZnP4/4 tetrahedra in rhombohe- NdZnPO structure types. For each structure one layer of edge-sharing dral NdZnPO only share three common edges (Fig. 1) and tetrahedra is emphasized. For details see text. they exhibit site symmetry 3m. These tetrahedral double layers are then alternately stacked with ONd4/4 tetrahedra Herein we focus on diverse structure chemical aspects in a rhombohedral fashion (Fig. 3). So far, this peculiar of such superconducting pnictides and pnictide oxides structure type has only been observed in quaternary zinc with a stronger emphasis on stacking variants (intergrowth phosphide oxides. CeZnPO and PrZnPO are dimorphic structures) and group-subgroup relations. [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. Basic crystal structures The four structure types discussed above have many representatives. The basic crystallographic data are sum- The basic building unit of the various ternary pnictide and marized in Refs. [8] and [37]. Besides these more or less d pnictide oxide crystal structures are layers of TPn4/4 tetra- simple structures, where the [FeAs] layers are separated dþ hedra. As emphasized in Fig. 1, these TPn4/4 tetrahedra just by the cations or the [REO] layers, a variety of share four common edges and the tetrahedra keep 44m2 stacking variants have recently been reported, where the site symmetry. These tetrahedral layers are separated and [FeAs]d layers show larger separation through insulating charge balanced either by Aþ, AE2þ,orRE3þ cations, by oxide slabs. The crystal chemical features of these com- [REO]þ layers, or by perovskite-related oxydic slabs. pounds are discussed in the following paragraph. In Fig. 2 we present the structures of LiFeAs [38, 39], The first reported compound with Fe2As2 layers sepa- LaFeAsO [40, 41], and BaFe2As2 [25, 42]. The basic crys- rated by thick perovskite-like oxide blocks was tallographic data of these arsenides and related pnictides Sr3Sc2O5Fe2As2 [47]. This was derived from the known are listed in Table 1. The three tetragonal structures have sulfide-oxide Sr3Sc2O5Cu2S2 [48], which crystallizes with similar negatively charged layers of FeAs4/4 tetrahedra. In the Sr3Fe2O5Cu2S2-type structure in the space group I4/mmm LiFeAs and LaFeAsO these layers are well separated by [49]. At that time, the latter compounds were considered double layers of lithium atoms, respectively the positively as candidates for new oxide superconductors. Figure 4a þ charged [LaO] layers. This is different in BaFe2As2. Here shows the crystal structure, where Fe2As2 layers and per- we observe large cages of coordination number 16 which ovskite-like Sr3Sc2O5 (SrjScO2jSrOjScO2jSr) layers are are filled by the barium atoms. The double layers of Liþ stacked along the c axis. Scandium has five oxygen neigh- also lead to a different packing of the layers as compared bors forming square ScO4/2O1/2 pyramids, which share the to BaFe2As2. apical oxygen atom. The distance to the arsenic atom is The Fe––As distances in the three structures vary be- 339 pm. This is not a significant bond, since typical Sc- tween 240 and 241 pm, close to the sum of the covalent As distances are around 270 pm and the sum of the van-

Fig. 2. The crystal structures of LiFeAs, LaFeAsO, and BaFe2As2. Lithium (lantha- num, 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. Superconducting pnictides and pnictide oxides with layered structures 437

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 I4/mmm 396.25(1) 1301.68(3) 0.2044 [25]

Sr3Sc2O5Fe2As2 I4/mmm 406.9 2687.6 0.4450 [47]

Sr2ScO3FeP P4/nmm 401.6 1554.3 0.2507 [58]

Sr2VO3FeAs P4/nmm 392.96 1567.32 0.2420 [61]

Sr2CrO3FeAs P4/nmm 391.12(1) 1579.05(3) 0.2416 [51]

Na2Ti2As2O I4/mmm 407.0(2) 1528.8(4) 0.2532 [70]

BaTi2As2O P4/nmm 404.7(3) 727.5(4) 0.1192 [72]

(SrF)2Ti2As2O I4/mmm 404.865(5) 1942.04(2) 0.3183 [73]

Sr2Mn2CuAs2O2 I4/mmm 407.913(6) 1858.26(3) 0.3092 [76]

Nd2Fe2Se2O3 I4/mmm 402.63(1) 1843.06(2) 0.2988 [79]

La5Cu4As4O4Cl2 I4/mmm 413.46(7) 4144(1) 0.7084 [80]

der-Waals radii is just 340 pm. One strontium atom has Insertion of further AMO3 perovskite layers lead to the the 12-fold oxygen coordination as in perovskites. The homologous series Can+1(Mg,Ti)nOyFe2As2 (n ¼ 3, 4, 5) second strontium atom encloses the Fe2As2 slabs as in the [53–56], which can also be written as A2MO2(AMO3)n ThCr2Si2-type structure, surrounded by four oxygen and (n ¼ 1 4) in order to include the first member four arsenic atoms. The Fe––As bond length is 243.5 pm Sr3Sc2O5Fe2As2 and to emphasize the stacking of the per- and the As––Fe––As angles are 113.3 (2) and 107.6 ovskite blocks. Bulk superconductivity up to 43 K has (4), thus the FeAs4 tetrahedra are slightly compressed been reported in Ca4(Mg,Ti)3O8Fe2As2 [55]. along the c axis. The structural features of the FeAs layers The closely related compounds A2MO3FePn (A ¼ Ca, in Sr3Sc2O5Fe2As2 are very similar to those of the other Sr, Ba; M ¼ Sc, Cr, V; Pn ¼ As, P) [57–59] crystallize parent compounds of iron arsenide superconductors, and with the Sr2GaO3CuS-type structure [60] in the space furthermore the formal charge (1) is identical. Surpris- group P4/nmm, shown in Fig. 4b. The distance between the ingly, this compound shows none of the crucial properties. Fe2Pn2 layers is about 1500 pm separated by perovskite-like Neither magnetic ordering nor any structural anomaly or blocks made of two MO4/2O sheets without connection via superconductivity could be detected in Sr3Sc2O5Fe2As2 the apical oxygen atoms, in contrast to Sr3Sc2O5Fe2As2. and Ba3Sc2O5Fe2As2 [50, 51]. Only traces of superconduc- Sr2ScO3FeP is superconducting below 17 K [58], which is tivity up to 20 K were observed in the Ti-doped samples the highest Tc in iron phosphides so far. Interestingly, Sr3(Sc0.8Ti1.2)O5Fe2As2 and Sr3(Sc0.6Ti1.4)O5Fe2As2 [52]. 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 experi- ments with Sr2CrO3FeAs revealed antiferromagnetic order- ing 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 Cr- doping 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 ef- fect through V3þ/V4þ mixed valence has been suggested from X-ray absorption spectroscopy [65]. Combined X-ray and neutron diffraction experiments verified the ideal stoi- chiometry, but showed also sensitivity to intrinsic V-dop- ing 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 Fig. 3. The rhombohedral crystal structure of NdZnPO. The layers of vanadium in the oxygen environment. The V-3d orbitals are edge-sharing ZnP4/4 and ONd4/4 tetrahedra are emphasized. For de- tails see text. removed from the Fermi level by the magnetic exchange 438 D. Johrendt, H. Hosono, R.-D. Hoffmann et al.

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 super- conductors [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 ex- periments revealed G-type magnetic ordering in the Mn2As2 layer (3.50(4) mB/Mn at 4 K) and non-magnetic b manganese in the MnO2 layer, which changes to an A- type ferrimagnetic structure in Sr2Mn2CuAs2O2. Herein, the Mn atoms in the Mn2As2 layers are ferromagnetically aligned (3.9(1) mB/Mn), but antiferromagnetically between a the layers [76, 77]. The structure of Nd2Fe2O3Se2 is simi- lar [78], build up by Nd2O2 and Fe2OSe2 layers, where iron is six-fold coordinated by two oxygen and four sele- nium 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 al- ternate with LaOCl blocks as shown in Fig. 4d.

Group-subgroup schemes – superstructures

Several of the iron pnictide and related structures (the par- ent structures of the superconducting phases) show small structural distortions due to the occurrence of a spin-den- sity-wave, ordered vacancies, or ordered oxygen displace- ments. Systemization of such superstructures is most effec- tive through group-subgroup schemes. In the present chapter we exemplarily present such schemes in the concise and compact Ba¨rnighausen formalism [81–83]. At room temperature BaFe2As2 [42] adopts the tetrago- c nal ThCr2Si2 type structure, space group I4/mmm. A struc- tural phase transition occurs at 140 K [25]. The BaFe2As2 d structure becomes orthorhombic, space group Fmmm, similar to the b-SrRh2As2-type [84]. This corresponds to a translationengleiche symmetryp reductionffiffiffi of index 2. The tetragonal mesh (we list a 2 for better compa- rison, i.e. F4/mmm) of 560.38 560.38 pm distorts to 561.46 557.42 pm. In parallel, the c lattice parameter Fig. 4. Crystal Structures of (a)SrSc O Fe As ,(b)SrVO FeAs, (c) 3 2 5 2 2 2 3 slightly contracts from the tetragonal (1301.68 pm) to the (SrF)2Ti2As2O and (d)La5Cu4As4O4Cl2. For details see text. orthorhombic (1294.53 pm) phase [25]. The decoupling of the tetragonal a lattice parameter is the only gain of free splitting. However, the progress is somewhat hampered by parameters for this phase transition. As is evident from the still relatively low quality of the Sr2VO3FeAs samples. Fig. 5, we do not gain further free x, y,orz parameters, Further related compounds with stacked layers extend but lower site symmetries for all atoms. Thus, the phase the pnictide oxide chemistry. Na2Ti2(As,Sb)2O [70] crys- transition leads to a slight flattening and distortion of the tallizes with a modified anti-K2NiF4 type structure (I4/ FeAs4/4 tetrahedra (Fig. 6). mmm) and exhibits anomalies in the magnetic susceptibil- A structural distortion at low temperature has also been ity and resistivity [71]. BaTi2As2O contains analogue, but reported for BaNi2As2 [85]. These authors reported the ecliptically stacked Ti2OAs2 layers [72]. A resistance low-temperature structure in the triclinic space group P11. anomaly around 200 K is suppressed by lithium doping in This is astonishing, since LT-BaNi2As2 would have the Ba1xLixTi2As2O, but no superconductivity was found. lowest symmetry of all BaAl4 superstructures; see the Ba¨r- (SrF)2Ti2Pn2O(Pn ¼ As, Sb) [73] is also derived from the nighausen tree in [86]. In most cases, the lowering of the þ Na2Ti2(As)2O-type structure through replacing Na by symmetry proceeds via few, often only one step. Superconducting pnictides and pnictide oxides with layered structures 439

Fig. 8. (color online) Ordering of the corner-sharing CdSe4/2 tetrahe- 1 dra in the layers around z ¼ 0 and z ¼ =2 in the La2CdSe2O2 struc- ture. Cadmium vacancies are marked with asterisks. For details see text.

shown in Fig. 7. Again, the FeAs4/4 tetrahedra are slightly flattened in LT-LaFeAsO with lower site symmetry. Fig. 5. Group-subgroup scheme in the Ba¨rnighausen formalism for The structure of La2CdSe2O2 [87, 88] also derives from the structures of HT-BaFe2As2 and LT-BaFe2As2. The index for the ZrCuSiAs type HT-LaFeAsO, however, with a distinct dif- translationengleiche symmetry reduction (t) and the evolution of the ference in the cadmium and selenium containing tetrahe- atomic parameters is given. dral layer. The formal ionic formula splitting for this com- 2þ 2 pound is [La2O2] [CdSe2] . In order to charge-balance 2þ the insulating [La2O2] layers, only half of the transition metal positions need to be filled with divalent cadmium. 2 The ordering pattern for the tetrahedral [CdSe2] 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

Fig. 6. The FeAs4/4 tetrahedra in LT- and HT-BaFe2As2. Relevant in- Ba2ZnO2Ag2Se2 [89], which crystallizes with a distortion teratomic distances, angles, and the site symmetries are given. variant of the Sr2Cu2CoO2S2 type structure [90–92]. This type is an intergrowth variant of [Ag2Se2] tetrahedral The nature of the BaFe2As2 structural phase transition is layers (similar to LaFeAsO and BaFe2As2) and perovskite- completely analogous to that in LaFeAsO [41] which con- related [Ba2ZnO2] slabs. In the Sr2Cu2CoO2S2 type subcell tains similar layers of condensed FeAs4/4 tetrahedra (vide ul- structure, space group I4/mmm, the zinc atoms show an tra). Staring from space group P4/nmm we observe a transla- unusual ZnO2 square plane (Fig. 10). However, in contrast tionengleiche symmetry reduction of index 2 (t2) to space to the Sr2Cu2CoO2S2 type compounds, if one places the group Cmme. The corresponding group-subgroup scheme is ZnO2 slab under tension (this is actually the case for the

Fig. 7. Group-subgroup scheme in the Ba¨rnighausen formalism for Fig. 9. Group-subgroup scheme in the Ba¨rnighausen formalism for the structures of HT-LaFeAsO and LT-LaFeAsO. The index for the the structures of HT-LaFeAsO and La2CdSe2O2. The index for the translationengleiche symmetry reduction (t) and the evolution of the klassengleiche symmetry reduction (k2) and the evolution of the atomic parameters is given. atomic parameters is given. 440 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 ar- rangement distorts and the structure forms discrete, linear 2 [ZnO2] units. The average structure in space group I4/mmm and the well ordered superstructure in space group Cmce are re- lated by a group-subgroup scheme (Fig. 11). The corre- sponding 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 stand- ardized 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 Fig. 11. Group-subgroup scheme in the Ba¨rnighausen formalism for Zn––O distances. In the subcell the average Zn––O dis- the structures of Sr2Cu2CoO2S2 and Ba2ZnO2Ag2Se2. The indices for tances within the ZnO2 squares are 214.2 pm. In the or- the translationengleiche and klassengleiche symmetry reductions and dered superstructure always two oxygen atoms move to- the evolution of the atomic parameters are given. In the last two col- umns the standardized and refined data are listed for better compari- wards the zinc atoms while the other two move away, son with the published data. For details see text. leading to shorter (2 189.6 pm) and longer (2 238.9 pm) Zn––O distances. Thus, Ba2ZnO2Ag2Se2 is a rare example A last example concerns the structure of 2þ 10 for the Zn d ion in linear coordination. As is evident Sr2MnO2Cu1.5S2 [32, 93]. At room temperature this sul- from the refined positional parameters, the maximum fide oxide is tetragonal, space group I4/mmm and shows a shifts from the subcell to the superstructure occur for the 75% random copper occupancy within the tetrahedral oxygen atoms, while the metal atoms almost remain at layer, similar to several CeCu1xSO samples [94, 95]. their original positions. The structural distortion is nicely Upon cooling the sample below 240 K, long-range order- underlined by bond valence sum (BVS) calculations, ing of filled (75%) and vacant (25%) copper sites occurs which show BVS ¼ 1.35 for the subcell and 1.63 for the (Fig. 12). The symmetry reduction proceeds via three superstructure [89]. steps (Fig. 13). The first two reductions, t2toFmmm 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. Superconducting pnictides and pnictide oxides with layered structures 441

Fig. 13. Group-subgroup scheme in the Ba¨r- nighausen formalism for the subcell (75% random copper occupation) and the super- structure 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 pub- lished data. For details see text. k2toBmem, lead to an intermediate model, where still all tron doping”, which suppresses the phase transition and (or half) of the copper sites would be filled. The third step superconductivity emerges at 26 K. Replacing lanthanum (k2) from Bmem to Ibam allows for a splitting of the 4a by smaller rare earth ions increases the Tc up to 55 K in Cu site into two fourfold Cu sites 4a and 4b, of which the Sm(O0.85F0.15)FeAs [18], which is the highest in iron latter remains unoccupied in the ordered state. The va- based superconductors so far. Subsequently, the iron ar- cancy formation has only tiny influence on the filled te- senides with ZrCuSiAs- and ThCr2Si2-type structures trahedral sites. One observes almost regular CuS4 tetra- turned out to allow substitutions of all atom sites. Substi- hedra in the ordered low-temperature structure. The copper tution of iron by small amounts of cobalt, e.g. in deficiency is driven by the manganese valence and an Ba(Fe1xCox)2As2 [97] or LaO(Fe1xCox)As [98] induced electron precise formulation (2Sr2þ)4þMn2.5þ(2O2)4– superconductivity at 22 and 14 K, respectively. This kind (1.5Cuþ)1.5þ(2S2)4– is adequate. The manganese valence of doping represents a remarkable difference to the cup- has been determined on the basis of XANES measure- rate compounds, where any, even small doping of the cop- ments [93]. per site is detrimental to superconductivity. Also reducing the negative charge of the (FeAs)d layer induces superconductivity in iron arsenides. Exam- Structural flexibility ples of such “hole doping” are the replacement of La3þ by 2þ Sr in (La1xSrx)OFeAs (Tc up to 25 K) [99] and substi- 2þ þ Superconductivity in iron based pnictides was first dis- tuting Ba by K in Ba1xKxFe2As2 (Tc up to 38 K) [23] þ covered in the stoichiometric phosphide-oxide LaOFeP or by Na in Ba1xNaxFe2As2 (Tc up to 34 K) [103]. Ana- [12], but the low Tc of 4 K caused not much attention at logous substitutions of the alkaline earth by alkali ions in that time. The breakthrough came in 2008 with the isoty- SrFe2As2 [101] and CaFe2As2 [102] have also been re- pic arsenide La(O1xFx)FeAs, where up to 20% of the ported. Finally, it was found that even substitutions with- oxide ions are substituted by fluoride ions [17]. Stoichio- out changing the total electron count can induce supercon- metric LaOFeAs is non-superconducting, but a poor me- ductivity. This “isovalent doping” is possible by replacing tal which exhibits a magneto-structural phase transition at iron with ruthenium in Ba(Fe1xRux)2As2 [103] or arsenic 150 K [96]. Fluorine substitution increases the negative with phosphorus in BaFe2(As1xPx)2 [104] and charge in the FeAs layers and is therefore called “elec- LaOFe(As1xPx) [105]. Typically, much higher degrees of 442 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 Ba1xKxFe2As2, but decreases significantly by substitution þ with smaller Na ions in Ba1xNaxFe2As2 [100].Itis quite remarkable that the critical temperature is not signifi- cantly changed by this rather large volume effect. The lat- ter finding also challenges the report about the coinci- dence of the structural changes in Ba1xKxFe2As2 with those in undoped BaFe2As2 under pressure, where super- conductivity emerges between 2.5 and 6 GPa [109].A special role of the As–Fe–As angle has been presumed Fig. 14. Phase diagram of Ba K Fe As 24 . 1x x 2 2 [ ] after Lee et al. [110] collected structural data of many pnictide superconductors and showed that the highest T ’s substitution (30–40%) are required at isovalent substitu- c occur if the tetrahedral angle is close to the ideal one. tion in comparison with electron- or hole-doping, where However, the true connection between the bond angle and 5–20% are sufficient in most cases. Despite these diversi- superconducting pairing mechanism is not yet clear. fied options of inducing superconductivity in parent com- A notable argument for competing structural and pounds with FeAs layers, the resulting phase diagrams are superconducting order parameters in underdoped remarkably similar and reminiscent to the cuprates. Super- Ba(Fe Co ) As arose from a remarkable accurate struc- conductivity emerges as the magneto-structural phase tran- 1x x 2 2 tural study by Nandi et al. [108]. According to this, the sition gets suppressed, and the superconducting domes orthorhombic distortion of the antiferromagnetic phase de- generally involve a large composition range as shown for creases below the onset temperature of superconductivity Ba K Fe As in Fig. 14 [24]. To what extend supercon- 1x x 2 2 as shown in Fig. 16. However, the detailed crystal struc- ductivity and magnetic ordering co-exist in the underdoped ture of the low-temperature phase is still unknown. Also region (x 0.15–0.25 in Fig. 14) is still controversial. the origin of the phase transition is not yet completely Findings from 57Fe-Mo¨ssbauer spectroscopy gave evidence clear, in particular because the structural distortion occurs of homogeneous co-existence, while 75As-NMR [106] and at temperatures well above the magnetic ordering in the mSR [107] data indicated mesoscopic phase separation in 1111-compounds, while both appear simultaneously in the magnetic and superconducting fractions. On the other undoped 122-materials. Lv et al. suggested ordering of the hand, homogeneous co-existence is generally accepted in iron d ,d orbitals [111], while nematic order of the mag- the Co-doped system Ba(Fe Co ) As [108] (see below). xz yz 1x x 2 2 netic moments was proposed by other authors [112]. While the effect of substitution on the electronic and Detailed studies of the effects of isovalent substitution magnetic properties has been intensively investigated with on the structures of CeOFe(As P ), Ba Sr Fe As and respect to the mechanism of superconductivity, not so 1x x 1x x 2 2 BaFe (As P ) have been reported [113, 114]. In the lat- much studies were published about the effects on the crys- 2 1x x 2 ter case, it has been shown that arsenic and phosphorus tal structure. Potassium doping in Ba K Fe As leads to 1x x 2 2 are not exactly at the same coordinates in the unit cell, a significant linear elongation of the c-axis. The Fe–As and that phosphorus doping is not a simple chemical pres-

Fig. 16. (color online) Orthorhombic distortion in underdoped Fig. 15. Structure parameters of Ba1xKxFe2As2 [24]. Ba(Fe1xCox)2As2 [111]. Superconducting pnictides and pnictide oxides with layered structures 443

Fig. 17. (color online) Correlation between TC and structural parameters in iron pni- cides(chalcogenides) superconductors. (a) TC vs. tetrahedral angle around Fe, (b) TC vs. pnictgen(chalcogen) height from the iron plane.

a b sure effect, but a subtle reorganization of the crystal struc- ture around the iron. Ogino et al. [56] examined the effect ture. of the thickness of the blocking layers between FeAs(P) layers on TC in iron pnictides with perovskite-like layers, finding that the TC is increased slightly but saturates Outlook around a thickness of 1.5 nm. The tetrahedral angle around iron or the pnictogen height from the iron plane is Approximately 30 months have passed since the first pa- only one structural parameter to predict the TC, although per of a La 1111 superconductor with TC ¼ 26 K was re- some exceptions are seen, at the present stage. The re- ported. More than 2000 papers have been already pub- searchers are concentrating on finding materials with high- lished to date. Although many iron-based superconductors er TC following this tentative guide, expecting totally new have been found, each of them contains a square net of high TC materials which are far from this rule. Strike Fe2þ ions in tetrahedral coordination with pnictogen ions while the iron is hot. The authors believe this old saying or chalcogenide ions. The electronic state near the Fermi is also true in the present case. level is almost governed by five Fe 3d orbitals and the contribution of the anion orbitals remains 10%. We may Acknowledgments. This work was supported by the Deutsche For- thus regard these materials as iron-based superconductors. schungsgemeinschaft through SPP 1458 Hochtemperatursupraleitung It has revealed that iron-based superconductors have sev- in Eisenpnictiden and the JSPS program. eral unique superconducting properties compared with high temperature cuprates and MgB2, i.e., high robustness to magnetic field, insensitiveness to magnetic impurities References and small anisotropy of the physical properties. 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