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 LaFeAsO1ÀxFx [17], and even and RExTyPnz pnictides (A ¼ alkali metal, AE ¼ alkaline 55 K was determined for SmFeAsO1ÀxFx [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, YIr2ÀxSi2þx, 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 Ba1ÀxKxFe2As2 [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 dÀ 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]dÀ 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.