SHORT COMMUNICATION DOI: 10.1002/zaac.201100529 On the Rare Earth Metal Bismuthide Oxides RE 2 BiO 2 (RE = Nd, Tb, Dy, Ho) Jürgen Nuss [a] and Martin Jansen* [a] Keywords: Rare earths; Bismuthide oxides; Solid-state reactions; Solid-state structures; X-ray diffraction Abstract. A series of new RE 2 BiO 2 compounds (RE = Nd, Tb, Dy, Ho) has been synthesized from elemental bismuth, REBi, and RE 2 O 3 in all-solid state reactions in sealed tantalum ampoules at 1770 K. The dark grey metallic compounds adopt the anti-ThCr 2 Si 2 type of struc- Introduction Rare earth metal pnictide oxides have attracted interest as potential thermoelectrics, in recent times. [1–3] On one hand, they contain some of the heaviest elements in the periodic table which reduces thermal lattice vibrations, on the other the com- bination with oxide may also minimize thermal conductivity, since the efficiency of phonon propagation is perturbed by structural complexity. [1,4] The rare earth metal pnictide oxides, Ce 2 SbO 2 and Ce 2 BiO 2 , were already reported by Benz in 1971, [5] the charge balance as adjusted by the presence of Ce 3+ and Ce 4+ , together with the formation of Pn 3– (Pn = Sb, Bi), was assumed to stabilize these compounds. Recent studies, including additional praseodymium representatives, have clearly shown that in all of these examples the rare earth metal is in the trivalent state, exclusively. This finding was evidenced by magnetic suscep- tibility measurements. [6] The electron count according to [RE 3+ ] 2 [Pn 2– ][O 2– ] 2 , in analogy to isoelectronic RE 2 TeO 2 , [7] seems to be valid for all of them. While semiconducting be- havior is observed for the antimonides, due to Sb–Sb bond formation, the bismuthides display metallic conductivity. The latter appear to formally contain unusual Bi 2– anions, without any effective chemical bond between them. [2] Consequently, there is no need for the presence of Ce 4+ or Pr 4+ , and it should be possible to extend this class of materials including other rare earth metals. The validity of this assumption was already affirmed by powder diffraction studies on RE 2 BiO 2 representatives with RE = Nd, Sm, Gd, Ho, Er, and Y. [2] However, for understanding the properties of this class of materials, more accurate structure data are desirable. In particular, the localization of the oxygen atoms in such heavy-element structures, using powder meth- ods, can be a rather difficult task. [8] * Prof. Dr. M. Jansen Fax: +49-711-689-1502 E-Mail: M.Jansen@fkf.mpg.de [a] Max-Planck-Institut für Festkörperforschung Heisenbergstr. 1 70569 Stuttgart, Germany Z. Anorg. Allg. Chem. 2012, 638, (3-4), 611–613 © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 611 ture (I4/mmm, tI10), according to X-ray single crystal structure analy- sis. As a main structural feature, there are PbO analogous slabs, [REO] + , and 4 4 nets of Bi 2– anions, alternately stacked along the c- axis. Herein we report on the synthesis and single crystal struc- ture refinements of new RE 2 BiO 2 representatives, with RE rep- resenting the trivalent rare earth metals neodymium, terbium, dysprosium, and holmium. Results and Discussion Starting from elemental bismuth, the bismuthides (REBi) and oxides (RE 2 O 3 ), Nd 2 BiO 2 , Tb 2 BiO 2 , Dy 2 BiO 2 and Ho 2 BiO 2 have been synthesized by all-solid state reactions. When exposed to humid air, the dark grey metallic compounds slowly corrode, and may start smoldering after mechanical treatment. The title compounds crystallize in the tetragonal space group I4/mmm (no. 139) adopting a structure analogous to anti- ThCr 2 Si 2 (Table 1, Table 2). The rare earth metal cations, which are occupying the Si positions, are surrounded by four oxygen and four bismuth atoms in the shape of square anti- prisms (Figure 1). The oxygen atoms (Cr positions) are tetra- hedrally coordinated by four rare earth metals forming PbO analogous slabs, and finally, the bismuth atoms are arranged in 4 4 nets, along (001), separating the REO slabs, whereby each bismuth atom is surrounded by eight RE atoms in the function of a slightly elongated cube. The distances between neighboring bismuth atoms range from 385.83(3) to 399.11(3) pm (Table 3), which correspond to the a-axis of the respective compound, are too long for as- suming a chemical bond. However, the bismuth atoms in each refined structure show significant anisotropy of thermal mo- tion, which is expressed by the ratio of the maximum and mini- mum displacement parameters U 11 /U 33 ≈ 2.2–4.3. This is indi- cating some local disorder, which gives freedom to assume real Bi–Bi distances shorter by approx. 28 pm (= 2√U 11 , cf. Table 2). This value is independently confirmed by applying a split model for the Bi position (8i site instead of 2a). In ele- mental bismuth, the Bi–Bi distances are 307 pm, and within 1D Bi – zigzag chains [9] or Bi 2 4– dumbbells [10] the distances are 322 and 329 pm, respectively, the latter are usually addressed as single bonds. In the 2D Bi – square sheets, present in EuBi 2 ,