Lead Rare-Earth Oxyhalides: Syntheses and Characterization of Pb 6 LaO 7 X (X ) Cl, Br) Oleg I. Siidra, ² Sergey V. Krivovichev,* ,² Thomas Armbruster, ‡ and Wulf Depmeier § Department of Crystallography, St. Petersburg State UniVersity, UniVersity Emb. 7/9, 199034 St. Petersburg, Russia, Laboratorium fu ¨r chemische und mineralogische Kristallographie, UniVersita ¨t Bern, Freiestrasse 3, CH-3102 Bern, Switzerland, and Institut fu ¨r Geowissenschaften, UniVersita ¨t zu Kiel, Olshausenstrasse 40, 24118 Kiel, Germany Received November 17, 2006 Yellowish elongated crystals of the two new compounds Pb 6 LaO 7 - Br (1) and Pb 6 LaO 7 Cl (2) have been obtained by the method of solid-state reactions. Both structures can be described in the terms of oxo-centered tetrahedra. The structures of 1 and 2 consist of [O 7 Pb 6 La] + chains that are built from oxocentered OA 4 (A ) Pb, La) tetrahedra. The halogen ions connect the chains through weak Pb-X bonds. An arrangement of eight OA 4 tetrahedra that all share the same central La atom forms a [O 8 Pb 10 La 3 ] 13+ cluster. The clusters are linked into chains, and additional OPb 4 tetrahedra are attached to the chains. Incorporation of Cl atoms instead of Br atoms into the structure causes a lowering of the symmetry from Cmcm to C2/m. Lead oxyhalides can form a variety of unusual structures because of the presence of a stereochemically active lone pair and represent a class of inorganic materials with possible applications as ionic conductors 1 and highly anisotropic nanomaterials. 2 They are also of great interest from the viewpoint of environmental chemistry 3 and mineralogy. 4 The formation and precipitation of lead oxide and hydroxide chlorides play an important role in the transport of lead from mines and mill tailings to the biosphere. To date, detailed chemical and structural information is available for pure oxychloride, 4a,f,5 oxybromide, 6 mixed oxyhalides, 7 and oxy- iodide 8 systems. The incorporation of rare-earth elements into structures of lead oxyhalides may lead to new structural topologies and is also of interest from the viewpoint of interesting physical properties because rare-earth metal halides are important to the development, fabrication, and application of electronic materials. 9 Here we report on the syntheses and structures of the new compounds Pb 6 LaO 7 X [X ) Br (1), Cl (2); Figure 1], high-temperature phases in the PbO-PbCl 2 -PbBr 2 -La 2 O 3 system that have been ob- tained by the solid-state reaction method. Single crystals of 1 were grown by mixing PbO, PbBr 2 , and La(NO 3 ) 3 in the ratio of 1:1:2. The produced mixture was loaded into a platinum crucible and kept at 910 °C for 1 h in air, followed by cooling to 685 °C with a cooling rate of 1°C min -1 and then cooling down to room temperature over 8.5 h. The product consisted of yellowish needles of 1, white transparent crystals of Pb 3 O 2 Br 2 10 in the mass of reddish-yellow isometric crystals of La 2 O 3 . 11 The crystals of 2 were produced by the same procedure, and PbCl 2 was used instead of PbBr 2 . The structure of 1 12 contains three symmetrically inde- pendent Pb sites, one La site, one Br site, and three O positions. In contrast, the structure of 2 12 contains five symmetrically independent Pb sites, one La site, one Br site, * To whom correspondence should be addressed. E-mail: skrivovi@ crystalspb.com. ² St. Petersburg State University. ‡ Universita ¨t Bern. § Universita ¨t zu Kiel. (1) Matsumoto, H.; Miyake, T.; Iwahara, H. Mater. Res. Bull. 2001, 36, 1177-1184. (2) Sigman, M. B., Jr.; Korgel, B. A. J. Am. Chem. 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