Journal of Solid State Chemistry 173 (2003) 203–208 Sr 18 Ru 1.9 Bi 4.1 O 33 : crystallization of a Ru(V)/Bi(V) oxide from molten hydroxide Marisol S. Martı´n-Gonza´lez, James L. Delattre, and Angelica M. Stacy  Department of Chemistry, University of California at Berkeley, 538A Latimer, Berkeley, CA 94720, USA Received 7 March 2002; received in revised form 22 November 2002; accepted 11 December 2002 Abstract Single crystals of a new complex oxide, Sr 18 Ru 1.9 Bi 4.1 O 33 , were precipitated from a mixture of molten alkali and alkaline earth metal hydroxides at 7501C. The structure was determined from a ruby-red crystal using single-crystal X-ray diffraction. Sr 18 Ru 1.88 Bi 4.12 O 33 crystallizes in the space group C2/c (monoclinic) and has unit-cell dimensions: a ¼ 10:2102ð11Þ A ˚ , b ¼ 17:882ð2Þ A ˚ , c ¼ 19:579ð2Þ A ˚ , and b ¼ 102:043ð2Þ1. The structure (refined to R 1 ¼ 4:5%; wR 2 ¼ 9:2%) is an unusual ABO 3 defect perovskite, with 1 12 th of the oxygen positions vacant. All the A sites and half of the B sites are occupied by Sr 2+ , while the remaining B sites are occupied by Bi 5+ or Ru 5+ . The oxygen atom vacancies are located within the Bi coordination sphere exclusively. The bonding in the BO 3 sublattice is less covalent than that in the perovskite archetype from which it is derived due to the presence of 50% Sr 2+ on the B sites. Thus, the structure of Sr 18 Ru 1.9 Bi 4.1 O 33 can also be viewed as being made up of isolated bismuthate anions (BiO 5 5 , BiO 5.5 6 and BiO 6 7 ) and ruthenate anions (RuO 6 7 ) separated by strontium cations. r 2003 Elsevier Science (USA). All rights reserved. Keywords: Molten hydroxide; Ruthenates; Bismuthates; Perovskites; Cryolite; Elpasolite 1. Introduction The perovskite structure constitutes one of the most interesting structures in solid-state chemistry and con- tinues to be investigated as a model for the study of fundamental properties in structural chemistry and physics. This is partly due to the extensive range of phase transitions that can be induced by composition changes, temperature, or pressure. The structure is known with the ABX 3 formula, where BX 6 octahedra are connected by corner-sharing X atoms in a three- dimensional framework with the A cations occupying positions coordinated by 12 X atoms. One of the many possible superstructures that can be derived from this framework [1] is cryolite (A 3 BX 6 ) [2]. This super- structure can also be written from the structural standpoint as A 2 (A 0 B)X 6 . The cation A occupies two sites in the cryolite structure: all the A positions of the perovskite structure with coordination number (CN)=12 plus half of the BX 6 positions with CN=6. The framework is then constructed from A 0 X 6 and BX 6 octahedra connected by corners. This cryolite structure can also be considered as part of the elpasolite family (A 2 BB 0 X 6 ) if A and B are the same cation [1,3,4]. Most of the cryolite-type compounds are halides [5], although oxides like R 3 WO 6 ,R 3 MoO 6 [6], Ca 5 Nb 2 TiO 12 [7], Ca 4 Nb 2 O 9 [8,9], or Sr 3 (Sr 1+x Nb 2x )O 93/2x [10] have been prepared. In the case of Sr 3 (Sr 1+x Nb 2x )O 93/2x , the structure forms with up to 8.3% oxygen vacancies which corresponds to a limiting composition of x ¼ 0:5 (Sr 6 Nb 2 O 11 ) [10]. We report here the properties of an oxygen-deficient mixed-metal (Ru 5+ , Bi 5+ ) perovskite, Sr 18 Ru 1.9 Bi 4.1 O 33 , which is related to the cryolite archetype. Sr 18 Ru 1.9 Bi 4.1 O 33 was prepared from a molten KOH-Sr(OH) 2 flux and structurally characterized using single-crystal X-ray diffraction. The use of molten hydroxides provides a highly oxidizing environment at moderate temperatures and offers a synthetic route to complex transition metal oxides. The materials that have been obtained by this method are sometimes metastable and often contain transition metals in unusual or elevated oxidation states as exemplified by Ni 4+ in Ba 6 Ni 5 O 15 [11], Bi 5+ in Ba 3 NaBiO 6 [12], Rh 5+ in Sr 3 NaRhO 6 [13], or Os 7+ in Ba 2 MOsO 6 (M=Li, Na) [14]. As described  Corresponding author. Fax: +1-510-642-8369. E-mail address: stacy@cchem.berkeley.edu (A.M. Stacy). 0022-4596/03/$-see front matter r 2003 Elsevier Science (USA). All rights reserved. doi:10.1016/S0022-4596(03)00025-2