CRYSTAL STRUCTURE OF 4H BaRuO₃: HIGH PRESSURE PHASE PREPARED AT AMBIENT PRESSURE. Seung-Tae Hong, Arthur W. Sleight, Department of Chemistry, Oregon State University, Corvallis, OR 97331-4003

Single crystals of four-layer hexagonal (4H) BaRuO₃ were grown at ambient pressure, and the crystal structure was determined (space group P6₃/mmc; a=5.7289(6) Å and c=9.5003(9) Å; Z=4). It is known that BaRuO₃ has the 9R polytype structure at atmospheric pressure, which transforms at 15 kbar to the 4H structure and further transforms to the 6H structure at 30 kbar. Single crystals of 4H BaRuO₃ were grown by using BaCl₂ as a flux with a nominal composition of Ba₆Ru₂W₂O_17-y. The mixture was heated to 1200deg.C and cooled slowly. Black BaRuO₃ crystals formed together with unidentified orange crystals. Most of the BaRuO₃ crystals have a 9R structure, while some of them turned out to have a 4H structure. The stoichiometry and possible W impurities were checked by microprobe analysis. The structure was determined by the interpretation of the Patterson map, which resulted in the positions of the metal atoms, and the subsequent difference Fourier maps located the positions of the oxygens. The final conventional R1(based on F_o) and wR2(based on F_o²) are 2.01 and 4.07%, respectively. The structure of 4H BaRuO₃ may be described as a perovskite-related compound with a four-layer stacking of BaO₃ layers in the sequence of hchc. This leads to two face-sharing octahedra; the strings are held together by corner sharing. All of the octahedral sites are occupied by ruthenium ions. Concerning the dimer units [Ru₂O₉], the Ru-Ru distance is 2.537(1) Å which is close to that in 9R BaRuO₃ (2.55 Å), and comparable with the value in La4Ru6O~g (2.488 18L), suggesting a possible metal-metal bonding. This result shows that a phase which has been regarded as a high pressure phase can in fact be prepared at one atmosphere when using appropriate synthesis conditions.