Crystal structure of 4H BaRuO3: High pressure phase prepared at ambient pressure

Single crystals of BaRuO3 were grown using a BaCl3 flux at ambient pressure, Most of the crystals had the nine-layer rhombohedral structure (9R) which is considered to be the stable form of BaRuO3 at ambient pressure, However, some BaRuO3 crystals in this preparation had the four-layer hexagonal (4H) structure which had been considered as a phase produced only at high pressure (15-30 kbar), The crystal structure of 4H BaRuO3 was refined for the first time, Single crystal X-ray diffraction data gave a space group of P6(3)/mmc (No. 194), a=5.729(1) Angstrom, c=9.500(1) Angstrom, Z=4, a final conventional R(based on F-0) of 2.01% and a wR2 (based on F-0(2)) of 4.07%. The structure of 4H BaRuO3 may be described as perovskite-related with a four-layer stacking of BaO3 layers in the sequence hchc. This leads to pairs of face-sharing RuO6 octahedra which are connected to other octahedra by corner sharing, The Ru-Ru distance in the Ru2O9 dimer of 4H BaRuO3 is 2.537(1) Angstrom which indicates the presence of a metal-metal bond. (C) 1997 Academic Press.