La₇Ru₃O₁₈ and La_4.87Ru₂O₁₂: Geometric frustration in two closely related structures with isolated RuO₆ octahedra

The crystal structures of La₇Ru₃O₁₈ and La_4.87Ru₂O₁₂ have been solved from powder neutron diffraction data (R(p) = 0.032) and single-crystal X-ray diffraction data (R(w) = 0.070 for all 3125 reflections), respectively. Although these compounds are both the first example of their structure types, La₇Ru₃O₁₈, La_4.87Ru₂O₁₂, and the known compound Sr₅Re₂O₁₂ all have closely related structures. La₇Ru₃O₁₈ crystallizes in the rhombohedral space group R3c, with cell constants of a = 9.83677(23) Å and c = 56.3493(16) Å in the hexagonal setting, and with Z = 12 formula units per cell. Monoclinic La_4.87Ru₂O₁₂ crystallizes in the P2₁/c space group with cell constants of a = 5.5798(6) Å, b = 10.1286(11) Å, c = 19.0095(20) Å, β = 90.815(4)°, and Z = 4. These structures contain isolated RuO₆ octahedra (d(Ru-Ru)~5.7 Å), which are organized into well-defined layers having trigonal or pseudo-trigonal symmetry. Furthermore, the three-dimensional patterning of Ru atoms is a nearly perfect close-packed arrangement, despite the large Ru-Ru distances. Magnetic measurements show that geometric frustration suppresses the ordering of the Ru spins and that monoclinic La_4.87Ru₂O₁₂ is more frustrated than rhombohedral La₇Ru₃O₁₈. (C) 2000 Academic Press.