La7Ru3O18 and La4.87Ru2O12: Geometric frustration in two closely related structures with isolated RuO6 octahedra

The crystal structures of La7Ru3O18 and La4.87Ru2O12 have been solved from powder neutron diffraction data (R-p = 0.032) and single-crystal X-ray diff raction data (R-w = 0.070 for all 3125 reflections), respectively. Although these compounds are both the first example of their structure types, La7Ru 3O18, La4.87Ru2O12, and the known compound Sr5Re2O12 all have closely relat ed structures. La7Ru3O18 crystallizes in the rhombohedral space group R3c, with cell constants of a = 9.83677(23) Angstrom and c = 56.3493(16) Angstro m in the hexagonal setting, and with Z = 12 formula units per cell, Monocli nic La4.87Ru2O12 crystallizes in the P2(1)/c space group with cell constant s of a = 5.5798(6) Angstrom, b = 10.1286(11) Angstrom, c = 19.0095(20) Angs trom, beta = 90,815(4)degrees, and Z = 4. These structures contain isolated RuO6 octahedra (d(Ru-Ru) similar to 5.7 Angstrom), which are organized int o well-defined layers having trigonal or pseudo-trigonal symmetry. Furtherm ore, the three-dimensional patterning of Ru atoms is a nearly perfect close -packed arrangement, despite the large Ru-Ru distances, Magnetic measuremen ts show that geometric frustration suppresses the ordering of the Ru spins and that monoclinic La4.87Ru2O12, is more frustrated than rhombohedral La7R u3O18. (C) 2000 Academic Press.