Neutron and X-ray powder diffraction study of LixRuO2 and LixIrO2: The crystal structure of Li0.9RuO2

Compounds formed by the insertion of lithium into the rutile structure hosts RuO2 and IrO2 were studied by X-ray and neutron powder diffraction techniques. Compositions in the range LixMO2, M = Ru or Ir, 0 < x < 1 are two-phase materials consisting of unreacted host, x = 0, and limiting compositions x = 0.9 in both cases. Preparation of compounds with x > 1 was unsuccessful. Li0.9RuO2 and Li0.9IrO2 have orthorhombic cells with a = 5.062(3), b = 4.967(4), c = 2.771(4) and a = 4.962(4), b = 4.758(4), c = 3.108(6), respectively. Compared to the host rutile (tetragonal) cells those of the insertion compounds are greatly expanded along [100] and [010], ∼0.5 Å for both, and contracted along [001], by ∼0.3 Å for Li0.9RuO2 and 0.05 Å for Li0.9IrO2. The space group for both insertion phases appears to be Pnnm, a subgroup of the rutile space group P42mnm. The structure of Li0.9RuO2 was solved from neutron diffraction data. Lithium exists as Li+ in octahedral sites. The LiO coordination is highly regular with two bonds at 2.05(1) Å and four at 2.08(2) Å. The overall structure is essentially an ordered NiAs-type very similar to but more regular than the previously reported LiMoO2. Attempts to solve the structure of Li0.9IrO2 from both X-ray and neutron powder data were unsuccessful due, presumably, to severe preferred orientation.