Structural and magnetic properties of orthorhombic LixMnO2

Rietveld refinement of the crystal and magnetic structures of LixMnO2 (x 1⁄4 0.98, 1.00, 1.02) are performed using neutron and X-ray measurements. A significant structural disorder due to the presence of manganese ions in lithium positions (MnLi) and lithium ions in manganese ones (LiMn) is found to be a common feature of Li0.98MnO2, Li1.00MnO2, and Li1.02MnO2. An essential anisotropy of the thermal-expansion coefficients of the lithium manganese oxides is observed in the temperature range of 1.5e 300 K. Furthermore, the distortion of the oxygen octahedral environment around the manganese ions decreases when the temperature lowers. This is attributed to the strong exchange interactions between parallel exchange-coupled Mn chains. First-principles calculations of the effective exchange-interaction parameters in Li16Mn16O32 confirm the essential antiferromagnetic interactions between the chains. In addition, a hypothetical (Li15Mn)Mn16O32 structure where a lithium atom located between the Mn double layers is replaced by a manganese atom is considered. The calculations reveal that the presence of such defects results in appearance of a ferromagnetic component that agrees with the magnetic measurements. 2006 Elsevier Masson SAS. All rights reserved.