Structure and Magnetism in λ-MnO2. Geometric Frustration in a Defect Spinel

λ-MnO2, a metastable form of manganese dioxide, retains the cubic spinel structure upon lithium removal from LiMn2O4 by soft chemical methods, either electrochemical or acid leaching. The minimum lithium content, achieved by the latter route at pH 1, is Li0.10MnO2, which is in reasonable agreement with previous reports. For lithium contents near the minimum value, long-range antiferromagnetic order sets in below T N = 32 K, and Curie−Weiss …