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 TN = 32 K, and Curie−Weiss susceptibility behavior is found above 125 K, with fitting constants, θc = −104(4) K and C = 1.97(2) emu-K/mol. This value of C is consistent with the lithium content found analytically. The susceptibility is remarkably field dependent in the temperature range near TN for some samples with larger lithium contents, which might be understood in terms of field-induced short-range ferromagnetic correlations. Neutron diffraction studies show a complex magnetic order described by a propagation vector k = (1/21/21/2)(128 Mn moments per magnetic unit cell) and confirm the TN = 3...