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

λ-MnO 2, a metastable form of manganese dioxide, retains the cubic spinel structure upon lithium removal from LiMn 2 O 4 by soft chemical methods, either electrochemical or acid leaching. The minimum lithium content, achieved by the latter route at pH 1, is Li 0.10 MnO 2, 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 susceptibility behavior is found above 125 K, with fitting constants, θ c = −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 T N 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 / 2 1 / 2 1 / 2 )(128 Mn moments per magnetic unit cell) and confirm the T N = 32 K. A model for the magnetic structure is proposed that is consistent with the neutron intensities. The complexity of the magnetic structure is consistent with the geometric frustration inherent in the Mn sublattice, which is comprised of a three-dimensional array of corner-sharing tetrahedra. The properties of λ-MnO 2 are compared and contrasted with those of β-MnO 2, with the rutile structure, and the pyrochlore Y 2 Mn 2 O 7, with the same topology for the Mn(4+) sublattice.