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

Abstract

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.

Magnetic properties of lithium intercalation compounds

“…The value of the ratio f=θ p /T N =2.5 is indicative of a frustrated antiferromagnet. The geometric frustration inherent in the Mn sublattice, which is comprised of a three-dimensional array of a corner-sharing tetrahedral, has been discussed by Greedan et al [26].…”

Magnetic Properties of Mn-Doped Amorphous SiO2Matrix

“…In this work two manganese oxides in the Mn-rich particles embedded in the amorphous SiO 2 matrix were identified, major phase λ-MnO 2 and minor phase Mn 3 O 4 . λ-MnO 2 is a metastable form of manganese dioxide whose thermodynamically stable structure is β-MnO 2 [5]. λ-MnO 2 phase is usually obtained by removal of Li from LiMn 2 O 4 by acid leaching or by electrochemical procedure [5,6], At higher temperatures Li x Mn 2 O 4 spinel has paramagnetic properties.…”

“…λ-MnO 2 is a metastable form of manganese dioxide whose thermodynamically stable structure is β-MnO 2 [5]. λ-MnO 2 phase is usually obtained by removal of Li from LiMn 2 O 4 by acid leaching or by electrochemical procedure [5,6], At higher temperatures Li x Mn 2 O 4 spinel has paramagnetic properties. At lower temperatures a complex antiferromagnetic (AFM) ordering in λ-MnO 2 [5], and very complex magnetic ordering in LiMn 2 O 4 [7] were reported.…”

Experimental and first-principles study of ferromagnetism in Mn-doped zinc stannate nanowires

“…Potential secondary phases containing Mn include Mn 0 , MnO and MnO 2 , which are paramagnetic or antiferromagnetic and do not contribute to ferromagnetism. [43][44][45] However, no secondary phases were observed within experimental detection capabilities.…”