The magnetic metal ions in the cubic pyrochlore Tb2M02O7 form an infinite three-dimensional network of corner-sharing tetrahedra with a very high potential for frustration in the presence of antiferromagnetism. We have performed neutron scattering measurements which show short-range spatial correlations that develop continuously with decreasing temperature, while the characteristic time scale for the fluctuating moments decreases dramatically below 77 -25 K. Therefore, this pure material, which possesses frustration that is purely geometrical in origin, displays a spin-glass state at low temperatures.

Powder neutron diffraction has been used to study the nature of the structural transition away from the Fd3m cubic structure upon cooling below ∼285 K in the spinel LiMn2O4. We report powder data taken between 10 K and 333 K and propose a large cell tetragonal structure in space group I41/amd for the material at 100 K. While complete segregation of the Mn3+ and Mn4+ ions is not possible in this space group, bond-valence analysis indicates that the distribution of Mn3+ and Mn4+ ions is not random and that there is a degree of charge segregation. Further, LiMn2O4 is also of interest because it is an example of a geometrically frustrated antiferromagnet. Direct current magnetic susceptibility measurements show field-cooled, zero-field-cooled irreversibility at ∼65 K and a maximum in zero-field-cooled data at ∼40 K. Neutron diffraction shows magnetic scattering in the form of a broad peak assigned to short-range order which develops above 100 K. Upon cooling to 60 K additional Bragg peaks are seen, signaling long-range magnetic order. The Bragg peaks grow at the expense of the diffuse feature on cooling to 10 K but the latter persists even at the lowest temperature studied which indicates that a significant fraction of the spins still remain disordered. The magnetic Bragg peaks index on a tetragonal cell which is 2a, 2b, and 4c with respect to the low-temperature tetragonal cell and contains 1152 spins. The large size and implied complexity of the magnetic structure is consistent with both charge segregation and significant further neighbor exchange interactions.

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.