The magnetic and crystal structures of the alpha-NaMnO2 have been determined by high-resolution neutron powder diffraction. The system maps out a frustrated triangular spin lattice with anisotropic interactions that displays two-dimensional spin correlations below 200 K. Magnetic frustration is lifted through magneto-elastic coupling, evidenced by strong anisotropic broadening of the diffraction profiles at high temperature and ultimately by a structural phase transition at 45 K. In this low-temperature regime a three-dimensional antiferromagnetic state is observed with a propagation vector k=(1/2,1/2,0).

The S=2 anisotropic triangular lattice alpha-NaMnO2 is studied by neutron inelastic scattering. Antiferromagnetic order occurs at T< or =45 K with opening of a spin gap. The spectral weight of the magnetic dynamics above the gap (Delta approximately equal to 7.5 meV) has been analyzed by the single-mode approximation. Excellent agreement with the experiment is achieved when a dominant exchange interaction (|J|/k(B) approximately 73 K), along the monoclinic b axis and a sizable easy-axis magnetic anisotropy (|D|/k(B) approximately 3 K) are considered. Despite earlier suggestions for two-dimensional spin interactions, the dynamics illustrate strongly coupled antiferromagnetic S=2 chains and cancellation of the interchain exchange due to the lattice topology. alpha-NaMnO2 therefore represents a model system where the geometric frustration is resolved through the lowering of the dimensionality of the spin interactions.

The stabilization of the n = 3 member of the Ruddlesden−Popper series has been investigated in the Sr−Nd−Fe−O system. A new series of phases closely derived from RP-type structures has been synthesized and characterized by X-ray diffraction and electron microscopy. The control of the oxygen stoichiometry first allowed us to isolate a new highly oxygen deficient, layered oxide, Sr3NdFe3O9-δ. The structural analysis has revealed that this oxide crystallizes in an orthorhombic lattice (a p√2 × a p√2 × 28 Å), distorted with regard to the ideal I-type symmetry of the RP-type structures. Interestingly, this oxygen-deficient n = 3 RP-member reacts at room temperature with ambient moisture to transform into a hydrated oxyhydroxide, Sr3NdFe3O7.5(OH)2·H2O. This second phase can be dehydrated topotactically in two steps by heating to 400 °C, leading to Sr3NdFe3O7.5(OH)2 and then Sr3NdFe3O8.5+δ, respectively. The reversible character of these hydration/hydrolysis reactions is also demonstrated.