The crystal structure and magnetic properties of a new layered iron oxyselenide Nd(2)Fe(2)O(3)Se(2) have been investigated through powder x-ray diffraction, magnetic susceptibility, specific heat, and (57)Fe Mössbauer spectrum measurements, and electronic structure calculations. This oxyselenide crystallizes in a tetragonal structure (space group I4/mmc) which is isostructural with La(2)Fe(2)O(3)Se(2). The lattice parameters were estimated to be a = 4.0263(1) Å and c = 18.4306(2) Å by the Rietveld analysis. Through its magnetic susceptibility, specific heat, and (57)Fe Mössbauer spectrum measurements, a long-range antiferromagnetic transition of the Fe(2+) ions was found at 88 K and the Nd(3+) ions exhibited Curie paramagnetic behavior down to 2 K. Two magnetic structure models for the Fe(2+) ions were suggested from the Mössbauer spectra.

The crystal, electronic, and magnetic structures of the cobalt oxyselenide La(2)Co(2)O(3)Se(2) were investigated through powder neutron diffraction measurements and band structure calculations. This oxyselenide crystallizes in a tetragonal layered structure with space group I4/mmm. The Co ion is sixfold-coordinated by two oxide ions and four selenide ions, and the face-sharing CoO(2)Se(4) octahedra form Co(2)OSe(2) layers. The band structure calculations revealed that the Co ion is in the divalent high-spin state. Rietveld analysis of the neutron diffraction profiles below 200 K demonstrated that the Co moments have a noncollinear antiferromagnetic structure with the propagation vector k = (½, ½, 0). The ordered magnetic moment was determined to be 3.5 μ(B) at 10 K, and the directions of the nearest-neighbor Co moments are orthogonal each other in the c plane.

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