STRUCTURE OF FEYB2S4 - ELECTRON-MICROSCOPY STUDY

The stoichiometric compound FeYb2S4 is in fact the Yb-rich end of a range of continuously variable compositions. In this range, two subranges are distinguished, corresponding to two different structures according to the proportion Fe/Yb: a rocksalt-type structure in the region of composition poor in Yb, and a spinel-type structure in the region rich in Yb. Electron diffraction of samples of various compositions picked up in the whole range show either sharp Bragg reflections characteristic of the spinel structure, or complex diffraction patterns with sharp reflections together with diffuse satellites and linear streaks (along [111]* directions), the sharp spots being characteristic of the rocksalt type; both diffraction patterns are often simultaneously observed, corresponding to a mixing of regions of different structures in the same sample. Electron imaging and high resolution greatly help the interpretation in showing in the rocksalt type of structure numerous random planar stacking faults perpendicular to [111], limiting more or less lozenge-shaped domains. A model is proposed for explaining this perturbed structure on the atomic level. The rocksalt-type structure with all its defects would appear as a transition structure tending to become an ideal spinel-type structure.