CRYSTAL-CHEMISTRY OF THE HALITE-RELATED CATION-DEFICIENT SUPERSTRUCTURE PHASE YB7-SE8 WITH DELTA=0.24 AND ITS RELATION TO YBSE(DELTA)

Systematic phase studies on the system YbSe-Yb2Se3 revealed besides th e two terminal compounds three intermediate phases Yb0.91Se, Yb0.83Se and Yb0.75Se, all of them without significant phase width, which is ch aracteristic for the so-called room temperature equilibrium. Lattice p arameters are given for all phases. Structure determinations from X-ra y powder diffraction data have been carried out for Yb0.9,Se=Yb7.24Se8 as well as for YbSe for comparison. The superstructure of Yb0.91Se is halite related (NaCl type) with ordering of vacancies in the cationic sublattice and partial ordering of Yb2+ and Yb3+. Therefore the ideal formula of the mixed valence compound is Yb7Se8=square1Yb52+Yb23+Se82 -. The actual phase Yb0.91Se has additional ytterbium statistically di stributed over the sites of vacancies. The change in the Yb-Se distanc e of 2.966 angstrom in YbSe to 2.830 and 2.938 angstrom in Yb7.24Se8 d efinitely proves the simultaneous appearance of Yb2+ and Yb3+, both oc tahedrally coordinated.