RATIONALIZATION AND PREDICTION OF RARE-EARTH SELENIDE SUPERSTRUCTURES

We apply second-moment scaled Huckel theory with inclusion of ionic terms to rationalize or predict superstructures found in elemental selenium, LnQ2 (Ln = La, Ce; Q = S, Se), Ln10Se19 (Ln = La, Ce, Pr, Nd, Sm), and RbDY3Se8. All these structures contain distortions of square lattices of chalcogen atoms. In the case of Ln10Se19 and RbDY3Se8 these lattice distortions are coupled to ordered defects in the chalcogen square lattice. On the basis of our energy calculations we propose eight ground state RbDY3Se8 superstructure patterns among the 5 x 10(5) possible alternatives. Finally we explain the results of our calculations using HOMO-LUMO and Madelung energy arguments.