ABcompounds with ScY and rare earth metals. II. FeB and CrB structures of monosilicides and germanides

Twenty new rare earth silicides and germanides of composition AB have been synthesized and their crystal structures have been determined.LaSi, CeSi, PrSi, NdSi, SmSi, GdSi, TbSi, DySi, HoSi, ErSi, LaGe, CeGe, PrGe crystallize in the FeB (B27) structure type.DySi, HoSi, ErSi, PrGe, NdGe, SmGe, GdGe, TbGe, DyGe, HoGe and ErGe crystallize in the CrB structure type.Polymorphism occurs in DySi, HoSi, ErSi and PrGe which crystallize in both of these structure types.The FeB-and CrB-type structures contain a common structural unit, the trigonal prism, which is stacked differently to form either structure.On this basis, a set of transformation equations relating the lattice constants of both structure types is derived.A survey of all known FeB-and CrB-type structures shows that these compounds occur only in two valence electron concentration ranges.The shape of the prism side within each range depends on the valence electron difference.