The stability range of the CaAl2Si2-type structure in case of LnAl2Ge2 compounds

AAl(2)GC(2) (A =Ca, Y, La, Nd, Gd, Tb, Lu) were synthesized by heating the elements at 1000-1300 K and characterized by single crystal X-ray methods. The isotypic compounds crystallize in the CaAl2Si2-type structure (space group P (3) over bar ml) with the lattice constants (A): CaAl2Ge2: a = 4.175(1), c = 7.173(2); YAl2Ge2: a = 4.205(l), c = 6.699(l); LaAl2Ge2: a = 4.297(l), c = 7.013(2); NdAl2Ge2: a = 4.269(1), c = 6.832(1); GdAl2Ge2: a = 4.253(2), c = 6.716(2); TbAl2Ge2: a = 4.238(1), e = 6.661 (1); LuAl(2)GC(2): a = 4.160(1), c = 6.615 (2). The electronic band structures (LMTO method) of CaAl2Ge2 and YAl2Ge2, as examples of an electrovalent and a none lectrovalent composition, are discussed with regard to bondings and electrical conductivity. Investigations of GdAl2-xZnxGe2 mixed crystals and of YAlMgGe2 show how the transition from an electrovalent to a nonelectrovalent composition affects the lattice and structural parameters. (C) 2002 Editions scientifiqucs et medicales Elsevier SAS. All rights reserved.