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

Abstract

A Al 2 Ge 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 CaAl 2 Si 2 -type structure (space group P 3 ̄ m1) with the lattice constants (Å): CaAl 2 Ge 2 : a =4.175(1), c =7.173(2); YAl 2 Ge 2 : a =4.205(1), c =6.699(1); LaAl 2 Ge 2 : a =4.297(1), c =7.013(2); NdAl 2 Ge 2 : a =4.269(1), c =6.832(1); GdAl 2 Ge 2 : a =4.253(2), c =6.716(2); TbAl 2 Ge 2 : a =4.238(1), c =6.661(1); LuAl 2 Ge 2 : a =4.160(1), c =6.615(2). The electronic band structures (LMTO method) of CaAl 2 Ge 2 and YAl 2 Ge 2 , as examples of an electrovalent and a nonelectrovalent composition, are discussed with regard to bondings and electrical conductivity. Investigations of GdAl 2– x Zn x Ge 2 mixed crystals and of YAlMgGe 2 show how the transition from an electrovalent to a nonelectrovalent composition affects the lattice and structural parameters. Graphic