The stability range of the CaAl_2Si_2-type structure in case of LnAl_2Ge_2 compounds

摘要

Aal_2Ge_2 (A = Ca, Y, La, Nd, Gd, Tb, Lu) were synthesized by heating the elements at l000-l300 K and characterized by single crystal X-ray methods. The isotypic compounds crystallize in the CaAl_2Si_2-type structure (space group P3m1) with the lattice constants (A): CaAl_2Ge_2: a = 4.175(1), c = 7.173(2); Yal_2Ge_2: a = 4.205(1), c = 6.699(1); LaAl_2Ge_2: a = 4.297(1), c = 7.013(2); NdAl_2Ge_2: a = 4.269(1), c = 6.832(1); GdAl_2Ge_2: a = 4.253(2), c = 6.716(2); TbAl_2Ge_2: a = 4.238(1), c = 6.661(1); LuAl_2Ge_2: a = 4.160(1), c = 6.615(2). The electronic band structures (LMTO method) of CaAl_2Ge_2 and Yal_2Ge_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_xGe_2 mixed crystals and of YalMgGe_2 show how the transition from an electrovalent to a nonelectrovalent composition affects the lattice and structural parameters.