Variations of the FeGa3 structure type in the systems CoIn3-xZnx and CoGa3-xZnx

摘要

We present an investigation of the quasibinary systems CoIn3-xZnx and CoGa3-xZnx which were structurally characterized by X-ray diffraction experiments and, in the case of CoGa3-xZnx, additionally by neutron powder diffraction experiments. The limiting compositions were found to be x=0.81(2) and x=0.73(2) for COIn3-xZnx and CoGa3-xZnx, respectively. The isotypic binary compounds CoIn3 and CoGa3 crystallize with the FeGa3 structure type (tetragonal, space group P42/mnm, Z=4) in which the p-block atoms form an array of columns of centered cubes defined by two different crystallographic sites. The substitution of In or Ga by Zn takes place in an ordered fashion and produces "colored" variants of the FeGa3 parent structure: In both systems Zn enters exclusively the position corresponding to the cube centers. Additionally, in CoIn3-xZnx this position is substituted in such a way that for a composition CoIn2.5Zn0.5, columns of Zn- and In- filled ln(8) cubes along the c axis alternate. The latter substitution pattern is accompanied by a symmetry lowering of the parent FeGa3 structure: The structure of CoIn3-xZnx is described by the space group P4(2)/m in which the cube center position is split into two separate sites. By performing first-principles electronic structure calculations we investigated the general bonding situation of the compounds CoIn3 and CoGa3 and the particular electronic effect when incorporating Zn. With respect to the density of states of the binary compounds the exchange of Ga or In by Zn virtually affects only the electronic states just below the Fermi level. On increasing Zn concentration a dip is created in the density of states which approximately coincides with the location of the Fermi level for an electron count corresponding to limiting composition of the two systems. (C) 2002 Elsevier Science (USA). [References: 24]