Systematic study of the UNiX2 ternary compounds (X=C, Si, Ge)

We attempted to grow UNiX2 ternary compounds by the arc melting method. UNiC2 and UNiGe2 may not melt congruently because both compounds contain impurity phases. UNiC2 crystallizes into the tetragonal UCoC2-type structure with space group P4/nmm, the annealed sample consists of UNiC2 and U2NiC3 as the main and secondary phases, respectively. The lattice constants were obtained to be a = 3.512 Å, c = 7.339 Å. UNiGe2 also contains two phases. We assumed that primary phase is isostructural to UNiSi2 that crystallizes into the orthorhombic CeNiSi2-type structure with the space group of Cmcm. On the other hand, it is reasonable to assume that UNi2Ge2 exists in the compound as secondary phase. The unit cell volume tends to increase as increasing the anomic number of X (C, Si, Ge). It comes from the fact that the atomic radius of X becomes larger as the atomic number becomes larger.