Order–disorder transition and valence state of ytterbium in YbAuxGa2−x (0.26≤x≤1.31)

The intermetallic compounds YbAuxGa2−x (0.26≤x≤1.31) were synthesized by melting of elemental components and subsequent annealing. The crystal structure of YbAu1.04Ga0.96 was investigated using single-crystal X-ray diffraction data: structure type TiNiSi, space group Pnma, a=7.1167(3) Å, b=4.5019(3) Å, c=7.7083(3) Å, RF=0.028 for 27 variables and 441 reflections. At 600 °C this compound is described as partially substituted TiNiSi type and shows a homogeneity range around the equiatomic composition YbAuxGa2−x (0.94≤x≤1.19). For the gallium- (0.26≤x≤0.83) and gold-rich (1.21≤x≤1.31) regions, the KHg2 type of crystal structure (space group Imma) with mixed Au/Ga occupation is found. A temperature-driven phase transition for the composition YbAuGa from ordered TiNiSi to disordered KHg2 structure type is observed at 629 °C. Yb LIII X-ray absorption spectra indicate an intermediate valence of +2.5 for Yb atoms in YbAuGa. For samples deviating from this composition a further reduced valence of Yb is observed. Magnetic susceptibility studies show a non-magnetic 4f14 ground state of Yb atoms with thermal fluctuations towards the 4f13 state.