U_1.33T₄Al₈Si₂ (T = Ni, Co): Complex Uranium Silicides Grown from Aluminum/Gallium Flux Mixtures

Two new quaternary analogs of the Gd₁₊Fe₄Si_10- structure type were grown from the reaction of uranium, silicon, and a transition metal (nickel or cobalt) in an excess of aluminum/gallium flux. The use of a mixed flux was found to be necessary for the formation of U_1.33T₄Al₈Si₂ (T = Ni, Co). Single crystal X-ray diffraction data shows the presence of disordered U/Si layers that are characteristic of this structure type; precession photographs indicate partial formation of a superstructure and stacking disorder along the c-axis. This disorder may be the cause of the spin glass behavior that is particularly evident in the nickel analog, which exhibits a spin freezing transition at T_F = 7 K. These compounds are resistant to chemical attack and oxidation and may be potential waste forms.