An_1.33T₄Al₈Si₂ (An = Ce, Th, U, Np; T = Ni, Co): Actinide Intermetallics with Disordered Gd₁₊Fe₄Si_10- Structure Type...

An_1.33T₄Al₈Si₂ (An = Ce, Th, U, Np; T = Ni, Co) were synthesized in metal flux reactions carried out in aluminum/gallium melts. In previous work, U_1.33T₄Al₈Si₂ (T = Co, Ni) analogues were formed by arc-melting U:T:Si and reacting this mixture in Al/Ga flux. However, in the current work, all compounds were synthesized by using AnO₂ reactants, taking advantage of the ability of the aluminum in the flux to act as both solvent and reducing agent. While reactions with T = Co yielded hexagonal Gd_1.33Fe₄Si₁₀-type quaternary phases for all An, reactions with T = Ni produced these compounds only with An = U and Np. For reactions with An = Ce and Th, the reactions led instead to the formation of AnNi_3-SiAl_4-Ga phases, with the tetragonal KCu₃S₄ structure type. Attempts to synthesize plutonium analogues Pu_1.33T₄Al₈Si₂ were also unsuccessful, producing the previously reported PuCoGa₅ and Pu₂Ni₅Si₆ instead. Magnetic data collected on the neptunium analogues Np_1.33T₄Al₈Si₂ (T = Ni, Co) show antiferromagnetic coupling at low temperatures and indicate a tetravalent state for the Np ions.