Crystal structure and physical properties of the new AcNi6Si6 compounds (Ac: Th, U)

The crystal structure of the new ternary ThNi6Si6 and UNi6Si6 intermetallic compounds has been investigated by means of both single crystal and powder X-ray diffraction followed by Rietveld refinement. Both compounds adopt the tetragonal CeNi6Si6-type structure, crystallizing in the space group P4/nbm (No. 125) (Pearson’s symbol tP52). This prototype is a ternary defect-derivative of the binary NaZn13-type. A group-theoretical analysis carried out in this work illustrates the structural relationship occurring among the aristotype NaZn13-type, the partially disordered CeNi6Si6 and the fully ordered YNi6Si6 hettotype crystal structures. Zero-field low-temperature electrical resistivity data show a metallic behaviour for both materials. DC magnetic measurements from room temperature down to 2 K reveal diamagnetic behaviour for ThNi6Si6, indicating that Ni atoms do not carry any magnetic moment. On the other hand, the effective magnetic moment per formula unit for UNi6Ni6 is found to be µeff = 3.07(1) µB, suggesting a trivalent (U3+) or tetravalent (U4+) state for U atoms with partial delocalization of 5 f electrons. No magnetic ordering down to 2 K is seen in UNi6Si6. No superconducting transition was detected, neither in UNi6Si6 nor in ThNi6Si6 in the available temperature interval (2 K ≤ T ≤ 300 K).