Quaternary neptunium compounds: Syntheses and characterization of KCuNpS₃, RbCuNpS₃, CsCuNpS₃, KAgNpS ₃, and CsAgNpS₃

The five quaternary neptunium compounds KCuNpS₃, RbCuNpS ₃, CsCuNpS₃, KAgNpS₃, and CsAgNpS₃ (AMNpS₃) have been synthesized by the reaction of Np, Cu or Ag, S, and K₂S or Rb₂S₃ or Cs₂S₃ at 793 K (Rb) or 873 K. These isostructural compounds crystallize as black rectangular plates in the KCuZrS₃ structure type in space group Cmcm of the orthorhombic system. The structure comprises MS₄ (M=Cu or Ag) tetrahedra and NpS₆ octahedra that edge share to form _∞² [MNpS₃^-] layers. These layers are separated by the alkali-metal cations. The Np-S bond lengths vary from 2.681(2) to 2.754(1) Å. When compared to the corresponding isostructural Th and U compounds these bond distances obey the expected actinide contraction. As the structure contains no S-S bonds, formal oxidation states of +1/+1/+4/-2 may be assigned to A/M/Np/S, respectively. From these results a value of 2.57 for the bond-valence parameter r₀ for Np ⁴⁺-S^2- has been derived and applied to the estimation of the formal oxidation states of Np in the binary Np_xS_y compounds whose structures are known.