Abstract
The five quaternary neptunium compounds KCuNpS3, RbCuNpS3, 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 comprisesMS4 (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) A° . 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
þ1/þ1/þ4/-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 Np4þ-S2- has been derived and applied to the estimation of the formal oxidation states of Np in the
binary NpxSy compounds whose structures are known.