Neptunium thiophosphate chemistry: Intermediate behavior between uranium and plutonium

Black crystals of Np(PS₄), Np(P₂S₆) ₂, K₁₁Np₇(PS₄)₁₃, and Rb₁₁Np₇(PS₄)₁₃ have been synthesized by the reactions of Np, P₂S₅, and S at 1173 and 973 K; Np, K₂S, P, and S at 773 K; and Np, Rb₂S₃, P, and S at 823 K, respectively. The structures of these compounds have been characterized by single-crystal X-ray diffraction methods. Np(PS₄) adopts a three-dimensional structure with Np atoms coordinated to eight S atoms from four bidentate PS₄^3- ligands in a distorted square antiprismatic arrangement. Np(PS₄) is isostructural to Ln(PS ₄) (Ln = La-Nd, Sm, Gd-Er). The structure of Np(P₂S ₆)₂ is constructed from three interpenetrating diamond-type frameworks with Np atoms coordinated to eight S atoms from four bidentate P₂S₆^2- ligands in a distorted square antiprismatic geometry. The centrosymmetric P₂S₆ ^2- anion comprises two PS₂ groups connected by two bridging S centers. Np(P₂S₆)₂ is isostructural to U(P₂S₆)₂. A₁₁Np ₇(PS₄)₁₃ (A = K, Rb) adopts a three-dimensional channel structure built from interlocking [Np₇(PS₄) ₁₃]^11--screw helices with A cations residing in the channels. The structure of A₁₁Np₇(PS₄) ₁₃ includes four crystallographically independent Np atoms. Three are connected to eight S atoms in bicapped trigonal prisms. The other Np atom is connected to nine S atoms in a tricapped trigonal prism. A₁₁Np ₇(PS₄)₁₃ is isostructural to A ₁₁U₇(PS₄)₁₃. From Np-S bond distances and charge-balance, we infer that Np is trivalent in Np(PS ₄) and tetravalent in Np(P₂S₆)₂ and A₁₁Np₇(PS₄)₁₃. Np exhibits a behavior intermediate between U and Pu in its thiophosphate chemistry.