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 PS43– ligands in a distorted square antiprismatic arrangement. Np(PS4) is isostructural to Ln(PS4) (Ln = La–Nd, Sm, Gd–Er). The structure of Np(P2S6)2 is constructed from three interpenetrating diamond-type frameworks with Np atoms coordinated to eight S atoms from four bidentate P2S62– ligands in a distorted square antiprismatic geometry. The centrosymmetric P2S62– anion comprises two PS2 groups connected by two bridging S centers. Np(P2S6)2 is isostructural to U(P2S6)2. A11Np7(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. A11Np7(PS4)13 is isostructural to A11U7(PS4)13. From Np–S bond distances and charge-balance, we infer that Np is trivalent in Np(PS4) and tetravalent in Np(P2S6)2 and A11Np7(PS4)13. Np exhibits a behavior intermediate between U and Pu in its thiophosphate chemistry.