Dichalcogenide Bonding in Seven Alkali-Metal Actinide Chalcogenides of the KTh₂Se₆ Structure Type

Abstract

The solid-state compounds CsTh(2)Se(6), Rb(0.85)U(1.74)S(6), RbU(2)Se(6), TlU(2)Se(6), Cs(0.88)(La(0.68)U(1.32))Se(6), KNp(2)Se(6), and CsNp(2)Se(6) of the AAn(2)Q(6) family (A = alkali metal or Tl; An = Th, U, Np; Q = S, Se, Te) have been synthesized by high-temperature techniques. All seven crystallize in space group Immm of the orthorhombic system in the KTh(2)Se(6) structure type. Evidence of long-range order and modulation were found in the X-ray diffraction patterns of TlU(2)Se(6) and CsNp(2)Se(6). A 4a × 4b supercell was found for TlU(2)Se(6) whereas a 5a × 5b × 5c supercell was found for CsNp(2)Se(6). All seven compounds exhibit Q-Q interactions and, depending on the radius ratio R(An)/R(A), disorder of the A cation over two sites. The electrical conductivity of RbU(2)Se(6), measured along [100], is 6 × 10(-5) S cm(-1) at 298 K. The interatomic distances, including those in the modulated structure of TlU(2)Se(6), and physical properties suggest the compounds may be formulated as containing tetravalent Th or U, but the formal oxidation state of Np in the modulated structure of CsNp(2)Se(6) is less certain. The actinide contraction from Th to U to Np is apparent in the interatomic distances.

Synthesis, structure, and electrical resistivity of Cs3U18Se38

“…This is supported by magnetic measurements [3]. In contrast, compounds with formulas AU 2 Q 6 generally contain some degree of Q-Q bonding [4]. The majority of compounds in the A/U/Q system have this formula.…”

“…3(d)). This coordination is found in the AAn 2 Q 6 compounds [4], and can be considered a slight distortion of a triangulated dodecahedron, found in the compounds MU 8 Q 17 (M ¼transition metal) [23,24]. The overall structure contains channels along the c axis, with the network around them composed of USe x coordination polyhedra.…”

“…AAn 2 Q 6 compounds crystallize in one of two structure types. AU 2 Se 6 (A ¼K, Rb, Cs) [4,6,7] crystallize in the KTh 2 Se 6 [8] structure type, in the orthorhombic space group Immm, as do RbTh 2 Se 6 [8], KNp 2 Se 6 , and CsNp 2 Se 6 [4]. CsTh 2 Te 6 crystallizes in a closely related structure [9] of the orthorhombic space group Cmcm, as does KTh 2 Te 6 [5].…”

“…The majority of compounds in the A/U/Q system have this formula. Other actinides crystallize in these structure types as well [4,5]. AAn 2 Q 6 compounds crystallize in one of two structure types.…”

“…Physical measurements suggest tetravalent uranium for these uranium compounds, but assigning such an oxidation state fails to achieve charge balance. The difficulties in assigning these oxidation states have been discussed in detail [4]. CsUTe 6 crystallizes in an unrelated chain structure and contains U in a tricapped-trigonal-prismatic coordination environment [10].…”