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

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posted on 20.09.2010, 00:00 authored by Daniel E. Bugaris, Daniel M. Wells, Jiyong Yao, S. Skanthakumar, Richard G. Haire, L. Soderholm, James A. Ibers
The solid-state compounds CsTh2Se6, Rb0.85U1.74S6, RbU2Se6, TlU2Se6, Cs0.88(La0.68U1.32)Se6, KNp2Se6, and CsNp2Se6 of the AAn2Q6 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 KTh2Se6 structure type. Evidence of long-range order and modulation were found in the X-ray diffraction patterns of TlU2Se6 and CsNp2Se6. A 4a × 4b supercell was found for TlU2Se6 whereas a 5a × 5b × 5c supercell was found for CsNp2Se6. All seven compounds exhibit Q−Q interactions and, depending on the radius ratio RAn/RA, disorder of the A cation over two sites. The electrical conductivity of RbU2Se6, measured along [100], is 6 × 10−5 S cm−1 at 298 K. The interatomic distances, including those in the modulated structure of TlU2Se6, 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 CsNp2Se6 is less certain. The actinide contraction from Th to U to Np is apparent in the interatomic distances.

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