Abstract
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.