Overview of the crystal chemistry of the actinide chalcogenides: Incorporation of the alkaline-earth elements

This review focuses on the results of exploratory syntheses of alkaline-earth-metal actinide chalcogenides Ak-An-Q (Ak = Ba, Sr; An = Th, U; Q = S, Se, and Te). About thirty new compounds are described. Although the basic building blocks of their structures are usually AnQ₆ octahedra and AkQ₈ bicapped trigonal prisms, these are combined in diverse ways to afford eleven new structure types. The structures reconfirm the prevailing presence of An⁴⁺ in chalcogenides, although some of the compounds discovered are mixed An⁴⁺/An⁵⁺ systems, and a few contain only An⁵⁺. The tendency of the chalcogens to form Q-Q bonds is again evident from the presence of S-S single bonds and infinite Te-Te-Te linear chains. The latter possess interatomic distances of lengths greater than that of a Te-Te single bond but less than that of a Te-Te van der Waals interaction. Assignment of formal oxidation states in compounds containing these chains is arbitrary at best. Addition of metal atoms (M) affords quaternary structures, some of which show remarkable flexibility in the positions of the An and M atoms, and in such compounds the nature of the M elements influences directly the dimensionality of the resultant structure. The presence of adventitious oxygen, often from etching of the fused-silica tubes by oxyphilic An elements, results in new quintary compounds that show remarkable structural variations with change of M. The compounds discussed have shown transport and electronic structures that range from metallic-like to semiconducting. We find, with the exception of BaUSe₃, when comparisons can be made that the values of the calculated band gaps are reasonably close but usually lower than the experimentally derived values. Thus the method used, in particular the HSE functional, has been generally successful on these 5f actinides. This is an important result because in the absence of suitable crystals, and hence experimental measurements, it still may be possible to offer credible predictions of some of the magnetic and electronic properties of the compounds.