Features of actinide contraction in AnХ crystals (X = S, Se or Te)

Using Voronoi–Dirichlet polyhedra, the features of actinide contraction in chalcogenides AnX, where An is Th, U, Np, Pu, Am, or Cm, and X is S, Se, or Te, were revealed and discussed. The characteristics of AnX and isostructural chalcogenides LnX, where Ln is lanthanide, were compared. It was found that, in contrast to LnX, which exhibit the effect of lanthanide contraction, in AnX crystals with increasing atomic number of An some characteristics change nonlinearly: in many cases the volume of Voronoi–Dirichlet polyhedra of An atoms as well as the length of An–X bonds and An–An contacts increase instead of decreasing. It was shown that the detected anomalies can be considered a consequence of the bonding 5f interactions, which are characteristic of U, Np, and Pu atoms, but are absent in the case of Th and transplutonium actinides. Using AnX and LnX as examples, a crystal-chemical method for evaluating the covalence of An–X and Ln–X bonds has been tested.