A REVISED AND EXPANDED STRUCTURAL HIERARCHY OF INORGANIC ACTINIDE COMPOUNDS

The behaviour of actinide elements (particularly Th, U, Np and Pu) in natural systems is of widespread interest, as some may occur naturally and all may occur as toxic environmental contaminants. Over the past three decades, the number of known inorganic oxide and oxy-salt phases containing stoichiometric quantities of the actinide elements has increased from a few dozen to well over 1000, with phases containing hexavalent uranium being by far the most abundant (totaling >700 of which ~100 are minerals). The entirety of these structures shows truly remarkable compositional variety (with nearly every element being represented) and topological diversity. Organizing crystal structures on a hierarchal basis is an effective way of distilling the information contained in a large number of structures. In a hierarchal classification, structures are divided into categories on the bases of topologically-similar features, defined by the polymerization of high-valence cations; for the actinides, these categories are isolated clusters, finite clusters, chains, sheets and frameworks. Our current work expands upon the previously-constructed hierarchies for actinide compounds (e.g., U⁶⁺, Burns 1999, 2005; Np^5/6+, Forbes et al. 2008) by considering the >400 as yet unclassified phases. We place particular emphasis on the U⁶⁺ phases, as they have nearly doubled in number since the publication of the most recent classification scheme and show the greatest number of novel structure types. Where relevant, we compare the structures of non-U phases (particularly Np and Pu) to these. For U⁶⁺ phases, the current breakdown of compounds is as follows: clusters (111 phases, >20 topologies), chains (78, >30), sheets (330, ~70), and frameworks (194, >50). Within each category, structures are separated according to topological details (e.g., chains consisting of just uranyl polyhedra vs. those consisting of multiple cations).