Description and classification of uranium oxide hydrate sheet anion topologies

The sheets of uranyl ions (U⁶⁺O₂)²⁺in the structures of all uranyl oxide hydrates (UOH) (and the structurally related α- and β-forms of U₃O₈) are based on only four structural unit chains. Each sheet type may be reduced to its underlying sheet anion topology to determine the chains present within each topology and to describe the structural relationships among these phases. Each sheet type is described by a chain stacking sequence. The four chain types required to construct the UOH sheet anion topologies are theH-chain, theR-chain, theP-chain, and the directional “arrowhead” chains denoted byUandD. TheH-chain is found only in the sheet anion topology of α-UO₂(OH)₂and consists of hexagonal sites sharing opposing edges. In α–UO₂(OH)₂, all hexagons are populated with uranyl ions. The “arrowhead” chain is composed of pentagonal sites populated with uranyl ions and sharing edges and alternating with vacant triangular sites. Arrowhead chains are present in the sheet anion topologies of all other UOH sheets. Arrowhead chains are directional and can occur in bothUandD“senses” within a single anion topology. The P-chain consists of edge-sharing pentagonal sites populated with uranyl ions forming a zigzag chain. TheP-chain is flanked on both sides by arrowhead chains of the same “sense”. The remaining structural unit is a discontinuous “chain” of rhombic sites. This “R-chain” is produced when nested; adjacentUandD“arrowhead” chains are translated diagonally. TheR-chain occurs in the sheet anion topologies of sheets which contain only 4-coordinate uranyl ions and those containing both 4- and 5-coordinate uranyl ions. The rhombic sites may be populated with a uranyl ion, a U⁴⁺or other cation and two apical oxygens, or they may be vacant.