Crystal chemistry,magnetic, and electrical properties of hexagonal plutonium oxide chalcogenides: Pu2O2X, X = O,S, Se

A study was made of the influence of the chalcogen X on the magnetic and electrical properties of Pu₂O₂X (X = O, S, Se) compounds and on bonding in these hexagonal compounds, isostructural with the corresponding rare-earth compounds. The comparison of the cell volumes of Nd₂O₂X and Pu₂O₂X compounds showed that the Pu³⁺ crystal radius decreased from β-Pu₂O₃ to Pu₂O₂Se as “5f delocalization” and 5f-p overlap increased. These plutonium compounds were all found to be antiferromagnetic. This was caused by superexchange coupling interactions via p orbitals of the oxygen and chalcogen ions. The Néel temperature increased from β-Pu₂O₃ (26 K) to Pu₂O₂S (28 K) then to Pu₂O₂Se (34 K) showing that 5f-p covalency was enhanced as the chalcogen electronegativity decreased and the p radial extent increased. The hexagonal β-Pu₂O₃ was found to be an insulator while both other compounds were semiconductors with gaps around 0.5 eV. These gaps were interpreted as the energy separation between the 6d-7s conduction band and the np band of the chalcogen X with some overlap of the occupied 5f states and the np band. A simple electron band scheme of these compounds is proposed on such assumptions.