Coordination model for the defect structure of hyperstoichiometric UO_2+x and U₄O₉

Abstract

Non-stoichiometric compounds or solids in which the ratios of the amounts of elements present are not integral may be characterized by the formula MX_x where x is non-integral and, in many crystalline compounds, may vary over quite a wide range without change of structure type. Ideally the thermodynamic stability and physical properties of such compounds should be related to the same atomic and electronic model but the ways in which various simple structures adapt to departures from ideal composition have not been adequately explored. Models based on a random distribution of point defects have proved successful but selected area electron diffraction and ultra high resolution electron microscopy have shown that their distribution is not simple. Interaction between defects produces microscopic sub-structures or clusters introducing specific local properties. Although our knowledge of the principal morphological features of sub-structures has advanced, their precise interatomic arrangements are generally unknown. Here we derive a model for non-stoichiometric oxides of uranium.