Structure and Luminescence Properties of U⁶⁺ Centers in Alkali and Alkaline Earth Fluorides

Abstract

Alkali and alkaline earth fluoride crystals doped with uranium and grown in air show luminescence emission due to transition between the molecular orbitals of U⁶⁺complexes. Investigations including polarized luminescence, pizospectroscopy, Stark effect, Zeeman effect, MCD, have established some important properties of these centers (symmetry, nature of emitting dipoles, nature of some levels) but they could not help us in deciding various structural models based on uranyl or uranate groups (UO₅F or UO₆in alkaline-earth fluorides). Since all the U⁶⁺complexes have a strong electron affinity, they could be transformed in U⁵⁺complexes by irradiating the crystal; the electrons created in this way are trapped in the first excited state of the center, which has predominant uranium 5f character without changing the symmetry and structure of the centers. By a combined study of the U⁵⁺complexes by EPR and of the luminescent properties of the parent U⁶⁺complexes some important conclusions are apparent: i) In alkali fluoride crystals the main luminescent U⁶⁺center has the structure UO₅F, ii) a variety of UO₆centers could be observed for which the extra-charge compensation is accomplished by substitutional cations, iii) in some circumstances, associations between the U⁵⁺complexes with defects created by irradiation have been observed, and iv) in CaF₂the main luminescent center is a trigonally distorted UO₆octahedron which replaces the cube CaF₈. A connection between the structural and luminescent properties of the U⁶⁺complexes was made. In connection with these results, the general problem of lasing in uranium compounds was briefly discussed.