Abstract

There are many different centres formed when uranium is incorporated into LiF or NaF in an oxidizing atmosphere.¹ Some of these centres are due to the presence of additional impurity ions, such as magnesium. It is also possible that lattice vacancies may be present. It was decided to calculate the formation energies of different models for centres in order to find the most likely candidate for the observed centres. In general, we require to calculate the interaction energies due to the presence of the defect and the lattice relaxations around the defects. It is assumed that changes in the electronic band structure due to the defect are negligible. This appears reasonable since screening and electronic relaxation will be negligible in an electronic insulator such as LiF. However the remaining calculations are still formidable as there are non-Coulomb short range interaction potentials as well as the Coulomb potentials. In order to make a preliminary survey of a large number of types of centre calculations were made using the Coulomb potential only and without lattice relaxation. Ions are assumed to substitute for lattice ions and are assigned the conventional ionic charges. Using the Madelung constant for the alkali halide lattice the calculations can be reduced to simple hand calculations. In practice we used a programme which could also cope with non-Coulomb potentials.² There is a problem in finding suitable short-range potentials and this more than complexity of calculation was the reason for their initial neglect. Calculations were made for 57 varieties of uranium centres in lithium fluoride which showed local charge compensation.

© 1984 Optical Society of America