Relaxations and their strain derivatives around impurity ions in MgO

Calculations of the relaxations of oxygen neighbours around Mn n+ , Fe n+ , Co n+ and Ni n+ impurities (n=2, 3 and 4) in MgO are presented. The values are used in an attempt to account for the enhancement of coupling to T 2u symmetry vibrations when the impurity carries excess charge. Although the correct trend is reproduced the point-ion coupling model fails to give the observed magnitude of the effect. Local hydrostatic, tetragonal and trigonal strains are related to the corresponding bulk strains. It is shown that for divalent impurities the fractional local strains are close to those in the bulk crystal but when the impurity carries extra charge large and symmetry-dependent reductions arise.