Structural and electronic properties of Fe 3+ -O 2− Pairs in KMF 3 lattices (M=Mg,Zn): Is the superposition model valid?

The geometrical and electronic properties of the tetragonal FeF 5 O 4 m complex which is likely formed in KMgF 3 and KZnF 3 fluoroperovskites have been investigated by means of Density Functional calculations. The properties of this centre are dependent on the Fe-O distance ( R ox ) and also on the distances R eq and R ax between Fe 3+ and the four equatorial F m ions and the apical F m ion respectively. The calculated equilibrium distance for the isolated Fe 3+ -O 2 m pair is equal to R ox =1.63 v + while when it is introduced in KMF 3 lattices ( M =Mg, Zn) R ox ～1.82 v + . Compared to FeF 6 3 m in KMgF 3 ( R eq = R ax =1.94 v + ) the obtained equilibrium distances R ax =2.05 v + and R eq =2.02 v + mean an outwards relaxation of 6% and 4% respectively induced by the F m M O 2 m substitution. The ratio of the associated superhyperfine ( shf ) constants of equatorial ( A eq ) and axial ( A ax ) F m ligands is A eq / A ax , 2 and thus account for an experimental fingerprint of this centre. The huge difference between A s (eq) and A s (ax) cannot be explained assuming that the law relating A s to the metal-ligand distance, R , for the octahedral FeF 6 3 m unit can be transferred to the present case involving a lower symmetry . The present analysis casts doubts on the conclusions previously reached using the empirical superposition model.