Ab Initio Simulation of the Short-Range Order Structure of Al Ions and Substitutional Fe,Cr Impurities in S.S.Spinel (MgAl_20_4)

摘要

The structure of the short-range surrounding of aluminum,ferric iron and chromium octahedrally coordinated cations in s.s.spinel MgAl_2O_4:(Fe,Cr) has been established in the framework of the cluster model.The total energy minimum of [~(VI)Me_(cent)~(IV)Mg_6~(VI)Al_6O~(38)]~(43-) clusters (Me_(cent) = Al,Fe,Cr) is realized when oxygens of the first coordination sphere of the central cation ~(VI)Me are displaced considerably relative to the initial ideal structure.The displacement depends on the central cation of the cluster.The estimated short-range order parameters including the oxygen parameter u of the spinel are in satisfactory agreement with experimental data.Comparable analysis of the electronic structure of Al,Fe,Cr cations in octahedral positions of the spinel in ideal and relaxed clusters showed that octahedral distortions should be taken into account when interpreting spectroscopic parameters of the natural spinels and evaluating the charge balance in their Structure.