Fe–Mg substitution in aluminate spinels. Effects on elastic properties investigated by Brillouin scattering

We investigated by a multi-analytical approach (Brillouin scattering, X-ray diffraction and electron microprobe) the dependence of the elastic properties on the chemical composition of six spinels in the series (Mg1−x,Fex)Al2O4(0 ≤ x ≤ 0.5). With the exception of C12, all the elastic moduli (C11, C44, KS0and G) are insensitive to chemical composition for low iron concentration, while they decrease linearly for higher Fe2+content. Only C12shows a continuous linear increase with increasing Fe2+across the whole compositional range under investigation. The high cation disorder showed by the sample with x = 0.202 has little or no influence on the elastic parameters. The range 0.202 < x < 0.388 bounds the percolation threshold (pc) for nearest neighbor interaction of Fe in the cation sublattices of the spinel structure. Below x = 0.202, the iron atoms are diluted in the system and far from each other, and the elastic moduli are nearly constant. Above x = 0.388, Fe atoms form extended interconnected clusters and show a cooperative behavior thus affecting the single-crystal elastic moduli. The elastic anisotropy largely increases with the introduction of Fe2+in substitution of magnesium in spinel. This behavior is different with respect to other spinels containing transition metals such as Mn2+and Co2+.