Effects of Zn Substitution on Structure Factors, Debye-Waller Factors and Related Structural Properties of the Mg<sub>1-x</sub>Zn<sub>x</sub>FeNiO<sub>4</sub> Spinels

Abstract

The effects of Zn 2+ ions substitutions on the Debye-Waller Factors, structure factor and other related structural properties of the Mg 1-x Zn x NiFeO 4 (where 0.0≤x≤1.0) spinels have been investigated using the XRD, TEM, SEM and FT-IR tools. The Mg 1-x Zn x NiFeO 4 samples were prepared using the conventional ceramic solid state sintering techniques at temperatures around 1100°C. The Mg 1-x Zn x NiFeO 4 spinels have predominantly inverse type structure with inversion factor, λ in the range 0.69 to 0.36. The X-ray diffraction (XRD) patterns of all compositions showed the formation of cubic spinel structure. The lattice constant "a" increases from 8.3397Å for MgFeNiO 4 to 8.3855Å for ZnFeNiO 4 spinels. The increases in lattice parameters have been attributed to the replacement of small Mg 2+ ions (0.66 Å) with the Zn 2+ (0.74 Å) ions of a larger ionic radius. The IR spectra confirm the existence of two main absorption bands υ 1 and υ 2 in the frequency range of (400-1000 cm-1), arising due to the tetrahedral (A) and octahedral (B) stretching vibrations respectively. Values of both υ 1 and υ 2 decrease as Zn content increases. The scanning electron microscope (SEM) and transmission electron microscope (TEM) images showed aggregates of stacked grains. The normalized XRD intensities of the main (hkl) planes were used in the estimation of the Debye-Waller factor. Values of the Debye-Waller factors were estimated to be in the range (0.77-1.44A 2). The calculated and observed relative intensities and areas of the most related plains to cation distributions (i.e.: the (220), (311), (222), (400), (422), (511) and (440) plains) were obtained by normalizing with respect to the most intensive reflection from the (311) plane. An inverse relation between the ordering, Q and inversion, λ factors exists in these partially inverse spinels. Both Q and λ decrease as Zn content (x) increases in the sample. The cation distributions indicate that the sample, MgFeNiO 4 with x=0, λ=2/3 and maximum configurational entropy Sc(=15.876 J/mol, K) should represents the sample of the complete randomness of cation distributions in these spinels and can be written as (Mg 1/3 Fe 2/3)[Mg 2/3 Fe 1/3 Ni 3/3)O 4. In general the variation of the different structural parameters with Zn content lie on two different regions, the first region for x values (0.0-0.6) the "highly normal" and the second region for x values (0.6-1.0) the "highly inverse" type structure.