Why MnIn2O4 spinel is not a transparent conducting oxide?

The title compound has been synthesized by a citrate technique. The crystal structure has been investigated at room temperature from high-resolution neutron powder diffraction (NPD) data. It crystallizes in a cubic spinel structure, space group Fd3̄m, Z=8, with a=9.0008(1) Å at 295 K. It exhibits a crystallographic formula (Mn0.924(2)In0.076(2))8a(In1.804(2)Mn0.196(2))16dO4, where 8a and 16d stand for the tetrahedral and octahedral sites of the spinel structure, respectively, with a slight degree of inversion, λ=0.08. MnIn2O4 shows antiferromagnetic interactions below TN≈40 K, due to the statistical distribution of Mn ions over the two available sites. Unlike the related MgIn2O4 and CdIn2O4 spinels, well known as transparent conducting oxides, MnIn2O4 is not transparent and shows a poor conductivity (σ=0.38 S cm–1 at 1123 K): the presence of Mn ions, able to adopt mixed valence states, localizes the charges that, otherwise, would be delocalized in the spinel conduction band.

Graphical AbstractFrom NPD data the crystallographic formula (Mn0.924(2)In0.076(2))8a(In1.804(2)Mn0.196(2))16dO4, shows a slight degree of inversion, λ=0.08 and a certain In deficiency. The presence of Mn ions, able to adopt mixed oxidation states, localize the charges that, otherwise, would be delocalized in the spinel conduction band; the presence of localized Mn2+ and Mn3+ ions provides the characteristic brown color.Figure optionsDownload as PowerPoint slideHighlights
► Accurate structural determination from NPD data: inversion degree (8%), and In deficiency.
► Bond-valence indicates Mn2+–Mn3+ ions; edge-sharing octahedra contain 90% In3++10% Mn3+ cations.
► Conductivity several orders of magnitude lower than those of MgIn2O4 or CdIn2O4.
► Variability of Mn oxidation states cancels any electron-doping effect, emptying conduction band of mobile charge carriers.
► Curie–Weiss behavior confirming the determined charge distribution.