Neutron structural characterization, inversion degree and transport properties of NiMn2O4 spinel prepared by the hydroxide route

The title compound has been synthesized by the hydroxide route. 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 Fd3¯m, Z = 8, with a = 8.3940(2) Å at 295 K. The crystallographic formula is (Ni0.202(1)Mn0.798(1))8a(Ni0.790(1)Mn1.210(1))16dO4 where 8a and 16d stand for the tetrahedral and octahedral sites of the spinel structure, respectively. There is a significant inversion degree of the spinel structure, λ = 0.80. In fact, the variable parameter for the oxygen position, u = 0.2636(4), is far from that expected (u = 0.25) for normal spinels. From a bond-valence study, it seems that the valence distribution in NiMn2O4 spinel is not as trivial as expected (Ni2+ and Mn3+), but clearly the tetrahedral Mn ions are divalent whereas the octahedral Mn and Ni are slightly oxidized from the expected +3 and +2 values, respectively. The mixed valence observed at the octahedral sites provides the charge carriers that, by a hopping mechanism between Mn3+/Mn4+ adjacent sites, leads to a significant conductivity, up to 0.85 S cm−1 at 800 °C in air.

A pure specimen has been synthesized by the hydroxide route. This spinel, studied by NPD, shows an important inversion degree, λ = 0.80. A bond-valence study shows that the tetrahedral Mn ions are divalent whereas the octahedral Mn and Ni are slightly oxidized from the expected 3+ and 2+ values, respectively. The mixed valence Mn3+/Mn4+ accounts for a hopping mechanism between adjacent octahedral sites, leading to a significant conductivity.Figure optionsDownload as PowerPoint slideHighlights
► A low-temperature hydroxide route allowed preparing almost pure specimens of NiMn2O4.
► NPD essential to determine inversion degree; contrasting Ni and Mn for neutrons.
► Bond valence establishes valence state of octahedral and tetrahedral Ni and Mn ions.
► Thermal analysis, transport measurements complement characterization of this oxide.
► A structure–properties relationship is established.