Abstract

Manganese nickel oxides with a spinel-related structure, NixMn3-xO4-delta (x = 0.5 or 0.75), were synthesized by an ultrasound-assisted sol-gel method. Inductively coupled plasma atomic optical emission spectroscopy and energy-dispersive X-ray spectroscopy analysis showed good agreement with the nominal oxide compositions. The Rietveld refinement of the X-ray data indicated that both Ni0.5Mn2.5O4-delta and Ni0.75MN2.25O4-delta crystallize in the Fd3m space group, having cell parameter a of 0.8439 and 0.8392 nm, respectively. Scanning electron microscopy images showed agglomerated particles with sizes ranging from 300 nm to 1.4 mu m, approximately. The magnetization curves showed paramagnetic behavior at room temperature, whereas ferromagnetic behavior was observed at 5 K. Zero-field-cooling and field-cooling magnetization measurements indicated the presence of three magnetic transitions, at T-C = T-1 = 107 K, T-2 = 98 K, and T-3 = 54 K for Ni0.5Mn2.5O4-delta and at T-C = T-1 = 101 K, T-2 = 93 K, and T-3 = 68 K for Ni0.75MN2.25O4-delta. X-ray photoelectron spectroscopy analysis revealed the presence of Ni2+, Mn2+, Mn3+, and Mn4+. The results provided by the different techniques allow us to propose the following cation distributions for these oxides: Mn0.82+Mn0.23+[Ni0.52+Mn1.043+Mn0.464+]O-3.98(2-) and Ni0.032+Mn0.972+[Ni0.722+Mn0.663+Mn0.624+]O-3.95(2-).