Synthesis and Properties of Layered-Structured Mn₅O₈ Nanorods

Mn 5 O 8 nanorods were prepared by a topotactic conversion of γ-MnOOH nanorod precursors in nitrogen at 400 °C. The as-prepared Mn 5 O 8 nanorods crystallized in a monoclinic structure (space group C 2/ m ) with unit cell dimensions a = 10.3784(2) Å, b = 5.7337(7) Å, c = 4.8668(6) Å, and β = 109.491(6)°, having a compositional formula Mn 2 2+ Mn 3 4+ O 8 . The structure allowed 18 Raman-active modes (10 A g + 8 B g ); 10 of these contributions were observed at 262, 300, 391, 429, 475, 533, 576, 615, 647, and 789 cm −1 . An intensive A g mode at 647 cm −1 was identified, representing a clear signature for probing the Mn 5 O 8 materials via Raman scattering. X-ray photoelectron spectroscopy studies revealed the distinctive spectral features of the Mn 5 O 8 due to the coexistence of divalent and tetravalent Mn ions. Magnetic measurements confirmed further that Mn 5 O 8 was a mixed valence oxide with an antiferromagnetic transition at about 133 K. The decreased Néel temperature of the Mn 5 O 8 nanorods suggested the possible presence of the finite size effect, which accounted also for the red-shift of the corresponding Raman bands in comparison with those of the bulk counterparts.