Tuning the structural stability and spin-glass behavior in α-MnO₂ nanotubes by Sn ion doping

A series of α-Mn_1-SnO₂ was synthesized by a simple hydrothermal method to shed light on the effect of substitution. Powder X-ray diffraction and scanning electron microscopy indicated that the particle size, crystal structure and morphology of the samples did not change with an increase of the Sn content. Sn, Mn, O and K elements were all uniformly distributed in the particles, which was observed using energy-dispersive X-ray spectroscopy. However, thermogravimetric analysis showed that the structural stability increased, and an increase of the Mn oxidation state from 3.8+ to nearly 4.0+ was observed by X-ray absorption spectroscopy. Besides, ¹¹⁹Sn Mössbauer spectroscopy revealed that the Sn ions are all 4+ and incorporate into the lattice by replacing the Mn ions. The DC and AC magnetic susceptibility measurements down to 2 K exhibited a spin-glass phenomenon, and the freezing temperature, T, decreased from 44 K to 30.5 K with increasing Sn content. This indicates that increased disorder by nonmagnetic substitution results in the enhancement of the frustration in the lattice. Meanwhile, with doping of Sn⁴⁺ ions, the Curie-Weiss temperature increased, indicating enhanced antiferromagnetic interaction. Although the mixed valence of Mn³⁺ and Mn⁴⁺ almost disappeared, the reduction of charge disorder did not lead to the magnetic ordering in the sample. Since the Sn⁴⁺ ions are diamagnetic and have the same magnetic effect as cation vacancies in the lattice, so it is reasonable to believe that the spin-glass transition in α-MnO₂ results from the cation vacancies rather than the mixture of Mn³⁺ and Mn⁴⁺.