Study On Structure And Ferromagnetism Of Metastable Tin Oxide

Stannic oxide（r-SnO₂）is a versatile electronic material.As an n-type semi-conductor with a direct band gap of 3.6 eV,it is widely used in transparent conductor,gas sensors and catalysts.Stannic oxide has rutile structure at ambient condition.However,a scrutinyite（α-PbO₂）structure has also been identified as s-SnO₂ transformed by r-SnO₂ under high pressure and elevated temperature.The latter,metastable phase mixed with r-SnO₂ can also be synthesized by oxidizing nanocrystalline tin monoxide,or epitaxial stannous oxide（SnO）film.Small variation in structure between two kinds of tin oxide would result in significant difference in properties.The loose building blocks of Sn-O₆ octahedron in s-SnO₂ provide more oxygen vacancies,making it with better properties in gas sensing and catalytic applications.In this paper,we will report our study on mixed phases of SnO₂,obtained by grinding micron-sized SnO into nano particles mechanochemically,followed by thermal processing in vacuum or air.Small amount of oxygen vacancies was found in ball-milled sample by electron paramagnetic resonance experiments.A relaxation was found related to the motion of oxygen ions at elevated temperature,indicating a transformation from static disorder to dynamic disorder of oxygen sublattice.We relied on solid-state ¹¹⁹Sn nuclear magnetic resonance to characterize the thermal processed samples and found a resonance peak at-615 ppm corresponding to s-SnO₂ for the first time.Through low spin-rate magic-angle-spinning experiments can we obtain the anisotropic chemical shifts of ¹¹⁹Sn in those phases.The assignments can be verified along with first-principles method.It was also reported for the first time that vacuum-annealing can create local structure similar to s-SnO₂ in nano-SnO through disproportionation reaction.A semi-empirical relationship between chemical shifts of ¹¹⁹Sn in different Sn-O compounds and their coordination parameter was concluded through our results and some data from published works.Annealing in vacuum can not only lead to disproportionation reaction of nano-SnO,but also introduce defects into the lattice,so as to generate room temperature ferromagnetism.Since the room temperature ferromagnetism of undoped hafnium oxide films was reported in 2004,the defect induced ferromagnetism in nano semiconductor has attracted much attention in the last decade or so because of its unique characteristics and potential application in spintronic devices.Nanocrystalline SnO powder has been prepared by ball-milling of micron-SnO precursor in a nitrogen atmosphere.A cation deficiency of 6.5%was found in the nano-SnO sample by X-ray photoelectron spectroscopy.After annealing at 350℃,450℃,or 550℃ under dynamic vacuum conditions,oxygen vacancies were found in the nano-SnO samples,as verified by electron paramagnetic resonance experiments.The samples displayed weak ferromagnetism;that with both 6.5%Sn and at least 7.5%O vacancies showed the maximum saturation magnetization,reaching almost 1.7×10^-3 emu/g at room temperature.The ferromagnetism induced by defects in nano-SnO is explained by the first-principles method.By establishing the defect model in the（3 × 3 × 2）supercell and carrying out theoretical calculation,we found that when tin vacancies exist in the lattice of SnO,oxygen vacancies tended to generate and gather around them.An L-shaped defect model composed of one tin and two oxygen vacancies has been established,and its ferromagnetism has been calculated by a first-principles method.We found that the L-shaped defect induced a magnetic moment of 0.55 μB in the SnO lattice,the main contribution to which came from a spin-polarized electron in the porbital of a corner Sn atom.