119Sn Mössbauer spectroscopy of Sn–O nanoparticles prepared by levitation-jet aerosol synthesis
119Sn Mössbauer spectroscopy of Sn–O nanoparticles prepared by levitation-jet aerosol synthesis. Kuznetsov M.V., Pankratov D.A., Morozov Yu.G., Parkin I.P., Safonov A.V., Belousova O.V. //Mendeleev Communications. 2021. V.31. №6. P.884-886.
Sn–O nanoparticles were prepared by levitation-jet aerosol synthesis and found to exhibit ferromagnetic behavior. X-ray powder diffraction analysis and 119Sn Mössbauer spectroscopy confirmed these nanoparticles consist of β-Sn, SnO and SnO2 phases. The maximum specific magnetization was observed for nanoparticles containing the SnO/SnO2 interface.
Tin dioxide (SnO2) is an n-type semiconductor with a wide bandgap and a pronounced potential in spintronic applications. Undoped SnO2 nanoparticles (NPs) were studied by various methods, including magnetic measurements, which revealed a low value of the maximum saturation magnetization at room temperature and convincingly indicated that room-temperature ferromagnetism is an intrinsic feature of NPs related to structural defects. This work used a levitation-jet generator to prepare Sn–O NPs and applied 119Sn Mössbauer spectroscopy to deepen such investigations.
In the region of low velocities of the Mössbauer spectra of the samples, resonance absorption of a complex shape was observed, which indicates that the experimental spectrum is a sum of partial subspectra. At the same time, the shape and intensity of the spectra are susceptible to temperature changes. Obviously, with a temperature change, the partial subspectra change their intensity independently of each other, which is the reason for the change in the shape of the entire spectrum. The spectra were described by a set of three subspectra of SnO2, SnO and β-Sn. The parameters of the first subspectrum were reliably determined and were relatively stable in all the samples. The other two subspectra are distinguished by similar values of isomeric shifts, which, due to the large width of the resonance lines, the small particle size and the high defectiveness of the structures, made it difficult to determine them reliably. In these cases, the parameters of the minor subspectra were fixed based on the data for sample T5, where the SnO parameters can be determined relatively reliably. The obtained Mössbauer parameters with the assumption of a temperature shift agree with the literature data. In the Mössbauer spectra of samples T2, T4 and T5 at room temperature, among other parameters, extended absorptions described by a very wide singlet were observed. They can be associated with the intermediate oxide (IO), the presence of NPs or both (taking into account the strong temperature dependence of the magnitude of this effect).
It is known that, due to the low Debye temperature, metallic β-tin exhibits a strong temperature dependence of the recoil-free fraction f in the Mössbauer effect (0.75, 0.446 and 0.039 at 4.2, 80 and 300 K, respectively). Moreover, for tin nanoparticles, an even more significant decrease in the f value can be observed. The recoil-free fraction f for SnO does not change so much with temperature (from 0.3 at 295 K to 0.8 at 4.2 K), but it is significant enough not to take it into account when determining the relative content of the components. For crystalline SnO2, the temperature dependence of the recoil-free fraction f is not very large (from 0.56 at 295 K to 0.77 at 4.2 K). However, for the amorphous and hydrated forms, it increases again. For the samples under discussion, even for the SnO2 subspectrum, the temperature dependence of the intensity of the resonance lines turned out to be noticeably stronger than could be expected based on the literature data for the recoil-free fraction f. This discrepancy may indicate a decrease in the recoil-free fraction f for nanosized particles of material, including SnO2. All this is the reason for the strong temperature dependence of the intensity of the corresponding subspectra. Hence, the most reliable data on the relative content of components in the samples under study are the data of low-temperature measurements. Therefore, even though the relative areas of the subspectra are significantly dependent on temperature, the relative areas of the subspectra determined at low temperatures are in good agreement with the XRD data.
In summary, it was demonstrated that ferromagnetic Sn–O nanoparticles prepared by the levitation-jet aerosol synthesis exhibit Mössbauer spectra with three main components: β-Sn, SnO and SnO2 and have the maximum specific magnetization for nanoparticles containing the SnO/SnO2 interface. We assume that certain localized states present at this interface, such as interacting oxygen vacancies, are responsible for the observed magnetic behavior of the nanoparticles under study.
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