The Study On Local Structure And Conduction Mechanism Of CeO₂-Based Oxygen Ion Conductor By Sm³⁺ Photoluminescence Spectroscopy

Doped ceria materials have important application prospects in the fields of oxygen permeable film,catalysis,solid oxide fuel cells and other fields due to their high oxygen ion conductivity,chemical and thermal stability.Its excellent properties mainly stem from the regulation of local crystal structure after various ions doping,so it is particularly important to accurately and sensitively detect its local structure.However,due to the introduction of defects such as oxygen vacancies（V₀^·）,acceptor ion doping always leads to materials that do not meet the conditions for uniform and uniform local structures.Therefore,traditional techniques such as X-ray diffraction（XRD）based on the diffraction principle cannot obtain the true local structure of the sample,and can only obtain an average result.Considering that Sm³⁺is one of the best dopants in the field of oxygen ion conductors,based on the photoluminescence（PL）spectra of Sm³⁺,this paper has studied the local structures in Sm³⁺doped ceria（SDC）materials.On this basis,the structural origin mechanism of concentration quenching of grain conductivity and the structural origin of the differences in grain conductivity when Sm³⁺and Gd³⁺are co-doped have been explained.The main research results are as follows:First,regarding the sensitive detection of the local structure of SDC.（1）SDC powders with different Sm³⁺doping concentrations at extremely low concentrations were prepared by the glycine nitrate method（GNP）.（1）Scanning electron microscopy（SEM）shows that the average grain size is about 50 nm and almost does not vary with doping concentration.（2）Raman spectroscopy shows that when the grain size is tens of nanometers,the F_2gpeak shifts towards a higher wave number and the half peak width decreases with the increase of the grain size.When the grain size is greater than 100 nanometers,the position and half peak width of F_2gdo not change with the grain size.（3）The PL spectrum further confirms the conclusion of the Raman spectrum,and it is also found that with the increase of Sm³⁺doping concentration,V₀^·is more likely to migrate from around Sm³⁺to around Ce⁴⁺.（2）Solid State Reaction（SSR）method has been used to prepare SDC powders with different calcination temperatures.（1）SEM shows that calcination at high temperatures below 800℃does not significantly change the average grain size of the powder（～50 nm）.When the calcination temperature is higher than 1000℃,the grain size increases significantly（from～50 nm to～1nm）as the calcination temperature increases.（2）Raman spectroscopy shows that thermal diffusion hardly promotes Sm³⁺doping into CeO₂lattice when the calcination temperature is below 1000℃,and there is almost no defect association;When the calcination temperature is higher than 1200℃,Sm³⁺will be significantly doped into the CeO₂lattice,resulting in enhanced defect association.（3）PL spectra showed that Sm³⁺gradually transited from"surface doping"to"bulk doping"with the increase of calcination temperature from 0℃to 800℃;From 800℃to 1200℃,more Sm³⁺enters the CeO₂lattice,andV₀^·is more easily located around Sm³⁺;Above 1200℃,more Sm³⁺enters the CeO₂lattice,introducing a large amount ofV₀^·,which will expand the CeO₂local lattice,thereby offsetting the distortion of the Sm³⁺local environment caused by"V₀^·and Sm³⁺association.".Second,the concentration quenching mechanism of grain conductivity was studied.SDC ceramic samples with different doping concentrations were prepared using GNP.The results showed that:（1）the crystal cell parameters increased with increasing doping concentration,while the average grain size decreased with increasing doping concentration.（2）The PL spectrum shows that when the Sm³⁺doping concentration is lower than 1%,with the increase of the doping concentration,the newly introduced V₀^·prefer to be located around Sm³⁺;When the doping concentration is higher than 5%,the number of V₀^·around Sm³⁺increases significantly as the doping concentration increases,resulting in"mutual squeezing between V₀^·".（3）The main reason for the concentration quenching of grain conductivity is that"after the total concentration ofV₀^·increases with the doping concentration,there is no enough space to help sharing the local distortion generated by each V₀^·",resulting in too large local distortion around eachV₀^·,which increases the enthalpy of migration.The ordering and formation ofV₀^·microregions further exacerbates this effect.Finally,from the perspective of differences in local structures,the reasons for the differences in grain conductivity when Sm³⁺and Gd³⁺are co-doped were studied.Sm³⁺and Gd³⁺co-doped ceria based ceramic samples with different doping ratios were prepared by GNP.The results shows that:（1）With the increase of Sm³⁺ratio,the crystal cell parameters and grain size increased.（2）The coordination preference of V₀^·is as follows:Sm³⁺>Ce⁴⁺>Gd³⁺,Sm³⁺has a more significant cell expansion than Gd³⁺,which reduces the³⁺-V₀^·spacing and decreases the defect association enthalpy.At the same time,the larger cell expansion caused by Sm³⁺is also beneficial to reducing the V₀^·migration enthalpy;Under the same defect association degree and lattice distortion,the electronegativity difference will make the migration enthalpy of Sm³⁺doping larger.