Study On The Defect Structure Of Solid Ionic Conductor Based On Tetrahedron By Solid State Nuclear Magnetic Resonance Spectroscopy

Defect structure are ubiquitous in solid materials including solid oxide fuel cell?SOFC?electrolyte materials.The characterization of defect structure and the relationship between defect structures and physical properties are the most difficult aspects in SOFC electrolyte materials research.The content of the defect structure is generally low.Diffraction method can only give the average structure,where the defect structure information is hidden.So it is difficult to extract the defect structure.The solid state nuclear magnetic resonance spectra are sensitive to local structures,and thus can be used for defect structure characterization.However the interpretation of solid-state NMR signals is mostly based on empirical knowledge,which is usually not accurate.In this paper,the defect structures in the medium temperature SOFC electrolyte materials La_1-xCa_xPO_4-0.5x,La_9.33+xSi₆O_26+1.5x and La_1-xBa_1+xGaO_4-0.5x systems with isolated tetrahedral structure are studied through combined solid-state nuclear magnetic resonance experiments spectrum and density functional theory?DFT?calculations.The major results of this paper are classified as the following three aspects:?1?For the La_1-xCa_xPO_4-0.5x system,the ³¹P NMR spectra of the samples treated with and without hydration showed similar results.The structural simulation of different models for La_0.958Ca_0.042PO_3.979 shows that the ³¹P chemical shielding value of³¹P in the P₂O₇ group of the defect structure is larger than that of ³¹P in the PO₄ group,and the chemical shift is in the relatively higher field direction.The comparison between the experimental results and the results of density functional theory shows that there is a defect structure in La_1-xCa_xPO_4-0.5x,and the chemical shift of ³¹P in HPO₄^2-is lower than that in PO₄^3-.In the experiment,the La_1-xCa_xPO_4-0.5x samples without hydrated treatment have a low doping content of Ca substitute La,and the H₂O in the air enters?P₂O₇?^··groups in the defect structure of La_1-xCa_xPO_4-0.5x,which transform to?HPO₄?^·units.Therefore,the ³¹P NMR spectra of La_1-xCa_xPO_4-0.5x samples with and without hydration showed similar results.?2?For the La_9.33+xSi₆O_26+1.5x system,the ²⁹Si NMR experimental spectra show that there are additional peaks near the main peak,that cannot be attributed to the bulk structure.We performed a DFT calculation on the La₁₀Si₆O₂₇ structure and found that the ²⁹Si chemical shift with SiO₅ groups appeared near-120 ppm.However there was no signal peak near-120 ppm in the ²⁹Si NMR experimental spectrum of La_9.33+xSi₆O_26+1.5x.We speculate that there is no steady-state SiO₅ group in the La_9.33+xSi₆O_26+1.5x system,and the signal peaks in the sub-strong peak region and the weakest peak region next to the main peak in the ²⁹Si NMR spectra may be ²⁹Si resonance signal peak of transient SiO₅ defect structure.?3?For the La_1-xBa_1+xGaO_4-0.5x system,the experimental results show that the solid solution limit is x=0-0.2.When the doping amount of La exceeds the solid solution limit,the second phase La₂Ba₆Ga₄O₁₅ is generated.In the ⁷¹Ga NMR spectra of La_1-xBa_1+xGaO_4-0.5x,no effective signal peak were collected.It was found that raw material Ga₂O₃ with different configurations and different instrument parameters has no significant influence on whether the ⁷¹Ga NMR signal peaks can be collected in La_1-xBa_1+xGaO_4-0.5x system under the 9.4 T magnetic field.Density functional theory calculations of LaBaGaO₄ show that the chemical shift of ⁷¹Ga is 260.158 ppm.Combined with the experimental results and density functional theory calculations,we hypothesized that due to the spin quantum number ⁷¹Ga I=3/2,⁷¹Ga is a quadrupolar nuclear.Affected by the second-order effect of quadrupolar,⁷¹Ga of nuclear magnetic resonance signal could be wide thus it is difficult to successfully capture ⁷¹Ga signal of LaBaGaO₄.Characterization under higher magnetic field,could be necessary as the second-order effect of quadrupolar will be weakened under high magnetic field intensity,which indirectly increase ⁷¹Ga NMR spectrum signal intensity.