Phase Transition and Oxygen Ion Diffusion in （La1-xLnx）2Mo2O9 （Ln=Nd, Gd, x=0.05-0.25） Using Dielectric Relaxation Method

Dielectric relaxation method was employed to study the properties of oxygen ion diffusion and phase transition in the oxide-ion conductors （Lal-xLnx）2Mo209 （Ln=Nd, Gd, x=0.05-0.25）. Two dielectric loss peaks were observed： peak Pd at about 600 K and peak P5 around 720 K. Peak Pd is a relaxational peak and associated with the short-range diffusion of oxygen ions, while peak P5 hardly changes its position and dramatically decreases in height with increasing frequency, exhibiting non-relaxational nature. With increasing Ln^3＋ concentration, the heights of peak Ph and Pd increase at first and then decrease after passing a maximum at 15% doping. It is suggested that peak P5 is related to the phase transition of a static disordered state to a dynamic disordered state in oxygen ions/vacancies distribution. It is found that the 15%Gd or 15%Nd doped La2Mo209 samples exhibit the highest conductivity in accordance with the highest height of peak Pd at this doping content.

Ultralow oxygen ion diffusivity in pyrochlore-type La_(2)(Zr_(0.7)Ce_(0.3))_(2)O_(7)

Thermally grown oxides(TGOs)at the ceramic top-coat/metallic bond-coat interface are a pressing chal-lenge in advanced thermal barrier coating(TBC)systems as they can affect the performance and ser-vice lifetime of TBCs.Thus,developing novel TBC materials with ultralow oxygen ion diffusivity is very urgent.In this study,we reported the diffusive properties of oxygen ions in a novel pyrochlore-type La_(2)(Zr_(0.7)Ce_(0.3))_(2)O_(7)(LZ7C3)material.The measured ionic conductivity and atomistic simulation revealed that the oxygen ion diffusivity in LZ7C3 grains is two orders of magnitude lower than that in conventional 8 wt.%yttria-stabilized zirconia(8YSZ)grains.This is due to the relatively high energy barrier for oxygen hopping in LZ7C3.In addition,it was found that enhancing the order distribution of cations is a strategy to reduce the intrinsic oxygen diffusion of pyrochlore-type oxides.On the other hand,we observed that La^(3+) cations segregate at the grain boundaries(GBs)of LZ7C3,which results in the electrostatic poten-tial at GBs being comparable to that in the bulk.Furthermore,we found that the oxygen ion diffusion is facilitated at the GBs of LZ7C3 due to the stretched O-Zr/Ce bond and the low coordination at GBs.How-ever,the segregations of Y^(3+)cations and the increase in the number of oxygen vacancies resulted in the formation of an electrostatic layer at the GBs of 8YSZ,which shielded the oxygen ion diffusion.Despite this,the oxygen ion diffusivity in LZ7C3 was still considerably less than that in conventional 8YSZ.This study offers a stepping stone toward utilizing pyrochlore-type LZ7C3 materials as advanced TBCs at high temperatures.