Phase Transition and Oxygen Ion Diffusion in (La_(1-x)Ln_x)_2Mo_2O_9 (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 (La_(1-x)Ln_x)_2Mo_2O_9 (Ln = Nd, Gd, x = 0.05-0.25). Two dielectric loss peaks were observed: peak Pd at about 600 K and peak Ph around 720 K. Peak Pd is a relaxational peak and associated with the short-range diffusion of oxygen ions, while peak Ph 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 Ph 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 La_2Mo_2O_9 samples exhibit the highest conductivity in accordance with the highest height of peak Pd at this doping content