Correlation of dynamical disorder and oxy-ion diffusion mechanism in a Dy, W co-doped La2Mo2O9 system: an electrolyte for IT-SOFCs

Abstract

In the present attempt, a Dy, W co-doped La₂Mo₂O₉ (LMX) system is explored to understand the order–disorder phase transition, dynamical disorder state and their influence on the oxy-ion diffusion mechanism. The X-ray diffraction study confirms the co-dopant induced suppression of the order–disorder phase transition temperature of LMX. The oxygen ion diffusion in the LMX matrix is through intrinsic oxygen vacancies. Disorder oxygen vacancies enhance the degree of freedom of oxy-ion diffusion; these are related to the dynamical disorder states in LMX. These disorder states are demonstrated by high temperature Raman spectra. Dynamical disordering of oxygen vacancies in co-doped LMX systems is revealed by studying the rate of change of intensity of the Mo–O bond vibration as a function of temperature; non-uniformity in the rate of change of intensity is correlated to dynamical disorder. The dielectric relaxation studied by using dielectric loss spectra reveals a single relaxation peak for the pure-LMX system, while two dielectric relaxation peaks are revealed for doped LMX systems. Oxygen vacancy reorientation associated with dielectric relaxation is correlated to the diffusion process between O(1) → O(2) and O(1) → O(3) oxygen ion-vacancy exchange sites in doped LMX systems, while it is O(1) through orderly arranged oxygen vacancies in the pure LMX system. To ascertain the relaxation dynamics of the bulk system, electric modulus formalism is helpful, M′′ data are fit by the Bergman function represented by the Kohlrausch–Williams–Watts (KWW) formula and non-Debye type relaxation is revealed for all systems. The activation energy of oxy-ion diffusion is reduced by a co-doping effect. Ionic conductivity extracted from complex impedance spectra indicates that oxy-ion conductivity in a co-doped LMX system is improved almost one order as compared to the pure system. The study reveals that a co-doped LMX system has the potential to be used as electrolytes for intermediate temperature solid oxide fuel cells (400–700 °C, IT-SOFCs).