Charge carrier relaxation phenomena and phase transition in La2Mo2O9 ceramics investigated by broadband impedance spectroscopy

Unique 2-electrode (100 mHz-10 GHz) and 4-electrode (100 mHz-2 MHz) impedance spectroscopy equipment enables investigation of the fine structure of oxygen ion conductive La2Mo2O9 (LMO) solid electrolytes ceramics' impedance response in temperatures ranging from 300 to 950 K. Temperature dependencies of the electrical parameters show abrupt jumps associated with the α↔β-LMO phase transition, which is also detected by X-ray powder diffraction and differential scanning calorimetry analyses. Electrical relaxation phenomena are analyzed in terms of the relaxation time distribution function. Four different relaxation processes are identified and attributed to processes in the ceramics' grains and grain boundary media. Detailed analysis of these processes reveals surprising findings about the nature of the α ↔ β-LMO phase transition, indicating that the abrupt change in LMO compound's resistivity is associated not only with a change in the relative volume of the different phases, but also with an alteration of the β-LMO phase itself; thus the α↔β-LMO phase transition is found to have properties of I and II order phase transitions. The methods applied in this paper could have wide application to the investigation of ionically conductive materials.