Phase transition and conduction mechanism study by overlapping large-polaron tunnelling model in Li2MoO4 ceramic compound

The Lithium-ionic conductor Li2MoO4 compound was prepared by the conventional solid-state reaction method. A Rietveld analysis of powder X-ray diffraction at room temperature has exhibited the title compound crystallizes in the trigonal system (space group) R (3) over bar with a phenacite-like structure. The calorimetric and Raman spectroscopy-studies revealed a single-phase transition at 635 K. The electrical conductivity was measured in the frequency range from 200 Hz to 1 MHz and temperatures between 578 and 723 K using impedance spectroscopy. Besides, the analysis of Nyquist plots revealed the contribution of two electrically active regions corresponding to the bulk mechanism and distribution of grain boundaries. The electrical conductivity for the grain (sigma(g)) follows an Arrhenius behavior with two different activation energies Ea(I) - 1.038 eV for T < 635 K and Ea(II) - 0.481 eV for T > 635 K. The frequency dependence of the conductivity was interpreted in terms of Jonscher's law. Temperature dependence of the power law exponent s suggests that the overlapping large-polaron tunneling (OLPT) model is the dominant transport process in this material. The optimum hopping length of the polaron (1.8 < R-omega < 3.4 angstrom), is large compared with the interatomic spacing (Li-O =1.964 angstrom; Mo-O = 1.8 angstrom).