Phase transition, impedance spectroscopy and conduction mechanism of Li0.5Na1.5WO4 material

In order to better characterize the tungsten-based samples, and to get better inside into this field, we synthesized the Li_0.5Na_1.5WO₄ compound and identified its purity by X-ray diffraction which showed that it crystallizes in the orthorhombic system with the Pmmm space group. Accordingly we used Raman spectroscopy, differential scanning calorimetry and impedance spectroscopy as to distinguish its properties. All WO₄²⁻ tetrahedral vibration modes appear in the Raman spectra. The calorimetric study displays two phase transitions at 498 K and 567 K. Impedance spectroscopy reveals the contribution of two electrically active regions corresponding to the bulk mechanism and distribution of grain boundaries which allows us to use two cells mounted in series each one of them is composed of the combination in parallel of a resistance R and a fractal capacitance CPE. The variation σ_dc as a function of the inverse of temperature confirms the presence of three phases for each one of them the conductivity is ensured by a specific conduction mechanism. It has been pointed out that the first phase is described by the correlated barrier hopping model (CBH), the second one by the over loping polaron tunneling model (OLPT) and the third one by the non small polaron tunneling model (SPT). A comparison with the compound of Li_1.5Na_0.5WO₄ confirms that the transport is dominated by the motion of the monovalent cations (Li⁺ and Na⁺) in the investigated material.