Optical and AC conductivity behavior of sodium orthosilicate Na2CoSiO4

Abstract Na2CoSiO4 was prepared by an improved solid-state (NCS-SS) and co-precipitation (NCS-CP) methods. The X-ray powder diffraction at room temperature revealed that these compounds crystallize in the orthorhombic system with Pna21 and Pbca space groups for NCS-SS and NCS-CP, respectively. Cristoballite structure presents a three-dimensional framework built of alternating corner-sharing CoO4 and SiO4 tetrahedra delimiting cavities, in which sodium Na+ ions are located. Infrared spectra at room temperature showed the presence of CoO4 and SiO4. The calculated values of the direct band gaps (Eg) were estimated at 2.22 eV and 2.17 eV for NCS-SS and NCS-CP, respectively, proving that NCS-CP has a better electronic conductivity than NCS-SS. The electrical data analysis of the impedance spectra revealed the presence of three contributions at different temperatures associated with two grains and grain boundary effects. In order to correlate the electric properties with NCS structure, the Na+ migration can move along [100], [010], [101] pathways for NCS-SS and [010], [001], [011] pathways for NCS-CP. Each NCS sample presented different activations energies due to the Na+ mobility pathway and NCS-SS had higher activation energies than those of NCS-CP due to different cavity forms. The temperature dependence of frequency exponent s showed that the conduction in Na2CoSiO4 was insured by the small polaron NSPT and CBH models, thus indicating that the migration of sodium can go through different paths. At high temperature (T>533 K), NCS-CP shows only one conduction process. Actually, it is an overlap of two processes that indicate either a cavity deformation or Na ions types becoming the same conduction process.