Investigations on the phase stability of Na+-conducting sodium dysprosium (phospho) silicates

Na+-conducting sodium dysprosium (phospho) silicates with the compositions Na5DySi4O12, Na3.9Dy0.6P0.3Si2.7O9 and Na3.9Dy0.6Si3O8.85 were synthesized by solid state route. The phase evolution in the three systems was studied as a function of heat treatment temperature from 600 °C to 1000 °C by X-ray diffraction technique. It was possible to achieve a NASICON Na5DySi4O12-type single phase with the compositions Na3.9Dy0.6P0.3Si2.7O9 and Na3.9Dy0.6Si3O8.85 at 800 °C. Heat treatment at a higher temperature (>850 °C) resulted in the formation of a less conductive Na9DySi6O18-type phase. Conventional and microwave sintering were carried out on Na3.9Dy0.6P0.3Si2.7O9 sample at 800 °C and the ionic conductivities were measured as a function of temperature from 75 °C to 325 °C. The microwave sintered sample exhibited better Na+ ionic conductivity of 0.42 × 10−2 S/cm when compared to that of conventionally sintered sample which was 0.43 × 10−3 S/cm at 325 °C.