Wet chemical synthesis and characterization of Na+-conducting sodium dysprosium silicates

The wet chemical synthesis and characterization of the NASICON family series, based on sodium dysprosium (phopho)silicates namely Na5DySi4O12, P doped Na5DySi4O12 (Na5DyPSi4O12), Na3.9Dy0.6P0.3Si2.7O9 and Na4.05Dy0.55P0.3Si2.7O9, were carried out. Thermal events of the as-dried mixture were analysed by thermogravimetry/differential scanning calorimetry (TG-DSC). Compacted disks of as-dried powder were subjected to conventional heat treatment from 400 to 1000 °C as well as to non-conventional heat treatments like microwave irradiation and spark plasma sintering at various temperatures. The phase evolution was studied as a function of heat treatment temperature by X-ray diffraction. It was found that the Na5DySi4O12 type (N5) crystal structure that was reported to exhibit high Na+-conductivity could not be achieved using either the conventional or even non-conventional heat-treatment techniques like microwave irradiation and spark plasma sintering, but could be obtained by doping the sodium dysprosium silicate with phosphorous. However, even after doping, the desired N5 phase was always found to co-exist along with N9 type (Na9DySi6O18) phase for Na5DySi4O12 and with N3 type (Na3DySi2O7) phase for Na3.9Dy0.6P0.3Si2.7O9 and Na4.05Dy0.55P0.3Si2.7O9 compositions up to a heat treatment temperature of 1000 °C.