Phase formation and ionic conduction in Na-doped Sr2MgSi2O7 melilite-type silicate

Na-doped melilite-type silicate Sr2MgSi2O7 was reported to display high oxide ion conductivity. In this work, through multiple complementary techniques including X-ray diffraction, scanning electron microscopy, transmission electron microscopy, solid-state Si-29 and Na-23 nuclear magnetic resonance spectroscopy, and DFT calculations of defect formation energies, we found that it is hard to replace Sr with Na in Sr2MgSi2O7, and the Sr2-xNaxMgSi2O7-0.5x compositions formed mixed crystalline Sr2MgSi2O7 and Na-containing glassy phases. AC impedance spectroscopy measurements indicated that the ionic conduction in Sr2-xNaxMgSi2O7-0.5x could be from Na+ ions in the glassy phase, instead of the oxide ions.

Automated summary

It was found that it is difficult to replace Sr with Na in Sr2MgSi2O7, forming mixed crystalline and Na-containing glassy phases in Sr2-xNaxMgSi2O7-0.5x compositions. The ionic conduction in Sr2-xNaxMgSi2O7-0.5x could potentially arise from Na+ ions in the glassy phase, as indicated by AC impedance spectroscopy measurements.