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

Abstract

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 ²⁹Si and ²³Na 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 Sr_2−xNa_xMgSi₂O_7−0.5x compositions formed mixed crystalline Sr₂MgSi₂O₇ and Na-containing glassy phases. AC impedance spectroscopy measurements indicated that the ionic conduction in Sr_2−xNa_xMgSi₂O_7−0.5x could be from Na⁺ ions in the glassy phase, instead of the oxide ions.