NMR investigations unveil phase composition−property correlations in Sr sub(0.55)Na sub(0.45)SiO sub(2.775) fast ion conductor

Abstract

This paper reports results of sup(23)Na and sup(29)Si solid-state NMR investigations carried out on sodium strontium silicate ion conductor, Sr sub(0.55)Na sub(0.45)SiO sub(2.775) and presents the first experimental evidence to show that different synthesis conditions induce multiple devitrified phases. Along with 1-dimensional NMR, sup(23)Na MQMAS spectra have been used to identify the phases corresponding to polymorphs of Na sub(2)Si sub(2)O sub(5), in addition to the crystalline SrSiO sub(3) and the glass/amorphous Na sub(2)Si sub(2)O sub(5) phases. The surprising observation of about an order of magnitude higher ionic conductivity achieved in devitrified samples is attributed to the growth of the crystalline delta-Na sub(2)Si sub(2)O sub(5) phase within the amorphous Na sub(2)Si sub(2)O sub(5) phase domains, identified using NMR. Together with XRD and conductivity measurement data, the study leads to the identification of the chemical phase composition and an understanding of the composition-property-structure correlation in this material. Present findings, while do not show any evidence of Na doping in the SrSiO sub(3) phase confirming earlier reports, explain the large discrepancy in the conductivity reported in the literature.