Tellurium bonding in amorphous Ge2Sb2Te4, Ge2Sb2Te5 and Ge2Sb2Te7: A 125Te NMR investigation

Abstract We have investigated the frequency-space nuclear magnetic resonance spectrum of 125Te in the amorphous forms of Ge2Sb2Tex (x = 4, 5, and 7). The results indicate that there is one distinct tellurium site in the a-Ge2Sb2Te4 and a-Ge2Sb2Te7 and two tellurium sites in the a-Ge2Sb2Te5. The width of the frequency-space spectra suggests that there is significant disorder about the average site for all cases. In light of recent extend X-ray absorption fine structure results, the a-Ge2Sb2Te4 and a-Ge2Sb2Te7 spectra may be the nuclear magnetic resonance fingerprint of three and twofold coordinated tellurium, respectively. The a-Ge2Sb2Te5 spectrum can be reproduced reasonably well with Gaussian components that are very similar to those components needed to model the a-Ge2Sb2Te4 and a-Ge2Sb2Te7 data. These consistencies in the modeling strongly suggest that the average Te structural units occurring in the latter two compounds are the most prominent Te sites in a-Ge2Sb2Te5.