125Te NMR for structural investigations in phase change materials: Optimization of experimental conditions coupled to NMR shift prediction

Phase Change Materials as those of the Ge-Sb-Te ternary system are of great interest for technological applications. Properties of these compounds are strongly related to presence of vacancies and structural investigations remain challenging. In this paper we evidence that 125Te NMR in natural abundance and using commercial systems at intermediate field (14.1 ​T) together with NMR parameters prediction can contribute to improve understanding of electronic structure of such systems. GeTe is a typical phase change material, whose structure contains germanium vacancies, even in its stoichiometric form, giving it metallic properties. Here, we use nominal Ge50Te50 and Ge48Te52 crystalline samples as an example to optimize the WURST-CPMG technique, a powerful technique to record wide NMR spectra which has not yet been used on 125Te. The goal was to minimize the time devoted to experiments as well as maximize the signal-to-noise ratio in order to detect small intensity signals directly linked to vacancies. Virtual Crystal Approximation (VCA) calculations performed with WIEN2K helped to interpret the NMR spectra. For Te-based crystalline conducting samples the best experimental results were obtained using 3.2 ​mm thin wall rotors with diluted samples 40 ​vol% GeTe-60 ​vol% SiO2. In addition to the WURST-CPMG technique, high resolution spectra using MAS as implemented in the pj-MAT technique allowed us to identify the distributions of chemical shift parameters in the high intensity contribution of the 1D spectra. The NMR spectra recorded on the samples showed that an addition of Tellurium in the stoichiometric Ge50Te50 sample leads to an important broadening of the spectrum together with a shift of the lines. According to VCA calculations it could be attributed to a distribution of concentrations of germanium vacancies in the sample and it would appear that Knight Shift but also Chemical Shift could contribute in similar proportion to the NMR line position when metavalent bonding is invoked. [Display omitted] •125Te NMR Investigation of Ge–Te crystalline materials, a model for phase-change materials.•125Te WURST-CPMG sequence optimized for conducting Te-containing materials, using commercial systems at intermediate fields.•VCA calculations in WIEN2K linking lineshape of the spectra to NMR parameters and charge carriers concentration distribution.•Use of the NMR pj-MAT sequence, relating the decrease of the anisotropy of 125Te environment with the amount of vacancie