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ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-021-21488-7 Fig. 2 Experimental Characterization of Na3-xY1-xZrxCl6. a XRD of the Na3-xY1-xZrxCl6 compositions, obtained in x = 0.125 increments. Asterisks indicate the presence of new peaks. b The corresponding room temperature conductivity values. c Arrhenius plot of Na2.25Y0.25Zr0.75Cl6 from experimental measurements. The activation energy (low-temperature regime) and room temperature conductivity values are consistent with the MTP results. Local Na environments and disorder. 23Na magic angle spin- ning (MAS) solid-state NMR spectra obtained on NYZCx (x = 0, 0.25, 0.5, 0.75, 1) are presented in Fig. 3a. The signal at about 7.2 ppm is attributed to NaCl(s) present as either an impurity or residual precursor phase in all NYZCx samples29. The NaCl content obtained from 23Na NMR signal integration was found to be 16.0 %, 10.0 %, 5.9 %, 6.2 % and 4.0 % for x=0, 0.25, 0.5, 0.75 and 1.0, respectively. The decrease in NaCl impurities with increasing Zr content is likely due to the concomitant decrease in Na content in NYZCx relative to NYC, which renders the for- mation of NaCl less favorable. While two crystallographically-distinct Na environments are expected in Na3YCl6 (as shown in Fig. 3b), the spectrum obtained on this compound (top spectrum in Fig. 3) exhibits at least five distinct resonances besides the NaCl impurity signal at about 7.2 ppm, which suggests the presence of local structural disorder and/ or non-stoichiometry. We have excluded the possibility of additional impurity phases in the sample, due to the lack of candidate Na-containing impurity phases with 23Na resonant frequencies and signal line-shapes matching those observed in the Na3YCl6 spectrum. The presence of well-defined peaks suggests the presence of a range of Na environments with varying numbers of Cl and Y (Na) nuclei in their first and second coordination shells, respectively, as in a non-stoichiometric material or a material with some disorder on the cation lattice. We note that a non-stoichiometric solid electrolyte phase will necessarily result from the presence of a separate NaCl component, unless some YCl3 impurity is also present in the sample. The latter phase is not observed with XRD, but 89Y NMR data would be needed to completely rule out the presence of amorphous YCl3 in the sample (the long relaxation time of 89Y spins, however, makes such experiments prohibitively time consuming). Non-stoichiometry will also lead to a range of bond angles and bond lengths around Na nuclei in the structure, which could partially account for peak broadening in the Na3YCl6 spectrum. Additional NMR experiments are underway, as well as first principles calculations of NMR parameters, to fully assign the data presented here. Previous NMR studies on solid NaYF430 and on the NaF-YF3 molten system31 have shown that the 23Na chemical shift becomes more negative as the number of Y3+ ions in the second coordination shell increases. In stoichiometric Na3YCl6, the Na0 site shown in Fig. 3b has four YCl6 neighboring octahedra, two of which are edge-sharing with the Y nucleus at a distance of 4.0–4.2 Ǻ from the central Na, and two of which are corner- sharing with Y-Na distances of 4.3 and 4.9 Ǻ. In contrast, the Na1 site has six corner-sharing YCl6 neighboring octahedra, with Y being 5.0–5.1 Ǻ away from the central Na. The presence of non-stoichiometry or disorder on the cation lattice will reduce the number of Y in the vicinity of the central Na and more strongly affect the chemical shift of Na0 (fewer Y3+ neighbors that are closer to the central Na) compared to that of Na1 (larger number of Y3+ neighbors further away from the central Na). It will also lead to additional 23Na NMR signals at more positive ppm values. With this in mind, we tentatively assign the most intense 23Na resonance at −8.1 ppm to Na nuclei in locally stoichiometric Na0 sites, the resonance at 3.6 p.p.m. to locally stoichiometric Na1 sites and the lower intensity peaks in- between these two resonances to Na nuclei in distorted and/or Y-deficient Na0 and Na1 sites. This assignment is consistent with the fact that there are twice as many Na0 sites than Na1 sites in the structure, such that the most populated Na local environment is expected to be the locally stoichiometric Na0 site. 4 NATURE COMMUNICATIONS | (2021)12:1256 | https://doi.org/10.1038/s41467-021-21488-7 | www.nature.com/naturecommunicationsPDF Image | cathode-solid electrolyte composite sodium-ion
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