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N7.09 - DFT Study of the Fast Ion Conductor LiAlLaZrO: The Energy Site Preference of Dopant Al and the Simulation of Al MAS NMR Spectra 
April 24, 2014   4:15pm - 4:30pm

Recent work has shown that many of the so-called “stuffed Li garnets”, namely those garnets having more than 3 Li cations in the formula unit, are excellent fast-ion conductors with values of approximately 10 S/cm at RT [1,2]. Their good chemical and thermal stability, as well as their wide energy potential window, are also important properties making them excellent candidates as possible electrolytes in all-solid-state Li-ion batteries. At room temperature, end-member LiLaZrO (LLZO) is tetragonal, 4/, with a lower conductivity compared to the cubic, -3, modification, which is only stable above 150 °C. It has been shown that the higher conducting cubic phase can be stabilized at room temperature through the doping of small amounts of Al [4] or other cations (e.g. Ga, Fe, etc.). Thus, the role of dopant cations and their effect on the conductivity and phase stability of LLZO needs to be carefully studied. We addressed two important issues:1.) The energy site preference of Al in LLZO.2.) A simulation of Al MAS NMR spectra of Al-doped LLZO to compare to published experimental results that are difficult to interpret.To do this, we are using DFT methods implemented in the Wien2k code [3]. We calculated the energetics and the NMR parameters for Al located in various structural sites in LLZO. Our results show that Al prefers the tetrahedrally coordinated 24 site and a distorted 4-fold coordinated 96 site in the space group, -3. Al has been reported to occur in regular octahedral coordination at a 48 site, but we find no evidence for this. Because the site energies for Al atoms are slightly displaced from the exact crystallographic sites (i.e., 24 and 96) are similar, this leads to a distribution of slightly different local oxygen coordination environments. Thus, broad Al NMR resonances result, reflecting the distribution of slightly different isotropic chemical shift and quadrupole splitting values.[1] Cussen, E. (20) 5167-5173.[2] Murugan, R.; Thangadurai, V.; Weppner, W. (119) 7925-7928.[3] Blaha, P.; Schwarz, K.; Madsen, G. K. H.; Kvasnicka, D.; Luitz, J. WIEN2K, Techn. Universitat, Wien, Austria (2001) ISBN: 3-9501031-1-1-2. p. 21-8.[4] Geiger, C. A.; Alekseev, E.; Lazic, B.; Fisch, M.; Armbruster, T.; Langner, R.; Fechtelkord, M.; Kim, N.; Pettke, T.; Weppner, W. (50) 1089-1097.

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