The solubility and site preference of Fe3+ in Li7−3xFexLa3Zr2O12 garnets

• Published in: Journal of solid state chemistry Vol. 230; pp. 266 - 271
• Main Authors: Rettenwander, D., Geiger, C.A., Tribus, M., Tropper, P., Wagner, R., Tippelt, G., Lottermoser, W., Amthauer, G.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01.10.2015; Academic Press
• Subjects: 57Fe Mössbauer spectroscopy; Chemical composition; Ion conductors; Li-oxide garnet; Synthesis; X-ray powder diffraction; Ion conductors; 57Fe Mössbauer spectroscopy; Li-oxide garnet; Chemical composition; Synthesis; X-ray powder diffraction
• ISSN: 0022-4596; 1095-726X
• DOI: 10.1016/j.jssc.2015.01.016

A series of Fe3+-bearing Li7La3Zr2O12 (LLZO) garnets was synthesized using solid-state synthesis methods. The synthetic products were characterized compositionally using electron microprobe analysis and inductively coupled plasma optical emission spectroscopy (ICP-OES) and structurally using X-ray powder diffraction and 57Fe Mössbauer spectroscopy. A maximum of about 0.25 Fe3+ pfu could be incorporated in Li7−3xFexLa3Zr2O12 garnet solid solutions. At Fe3+ concentrations lower than about 0.16pfu, both tetragonal and cubic garnets were obtained in the synthesis experiments. X-ray powder diffraction analysis showed only a garnet phase for syntheses with starting materials having intended Fe3+ contents lower than 0.52 Fe3+ pfu. Back-scattered electron images made with an electron microprobe also showed no phase other than garnet for these compositions. The lattice parameter, a0, for all solid-solution garnets is similar with a value of a0≈12.98Å regardless of the amount of Fe3+. 57Fe Mössbauer spectroscopic measurements indicate the presence of poorly- or nano-crystalline FeLaO3 in syntheses with Fe3+ contents greater than 0.16 Fe3+ pfu. The composition of different phase pure Li7−3xFexLa3Zr2O12 garnets, as determined by electron microprobe (Fe, La, Zr) and ICP-OES (Li) measurements, give Li6.89Fe0.03La3.05Zr2.01O12, Li6.66Fe0.06La3.06Zr2.01O12, Li6.54Fe0.12La3.01Zr1.98O12, and Li6.19Fe0.19La3.02Zr2.04O12. The 57Fe Mössbauer spectrum of cubic Li6.54Fe0.12La3.01Zr1.98O12 garnet indicates that most Fe3+ occurs at the special crystallographic 24d position, which is the standard tetrahedrally coordinated site in garnet. Fe3+ in smaller amounts occurs at a general 96h site, which is only present for certain Li-oxide garnets, and in Li6.54Fe0.12La3.01Zr1.98O12 this Fe3+ has a distorted 4-fold coordination. Cubic nominally Li7La3Zr2O12 (LLZO) garnet is a promising candidate to be used as a solid electrolyte in Li-ion batteries. A series of Fe3+-bearing LLZO garnets was synthesized and characterized compositionally and structurally. 57Mössbauer measurements were made to determine where Fe is incorporated in the crystal structure. X-ray diffraction, electron microprobe, ICP-OES and 57Mössbauer measurements are needed to obtain a full description of the synthetic products, some of which contain small amounts of nano- or poorly crystalline FeLaO3 [Display omitted] •A series of Fe3+-bearing Li7La3Zr2O12 (LLZO) garnets was synthesized and characterized compositionally and structurally.•57Mössbauer measurements were made to determine where Fe is incorporated in the crystal structure.•Most Fe3+ substitutes for Li+ in LLZO at the 24d and 96h sites in the cubic phase (Ia-3d).•No more than about 0.25 Fe3+ pfu can be incorporated into the LLZO garnet structure.•X-ray powder diffractions measurements indicate the presence of both cubic and tetragonal garnets phases in some syntheses.•The probable presence of small amounts of poorly or nano-crystalline FeLaO3 can only be identified by Mössbauer spectroscopy.