The solubility and site preference of Fe³⁺ in Li_7-3xFe_xLa₃Zr₂O₁₂ garnets

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 Li_7-3xFe_xLa₃Zr₂O₁₂ garnet solid solutions. At Fe³⁺ concentrations lower than about 0.16 pfu, 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≈12.98 Å 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 Li_7-3xFe_xLa₃Zr₂O₁₂ garnets, as determined by electron microprobe (Fe, La, Zr) and ICP-OES (Li) measurements, give Li_6.89Fe_0.03La_3.05Zr_2.01O₁₂, Li_6.66Fe_0.06La_3.06Zr_2.01O₁₂, Li_6.54Fe_0.12La_3.01Zr_1.98O₁₂, and Li_6.19Fe_0.19La_3.02Zr_2.04O₁₂. The ⁵⁷Fe Mössbauer spectrum of cubic Li_6.54Fe_0.12La_3.01Zr_1.98O₁₂ 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 Li_6.54Fe_0.12La_3.01Zr_1.98O₁₂ this Fe³⁺ has a distorted 4-fold coordination.