The solubility and site preference of Fe in Li Fe LaZrO garnets.

A series of Fe-bearing LiLaZrO (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 Fe Mössbauer spectroscopy. A maximum of about 0.25 Fe pfu could be incorporated in Li Fe LaZrO garnet solid solutions. At Fe 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 Fe contents lower than 0.52 Fe pfu. Back-scattered electron images made with an electron microprobe also showed no phase other than garnet for these compositions. The lattice parameter, , for all solid-solution garnets is similar with a value of ≈12.98 Å regardless of the amount of Fe. Fe Mössbauer spectroscopic measurements indicate the presence of poorly- or nano-crystalline FeLaO in syntheses with Fe contents greater than 0.16 Fe pfu. The composition of different phase pure Li Fe LaZrO garnets, as determined by electron microprobe (Fe, La, Zr) and ICP-OES (Li) measurements, give LiFeLaZrO, LiFeLaZrO, LiFeLaZrO, and LiFeLaZrO. The Fe Mössbauer spectrum of cubic LiFeLaZrO garnet indicates that most Fe occurs at the special crystallographic 24 position, which is the standard tetrahedrally coordinated site in garnet. Fe in smaller amounts occurs at a general 96 site, which is only present for certain Li-oxide garnets, and in LiFeLaZrO this Fe has a distorted 4-fold coordination.