The solubility and site preference of Fe(3+) in Li(7−3x)Fe(x)La(3)Zr(2)O(12) garnets

A series of Fe(3+)-bearing Li(7)La(3)Zr(2)O(12) (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 (57)Fe Mössbauer spectroscopy. A maximum of about 0.25 Fe(3+) pfu could be incorporated in Li(7−3x)Fe(x)La(3)Zr(2)O(12) garnet solid solutions. At Fe(3+) 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(3+) contents lower than 0.52 Fe(3+) pfu. Back-scattered electron images made with an electron microprobe also showed no phase other than garnet for these compositions. The lattice parameter, a(0), for all solid-solution garnets is similar with a value of a(0)≈12.98 Å regardless of the amount of Fe(3+). (57)Fe Mössbauer spectroscopic measurements indicate the presence of poorly- or nano-crystalline FeLaO(3) in syntheses with Fe(3+) contents greater than 0.16 Fe(3+) pfu. The composition of different phase pure Li(7−3x)Fe(x)La(3)Zr(2)O(12) garnets, as determined by electron microprobe (Fe, La, Zr) and ICP-OES (Li) measurements, give Li(6.89)Fe(0.03)La(3.05)Zr(2.01)O(12), Li(6.66)Fe(0.06)La(3.06)Zr(2.01)O(12), Li(6.54)Fe(0.12)La(3.01)Zr(1.98)O(12), and Li(6.19)Fe(0.19)La(3.02)Zr(2.04)O(12). The (57)Fe Mössbauer spectrum of cubic Li(6.54)Fe(0.12)La(3.01)Zr(1.98)O(12) garnet indicates that most Fe(3+) occurs at the special crystallographic 24d position, which is the standard tetrahedrally coordinated site in garnet. Fe(3+) in smaller amounts occurs at a general 96h site, which is only present for certain Li-oxide garnets, and in Li(6.54)Fe(0.12)La(3.01)Zr(1.98)O(12) this Fe(3+) has a distorted 4-fold coordination.