From Tetrahedral to Octahedral Iron Coordination: Layer Compression in Topochemically Prepared FeLa₂Ti₃O₁₀

Synthesis, characterization, and thermal modification of the new layered perovskite FeLa₂Ti₃O₁₀ have been studied. FeLa₂Ti₃O₁₀ was prepared by ion exchange of the triple-layered Ruddlesden-Popper phase Li₂La₂Ti₃O₁₀ with FeCl₂ at 350 °C under static vacuum. Rietveld refinement on synchrotron X-ray diffraction data indicates that the new phase is isostructural with CoLa₂Ti₃O₁₀, where Fe^II cations occupy slightly compressed/flattened interlayer tetrahedral sites. Magnetic measurements on FeLa₂Ti₃O₁₀ display Curie-Weiss behavior at high temperatures and a spin-glass transition at lower temperatures (<30 K). Thermal treatment in oxygen shows that FeLa₂Ti₃O₁₀ undergoes a significant cell contraction (Δc ≈ -2.7 Å) with a change in the oxidation state of iron (Fe²⁺ to Fe³⁺); structural analysis and Mössbauer studies indicate that upon oxidation the local iron environment goes from tetrahedral to octahedral coordination with some deintercalation of iron as Fe₂O₃ to produce Fe_0.67La₂Ti₃O₁₀.