TEM Studies: The Key for Understanding the Origin of the 3.3 V and 4.5 V Steps Observed in LiMn2O4-based Spinels

The pressure dependence of the crystal structure of Sr2Fe2O5 has been studied at room temperature by angle-dispersive synchrotron X-ray powder diffraction and 57Fe M?ssbauer spectroscopy using diamond anvil cell techniques. Between 11 and 14 GPa the low-pressure brownmillerite-type structure with an equal number of octahedrally and tetrahedrally coordinated Fe3+ sites is transformed into a tetragonal perovskite-like structure. The M?ssbauer spectra of the high-pressure phase reveal two subspectra with an intensity ratio of about 3:1. The M?ssbauer parameters are consistent with two fivecoordinated Fe3+ sites differing in the degree of structural distortion of the FeO5 polyhedra. Laser annealing of the highpressure phase at 15 GPa induces a transformation from the tetragonal perovskite into an orthorhombic phase that is isostructural with Sr2Mn2O5. This structure consists of a network of corner-sharing FeO5 square pyramids. The coordination number of the Sr2+ ions has increased from 8 in the brownmillerite structure to 10 in the Sr2Mn2O5-type phase.