Pair distribution function analysis and Mössbauer study of defects in microwave-hydrothermal LiFePO 4

Olivine-type LiFePO 4 is nowadays one of the most important cathode materials of choice for high-energy lithium ion batteries. Its intrinsic defectivity, and chiefly the so-called lithium iron anti-site, is one of the most critical issues when envisaging electrochemical applications. This paper reports a combined diffractometric (Synchrotron Radiation XRD with Rietveld and PDF analyses) and spectroscopic (Mössbauer) approach able to give a thorough characterization of the material defectivity. Such analytical procedure has been applied to a sample prepared following an innovative microwave-assisted hydrothermal synthesis route that, in a few minutes, allowed us to obtain a well crystallized material. PDF analysis, which is applied for the first time to this type of battery material, reveals the presence of disorder possibly due to Li/Fe exchange or to a local symmetry lowering. A 5% amount of iron on the lithium site has been detected both by PDF as well as by Mössbauer spectroscopy, which revealed a small percentage of Fe 3+ on the regular sites. © 2012 The Royal Society of Chemistry

Static and dynamic magnetic properties of an [Fe13] cluster

The static and dynamic magnetic properties of an [Fe13] cluster were investigated using several experimental techniques. The cluster crystallizes in a cubic space group, but careful investigation of the crystallographic data revealed that the symmetry is distorted locally. dc magnetic susceptibility measurements showed the presence of competing antiferromagnetic isotropic exchange interactions leading to a high-spin ground state and many low-lying excited spin states, which was confirmed by inelastic neutron scattering measurements. From high-field electron paramagnetic resonance measurements a small zero-field splitting of the spin ground state was inferred, which supports the local symmetry distortion found in the crystallographic studies. The spin dynamics slows down at sub-kelvin temperatures, where ac susceptibility measurements indicated that part of the sample shows superparamagnetic-like behavior and the other part relaxes through magnetization tunneling. The Mössbauer data confirmed the slowing down of the spin dynamics, indicating that this occurs mainly in the peripheral spins