New insights on the structural formation of polyanionic phosphate compounds revealed by new Mossbauer and XRD measurements

The exploration of optional electrode materials for lithium and sodium ion batteries, still an active field with continuous research efforts. Based on several advantages, polyanionic phosphate compounds M_0.5TiOPO₄ (M= Fe, Ni, Co, Cu), as negative electrode materials of Lithium-ion batteries, were the subject of a limited number of experimental studies.
Motivated by these studies, intensive efforts have been devoted by our group to understand crystal and electronic structure changes, upon lithium ions insertion/extraction. As a result, the Iron-57 Mössbauer measurements of Fe_0.5TiOPO₄ (pristine material) report new findings revealing the presence of two iron distinct environments, in contradiction with previously reported structural observations [1]. In addition, based on an XRD study with a Rietveld refinement, the newly found crystal structure of Fe_0.5TiOPO₄ shows that the Fe²⁺ cations are located in two different triangular based antiprism crystallographic sites. More importantly, this finding is further supported by DFT calculations that have showed the possibility of co-existence of several host sites for Fe²⁺.