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⁺.