Mössbauer analysis of silicate Li2FeSiO4 and delithiated Li2−xFeSiO4 (x = 0.66) compounds

Abstract

Lithium iron silicate compounds of Li2FeSiO4 and partially delithiated Li2−xFeSiO4 (x = 0.66) were synthesized by vacuum-sealed solid-state and chemical delithiation reactions, and their magnetic properties were characterized based on Mössbauer analysis. Crystal structures of both Li2FeSiO4 and Li2−xFeSiO4 (x = 0.66) compounds are found to be γs-type (P21/n) monoclinic structures with difference in the lattice parameters due to lithium delithiation. Mössbauer spectrum of Li2FeSiO4 below TN1 = 20 K exhibits eight Lorentzians of Fe2+ with antiferromagnetic ordering. However, the spectrum of intermediate Li2−xFeSiO4 (x = 0.66) compound shows the appearance of magnetically ordered Fe3+ sextet below TN2 = 28 K. The temperature-dependent isomer shift of Li2−xFeSiO4 indicates the coexistence of nonequivalent Fe2+/Fe3+ valence states with the partial oxidation of FeO4, enhanced by the lithium ion deficiency. Also, we have observed a considerable change in electric quadrupole interaction between Fe2+/Fe3+ ions in Li2−xFeSiO4, when compared to that of Li2FeSiO4, due to the different lattice and valence electron contributions, being originated from crystalline and valence transitions caused by the lithiation/delithiation process.