Correlation between Structural Features and Ionic Transport in Lithium-Ion Conducting Glass-Ceramics from the Li1+xCrxGeTi1-x(PO4)(3) System

NASICON-structured glass-ceramics based on the Li1+xCrxGeTi1-x (PO4)(3) (0 <= x <= 1) system are synthesized by the melt-quenching method followed by glass crystallization. Since Ti4+ and Cr3+ have approximately the same crystal radius in octahedral coordination, this aliovalent substitution should avoid considerable changes in the cell volume, thus allowing us to evaluate the true contribution of the substitution of a tetravalent cation by a trivalent one on other structural factors. The crystal structure of these glass- ceramics is investigated by X-ray and high-resolution neutron diffraction while the electrical properties are accessed by impedance spectroscopy. Fourier differences and bond valence energy landscape analyses are used to determine the additional Li position and occupancies caused by the aliovalent substitution of Ti4+ by Cr3+. The most important change in the structural features caused by the increase in the lithium content in the chemical formula is the occupation of the 36f position followed by the partial depopulation of the 6b sites. Furthermore, the grain-related ionic conductivity and activation energy for lithium conduction are notably dependent on the aliovalent substitution. The correlations between these two main findings are discussed herein.