Elucidating the local structure of Li1+xAlxTi2-x(PO4)3 and Li3AlxTi2-x(PO4)3 (x=0, 0.3) via total scattering

Li1+xAlxTi2-x(PO4)(3) (LATP) and Li3AlxTi2-x(PO4)(3) (x = 0, 0.3) are promising candidates in all-solid-state batteries due to their high room temperature conductivity of 10(-3) S cm(-1) and air- and moisture-stability. They also exhibit unusual thermal expansion properties, with Li1+xAlxTi2-x(PO4)(3) showing near-zero thermal expansion along the a axis while Li3AlxTi2-x(PO4)(3) exhibits polynomial positive thermal expansion along the a axis and polynomial negative thermal expansion along the c axis. A crucial component to understanding these properties is understanding the local structure. Total scattering is a powerful analytical technique as it provides information on the long-range, average structure as well as the local structure. Here, we report the first X-ray and neutron total scattering experiments performed on Li1+xAlxTi2-x(PO4)(3) and Li3AlxTi2-x(PO4)(3) (x = 0, 0.3). We show that the PO4 and TiO6 polyhedra experience very little expansion of the P/Ti-O bonds up to 800 degrees C, nor is there much expansion when the Li content increases significantly. The minor thermal expansion of the nearest-neighbor bonds of the polyhedra is revealed to be the reason behind the unusual thermal expansion properties, causing the near-zero thermal expansion along a in Li1+xAlxTi2-x(PO4)(3) and moving as whole units in Li3AlxTi2-x(PO4)(3). The structural robustness of the framework is also the reason for the increased conductivity as Li content increases, as the framework remains undistorted as Li content increases, permitting Li-ion mobility as the number of charge carriers increases. This suggests that phosphate-based framework materials beyond LATP would also be a good material space to explore for new Li-ion (and other ion-) conducting materials.