Composition and conduction mechanism of the NASICON structure X-ray diffraction study on two crystals at different temperatures

The results of a single crystal X-ray structure determination of NASICON at 495K are presented. The crystal symmetry is rhombohedral at this temperature, but monoclinic at room temperature. The structure refinement yielded the following composition: Na/3.1/Zr/1.78/Si/1.24/P/1.76/0/12/. This compound with NASICON structure is not located on the NASICON section given in the literature and this leads us to propose a new compositional plane. The conduction path involves both sodium sites and could be unambiguously derived from the joint probability density (PDF) of the Na ions. The potential barrier calculated from the PDF has about the same value as the activation energies measured on NASICON ceramics above the phase transition. These results are compared with similar studies on a pure Si NASICON crystal at temperatures up to 933K where the same conduction mechanism is found.