A microcontact impedance study on NASICON-type Li_1+xAl_xTi_2-x(PO₄)₃ (0 ≤ x ≤ 0.5) single crystals

We successfully demonstrated the applicability of microcontact impedance spectroscopy (MC IS) on Li⁺ conducting solid electrolytes and measured the Li⁺ bulk conductivity (σ_b) of LiTi₂(PO₄)₃ (LTP) and Li_1+xAl_xTi_2-x(PO₄)₃ (LATP) single crystals independent of microstructural effects (e.g., grain boundaries, pores, and density). The crystals had a size of about 100 μm in each direction and crystallized with NASICON-type structure (R3¯c). Finite element calculations were performed to validate the impedance data analysis. A strong increase in σ_b in the order of three magnitudes (3.16 × 10^-6 to 1.73 × 10^-3 S cm^-1) was found after incorporating 0.1 mol Al³⁺ per formula unit into LTP. Moreover, since the crystal structural changes are almost linear in the LATP system up to x = 0.5, the increase of σ_b is most probably related to additional Li⁺ sites at the M₃ (36f) position. The additional Li⁺ leads to a displacement of Li⁺ occupying the M₁ (6b) sites towards the nearest-neighboring M₃ position, and therefore opens the fast-conducting pathway within the NASICON structure. A significant change in σ_b was also observed as the Al³⁺ content further increased (x = 0.1 to 0.5). The highest σ_b value of 5.63 × 10^-3 S cm^-1 was obtained for samples with x = 0.4.