Arrhenius Behavior of the Bulk Na-Ion Conductivity in Na₃Sc₂(PO₄)₃ Single Crystals Observed by Microcontact Impedance Spectroscopy

NASICON-based solid electrolytes with exceptionally high Na-ion conductivities are considered to enable future all solid-state Na-ion battery technologies. Despite 40 years of research the interrelation between crystal structure and Na-ion conduction is still controversially discussed and far from being fully understood. In this study, microcontact impedance spectroscopy combined with single crystal X-ray diffraction, and differential scanning calorimetry is applied to tackle the question how bulk Na-ion conductivity σ_bulk of sub-mm-sized flux grown Na₃Sc₂(PO₄)₃ (NSP) single crystals is influenced by supposed phase changes (α, β, and γ phase) discussed in literature. Although we found a smooth structural change at around 140 °C, which we assign to the β → γ phase transition, our conductivity data follow a single Arrhenius law from room temperature (RT) up to 220 °C. Obviously, the structural change, being mainly related to decreasing Na-ion ordering with increasing temperature, does not cause any jumps in Na-ion conductivity or any discontinuities in activation energies E_a. Bulk ion dynamics in NSP have so far rarely been documented; here, under ambient conditions, σ_bulk turned out to be as high as 3 × 10^-4 S cm^-1 at RT (E_{a, bulk} = 0.39 eV) when directly measured with microcontacts for individual small single crystals.