Sodium ion transport in polymorphic scandium NASICON analog Na3Sc2(PO4)3 with new dielectric spectroscopy approach for current-constriction effects

Abstract

Conductivity, calorimetry, and in-situ XRD can be closely interrelated with each other in terms of the sodium ion ordering and rearrangements in NASICON structure. An intermediate phase transition at 103°C, not associated with heat effects, is evidenced for the first time, in addition to α–β and β–γ phase transitions at 64°C and 166°C. AC characterization of polycrystalline Na3Sc2(PO4)3 has been performed over the wide temperature range from 225 to −100°C. Strongly dispersive spectra of the low temperature monoclinic phase were systematically described by the total sample resistance connected by the several capacitive responses all in parallel: pure dielectric geometric capacitance and two Havriliak–Negami (HN) dielectric functions with opposite skewnesses. The capacitance strengths of HN functions are inversely proportional to the temperature and the relaxation times represent the thermally activated mobile charge carrier transport. While the higher frequency response with a negative skewness as γ=0.388 and β=1 is of a universal character common with many other solid electrolytes, known as K1 model, the lower frequency response with a positive skewness as γ⋅β≈1 where β≈0.59 and γ≈1.67 describes the current-constriction effects occurring in the polycrystalline Na3Sc2(PO4)3. The current-constriction effects are described by a homogeneous polarization throughout the sample with 4 times the hopping distance and 5 times the relaxation time of the K1 response. 14 model parameters competently simulate the AC behavior over a wide temperature and frequency range.