Investigation of Na ⁺ Ion Conduction, NTCR Behavior, and Dielectric Relaxation Mechanisms in Na ₂ NiP ₂ O ₇...

Sodium-ion pyrophosphate Na₂NiP₂O₇ (NNPO) was synthesized by a conventional solid-state reaction and crystallizes in a triclinic structure with an average crystallite size of ≈39.5 nm. SEM/EDS analysis indicates a plate- and needle-like morphology indicative of anisotropic growth, confirming phase purity. Raman spectroscopy identifies the [P₂O₇]^4- pyrophosphate groups. Dielectric studies reveal exceptionally high low-frequency permittivity (ε' ≈ 2 × 10⁷ at 673 K) driven by interfacial polarization, with tan δ peaks (∼80) shifting to higher frequencies upon heating, indicative of thermally activated non-Debye relaxation. Capacitance values exceeding 2 × 10^-4 F at high temperature and low frequency further reflect strong space-charge and defect-assisted polarization. Cole-Cole plots exhibit depressed semicircles and low-frequency tails, modeled by (Rg//Cg//CPEg) + (Rgb//CPEgb) + CPEe circuits, revealing Negative Temperature Coefficient of Resistance (NTCR) behavior via decreasing grain/grain boundary resistances with temperature. Activation energies derived from DC conductivity are 0.803 ± 0.008 eV (grains), 0.884 ± 0.006 eV (grain boundaries), and 0.832 ± 0.001 eV (total), consistent with thermally activated charge transport. Relaxation dynamics derived from modulus spectroscopy (ln-(ωp.T) vs 1000/T) yield an activation energy of 0.846 ± 0.016 eV, in excellent agreement with the total DC conduction value, thereby confirming that thermally activated hopping governs both processes. The AC conductivity behavior is described by the Correlated Barrier Hopping (CBH) model. Overall, these findings demonstrate the potential of NNPO, positioning it as a promising stable solid electrolyte for Na-ion energy storage devices.