Utilizing non-stoichiometry in Nd2Zr2O7 pyrochlore: exploring superior ionic conductors

Abstract

To explore materials with high ionic conductivity without any heterovalent substitution, a non-stoichiometric Nd_2−xZr_2+xO_7+x/2 (−0.2 ≤ x ≤ 0.4) pyrochlore system was synthesized via a combustion process. All the compositions were characterized by X-ray diffraction and revealed perfect pyrochlore-type compounds. Interestingly, Raman spectroscopy exhibited an enhanced oxygen disorder in Nd_1.6Zr_2.4O_7.2 (x = 0.4) relative to the rest of the compositions, which was manifested in a substantially high pre-exponential factor for this composition. Detailed AC impedance analysis revealed that the ionic conductivity for Nd_1.6Zr_2.4O_7.2 was higher by two orders of magnitude despite the similar structures adopted by all the other compositions. X-ray photoelectron spectroscopy provided support for the high ionic conductivity observed. Modulus spectroscopic analysis related the microscopic energy barrier to the macroscopic activation energy. The highlight of the study was the high ionic conductivity (1.78 × 10⁻² S cm⁻¹ at 973 K) for Nd_1.6Zr_2.4O_7.2, which is comparable to the highest values reported for a zirconia-based system at this temperature. This was attributed to the optimized hybrid structure consisting of fluorite-type disordered zones interspersed in the ordered pyrochlore structure obtained under suitable annealing conditions. The present study highlights the utilization of non-stoichiometry in the pyrochlore structure to tailor its ionic conductivity. Such high ionic conductivity observed in Nd_1.6Zr_2.4O_7.2 makes it a superior candidate for electrolyte materials as it is endowed with the stability of zirconia-based systems and has ionic conductivity comparable to ceria-based systems.