Correlation between Structural Changes and Electrical Transport Properties of Spinel ZnFe₂O₄ Nanoparticles under High...

The structural phase transition of synthetic ZnFe₂O₄ nanoparticles (ZFO NPs) is investigated as a function of pressure up to 40.6 GPa at room temperature for the first time, and its associated intriguing electrical transport properties are resolved from in situ impedance spectra and magnetoresistivity measurements. Significant anomalies are observed in the properties of the grain boundary resistance ( R_gb), the relaxation frequency ( f_max), and the relative permittivity (ε_r) in the ZFO NPs under the pressures around 17.5-21.5 GPa. These anomalies are believed to be correlated with a cubic-to-orthorhombic phase transition of ZnFe₂O₄ at the pressures between 21.9 and 25.7 GPa, which is found to be partially reversible. The pressure-tuned dielectric properties are measured for the cubic and the orthorhombic phases of ZFO, respectively. Remarkably, R_gb decreases up to 6 orders of magnitude as a function of pressure in the cubic phase. The dielectric polarization is obviously strengthened with increased f_max and decreased ε_r with pressure in the orthorhombic phase. Furthermore, it is confirmed that the external pressure effectively improves the electrochemical stability of the sample based on the cycled measurements of the impedance spectra at various pressures. The changes in the complex permittivity (ε', ε″) and the dielectric loss tangent (tan δ) with frequency reveal the irreversible increase in the dielectric loss accompanied by phase transition. The MR measurements indicate that ZFO NPs are superparamagnetic under high pressure of up to 40 GPa. The transmission electron microscopy images reflect the decrease in the grain boundary number and some local amorphization of grains after compression, which provides good explanations for the changes in the electrical transport properties as a function of pressure. Herein, the structural and electrical properties of ZnFe₂O₄ NPs generated are preserved by quenching the high-pressure phase to ambient conditions, thus providing great choices of ferrites materials for a variety of applications.