Tailoring Dielectric Permittivity in Gd_xCe_1-xO_2-δ Films by Ionic Defects

Fluorite-based crystalline materials have the general chemical formula AX₂ (A = Ca, Hf, Zn, Zr, Ce, while X = F, O) and find wide application in fuel cells, electroceramics, oxygen sensors and exhaust reduction systems. Despite their rather simple and prototypical crystallographic structure, “fluorites” remain the subject of much academic research and unexpected scientific discoveries [1], such as ferroelectricity in HfO₂-based thin films or large electromechanical coupling in polycrystalline Gd-doped CeO_2-<i>x</i> (CGO) ceramics/films [2]. Moreover, a recent discovery has demonstrated the potential to induce significant piezoelectric effects in nominally centrosymmetric CGO by breaking symmetry through the control of ionic defects, specifically oxygen vacancies (V_O) [2]. It was found that the motion and redistribution of defects induced by an electric field results in notable changes in the dielectric permittivity of the system, which are closely associated with the magnitude of the electromechanical effects [3]. Hence, controlling the dynamic V_O in the CGO and similar oxides is key for achieving significant permittivity and electromechanical effects. In this work, we effectively tuned the V_O contents in epitaxial CGO(001) films grown on Nb-doped SrTiO₃(001) single crystal using pulsed laser deposition. Our findings show that the control of V_O contents in the epitaxial films is highly limited, regardless of the oxygen environment (oxygen partial pressure, <i>P</i>_O), during high-temperature growth (e.g., <i>T</i> = 700 °C). In contrast, an effective control of V_O contents in the epitaxial films was achieved by utilising a two-step growth technique. Subsequently, we noted a strong variation in the V_O content in the engineered CGO films and the emergence of significant apparent dielectric permittivity, thus enabling the generation of substantial electromechanical coupling and piezoelectric effect.<br/><br/><b>Reference</b><br/>[1] U. Schroeder, et al., <i>Nat. Rev. Mater.</i>, <i>7</i>(8), 653–669 (2022).<br/>[2] D.-S. Park, et al., <i>Science</i>, <i>375</i>, 653–657 (2022).<br/>[3] D. Damjanovic, <i>Rep. Prog. Phys.</i>, <i>61</i>(9), 1267–1324 (1998).