Invited
Strongly correlative ionic transport triggered by oxygen-vacancy ordering transition in a doped multiferroic

Long-range correlated fluctuations in order parameters occur at phase transition points, revealing emergent functionalities such as the morphotropic-phase-boundary driven high-k dielectrics, large piezoelectricity, and quantum paraelectricity. Likewise, the correlation length of thermal and quantum fluctuations of atomic positions can be large at the critical point, leading to rapid and coherent ionic conduction. However, the lack of a lattice system that exhibits competing defect orderings has hindered the study of the feasibility of this general thermodynamic wisdom. Here, we demonstrate a route toward coherent thermionic conduction in solids, by exploiting the lowered activation energy associated with oxygen transport in Ca-substituted bismuth ferrite (Bi_1-xCa_xFeO_3-d) thin films [1]. Our demonstration relies on the finding that a compositional phase boundary between two isostructural phases with oxygen vacancy channel orderings competitively along the or crystal axes emerges at a chemical doping ratio (x_Ca ~ 0.45). Regardless of the atomic-scale irregularity in defect distribution at the compositional boundary, the activation energy is largely suppressed to 0.43 eV, compared with the ~0.9 eV measured in otherwise rigid phases. From first-principle calculations, we clarify that the effective short-range attraction between two positively charged oxygen vacancies sharing lattice deformation is the driving force of the long-range order and collective diffusion in the system. The collective phase evolution is necessary to understand the suppression of ionic activation energy. Our findings open a new avenue into strongly correlative ionics.

[1] Ji Soo Lim, Ho-Hyun Nahm, Marco Campanini, Jounghee Lee, Young-Jin Kim, Heung-Sik Park, Jeonghun Suh, Jun Jung, Yongsoo Yang, Tae Yeong Koo, Marta D. Rossell, Yong-Hyun Kim, and Chan-Ho Yang, Critical ionic transport across an oxygen-vacancy ordering transition, Nature Communications 13, 5130 (2022)