Description

Topotactic transformations in complex transition-metal oxides provide a powerful route to tune functionality while preserving the underlying crystallographic framework [1]. By selectively modifying oxygen stoichiometry, these reactions can induce major changes in crystal structure, as well as electronic and magnetic ground states while maintaining the overall epitaxial crystal orientation of the thin film. This makes such phase transformations especially attractive for the design of novel material phases inaccessible via standard synthesis methods and for enabling property control through the reversible nature of such reactions.
This PhD project will investigate two representative oxide systems that undergo distinct oxygen-driven topotactic transitions. The first is the rare-earth (R) nickelate family, where reduction from the perovskite phase RNiO3 to the infinite-layer phase RNiO2 (see Figure 1) converts the correlated metal into a superconducting film [2,3]. The second is the layered Carpy–Galy phases A2B2O7 (with A=Sr, La and B=Ti, Nb, Ta) [4], which are insulating and ferroelectric but can be transformed into paraelectric metallic perovskite ABO3 phases.
The project aims to understand and engineer these transformations at the atomic scale in epitaxial thin films. Key ingredients will include the precise control of growth conditions, where oxygen content, strain state, and thickness can be finely tuned. A central question will be how oxygen removal proceeds across the film thickness, how completely the reaction takes place, and which local defects, stacking faults, or intermediate configurations accompany the transition. These transformations will be discussed in relation to the evolution of the thin-film physical properties, with particular emphasis on the emergence of superconducting domains in nickelates and the potential engineering of magnetotransport in Carpy–Galy phases.

Skills required

Academic Background: Master’s degree (or equivalent) in Physics, Materials Science, Chemistry, Nanoscience, or related fields. Technical Skills: Prior experience with transmission electron microscopy, scanning probe microscopy, spectroscopic methods and/or structural characterization techniques such as XRD, RSM is appreciated. Familiarity with complex transition metal oxides, magneto-electric characterization, vacuum-based deposition methods such as pulsed laser deposition or sputtering will be considered a plus. Applicant soft skills: Motivation and willingness to carry out experimental work, good organization skills, perseverance, teamwork.

Bibliography

[1] Z. Meng et al., Advanced Functional Materials, 33, 2305225 (2023).
[2] A. Gutierrez-Llorente et al., Advanced Science 11, 2309092 (2024).
[3] D. Zhang et al., Communications Materials 6, 293 (2025).
[4] E. Gradauskaite et al., Advanced Materials, 2416963 (2025).
[5] A. Raji et al., Advanced Functional Materials, 34, 2409930 (2024).

Keywords

Complex transition metal oxides, ferroelectricity, scanning transmission electron microscopy, electron spectroscopy, superconductivity