Topotactic transformations of single-crystals

Synthetic routes to materials in which anions are partially removed, inserted, or exchanged are rapidly gaining attention in solid-state physics and chemistry. Along these lines, especially topotactic transformations, involving structural changes between related crystal structures, have open up possibilities to prepare entirely novel families of quantum materials. Whereas most such “soft chemistry” experiments have been carried out on polycrystalline powders or thin-films, the topotactic modification of single-crystals has been realized only sparsely. Recently, we demonstrated that single-crystals of the perovskite material La_1-xCa_xNiO₃ can be transformed to the infinite-layer phase La_1-xCa_xNiO₂, using CaH₂ as the reducing agent. This first successful reduction of a bulk perovskite crystal to an infinite-layer crystal provides new perspectives for the transformation of three-dimensional to quasi-two-dimensional materials.

Further investigation of infinite-layer La_1-xCa_xNiO₂ crystals is particularly interesting, since superconductivity and antiferromagnetic spin excitations were recently discovered in infinite-layer nickelate thin-films with a similar stoichiometry. The discovery of superconductivity in nickelates is the result of a decades-long quest to create a new superconductor that mimics the properties of cuprate superconductors, which are known for their exceptionally strong superconductivity. Nevertheless, recent spectroscopic studies indicated that the electronic structure of infinite-layer nickelate includes significant distinctions to cuprates, and the nature of superconductivity in nickelates has not be clarified yet.

In the scope of this undergraduate research project, the student will be involved in the synthesis of perovskite nickelate single-crystals and their topotactic reduction to the infinite-layer phase. Furthermore, the student will learn how to use state-of-the-art characterization methods, such as magnetic susceptibility and electrical transport measurements at the Max Planck Institute for Solid State Research. The opportunity to participate in neutron spectroscopy experiments on topotactically transformed crystals and powders at major facilities, such as FRM II (Garching, Germany) and ILL (Grenoble, France), might be possible. Ideally, the research will contribute to a deeper understanding of infinite-layer nickelates and other topotactically reduced materials.

Fig. 1 | Infinite-layer nickelates RNiO₂ (R = La, Pr, Nd) can be obtained from perovskite nickelates RNiO₃ via a soft-chemistry topotactic reduction using CaH₂ as a reducing agent.