Advances and Challenges in The Single-Crystal Synthesis of Infinite-Layer and Ruddlesden-Popper Phase Nickelates

Unconventional superconductivity remains a key focus in condensed matter research, traditionally centered on material classes such as cuprates, iron pnictides, and heavy fermion compounds. Recently, rare-earth nickel oxides have emerged as a new class with potential for unconventional superconducting behavior. Within this nickelate family, two types of structures have drawn particular interest. The first type comprises nickelates with the infinite-layer crystal structure, such as Nd_0.8Sr_0.2NiO₂, showing superconducting transition temperatures (<i>T</i>_c) up to 20 K. The second type encompasses Ruddlesden-Popper phase nickelates, such as La₃Ni₂O₇, which under hydrostatic pressure manifest a remarkably high <i>T</i>_c of 80 K. Despite these promising observations, the possibly distinct mechanisms driving the superconductivity in these two types of nickelates are not yet fully understood.<br/><br/>To address this, further investigations employing advanced spectroscopic methods are warranted, which are capable of probing the complex interplay between electronic, magnetic, and lattice degrees of freedom typically found in unconventional superconductors. Accordingly, a tailored sample synthesis is desirable to meet the specific requirements of each spectroscopic technique, such as large sample masses for neutron scattering and cleavable single crystals for surface sensitive techniques.<br/><br/>Yet, both nickelate families present unique challenges in material synthesis that have previously hindered the application of certain spectroscopic methods. For infinite-layer nickelates, synthesis can only be realized via a topotactic reduction of the parent perovskite phase. This process is invasive for the sample surface and has previously been executed primarily on thin films and polycrystalline powders. In this talk, I will discuss our advances in the topotactic reduction of single-crystalline samples [1,2]. Furthermore, we have performed resonant ineleastic x-ray scattering (RIXS) experiments on these crystals, providing unprecedented insights into spin excitations and charge ordering of the bulk phase [3]. The topotactic infinite-layer crystals are also suitable for future experiments using neutron or surfaces sensitive spectroscopies.<br/><br/>In the case of La₃Ni₂O₇, sizable single crystals can be readily grown using the optical floating zone method and do not necessitate topotactic treatment. However, we recently observed that these crystals exhibit multiple crystallographic phases and a pronounced sensitivity to oxygen stoichiometry, affecting their physical properties. I will delineate how a close feedback loop between advanced characterization methods and iterative adjustments of growth parameters facilitates control over the material's phases, thereby enabling the realization of single crystals optimized for subsequent spectroscopic studies.<br/><br/>[1] P. Puphal, Y.-M. Wu, K. Fürsich, H. Lee, M. Pakdaman, J. A. N. Bruin, J. Nuss, Y. E. Suyolcu, P. A. van Aken, B. Keimer, M. Isobe, and M. Hepting, Topotactic transformation of single crystals: From perovskite to infinite-layer nickelates, Sci. Adv. <b>7</b>, eabl8091 (2021).<br/><br/>[2] P. Puphal, B. Wehinger, J. Nuss, K. Küster, U. Starke, G. Garbarino, B. Keimer, M. Isobe, and M. Hepting, Synthesis and physical properties of LaNiO2 crystals, Phys. Rev. Materials <b>7</b>, 014804 (2023).<br/><br/>[3] S. Hayashida, V. Sundaramurthy, P. Puphal, M. Garcia-Fernandez, Ke-Jin Zhou, M. Isobe, Y.-M. Wu, Y. E. Suyolcu, P. A. van Aken, M. Minola, B. Keimer, and M. Hepting, (unpublished)