8:15 PM - CT04.05.01
Phase Selection of Nickel Sulfides via Precise Oxidation State Control in Molten Hydroxides—A High-Temperature Aqueous Analogue
Xiuquan Zhou1,David Mandia1,Duck-Young Chung1,Mercouri Kanatzidis1,2
Argonne National Laboratory1,Northwestern University2
High-temperature solutions are promising for discovery of novel materials with interesting properties relevant to superconductivity, magnetism, energy conversion, etc. Despite highly effective for exploratory synthesis, they are much less predictable and offer little to no control of the oxidation state compared to aqueous solutions. Here, we demonstrate that molten hydroxides not only offers crystal growth but also exhibit similar acid-base chemistry like water. Although never before they have been used for the synthesis of a chalcogenide, we found it was surprisingly powerful. By precise oxidation state control in hydroxide mediums, not only we were able to grow single crystals of all known ternary K-Ni-S, we have successfully isolated several new phases. Among them, we have identified a new low-valence nickel-rich sulfide, KNi4S2 and discovered polytypism in the kinetically stablized K2Ni3S4. By controlling the polytypism in K2Ni3S4, we could obtain a dilute Kagome lattice, which could be a new spin liquid materials. In addition, using KNi4S2 as a template, we obtained a new layered binary Ni2S by deintercalating K and a LiOH-intercalated Ni2S by exchanging K with LiOH. This new Van der Waals building block of Ni2S proves to be a new host layer for intercalation chemistry. The rich acid-base chemistry in molten hydroxides can lead to rational discovery of new materials.
This new compound KNi4S2 share great structural similarities to the tetragonal KNi2S2. However, unlike KNi2S2, it consists of doubly stacked Ni square sheets between two sulfur square sheets instead of edge-sharing NiS4 tetrahedra. Thus, each Ni forms both Ni-S ionic bonding and Ni-Ni metallic bonding. In addition to its exotic crystal structure, this low-valence Ni compound also shares great similarities with nickelate superconductors. Recently, new excitement emerges following the report of superconductivity (Tc up to 15 K) in a heterostructure of NiO2 infinite-layer on pervoskite.1 One of the most extraordinary features of this nickelate superconductor is its oxidation state, +1.0-1.2, which is nearly isoelectronic with the holedoped high-Tc cuprate superconductors.2 However, unlike the parent phase of the curpate superconductors (antiferromagnetic), no long-range magnetic ordering has yet observed in the nickelate system. Thus, it is of great importance to elucidate the underlying magnetic ordering or lack thereof in the low-valence nickelate system. Here, our new K2Ni3S4 compound not only shares similar Ni-square net with both the nickelate and Fe-based superconductors, but also offers a rare opportunity to study the long-range magnetic ordering in low-valence Ni compounds.
To fully understand their electronic properties, we carried out density functional theory (DFT) calculations. Very interestingly, after deintercalation the binary Ni2S does not show a simple change in electron-filling. Instead, close to its Fermi level, an electron pocket and almost a hole pocket appear at the G and M points, respectively, whereas there was only an electron pocket at the G point for the parent KNi4S2. This feature highly resemble the Fe-based superconductors especially the 122-type KFe2Se2 and the LiOH-intercalated FeSe. Although Ni2S is not superconducting, but the magnetism is suppressed after the deintercalation. In addition, the Fermi surface nesting at the M point is not complete as there is no hole pocket for Ni2S. However, if we can future oxidize Ni to between Ni +1-1.5, then a electron and hole pocket pair could be created and lead to superconductivity. This will link Fe-based superconductors to the new nickelate superconductors.
1. Li, D.; Lee, K.; Wang, B. Y.; Osada, M.; Crossley, S.; Lee, H. R.; Cui, Y.; Hikita, Y.; Hwang, H. Y. Nature 2019, 572, 624–627.
2. Bednorz, J. G.; Muller, K. A. Zeitschrift fur Physik B Condensed Matter 1986, 64, 189–193.