New Compounds and Phase Selection of Nickel Sulfides via Oxidation State Control in Molten Hydroxides

Molten salts are promising reaction media candidates for the discovery of novel materials; however, they offer little control over oxidation state compared to aqueous solutions. Here, we demonstrated that when two hydroxides are mixed, their melts become fluxes with tunable solubility, which are surprisingly powerful solvents for ternary chalcogenides and offer effective paths for crystal growth to new compounds. We report that precise control of the oxidation state of Ni is achievable in mixed molten LiOH/KOH to grow single crystals of all known ternary K-Ni-S compounds. It is also possible to access several new phases, including a new polytope of β-K₂Ni₃S₄, as well as low-valence KNi₄S₂ and K₄Ni₉S₁₁. KNi₄S₂ is a two-dimensional low-valence nickel-rich sulfide, and β-K₂Ni₃S₄ has a hexagonal lattice. Moreover, using KNi₄S₂ as a template, we obtained a new layered binary Ni₂S by topotactic deintercalation of K. The new binary Ni₂S has a van der Waals gap and can function as a new host layer for intercalation chemistry, as demonstrated by the intercalation of LiOH between its layers. The oxidation states of low-valence KNi₄S₂ and Ni₂S were studied using X-ray absorption spectroscopy and X-ray photoelectron spectroscopy. Density functional theory calculations showed large antibonding interactions at the Fermi level for both KNi₄S₂ and Ni₂S, corresponding to the flat-bands with large Ni-d character.