Metathesis as an alternative synthesis route to layered sulfides A(LiZn)S2 (A = alkali-metal) with unexpected colors

Abstract

Pure powders of Na(LiZn)S₂ can be obtained through a solid-state reaction, and A(LiZn)S₂ (A = K, Rb, Cs) result from metathesis reactions between alkali-metal chlorides and the same constituents used to prepare the Na(LiZn)S₂ powder. Hence, the metathesis reaction enables extended sulphide chemistry without the use of either H₂S gas or very reactive starting materials. By the metathesis reaction it was possible to obtain relatively pure Cs(LiZn)S₂. Trigonal Na(LiZn)S₂ and tetragonal A(LiZn)S₂ (A = K, Rb, Cs) exhibit significant structural similarities, having (LiZn)S₂ layers that are separated by alkali-metals (Na–Cs). Against expectations, Cs(LiZn)S₂ is orange red in colour, Rb(LiZn)S₂ is strongly yellow, K(LiZn)S₂ is pale yellow, and Na(LiZn)S₂ is colourless. Ultraviolet-visible spectroscopy data on Cs(LiZn)S₂ and Na(LiZn)S₂ contain several shoulders apart from apparent band-edges close to 3.3 eV. In the former, it seems as if the optical excitations range all the way into green (∼600 nm), which concurs with the observed red colour. Nuclear magnetic resonance investigations on cores ¹³³Cs, ²³Na, and ⁷Li suggest that these ions are firmly held in the atomic lattice, as judged by the resonance frequency widths and relatively long nuclear spin relaxation times (T₁), ranging from 10 to 200 seconds for ²³Na and ⁷Li. So there should be only electronic excitations in these compounds. Band-structure calculations of Li–Zn ordered versions of the lattices suggest a direct band-gap in both compounds, corresponding to an excitation from sulphur to zinc. The theoretical band-gaps amount to 2.54 eV for CsLiZnS₂ and 1.85 eV for NaLiZnS₂, and the steep edges in the density of states are found at 3.3 eV for both cases. As no Li–Zn ordering is observed by X-ray diffraction, there must be an inherent atomic disorder. By theoretical simulations, local Li–Zn anti-site orderings were introduced and the resulting electronic structure was evaluated. However, the simulated optical behaviour could only tentatively explain the spectroscopic data of Na(LiZn)S₂; the orange red colour of Cs(LiZn)S₂ must be an even more complex phenomenon, as the Li–Zn simulations were insufficient to explain the relatively strong optical activity in the range between 400 and 600 nm.