Charge compensation in a layered van der Waals NiPS3 host through various cationic intercalations

Abstract

The layered structure of van der Waals compounds enables facile insertion of guest species between layers, resulting in material multifunctionality through easily modifying its physical properties. Isostructural MPS₃ compounds with 3d transition metal cations such as Mn, Fe, Co and Ni can serve as hosts for relatively small alkali metals as well as larger organic molecules. NiPS₃ is the most exotic representative among them, because despite 30 years of intense research, its electronic structure still evokes numerous questions, not to mention the electronic structure of intercalated NiPS₃. There are two possibilities for electron transfer in semiconducting NiPS₃ upon insertion of electron-donating species, either to a discrete Ni atomic level, or to a molecular level of the (P₂S₆)⁴⁻ unit. We performed a systematic structural and spectroscopic study of NiPS₃ upon electrochemical intercalation of Li, Na and 1-ethyl-3-methylimidazolium (EMIM) cations. Up to 0.5 Li or 0.5 Na per NiPS₃ formula unit can be inserted into free octahedral spaces in the interlayers without visible changes in the diffraction pattern of the host. In contrast, more than 1 EMIM per NiPS₃ unit can be intercalated between host layers leading to a significant interlayer distance expansion from 6.33 Å to 11.3 Å. The charge compensation was found to be different for the three intercalants: upon Li insertion, the electron density increases on the (P₂S₆)⁴⁻ unit and Ni remains redox-inactive, while intercalation of Na leads to reduction of Ni. In contrast, uptake of larger EMIM cations does not result in any changes in Ni, S and P K-edge near edge XANES spectra of NiPS₃ and results in only very little change in their extended X-ray absorption fine structure spectra. It is likely that there is an electrochemical reduction of EMIM cations to heterocyclic carbenes with their possible dimerization. The impact on magnetization of Li and EMIM intercalation was also studied.