Probing electron–phonon coupling in magnetic van der Waals material NiPS₃: A non-magnetic site-dilution study

NiPS 3 is a van der Waals antiferromagnet which has been shown to exhibit spin-phonon and spin-charge coupling in the antiferromagnetically ordered state below T N = 155 K. It is also a rare Ni-based negative charge-transfer-type insulator with Ni valence in a linear superposition state ψ = α d 8 + β d 9 L − + γ d 10 L − 2 , where L − is the ligand hole. Here, we study high-quality single-crystals of Ni 1− x Zn x PS 3 (0 < x < 0.2) using temperature-dependent specific heat and Raman spectroscopy probes. We show that in pristine NiPS 3 , the phonon mode at 176 cm −1 (P 2 ), associated with the vibrations of Ni ions, exhibits a distinct Fano asymmetry. The Fano resonance is particularly pronounced in the paramagnetic phase above T N , which was further confirmed by temperature dependent Raman data on the Zn-doped crystals. In the Zn-doped crystals, while the magnetism weakens following the mean-field prediction for site-dilution in a honeycomb lattice, the Fano coupling | 1 / q | strengthens, increasing monotonically with increasing Zn-doping. The x-ray photoemission spectra suggest an increase in the weight of the d 9 and d 10 components in the Zn-doped crystals. These observations indicate the presence of strong electron–phonon coupling in Ni 1− x Zn x PS 3 , in addition to the spin-phonon, and spin-charge coupling previously reported.