High-Pressure Electronic and Vibrational Properties of NiMoO$_ 4$ in the α and ω Phases: Experimental and Density Functional Theory Study

This work investigates the electronic and vibrational properties of NiMoO$_4$ single crystals by optical absorption and Raman spectroscopy under high pressure (0-20 GPa). We focus on the alpha-NiMoO$_4$ phase and the metastable omega-NiMoO$_4$ phase, which forms under compression above 14.5 GPa, thereby enriching the polymorphism of this material. Experimental Raman frequencies and their pressure dependence show excellent agreement with density functional theory (DFT) calculations, enabling precise mode assignment for the monoclinic C2/c (alpha-phase) and P2/c (omega-phase) structures. We provide consistent links between structure, vibrations, and electronics for alpha- and omega-NiMoO$_4$-essential for applications relying on nanoscale materials where phase mixtures (mostly alpha and beta phases) are common. Optical absorption spectra reveal five weak subgap bands associated with Ni$^{2+}$ d-d transitions, together with an absorption edge corresponding to an indirect band gap of 2.99 eV for the alpha-phase and a direct band gap of 1.7 eV for the metastable omega-phase at ambient conditions-thus clarifying the semiconducting nature of the omega-phase, contrary to prior claims of metallicity. Using pressure-tuned Tanabe-Sugano analysis, we unambiguously assign these subgap features. Notably, some previously undetected Ni$^{2+}$ bands become visible under compression, as pressure enables adequate tuning of excited states to reach Fano resonance conditions. In addition, we provide a detailed experimental characterization of the pressure-induced alpha -> omega transition. Remarkably, the high-pressure omega phase is quenchable to ambient conditions, where it remains metastable. We explain the large pressure hysteresis (alpha -> omega at similar to 15 GPa vs DFT-predicted stability above similar to 1 GPa) by its high activation energy and a volume change of 13%. The omega phase exhibits enhanced absorption characteristics-with a direct band gap of 1.7 eV-making it highly attractive for photoactive applications. Our combined experimental and DFT results provide a consistent interpretation of the pressure-driven evolution of structure, vibrations, and electronic properties in this material system.