Electronic and vibrational properties of the two-dimensional Mott insulator<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:msub><mml:mi mathvariant="normal">V</mml:mi><mml:mrow><mml:mn>0.9</mml:mn></mml:mrow></mml:msub><mml:msub><mml:mi>PS</mml:mi><mml:mn>3</mml:mn></mml:msub></mml:mrow></mml:math>under pressure

We present a Raman spectroscopic study of the layered antiferromagnetic Mott insulator ${\mathrm{V}}_{0.9}{\mathrm{PS}}_{3}$ and demonstrate the evolution of the spectra with applied quasihydrostatic pressure. Clear features in the spectra are seen at the pressures identified as corresponding to a structural transition between 20 and 80 kbar and the insulator-metal transition at 120 kbar. The feature at 120 kbar can be understood as a stiffening of interplanar vibrations, linking the metallization to a crossover from two- to three-dimensionality. Theoretical ab initio calculations, using the previously determined high-pressure structures, were able to reproduce the measured spectra and map each peak to specific vibration modes. We additionally show calculations of the high-pressure band structure in these materials, where the opening of a band gap with an included Hubbard $U$ term and its subsequent closing with pressure are clearly demonstrated. This little-studied material shows great promise as a model system for the fundamental study of low-dimensional magnetism and Mott physics.

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