Electronic and vibrational properties of the two-dimensional Mott insulator ${\mathrm{V}}_{0.9}{\mathrm{PS}}_3$ 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.

Figure 1

Crystal structures and parameters of the high- and low-pressure phases of ${\mathrm{V}}_{0.9}{\mathrm{PS}}_3$, taken from [27]. Crystal structure of ${\mathrm{V}}_{0.9}{\mathrm{PS}}_3$ (a) at 11 kbar, the LP phase, and (b) at 177 kbar, the HP-I phase. The HP-I structure has $\beta$ close to $90{}^{\circ }$ so atoms in each plane are aligned with their equivalents in neighboring planes.

Figure 2

Normalized Raman intensities for powder ${\mathrm{V}}_{0.9}{\mathrm{PS}}_3$ taken at room temperature at increasing pressures. The $y$ intercept of each data set has been set to its corresponding pressure value. Four peaks, labeled with arrows, can be identified at ambient pressure. Peak p3 is gradually suppressed and disappears around 90 kbar.

Figure 3

Raman peak positions for powder ${\mathrm{V}}_{0.9}{\mathrm{PS}}_3$ taken at room temperature at increasing pressures, extracted from fits to Fig. 2. All peaks shift upwards in wave number as the pressure is increased, in line with lattice compression, but peak p3 vanishes around 90 kbar and a change of slope in the pressure dependence of peak p2 is evident at around 120 kbar. The equations of the fits are as follows (pressures in kbar, peak positions in ${\mathrm{cm}}^{-1}$): p1, $0.149p+176.3$; p2, $0.514p+251.6$ and, at high pressure, $0.369p+269.2$; p3, $0.376p+275.2$; and p4, $0.125p+366.9$.

Figure 4

Calculated Raman spectra for ${\mathrm{V}}_{0.9}{\mathrm{PS}}_3$ at (a) 11 kbar and (b) 177 kbar, using the structural information from [27]. The peaks can be matched to those shown in Fig. 2, as annotated, except for two small peaks in the high-pressure phase, marked with unlabeled black arrows. The preferred orientation and imperfect powder averaging mean that the experimental and theoretical peak heights will not agree.

Figure 5

Visualizations, looking along the $ab$ planes, with unit cell boundaries shown in light gray, from calculation of Raman vibrational modes: (a) p1, (b) p2, (c) p3, and (d) p4 at 11 kbar. Mode p2 stands out as being an out-of-plane motion along the $c^{*}$ axis.

Figure 6

Calculated partial and total spin-polarized densities of states for ${\mathrm{V}}_{0.9}{\mathrm{PS}}_3$ at 11 and 177 kbar, using structural information from [27]. These are calculated using a Hubbard $U$ of 3 eV for the vanadium on-site repulsion. Reducing this quickly closes the ambient pressure band gap. Inset: Zoomed-in view of the closed gap region from 0 to 2 eV.

Figure 7

Calculated spin-polarized band structures (up spin states in blue and down spin states in red) of ${\mathrm{V}}_{0.9}{\mathrm{PS}}_3$ with a Hubbard $U$ of 3 eV (a) at 11 kbar (band gap of 1.495 eV) and (b) at 177 kbar.

Figure 8

Calculated spin-polarized band structures (up spin states in blue and down spin states in red) of ${\mathrm{V}}_{0.9}{\mathrm{PS}}_3$ with a Hubbard $U$ of 1 eV (a) at 11 kbar (band gap of 0.226 eV) and (b) at 177 kbar.