Pressure-dependent excitonic instability and structural phase transition in ${\mathrm{Ta}}_2{\mathrm{NiS}}_5$: Raman and first-principles study

${\mathrm{Ta}}_2{\mathrm{NiS}}_5$, a semiconductor at ambient conditions, does not exhibit an excitonic insulating state like its selenium counterpart ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$, owing to its large band gap. Using a combination of Raman spectroscopy and analysis with first-principles effective Hamiltonian, we explore its instability toward an excitonic insulating state as a function of pressure, and affirm that excitonic insulating state does not get stabilized in ${\mathrm{Ta}}_2{\mathrm{NiS}}_5$ with pressure. We observe pressure-induced structural phase transition from its orthorhombic $Cmcm$ structure to another orthorhombic $Pmnm$ structure, with onset at $\sim$ 4.2 GPa and this transition gets completed at $\sim$ 6 GPa. We observe Raman signatures of an additional phase transition at $\sim$ 10.8 GPa, which is suggested to be associated with a semiconductor to metal transition.

Figure 1

Pressure evolution of Raman spectra in the spectral range of (a) 30–115 ${\mathrm{cm}}^{-1}$, (b) 115–205 ${\mathrm{cm}}^{-1}$, (c) 205–380 ${\mathrm{cm}}^{-1}$, and (d) 380–500 ${\mathrm{cm}}^{-1}$ respectively. Solid lines (red and blue) are the Lorentzian fits to experimental data points (black). Disappearance/appearance of new modes is indicated by asterisk symbol and arrows respectively. The top panel shows the Raman spectrum at $\sim$ 0.1 GPa in the return pressure cycle, indicated by R next to the pressure value.

Figure 2

(a) Raman shift versus pressure plot. Solid red lines are linear fits to the observed frequencies. The numbers next to the straight lines are the fitted value of $\mathrm{d}\omega$/dP in ${\mathrm{cm}}^{-1}$/GPa. The vertical shaded region from $P\sim$ 4.2 to 6.0 GPa denotes the onset and completion of first phase transition and the black dashed line at $P\sim$ 10.8 GPa denotes the second phase transition. (b) Calculated frequencies of Raman active phonon modes at $\mathrm{\Gamma }$ point, numbers denote pressure slope of linear fit which is in good agreement with experiments. The blue dashed line denotes the theoretically calculated first transition at $P\sim$ 3.8 GPa. Different symbols and colors are used to denote phonon modes having specific symmetry irreducible representations of the respective space groups. Please note, we provide calculated Raman spectra till $P$ = 7 GPa, because the high-pressure phase ($P>10$ GPa) remains unidentified.

Figure 3

(a) Conventional and (b) primitive unit cells of ${\mathrm{Ta}}_2{\mathrm{NiS}}_5$ in $Cmcm$ crystal structure, and (c) Brillouin zone of primitive cell with high symmetry path shown in green, (d) electronic structure exhibits a gap of $E_g=$ 0.39 eV at the $\mathrm{\Gamma }$ point, and orbital-projected density of states show Ta-$5d$ orbitals dominate the states at conduction band minimum (CBM) and Ni-$3d$, S-$3p$ orbitals contribute primarily to the states at the valence band maximum (VBM).

Figure 4

(a) Difference in zone center frequencies of phonons of orthorhombic ($Cmcm$) ${\mathrm{Ta}}_2{\mathrm{NiS}}_5$ at $P=0$ GPa calculated using Hubbard-$U=0$ eV and 6 eV (at Ta $5d$ orbitals). Symmetry irreducible representations (irreps.) are given for the modes exhibiting frequency changes larger than 20 ${\mathrm{cm}}^{-1}$. Modes marked with colored circles in (a) are visualized in (b), (c), and (d). Black arrow denotes displacement of ions. Ta, Ni and S are denoted by blue, red and green balls, respectively, and solid black box displays conventional unit cell of the orthorhombic $Cmcm$ crystal structure.

Figure 5

(a) Evolution of relative enthalpies of $Pmnm$ (HP1) and $P2/n$ (HP1') crystal structures of ${\mathrm{Ta}}_2{\mathrm{NiS}}_5$ with pressure. There is a transition to HP1 structure at $P\sim$ 3.8 GPa (observed experimentally at room temperature) and transformation to HP1' structure is around 3.0 GPa (at low temperature). (b) Schematic phase diagram of ${\mathrm{Ta}}_2{\mathrm{NiS}}_5$ sketched based on our calculations.

Figure 6

Variation in the estimated ${\sum }_{\nu }\frac{F_{\nu }^2}{2K_{\nu }{E}_g}$ of ${\mathrm{Ta}}_2{\mathrm{NiS}}_5$ (blue) and ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ (red) in $C2/c$ crystal structure with pressure confirms no onset of excitonic instability of ${\mathrm{Ta}}_2{\mathrm{NiS}}_5$ at low pressures in contrast to ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ (please note, EI phase is stable in the region above the horizontal dotted line).