Pressure induced electronic topological transitions in the wide band gap semiconductor Sb 2 S 3 (E g = 1.7-1.8 eV) with similar crystal symmetry (SG: Pnma) to its illustrious analog, Sb 2 Se 3 , has been studied using Raman spectroscopy, resistivity and the available literature on the xray diffraction studies. In this report, the vibrational and the transport properties of Sb 2 S 3 have been studied up to 22 GPa and 11 GPa, respectively. We observed the softening of phonon modes A g (2), A g (3) and B 2g and a sharp anomaly in their line widths at 4 GPa. The resistivity studies also shows an anomaly around this pressure. The changes in resistivity as well as Raman line widths can be ascribed to the changes in the topology of the Fermi surface which induces the electron-phonon and the strong phonon-phonon coupling, indicating a clear evidence of the electronic topological transition (ETT) in Sb 2 S 3 . The pressure dependence of a/c ratio plot obtained from the literature showed a minimum at ~ 5 GPa, which is consistent with our high pressure Raman and resistivity results. Finally, we give the plausible reasons for the non-existence of a non-trivial topological state in Sb 2 S 3 at high pressures.

We explore the response of Ir 5d orbitals to pressure in β-Li2IrO3, a hyperhoneycomb iridate in proximity to a Kitaev quantum spin liquid (QSL) ground state. X-ray absorption spectroscopy reveals a reconstruction of the electronic ground state below 2 GPa, the same pressure range where x-ray magnetic circular dichroism shows an apparent collapse of magnetic order. The electronic reconstruction, which manifests a reduction in the effective spin-orbit (SO) interaction in 5d orbitals, pushes β-Li2IrO3 further away from the pure J eff = 1/2 limit. Although lattice symmetry is preserved across the electronic transition, x-ray diffraction shows a highly anisotropic compression of the hyperhoneycomb lattice which affects the balance of bond-directional Ir-Ir exchange interactions driven by spin-orbit coupling at Ir sites. An enhancement of symmetric anisotropic exchange over Kitaev and Heisenberg exchange interactions seen in theoretical calculations that use precisely this anisotropic Ir-Ir bond compression provides one possible route to realization of a QSL state in this hyperhoneycomb iridate at high pressures.The novel electronic ground states of 5d-based compounds driven by spin-orbit interactions continue to provide an excellent playground for the realization of unconventional quantum phases of matter including topological insulators [1-4] and quantum spin-liquids (QSLs) [5][6][7]. One example of the latter is the non-trivial QSL ground state of the Kitaev model [8], a rare example of a solvable interacting quantum model with Majorana fermions as its elementary excitations. Material candidates for possible realization of the Kitaev model include honeycomb-based-lattice systems with strong spin-orbit coupling [6,9], such as the two and three-dimensional honeycomb iridates, α-Li(Na) [7,[20][21][22] as well as α-RuCl 3 [23,24]. However, it is experimentally established that these materials order magnetically at low temperatures [17,18,20,[25][26][27], spoiling numerous attempts to realize the Kitaev QSL. Hence, tuning structure and related intricate interactions present in these materials through chemical or physical pressure provides a potential route to introduce magnetic frustration and realize novel phases of matter.In this Letter we have investigated the electronic and structural response of β-Li 2 IrO 3 to high pressure. Xray absorption near edge structure (XANES) measurements at Ir L-edges reveal a dramatic suppression of the isotropic Ir (L 3 /L 2 ) branching ratio at P ∼ 1.5 GPa, signaling a reduction in the effective strength of spinorbit interactions in the 5d band. This is the same pressure at which net magnetization in applied field collapses [17]. The reconstructed electronic state preserves the L z / S z orbital-to-spin moment ratio of Ir magnetic moments and the insulating ground state indicating that spin-orbit interactions and Mott physics con-

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We report the effect of strong spin orbit coupling inducing electronic topological and semiconductor to metal transitions on the thermoelectric material AgBiSe2 at high pressures. The synchrotron X-ray diffraction and the Raman scattering measurement provide evidence for a pressure induced structural transition from hexagonal (α-AgBiSe2) to rhombohedral (β-AgBiSe2) at a relatively very low pressure of around 0.7 GPa. The sudden drop in the electrical resistivity and clear anomalous changes in the Raman line width of the A1g and Eg(1) modes around 2.8 GPa was observed suggesting a pressure induced electronic topological transition. On further increasing the pressure, anomalous pressure dependence of phonon (A1g and Eg(1)) frequencies and line widths along with the observed temperature dependent electrical resistivity show a pressure induced semiconductor to metal transition above 7.0 GPa in β-AgBiSe2. First principles theoretical calculations reveal that the metallic character of β-AgBiSe2 is induced mainly due to redistributions of the density of states (p orbitals of Bi and Se) near to the Fermi level. Based on its pressure induced multiple electronic transitions, we propose that AgBiSe2 is a potential candidate for the good thermoelectric performance and pressure switches at high pressure.

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Optimally doped iron-chalcogenide superconductor Fe1.03Se0.5Te0.5 has been investigated under high pressures using synchrotron-based x-ray diffraction and mid-infrared reflectance measurements at room temperature. The superconducting transition temperature (Tc) of the same sample has been determined by temperature-dependent resistance measurements up to 10 GPa. The tetragonal phase (P4/nmm) is found to exist in phase-separated states where both the phases have remarkably high compressibility. A first-order structural transition to the orthorhombic phase (Pbnm) is reported above 10 GPa. For the tetragonal phase, a strong correlation is observed between the Fe(Se,Te)4 tetrahedral deformation and the sharp rise of Tc up to ∼ 4 GPa, above which Tc shows marginal pressure dependence at least up to 10 GPa. The evolution with pressure of the optical conductivity shows that with increasing pressure the tetragonal phase approaches towards a conventional metallic state. Above ∼ 6 GPa, the Drude term reduces drastically, indicating poor metallic character of the high-pressure orthorhombic phase.

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The structural and vibrational properties of the spin-gapped system Cu(2)PO(4)(OH) have been investigated at room temperature under high pressure up to ~20 GPa by Raman scattering and synchrotron-based x-ray diffraction and infrared (IR) spectroscopic measurements. The orthorhombic phase (space group Pnnm, z = 4) remains stable up to at least 7 GPa where it undergoes a weakly first order structural transition (with negligible volume drop) to a monoclinic phase (space group P2(1)/n, z = 4) with an abrupt monoclinic distortion. Refinement of atomic positions has been performed for the low pressure phase. The conspicuous changes in the vibrational spectra (Raman as well as far-IR) confirm this phase transition. At further higher pressures the monoclinic angle increases rapidly and the system transforms irreversibly into a disordered phase. Detailed vibrational analyses have been performed in the orthorhombic phase and pressure-induced structural evolution has been correlated with the vibrational modes corresponding to the Cu-O bonds. A strong negative pressure dependence of hydroxyl mode frequencies (as observed from the mid-IR absorption spectra) supports the pressure-induced structural disordering at higher pressures.