Electronic Topological Transition in Ag2Te at High-pressure

Abstract

Recently, Ag2Te was experimentally confirmed to be a 3D topological insulator (TI) at ambient pressure. However, the high-pressure behaviors and properties of Ag2Te were rarely reported. Here, a pressure-induced electronic topological transition (ETT) is firstly found in Ag2Te at 1.8 GPa. Before ETT, the positive pressure coefficient of bulk band-gap, which is firstly found in TIs family, is found by both first-principle calculations and in situ high-pressure resistivity measurements. The electrical resistivity obtained at room temperature shows a maximum at 1.8 GPa, which is nearly 3.3 times to that at ambient pressure. This result indicates that the best bulk insulating character and topological nature in Ag2Te can be obtained at this pressure. Furthermore, the high-pressure structural behavior of Ag2Te has been investigated by in situ high-pressure synchrotron powder X-ray diffraction technique up to 33.0 GPa. The accurate pressure-induced phase transition sequence is firstly determined as P21/c → Cmca → Pnma. It is worth noting that the reported isostructural P21/c phase is not existed, and the reported structure of Cmca phase is corrected by CALYPSO methodology. The second high-pressure structure, a long puzzle to previous reports, is determined as Pnma phase. A pressure-induced metallization in Ag2Te is confirmed by the results of temperature-dependent resistivity measurements.

Pressure-induced charge density wave phase in Ag2−δTe

“…This is followed by structural transitions to metallic phases III and IV. The recent measurements of transport properties under pressure by Zhang et al 11 and Zhu et al 12 showed a collapse of the resistivity at the isostructural transition, while the temperature dependence confirmed a semiconducting behavior up to 11 or slightly beyond 12 the isostructural transition and fully metallic beyond it. At low pressures the MR has a quadratic dependence on temperature followed by linear dependence without saturation, changing to purely quadratic as the pressure is increased.…”

“…Since the P21/c structure is maintained through this transition, it must correspond to the electronic isostructural transition identified in previous work. 11,12 The unit cell volume plotted in Fig. 1(c) shows discontinuities at the two structural phase transitions and the inset one at between 1.9 and 2.3 GPa due to the isostructural transition as in Ag2Te.…”

“…2, left) at 1.7 GPa and 3.4 GPa indicate P21/c and Cmca phases, respectively, in accord with previous work. [10][11][12] The structure of the highest pressure phase is controversial: we follow Zhang et al 11 in refining it in the Pnma structure rather than the C2/m adopted by Zhu et al 12 Rietveld refinements at 21.9 GPa and 39.4 GPa ( Fig Lattice parameters derived from Rietveld fits at each measured pressure ( Fig. 1(b)) indicate clearly the P21/c ® Cmca transition at 2.9 GPa and the Cmca ® Pnma transition at around 12.9 GPa.…”

“…8,9 This work was followed by several high-pressure x-ray diffraction studies combined with firstprinciples electronic structure calculations, identifying four phases as the pressure is increased. [10][11][12] Phase I was determined to be semiconducting with topological conducting surface states, transforming into a semi-metallic Phase II through an isostructural transition identified as an electronic topological transition at 1.8-2.0 GPa. This is followed by structural transitions to metallic phases III and IV.…”

Topological nodal-line semimetal in nonsymmorphic Cmce -phase Ag2S

“…31 As temperature increases, Ag 2 S undergoes phase transitions into the body-centered cubic α phase (the so-called argentite) at 453 K or the face-centered cubic γ phase at 859 K, which are electrical conductors. 32,33 In addition to these phases of silver chalocogenides, recently several new allotropes have been theoretically or experimentally discovered, [34][35][36][37] expanding rich physical properties and potential applications in the family of silver chalcogenides.…”