Pressure induced Ag2Te polymorphs in conjunction with topological non trivial to metal transition

Abstract

Silver telluride (Ag2Te) is well known as superionic conductor and topological insulator with polymorphs. Pressure induced three phase transitions in Ag2Te have been reported in previous. Here, we experimentally identified high pressure phase above 13 GPa of Ag2Te by using high pressure synchrotron x ray diffraction method in combination with evolutionary crystal structure prediction, showing it crystallizes into a monoclinic structure of space group C2/m with lattice parameters a = 6.081Å, b = 5.744Å, c = 6.797 Å, β = 105.53°. The electronic properties measurements of Ag2Te reveal that the topologically non-trivial semiconducting phase I and semimetallic phase II previously predicated by theory transformed into bulk metals for high pressure phases in consistent with the first principles calculations.

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.…”

“…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. 12 It should be noted that the recent experimental studies and first-principles calculations were carried out on nominally stoichiometric Ag2Te, and the theoretical work assumed that the effect of doping was to shift the Fermi level through the rigid band structure, without consideration of defect states within the gap. In the present work, we take advantage of meticulous preparation of single-phase self-doped chalcogenides recently developed by Schnyders 13 to synthesize samples of Ag2-dTe, d = 2.0´10 -4 , that are not only free of the secondary metallic phase assumed by the earlier theories but in addition have a well-characterized silver deficit.…”