Pressure-induced structural transitions and metallization in Ag${}_2$Te

High-pressure in situ synchrotron x-ray diffraction experiments were performed on Ag${}_2$Te up to 42.6 GPa at room temperature, and four phases were identified. Phase I (β–Ag${}_2$Te) transformed into isostructural phase II at 2.4 GPa, and phase III and phase IV emerged at 2.8 and 12.8 GPa, respectively. Combined with first-principles calculations, we solved the phase II and phase III crystal structures and determined the compressional behavior of phase III. Electronic band structure calculations show that the insulating phase I with a narrow band gap first transforms into the semimetallic phase II with the perseverance of topologically nontrivial nature and then to the bulk metallic phase III. Density of states calculations indicate the contrasting transport behavior for Ag${}_{2-\delta }$Te and Ag${}_{2+\delta }$Te under compression. Our results highlight pressure's dramatic role in tuning Ag${}_2$Te's electronic band structure and its novel electrical and magnetotransport behaviors.

Figure 1

(a) Representative XRD patterns for Ag${}_2$Te under pressure up to 42.6 GPa (λ $=$ 0.373 79 Å). Either new peaks or peaks with different intensity are marked by circles, diamonds, and asterisks for phases II, III, and IV, correspondently. Arrows indicate diffraction peaks from the Ne pressure medium. (b) Schematic view of Ag${}_2$Te phase I, II, and III structures.

Figure 2

Selected Ag1 to nearby Te distance vs pressure. Filled and half-filled circles are from experiments with Ne and silicone oil, respectively, as the pressure medium. Errors given by the GSAS EXPGUI package are smaller than the marker sizes.

Figure 3

Volume per Ag${}_2$Te formula unit vs pressure. The inset shows the normalized cell parameters $a$/$a_0$, $b$/$b_0$, and $c$/$c_0$. Filled and half-filled circles are from experiments with Ne and silicone oil, respectively, as the pressure medium. Errors given by the GSAS EXPGUI package are smaller than the marker sizes.

Figure 4

Calculated band structure of (a) phase I, (b) phase III, and (c) phase III. The red dots indicate the projection of the Ag $s$ orbital; in (a) and (b), it goes down to occupied from unoccupied around $\Gamma$, which indicates the band inversion. In (d), Fermi level is shifted to 0 eV. These band structures present (a) phase I with a narrow band gap, (b) the semimetallic phase II, and (c) the metallic phase III. (d) Calculated DOS of phases I, II, and III.