Laser-induced crystallization and phase transitions of ${\mathrm{As}}_2{\mathrm{Se}}_3$ under high pressure

Utilizing the synchrotron x-ray powder diffraction ${\mathrm{As}}_2{\mathrm{Se}}_3$ has been investigated under high pressure in a diamond anvil cell with hydrogen and neon applied as the pressure media. For both systems the amorphous samples were compressed to ca. 2–3 GPa and heated in situ by a laser, which led to their crystallization in the $R$-3 $m, {\mathrm{Bi}}_2{\mathrm{Te}}_3$-type phase. During further compression this phase transforms to the $C2/m, \beta \text{−}{\mathrm{Sb}}_2{\mathrm{Te}}_3$-type structure, and they coexist within the pressure range of 21.5–33.0 GPa. The latter phase is observed up to the highest achieved pressure of 52.0 GPa, revealing no signs of reaction with the hydrogen pressure-transmitting medium. The pressure evolution of the volume of the reported phases has been described by the third-order Birch-Murnaghan equation of state. The relative stability of the experimentally detected crystalline phases is confirmed by the density functional theory calculations. The pressure-driven evolution of the As–Se distances and coordination number has been discussed in comparison to the recent findings concerning the amorphous $a\text{−}{\mathrm{As}}_2{\mathrm{Se}}_3$ phase.

Figure 1

The crystal unit cells and the coordination environments of As in the most important structures of ${\mathrm{As}}_2{\mathrm{Se}}_3$ and its heavier siblings considered in this work. As—purple balls; Se—yellow balls.

Figure 2

The x-ray powder diffraction patterns of ${\mathrm{As}}_2{\mathrm{Se}}_3$ compressed in (a) $\mathrm{Ne}\left(\lambda =0.3344\AA \right)$; (b) ${\mathrm{H}}_2\left(\lambda =0.3547\AA \right)$. The patterns from amorphous samples, measured before laser heating, are shown at the bottom of each dataset (and also from liquid Ne pressure medium below 4 GPa, if applicable).

Figure 3

The molecular volume of phases III and IV of ${\mathrm{As}}_2{\mathrm{Se}}_3$ as a function of pressure. The refined values have been presented as the filled symbols; the volumes estimated on the basis of measured $d$ spacings are shown as the open symbols.

Figure 4

(a) The calculated enthalpy (per formula unit, f.u.) of several crystalline phases known for ${\mathrm{As}}_2{\mathrm{Se}}_3$ and related compounds. The values of enthalpy have been normalized to the enthalpy of phase III. (b) The comparison of the calculated volume and the equation of state derived from the experimental data.

Figure 5

The summary of the As–Se distances (intra- and interlayer) in the most relevant phases of ${\mathrm{As}}_2{\mathrm{Se}}_3$: phase I at 0 GPa, number of As–Se contacts, $N\left(\mathrm{As}–\mathrm{Se}\right)=3$; phase III at 3–25 GPa, $N\left(\mathrm{As}–\mathrm{Se}\right)=6$; phase IV, $N\left(\mathrm{As}–\mathrm{Se}\right)=7–8$. The experimental data are shown as the black marks.