Synchrotron infrared spectroscopy of the pressure-induced insulator-metal transitions in glassy ${\mathrm{As}}_2{\mathrm{S}}_3$ and ${\mathrm{As}}_2{\mathrm{Se}}_3$

Optical reflectivity and absorption measurements of glassy ${\mathrm{As}}_2{\mathrm{S}}_3$ and ${\mathrm{As}}_2{\mathrm{Se}}_3$ from 0.18 to 1.4 eV to 57 GPa reveal closure of the optical gap in both materials. The transition to the metalliclike response is continuous with pressure. The insulator-metal transition can be understood in terms of the delocalization of nonhybridized S and Se $p$ states (lone pairs), as supported by the optical reflectivity data and theoretical calculations that show delocalization and evolution of three-dimensional electronic states in crystalline ${\mathrm{As}}_2{\mathrm{Se}}_3$.

Figure 1

(Color online) (a) Electronic spectra of crystalline and covalently bonded disordered (glassy) ${\mathrm{As}}_2{\mathrm{S}}_3$ according to x-ray photoemission spectroscopy measured in Ref. 4. We also indicate the distribution of the electronic states following the general considerations of Ref. 4. The reflectivity spectra from Ref. 5 (symbols) and Lorentz oscillator fits (solid lines, see text for details) are shown in the lower panel.

Figure 2

(Color online) Structure of $P{2}_1∕c$ crystalline ${\mathrm{As}}_2\mathrm{S}{\left(\mathrm{Se}\right)}_3$. The disordered or glassy state can be locally considered as retaining its crystalline counterpart’s SRO and partially MRO: valence bond lengths, and bond angles are just slightly distorted, and the layered structure is partially preserved (Refs. 29, 30).

Figure 3

(Color online) Reflectivity and absorption spectra of ${\mathrm{As}}_2{\mathrm{Se}}_3$. The region of second-order absorption in the diamond anvils at $0.25–03\mathrm{eV}$ is not accessible. The red dashed lines are Drude–Lorentz fits in the metallic phase.

Figure 4

Pressure dependence of the energy gap in ${\mathrm{As}}_2{\mathrm{S}}_3$ (large closed circles, left axis) and ${\mathrm{As}}_2{\mathrm{Se}}_3$ (large closed squares, left axis) from absorption measurements. Absorption data below $10\mathrm{GPa}$ for ${\mathrm{As}}_2{\mathrm{S}}_3$ are from Ref. 8. Absorption data below $7\mathrm{GPa}$ and energy gap from transport measurements (small circles connected with dotted line, left axis) for ${\mathrm{As}}_2{\mathrm{Se}}_3$ are from Ref. 9. The gap for ${\mathrm{As}}_2{\mathrm{Te}}_3$ from transport measurements in Ref. 31 (small circles connected by solid line, left axis) is shown for comparison. Drude contribution to reflectivity is shown for ${\mathrm{As}}_2{\mathrm{S}}_3$ (large open circles, right axis) and ${\mathrm{As}}_2{\mathrm{Se}}_3$ (large open squares, right axis).

Figure 5

Electronic band structure and density of states of the crystalline ${\mathrm{As}}_2\mathrm{S}{\left(\mathrm{Se}\right)}_3$ computed at 0 and $45\mathrm{GPa}$. ${\mathrm{As}}_2{\mathrm{Se}}_3$ is a low-dimensional insulator at low pressures and a three-dimensional metal at high pressures. $Y=(1∕200)$, $A=(1∕21∕20)$, $B=(01∕20)$, $Z=\left(001.2\right)$, $C=(1∕201∕2)$, $E=(1∕21∕21∕2)$, and $D=(01∕20)$.

Figure 6

Electronic susceptibility ${ϵ}_1-1=4\pi \chi$ (closed symbols, left axis) in the insulating phase and conductivity (open symbols, right axis) in the metallic phase as follows from the Drude–Lorentz model fits for ${\mathrm{As}}_2{\mathrm{Se}}_3$.