AsS Melt Under Pressure: One Substance, Three Liquids

An in situ high-temperature—high-pressure study of liquid chalcogenide AsS by x-ray diffraction, resistivity measurements, and quenching from melt is presented. The obtained data provide direct evidence for the existence in the melt under compression of two transformations: one is from a moderate-viscosity molecular liquid to a high-viscosity nonmetallic polymerized liquid at $P\sim 1.6–2.2\mathrm{GPa}$; the other is from the latter to a low-viscosity metallic liquid at $P\sim 4.6–4.8\mathrm{GPa}$. Upon rapid cooling, molecular and metallic liquids crystallize to normal and high-pressure phases, respectively, while a polymerized liquid is easily quenched to a new AsS glass. General aspects of multiple phase transitions in liquid AsS, including relations to the phase diagram of the respective crystalline, are discussed.

Figure 1

Pressure-temperature phase diagram of AsS in crystalline and liquid states. Opened circles correspond to the experimental melting points of AsS crystal, while the thin line and thick lines are approximations of melting line and boundaries between liquid states, respectively. Solid triangles correspond to phase transitions between crystalline phases, determined at the near isobaric heating. Semisolid triangle corresponds to metallization of liquid AsS. Dashed lines are approximation of experimental transition lines (kinetically dependent) from AsS I (right line) and AsS II (left line) crystalline phases, while dashed-dotted line is approximation of I-II equilibrium line.

Figure 2

EDXD experimental spectra recorded at different scattering $2\theta$ angles [(a) $2\theta =3°$, (b) $2\theta =4°$, (c) $2\theta =6°$, and (d) $2\theta =10°$] and normalized by the amplitudes 1st diffraction peak (a)–(c) and 2nd diffraction peak (d), respectively. The initial EDXD data, recorded with the $\approx 40\mathrm{eV}$ energy step, have been smoothed by the Fourier transformation filter and converted from the energy abscissa scale to the wave scattering vector $Q$. The asterisk on the plot (a) marks the peaks at $Q\approx 1.8{\AA }^{-1}$, appeared in the spectra due to contribution of the (002) reflections from the graphite heater. Double structure of the 2nd diffraction peak is due to instrumental factor. Plot (b) shows changes of the prepeak, while examples of the whole spectra are shown in the inset. The thin and thick lines correspond to different states of the AsS melt (see the text).

Figure 3

Typical structure factors for different states of the AsS melt, including the molecular liquid at 1 GPa (thin line), polymeric liquid at 3 GPa (intermediate-thickness line), and metallic liquid at 4.9 GPa (thick line). All structure factors are obtained for the experimental sets of spectra recorded at temperatures exceeding the melting point by 70 K.

Figure 4

Example of electric voltage-current dependencies recorded at 3.5 GPa (diamonds) and 5.5 GPa (circles) for the “graphite ampoule plus AsS sample” assemblies. The close and open symbols correspond to the temperature increase and decrease, respectively. At pressures $P>4.8\mathrm{GPa}$, a significant current increase was observed at the melting temperature, suggesting the metallic state ($\sigma \sim {10}^3{\Omega }^{-1}{\mathrm{cm}}^{-1}$) of the liquid at these pressures. The width of the voltage-current dependency anomaly, corresponding to the melting, is associated with the temperature hysteresis on the sample. Sample temperatures at increasing current are indicated on the diagram.