Synthesis and properties of selenium trihydride at high pressures

The chemical reaction products of molecular hydrogen $\left({\mathrm{H}}_2\right)$ with selenium (Se) are studied by synchrotron x-ray diffraction (XRD) and Raman spectroscopy at high pressures. We find that a common ${\mathrm{H}}_2\mathrm{Se}$ is synthesized at 0.3 GPa using laser heating. Upon compression at 300 K, a crystal of the theoretically predicted Cccm ${\mathrm{H}}_3\mathrm{Se}$ has been grown at 4.6 GPa. At room temperature, ${\mathrm{H}}_3\mathrm{Se}$ shows a reversible phase decomposition after laser irradiation above 8.6 GPa, but remains stable up to 21 GPa. However, at 170 K Cccm ${\mathrm{H}}_3\mathrm{Se}$ persists up to 39.5 GPa based on XRD measurements, while low-temperature Raman spectra weaken and broaden above 23.1 GPa. At these conditions, the sample is visually nontransparent and shiny suggesting that metallization occurred.

Figure 1

Microphotographs of the DAC cavities at elevated pressures demonstrating the formation of H-Se compounds. (a) Solid ${\mathrm{H}}_2\mathrm{Se}$ is grown (circled). (b) Cccm ${\mathrm{H}}_3\mathrm{Se}$ is grown pictured as a bar in the middle of the cavity. (c) Supposedly a metallic ${\mathrm{H}}_3\mathrm{Se}$ is formed; the crystal bridged the cavity sides and deformed. It reflects light in the areas where the surface is perpendicular to the optical axis and is dark (nontransparent for the transmitted light) where it is tilted.

Figure 2

Raman spectra of H-Se compounds at different pressures and temperatures in experiment A. Please note the vibron modes (a triplet) of bulk unreacted ${\mathrm{H}}_2$ at 0.3 GPa. At higher pressures they become a single band shifting to higher frequencies outside the presented frequency range.

Figure 3

The pressure dependence of the Raman frequencies of H-Se compounds as a function of pressure at 300 K and in the low-temperature experiments A and B emphasizing the difference in vibrational properties of the ${\mathrm{H}}_3\mathrm{Se}$ compound compared with ${\mathrm{H}}_2\mathrm{Se}$, ${\mathrm{H}}_2$, and (${\mathrm{H}}_2{\mathrm{Se})}_2{\mathrm{H}}_2$ at room temperature [32]. The black, blue, and red symbols correspond to the Raman bands observed in the ${\mathrm{H}}_2\mathrm{S}$ and ${\mathrm{H}}_3\mathrm{S}$ compounds at 300, 200, and 100 K, respectively. The gray symbols are the spectral positions of the bulk unreacted ${\mathrm{H}}_2$ vibron; a solid line through these data is the literature data for ${\mathrm{H}}_2$ [40].

Figure 4

XRD pattern of the H-Se compound at 7.0 GPa and 300 K. The x-ray wavelength is 0.3344 Å. Red vertical ticks with the peak indexing are presented for the Cccm ${\mathrm{H}}_3\mathrm{Se}$ [22] and blue vertical ticks - for Se-I [41]. The gasket is made of Re. The inset is a two-dimensional diffraction image in the radial coordinates (cake).

Figure 5

XRD of a single-crystal like ${\mathrm{H}}_3\mathrm{Se}$ at 39.5 GPa. The x-ray wavelength is 0.3344 Å. The bottom panel is the integrated one-dimensional pattern. The blue ticks indicate the peak positions of the predicted Cccm ${\mathrm{H}}_3\mathrm{Se}$ (the values of $a$ and $b$ lattice parameters are indistinguishable) and the black ticks of hcp hydrogen (phase I). The inset shows a microphotograph of the sample before cooling; the red square has a side of approximately 20 μm. The top panel is a two-dimensional diffraction image in the radial coordinates (cake), the single crystal spots of the sample are marked by red squares and those of bulk ${\mathrm{H}}_2$ by the blue rectangle. Please note that in this low-temperature experiment we could only rotate the sample within ±2 deg because of a cryostream nozzle attached to the DAC, limiting the number of observed reflections.

Figure 6

Equation of state of Cccm ${\mathrm{H}}_3\mathrm{Se}$ at 170 K of this work in comparison to that of experiment at room temperature [32] and theory [22] and also to the EOS of Cccm ${\mathrm{H}}_3\mathrm{S}$ at 170 K [17]. The error bars are within the symbol dimensions.

Figure 7

Decomposition and recovery of ${\mathrm{H}}_3\mathrm{Se}$ at 300 K revealed by the disappearance and appearance back again of the Raman peaks of the compound (marked by the red arrows). The asterisk (*) indicates the peak of Se. The Raman peaks of the surrounding bulk hydrogen remain unaffected.

Figure 8

Raman spectra of H-Se compounds at elevated pressures at room temperature. The excitation wavelength is 532 nm.

Figure 9

Raman spectra of H-Se compounds at different pressures and temperature in experiment B. The excitation wavelength is 532 nm. As in Fig. 2, please note the vibron modes of bulk ${\mathrm{H}}_2$ at 0.6 GPa. At higher pressures they become a single band shifting to higher frequencies outside the presented frequency range. The red arrows mark the positions of the Se-H bending mode.