Novel Pressure-Induced Interactions in Silane-Hydrogen

We report novel molecular compound formation from silane-hydrogen mixtures with intermolecular interactions unprecedented for hydrogen-rich solids. A complex ${\mathrm{H}}_2$ vibron spectrum with anticorrelated pressure-frequency dependencies and a striking H-D exchange below 10 GPa reveal strong and unusual attractive interactions between ${\mathrm{SiH}}_4$ and ${\mathrm{H}}_2$ and molecular bond destabilization at remarkably low pressure. The unique features of the observed ${\mathrm{SiH}}_4({\mathrm{H}}_2{)}_2$ compound suggest a new range of accessible pressure-driven intermolecular interactions for hydrogen-bearing simple molecular systems and a new approach to perturb the hydrogen covalent bond.

Figure 1

(a) Proposed $P\mathrm{\text{−}}x$ phase diagram for ${\mathrm{SiH}}_4+{\mathrm{H}}_2$. Fluid 1 $(F_1)={\mathrm{SiH}}_4+{\mathrm{H}}_2$, solid 1 $(S_1)=\mathrm{\text{solid silane}}$, solid 2 $(S_2)=\mathrm{\text{solid}}{\mathrm{H}}_2$, solid 3 $(S_3)={\mathrm{SiH}}_4({\mathrm{H}}_2{)}_2$. The dotted horizontal line shows the reported phase transition from silane III to silane IV [17]. Photomicrographs for each region of the phase diagram are included. We cannot rule out additional stability fields in certain regions of the phase diagram. (b) Representative Raman spectra for an ${\mathrm{H}}_2$-rich sample during compression.

Figure 2

(a) Frequency of Raman (filled symbols) and IR (open symbols) ${\mathrm{H}}_2$ vibrons as a function of pressure. Regions I, II, and III indicate fluid, mixed fluid-solid, and solid phases (298 K). Dashed lines are drawn through the data to guide the eye. Solid lines represent Raman and IR data [5] for bulk ${\mathrm{H}}_2$. Note the vibron “band” for ${\mathrm{SiH}}_4+{\mathrm{H}}_2$ falls completely outside the “band” for bulk ${\mathrm{H}}_2$. (b) Raman and transmission IR spectra for ${\mathrm{SiH}}_4({\mathrm{H}}_2{)}_2$ as a function of pressure. Asterisks denote contributions from solid ${\mathrm{H}}_2$, and another weak band is present as indicated by measurements on ${\mathrm{SiH}}_4$-rich samples and ${\mathrm{D}}_2/\mathrm{HD}$ measurements. Spectra are scaled by indicated values due to the decrease in intensity of Raman scattered light and IR transmission with increasing pressure.

Figure 3

(a) Synchrotron x-ray ($\lambda =0.4246\AA$) diffraction pattern of ${\mathrm{SiH}}_4({\mathrm{H}}_2{)}_2$ obtained at 6.8 GPa (300 K) (points) and fit to data using the Le Bail intensity extraction method with $a=6.426(2)\AA$. At this pressure the valence electron density of hydrogen within the solid corresponds to that of bulk ${\mathrm{H}}_2$ compressed to $\sim 9\mathrm{GPa}$. The tick marks indicate allowed reflections for the $F\overline{4}3m$ structure. (b) Experimental EOS (volume per formula unit) for ${\mathrm{SiH}}_4({\mathrm{H}}_2{)}_2$. Filled circles and open circles are data obtained upon compression and decompression, respectively. The labeled curves represent the volumes obtained for the assemblage of four ${\mathrm{SiH}}_4$ and eight ${\mathrm{H}}_2$ using volumes of bulk silane [20] and hydrogen [21] and the assemblage of four ${\mathrm{H}}_2$ and crystalline Si(V) [22]. The data were fit to a second order Birch-Murnaghan EOS with $V_0=410(12){\AA }^3$, $K_0=6.8(7)\mathrm{GPa}$ with $K_0^{\prime }$ fixed at 4. (c) Proposed structure with large and small spheres as ${\mathrm{SiH}}_4$ and ${\mathrm{H}}_2$, respectively.

Figure 4

Raman spectra at $\sim 8\mathrm{GPa}$ for ${\mathrm{SiH}}_4(X{)}_2$ where $X$ is ${\mathrm{D}}_2$, HD, or ${\mathrm{H}}_2$. The ${\mathrm{D}}_2$ and HD spectra were obtained from the same measurement $\sim 24\mathrm{h}$ after the formation of ${\mathrm{SiH}}_4({\mathrm{D}}_2{)}_2$. The ${\mathrm{H}}_2$ spectrum is shown for comparison.