High pressure chemistry in the H2-SiH4 system

Abstract

Understanding the behavior of hydrogen-rich systems at extreme conditions has significance to both condensed matter physics, where it may provide insight into the metallization and superconductivity of element one, and also to applied research areas, where it can provide guidance for designing improved hydrogen storage materials for transportation applications. Here we report the high-pressure study of the SiH4-H2 binary system up to 6.5 GPa at 300 K in a diamond anvil cell. Raman measurements indicate significant intermolecular interactions between H2 and SiH4. We found that the H2 vibron frequency is softened by the presence of SiH4 by as much as 40 cm(-1) for the fluid with 50 mol% H2 compared with pure H2 fluid at the same pressures. In contrast, the Si-H stretching modes of SiH4 shift to higher frequency in the mixed fluid compared with pure SiH4. Pressure-induced solidification of the H2-SiH4 fluid shows a binary eutectic point at 72(+/-2) mol% H2 and 6.1(+/-0.1) GPa, above which the fluid crystallizes into a mixture of two nearly end-member solids. Neither solid has a pure end-member composition, with the silane-rich solid containing 0.5-1.5 mol% H2 and the hydrogen-rich solid containing 0.5-1 mol% SiH4. These two crystalline phases can be regarded as doped hydrogen-dominant compounds. We were able to superpressurize the sample by 0.2-0.4 GPa above the eutectic before complete crystallization, indicating extended metastability.

Fig. 1.

Photomicrographs showing evolution of H₂-SiH₄ mixtures with pressure in a DAC at 300 K: Left (A–C) show the sample with 5:1 H₂:SiH₄ starting composition. Right (D–F) show 1:1 H₂:SiH₄ sample. As pressure was increased (B) an H₂-dominant phase (H-solid) and (E) SiH₄-dominant phase (S-solid) solidified from the initially fluid samples. (C and F) These panels show the completely solidified samples above the eutectic point.

Fig. 2.

Binary P-x phase diagram of H₂-SiH_4. Circles are measured from liquid phase, and diamonds are from solid. Red symbols show data for the 5:1 H₂:SiH₄ sample, and blue symbols are from 1:1 sample. Data above the eutectic pressure are a result of superpressurization of the sample. Possible extension to the freezing pressures for pure H₂ (8) and SiH₄ (2) are shown by dashed lines.

Fig. 3.

Representative Raman spectra for the SiH₄ ν₁, ν₂, and ν₃ modes and H₂ Q₁(1) vibron in both fluid and solid H₂-SiH₄ phases of the 5:1 and 1:1 H₂:SiH₄ samples. (A) From bottom to top, spectra of Si-H stretching modes for the SiH₄ in the H-solid at 6.1 GPa, which contains 0.4 mol% SiH_4, in 50 mol% SiH₄ fluid at 4.1 GPa, and in the S-solid at 6.1 GPa with 98.7 mol% SiH₄. (B) From bottom to top, spectra for the H₂ vibron for the H₂ in S-solid at 5.6 GPa with 0.6 mol% H₂, in the 5:1 H₂:SiH₄ fluid at 5.2 GPa, which contains 83.3% H₂, and in the H-solid at 6.0 GPa, which contains 98.5 mol% H_2.

Fig. 4.

Evolution of the Raman spectra of the fluid portion of the 5:1 H₂:SiH₄ sample with increasing pressure. Left series shows the SiH₄ ν₁, ν₃ modes, right series the H₂ vibron. The sample hits the liquidus just >5.8 GPa.

Fig. 5.

Evolution of the Raman spectra of the fluid portion of the 1:1 H₂:SiH₄ sample with increasing pressure. Left series shows the SiH₄ ν₁, ν₃ modes, right series the H₂ vibron. The sample hits the liquidus just >5.4 GPa.

Fig. 6.

Raman shift of SiH₄ ν₁ modes in H₂ environment as a function of pressure. Circles represent liquid phase, diamonds refer to solid. Red data are for the 5:1 H₂:SiH₄, blue for the 1:1 sample. Vertical blue and red lines indicate the pressure where the first solid forms. Vertical black line indicates the crossover in the liquid phase data that occurs at the eutectic pressure. Fluid data above this pressure is a result of superpressurization. Black symbols show pure fluid SiH₄ data (2).

Fig. 7.

Raman shift of H₂ vibron in SiH₄ environment as a function of pressure. Circles represent liquid phase, diamonds refer to data from solid. Red data are for the 5:1 H₂:SiH₄ sample, blue for the 1:1 sample. Vertical blue and red lines indicate the pressure where the first solid forms. Vertical black line indicates the crossover in the liquid phase data that occurs at eutectic pressure. Fluid data above this pressure is a result of superpressurization. For comparison, data for pure H₂ (8) are shown in black with dashed line representing liquid and solid black line for the solid.

Fig. 8.

Calibration for the linear relationship between the Raman intensity ratio (RIR) and the SiH₄/H₂ molar ratio of liquid composition (C) of the starting samples. Filled circles (blue in the case of 5:1 H₂:SiH₄ and red for 1:1) are data points for the sample before crystallization. Unfilled circles are data whose compositions have been determined with the RIR. The uncertainty for the H₂ molar fraction is 1.5%.