Research Article

High pressure chemistry in the H2-SiH4 system

Shibing Wang, Ho-kwang Mao, Xiao-Jia Chen, and Wendy L. Mao
PNAS September 1, 2009 106 (35) 14763-14767; https://doi.org/10.1073/pnas.0907729106

  1. Contributed by Ho-kwang Mao, July 17, 2009 (received for review June 5, 2009)

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.

Group IVa hydrides (i.e., CH4, SiH4, GeH4, SnH4) have the highest atomic fraction (80%) of hydrogen among elemental hydrides and were predicted to metallize into hydrogen-dominant metallic alloys at lower pressures compared with pure hydrogen (1). Recent experiments on silane (SiH4) have confirmed such predictions: Synchrotron infrared reflectivity and electrical conductivity measurements indicate its metallization at ≈50–60 GPa (2, 3), and SiH4 becomes superconducting at a transition temperature of 17 K at 96 GPa (3). Interactions between elemental hydrides and additional molecular hydrogen at high pressure are a rapidly growing area of research (4, 5). Formation of numerous stoichiometric compounds demonstrates the complicated interactions between hydrogen and other molecular species in condensed phases. H2 and H2O form clathrates and filled ices that can be quenched to ambient pressure at low temperature (6). Methane (CH4) was discovered to form at least four stoichiometric compounds with hydrogen at pressures up to 10 GPa (7). The high pressure behavior of the H2-SiH4 is of interest, in part because potential phases in this system may store a significant amount of molecular hydrogen and mimic the behavior of pure H2 and its possible metallization but at lower pressures. Here we report our study of the phase diagram of H2-SiH4 system to 6.5 GPa at room temperature. Samples with two premixed starting compositions, 5:1 and 1:1 molar H2:SiH4 ratios, were loaded as a well-mixed fluid phase in a diamond anvil cell and were monitored in situ using optical microscopy and Raman spectroscopy.

Results and Discussion

The 5:1 H2:SiH4 sample starts as a homogeneous, colorless fluid at 1 GPa (Fig. 1A), a hydrogen-dominant phase (H-solid) first appears when pressure was raised to 5.8 GPa, higher than the freezing pressure for pure H2 of 5.5 GPa (8). As the H-solid grows from the fluid with further pressure increase (Fig. 1B), the H2 content (initially 83.3 mol%) of the fluid decreases to a minimum value of 64 mol% (estimated from the Raman spectra, see Materials and Methods). At pressures >6.5 GPa, the remaining fluid suddenly and completely solidifies (Fig. 1C), into a mixture of a SiH4-dominant phase (S-solid) and H-solid, clearly indicating the eutectic behavior. The darkened appearance is because of light scattering off the grain boundaries between the two different phases that have different refractive indices. The 1:1 H2:SiH4 sample also starts as a homogeneous, colorless fluid (Fig. 1D). The S-solid first crystallizes from the fluid at 5.4 GPa, significantly higher than the freezing pressure reported for pure SiH4 at 4 GPa (2). With increasing pressure, the remaining fluid phase becomes increasingly H2-rich as more S-solid crystallizes (Fig. 1E), indicating behavior on the other side of the eutectic from the 5:1 H2:SiH4 sample. At 6.4 GPa, the remaining fluid suddenly and completely solidifies (also with darkened appearance; Fig. 1F).

Fig. 1.
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.

The observations for these two compositions indicate that SiH4-H2 system is consistent with a simple binary eutectic phase diagram (Fig. 2). Kinetics effects were found to be significant in the solidification process. We observed superpressurization phenomenon in our system whereby we had to increase pressure by 0.2–0.4 GPa above the eutectic pressure before the entire system crystallized, analogous to the supercooling effect observed in the freezing of fluids. The two liquidus curves actually cross, i.e., the H2-rich liquidus extends to as low as 62 mol% H2, and the H2-poor liquidus extends to as high as 74 mol% H2 before the second solid appears. From the point where the pressure-composition (P-x) liquidus curves of the two fluids intersect, we were able to determine the eutectic pressure and composition as being, 6.1(±0.1) GPa and 72(±2) mol% H2 at 300 K.

Fig. 2.
Fig. 2.

Binary P-x phase diagram of H2-SiH4. 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.

The main Raman features for SiH4 are the ν1(A1) and ν3(F2) vibrational modes of SiH4 that overlap at ≈2,200 cm−1 and the ν2(E) mode at ≈900 cm−1 (2, 9). These features become much sharper when SiH4 solidifies. Representative spectra of SiH4 are shown in Fig. 3A: The S-solid contains 1.3 mol% H2, the liquid phase at 4.1 GPa 50 mol% H2, and the H-solid 99.6 mol% H2. Fig. 3B shows the high pressure H2 vibrons in the H-solid (98.5 mol% H2, 6 GPa), H2-SiH4 fluid (83.3 mol% H2, 5.2 GPa), and the S-solid (0.6 mol% H2, 5.6 GPa). The Raman spectra of the liquid phases of H2-SiH4 for the two samples are shown in Fig. 4 (5:1 H2:SiH4) and Fig. 5 (1:1 H2:SiH4). The peak intensities in the spectra are normalized to the acquisition time (laser power was held constant in our experiments).

Fig. 3.
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% SiH4, 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% H2.

Fig. 4.
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.
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.

Figs. 4 and 5 present the evolution of the fluid Raman peaks as a function of pressure. The H2 Q1(1) vibron of both fluid samples broadens with increasing pressure and reaches 30 cm−1 at 6 GPa. It sharpens greatly in the crystalline H-solid (FWHM = 7 cm−1 with instrument resolution of 4 cm−1; see Materials and Methods). Compared with the corresponding pure H2 liquid (15 cm−1) and solid (5.6 cm−1) (8), our observation suggests the strong interaction between the SiH4 and H2 components. Likewise in both fluid samples, the SiH4 ν1 and ν3 modes broaden and diverge from each other as pressure increases leading to the broadening of the feature at ≈2,200 cm−1. The behavior of the hydrogen protons in our samples were consistent with that of pure hydrogen and were not affected by the SiH4 composition within measurement error.

The Raman intensity, frequency, and FWHM of the SiH4 ν1, ν3, and H2 Q1(1) modes are highly characteristic of the solid and fluid phases and their compositions. For the fluid across the H2-SiH4 binary system, the SiH4 ν1, ν3 frequency decreases and H2 Q1(1) vibron frequency increases with increasing H2 content (Figs. 6 and 7). The liquidus pressure of 5.8 GPa for the 5:1 H2:SiH4 sample can thus be precisely identified by intensity and frequency decrease of the H2 vibron and intensity increase and frequency decrease of SiH4 ν1, ν3. Conversely, the liquidus pressure of 5.4 GPa for the 1:1 H2:SiH4 sample can be precisely identified by intensity and frequency increase of the H2 vibron and intensity decrease and frequency increase of SiH4 ν1, ν3. The sharp change in the slopes of the pressure dependence with Raman shift can be readily observed at the liquidus pressures (Figs. 6 and 7), above which the fluids no longer remain at the original bulk composition, but change with the fractionation of the H-solid or S-solid. The fractionating fluid curves for the two compositions intersect at the eutectic point, providing additional strong support to the binary eutectic at 6.1 ± 0.1 GPa, and then continue to cross over as a result of the metastable superpressurization phenomenon (Figs. 6 and 7).

Fig. 6.
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.
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.

The compositions of the H-solid and S-solid are close to the end-members but contain a small and variable amount of the opposite components. The H2 content of the S-solid in equilibrium with the fluid <6 GPa is 1.5 mol%. This drops to 0.5 mol% in equilibrium with the H-solid >6.1 GPa. Conversely, the SiH4 content in the H-solid in equilibrium with the fluid <6 GPa is 2 mol%. This drops to 0.5 mol% in equilibrium with the S-solid >6.1 GPa. These small but real variations are significant. The H2 vibron frequency shift of the S-solid as a function of pressure shows a sharp kink at the eutectic pressure (blue diamonds in Fig. 7), confirming the small compositional change in the S-solid at the eutectic.

The addition of the minor components has a remarkable impact on the crystalline phases making S-solid and H-solid significantly different from the pure SiH4 and H2 solids. Pure silane crystallizes at 4 GPa into solid phase III and transforms to solid phase IV at 6.5 GPa, which is stable up to 10 GPa (2). When the S-solid began to crystallize at 5.4 GPa from the 1:1 H2:SiH4 fluid, however, the Raman spectra of the SiH4 ν1, ν3, and ν2 modes were similar to the pure SiH4 phase IV (2). We did not observe the equivalent phase III spectra over the range studied. The Raman vibron frequency of the minor H2 component in the S-solid is 30–40 cm−1 lower than that of pure H2 (Fig. 7). These distinctive features make it very easy to characterize that S-solid as a compound of silane and hydrogen, rather than a mixture of two end-member phases.

The ν1, ν3, and ν2 modes of the minor SiH4 component in the H-solid that crystallized from the 5:1 H2:SiH4 sample are similar to that of the S-solid or pure silane phase IV in terms of peak shape, but are 30 cm−1 higher in frequency (Fig. 6), and thus clearly distinguishable from S-solid and SiH4 phase IV. The minor SiH4 component also has a significant effect on Raman vibron of the H2 vibron of the H-solid as shown in its frequency decrease of 6 cm−1 compared with the pure solid H2. Again, these features establish the distinction between the H-solid and pure H2 solid.

Conclusions

We used optical microscopy and Raman spectroscopy to study the H2-SiH4 binary system at pressures up to 6.5 GPa. Crystallization from the fluid, the H2-SiH4 system, shows an apparently simple binary eutectic phase diagram consisting of a fluid and two near end-member solids, S-solid and H-solid, with limited solid solubility between SiH4 and H2. No solid phases with intermediate composition are observed within the pressure range studied. Monitoring the Raman peaks of H2 and SiH4 in different fluids and solids visible through optical microscopy, in two samples with starting compositions of 5:1 and 1:1 molar ratios, we determined its P-x phase diagram with liquidus curves leading to the binary eutectic point at 6.1(±0.1) GPa and 72(±2) mol% H2 at 300 K. The eutectic pressure determination based on the change of H-H and Si-H Raman peaks intensity ratio is in agreement with several independent determinations from kinks in the Raman shifts of the H-H vibron frequency and Si-H Raman frequencies in the two liquids and the kink in the Raman shifts of the H-H vibron frequency in the S-solid with pressure. Superpressurization is significant when the mixtures fully solidify, indicating important kinetics effects in the H2-SiH4 system. Overshooting of the eutectic by 0.2–0.4 GPa is evident as shown by the crossovers of P-x plot and P-ν plots of the Si-H stretching modes and H2 vibron for the 5:1 and 1:1 H2:SiH4 starting compositions. Metastability and sluggish reaction are a key favorable condition for the possible existence and retaining of additional phases in the system and for potentially applications.

Strong intermolecular interaction between the two species was observed. The Raman spectra for the H2 vibron in both the H2-SiH4 fluid and solid phases show substantial red shift and broadening compared with pure H2. This softening becomes larger with increasing SiH4 content. Conversely the ν1, ν3 Si-H stretching modes show substantial blue shift compared with pure silane in an H2-rich environment. Most intriguingly, the H-solid and S-solid are different from the respective end-member solids. In both phases, addition of minor components of the opposite compound has a substantial effect on the bonding and phase stability. The original Ashcroft concept (1) only requires a hydrogen-dominant material that may become a metallic alloy and the second component (or dopant) may participate in the common overlapping bands. It has been well-established that a minor composition change can have major effects on electron properties. For instance, YH3–δ can be switched back-and-forth sharply between insulator and metal by the hydrogen content change (δ) of several percent, which triggers a phase change (10), and the diamond goes through an insulator-superconductor transition by doping with percent-level boron without a structure change (11). Both the H-solid and S-solid are more hydrogen-dominant than pure Group IV hydrides, and the 0.5–2% dopants are sufficient to contribute to overlapping bands, making them interesting candidates for further investigation of hydrogen metallization and superconductivity at higher pressure.

Materials and Methods

1. Diamond Anvil Cell Sample Loading.

Two gas mixtures of SiH4 and H2 gas with 50 and 83 mol% H2 were premixed by Voltaix Product and certified with ±1% composition accuracy. They were loaded in diamond anvil cells (DACs) using the gas loading system at the Geophysical Laboratory. Small pieces of ruby were placed in the sample chamber for pressure calibration, and the entire DAC was placed in a large gas pressure vessel. The highly uniform gas mixture was pumped into the vessel to a nominal pressure of 100 MPa, which fills the DAC sample chamber (which was left slightly opened) as well as its surroundings. A feed-through mechanism was then applied to close the DAC sample chamber and seal the gas samples inside the gasket. The gaskets were made of a Be-Cu alloy, which was chosen for its superior ability in preventing H2 loss. After loading, venting of the excess flammable silane-hydrogen gas mixture in the gas loading vessel was controlled by passing the exhaust through water. After sealing the samples in the gasket, the DACs were removed from the gas vessel. Both samples started as well-mixed fluid phase, as in Fig. 1 A and D. The diamonds in both cells had culets 0.5 mm in diameter, and the diameter of the sample chamber was ≈150 μm.

2. Raman Spectroscopy and Optical Microscopy Measurements.

We used Raman spectroscopy to quantitatively monitor the behavior of two components in the systems. The spectra were measured in a back scattering geometry with excitation wavelength of 487.987 nm. The energy resolution for all of the spectra is 4 cm−1. Pressure is determined by the shift of the ruby R1 fluorescence line (12) using the same system. Because the peak intensity for a specific Raman feature is proportional to the amount of that component in the phase being measured, we estimated the composition of the solid and fluid SiH4 and H2 phases by comparing the intensity (integrated area) ratio for the Si-H stretching modes of SiH4 and Q1 vibron of H2 with that of a known composition, i.e., the starting compositions (Fig. 8). The SiH4/H2 Raman intensity ratio (RIR) was fit to a linear relationship with the SiH4/H2 molar ratio of liquid composition (C) of the starting samples, and the calibration line was found to be: Embedded Image Raman intensity has been used successful for determinations of composition for the H2-CH4 fluid (7) and the (H2)4CH4 crystalline solid (13). This method depends on using molecular Raman modes whose intensity is insensitive to the chemical environment that is a good assumption for the high frequency H-H molecular vibration and the Si-H stretch in the fluid phase using the calibration. If the molecular polarizability of these molecular vibration changes drastically in S- and H-solids, the calibration slope may change. However, our observations of relative concentration change within each solid are still valid.

Fig. 8.
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%.

The shift of the Raman peaks (Figs. 6 and 7) is also consistent with the composition determination from the peak intensities (Fig. 2). Because of the difference in refractive indices of the solid and fluid phases, they can be clearly distinguished using optical microscopy. We used direct visual observation to determine the pressure at which phase separation occurred and when the eutectic point was reached.

Acknowledgments

We thank J. Shu and M. Somayazulu for their assistance loading the samples and the Geophysical Laboratory for the use of their gas-loading and Raman spectroscopy facilities. This work is supported by the Department of Energy through the Stanford Institute for Materials and Energy Science (DE-AC02–76SF00515). Work at Carnegie Institution is supported by National Science Foundation Grant EAR-0810255 and NASA Grant NNX08AL2TG.

Footnotes

  • 1To whom correspondence may be addressed. E-mail: shibingw{at}stanford.edu or hmao{at}gl.ciw.edu

  • Author contributions: S.W., H.-k.M., X.-J.C., and W.L.M. designed research; S.W. performed research; S.W., H.-k.M., and W.L.M. analyzed data; and S.W. wrote the paper.

  • The authors declare no conflict of interest.

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High pressure chemistry in the H2-SiH4 system
Shibing Wang, Ho-kwang Mao, Xiao-Jia Chen, Wendy L. Mao
Proceedings of the National Academy of Sciences Sep 2009, 106 (35) 14763-14767; DOI: 10.1073/pnas.0907729106
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