High-Pressure Synthesis, Amorphization, and Decomposition of Silane

By compressing elemental silicon and hydrogen in a diamond anvil cell, we have synthesized polymeric silicon tetrahydride (${\mathrm{SiH}}_4$) at 124 GPa and 300 K. In situ synchrotron x-ray diffraction reveals that the compound forms the insulating $I{4}_1/a$ structure previously proposed from ab initio calculations for the high-pressure phase of silane. From a series of high-pressure experiments at room and low temperature on silane itself, we find that its tetrahedral molecules break up, while silane undergoes pressure-induced amorphization at pressures above 60 GPa, recrystallizing at 90 GPa into the polymeric crystal structures.

Figure 1

Powder x-ray diffraction spectra showing synthesis of the polymeric silicon tetrahydride. (Lower panel) Powder diffraction spectra of hcp and fcc phases of pure Si with two weak diffraction peaks from hcp phase of molecular hydrogen marked as “${\mathrm{H}}_2$”, before the reaction took place. “$*$” denotes the remaining peak from hcp Si. (Upper panel) Powder diffraction spectrum and the full structure Rietveld refinement of the synthesized silicon tetrahydride in an $I{4}_1/a$ phase (upper tick marks) in the presence of the remaining fcc phase of Si (lower tick marks) and hcp phase of molecular hydrogen (two strongest peaks marked on the spectrum as ${\mathrm{H}}_2$). The crosses, upper solid line and lower solid line represent experimental, modeled and difference spectra, respectively. Inset in the upper panel shows a Raman spectrum of the $I{4}_1/a$ phase of silicon tetrahydride. The stars indicate the peaks from diamond, and the strongest Raman line from the silicon tetrahydride corresponding to the Si-H stretching mode is denoted as $A_g$.

Figure 2

Schematic illustration of the transformation of the molecular silane into a polymeric atomic arrangement. (Left) Four molecules of the phase V at 25 GPa are shown that constitute one unit cell of the ${\mathrm{SnBr}}_4$-type structure. Above 60 GPa, the molecules dissociate and the sample becomes amorphous, crystallizing (right) above 90 GPa into a polymeric phase with an eight-coordinated silicon, with two different crystal structures observed (upper) $I{4}_1/a$ and (lower) $I\overline{4}2d$. The lattice parameters and atomic coordinates of Si are obtained in the present work from Rietveld refinement of diffraction spectra at 108 GPa with hydrogen positions taken from Ref. 7. Bonds up to 1.8 Å are shown.

Figure 3

Representative Raman and x-ray powder diffraction patterns of ${\mathrm{SiH}}_4$ at different pressures and temperatures. (Left) Powder x-ray diffraction spectra of silane collected on pressure increase at 90 and 300 K show transformation from phase V to phase VI, then to an amorphous state recrystallizes into a mixture of two new phases at higher pressures. The tick marks below the spectra show the calculated peak positions for the ${\mathrm{SnBr}}_4$ structure for phase V and the $I{4}_1/a$ (lower tick marks) and $I\overline{4}2d$ (upper tick marks) structures for the upper spectrum of the mixture of two silane phases. “$+$” and “$*$” denote the two strongest peaks from Re (gasket material) and Re hydride, formed after the reaction with atomic hydrogen released due to a partial decomposition of silane at lower pressure (see also Ref. 16). (Right) Raman spectra of silane at various pressures and temperatures, showing the absence of Raman peaks from the amorphous phase.

Figure 4

Proposed phase diagram of silane (pressure logarithmic scale) in a wide pressure and temperature range. The data points are shown from the present study, that reveal the existence of phases V and VI (tetrahedral molecular phases), an amorphous state and the $I{4}_1/a$ and $I\overline{4}2d$ (polymeric) phases.