High pressure chemistry in the H ₂ -SiH ₄ 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 hydrogendominant compounds. We were able to superpressurize the sample by 0.2-0.4 GPa above the eutectic before complete crystallization, indicating extended metastability.hydrogen ͉ phase diagram ͉ silane G roup IVa hydrides (i.e., CH 4 , SiH 4 , GeH 4 , SnH 4 ) 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 (SiH 4 ) have confirmed such predictions: Synchrotron infrared reflectivity and electrical conductivity measurements indicate its metallization at Ϸ50-60 GPa (2, 3), and SiH 4 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. H 2 and H 2 O form clathrates and filled ices that can be quenched to ambient pressure at low temperature (6). Methane (CH 4 ) was discovered to form at least four stoichiometric compounds with hydrogen at pressures up to 10 GPa (7). The high pressure behavior of the H 2 -SiH 4 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 H 2 and its possible metallization but at lower pressures. Here we report our study of the phase diagram of H 2 -SiH 4 system to 6.5 GPa at room temperature. Samples with two premixed starting compositions, 5:1 and 1:1 molar H 2 :SiH 4 ratios, were loaded as a well-mixed fluid phase in a diamond anvil cell and were monitor...

High temperature superconductivity in sulfur and selenium hydrides at high pressure

“…The first system explored experimentally was silane (SiH 4 ) [17]. Soon after, many others materials have been explored experimentally [18][19][20][21] and theoretically [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40]. …”

Perspective: Crystal structure prediction at high pressures

“…HP synthesis of functional materials with desirable chemical or physical properties via chemical reaction of distinct species has recently received great attentions. Experimentally, some of successful examples can be referred to the syntheses of mechanically important PtN 2 7 and IrN 2 , 8 and hydrogen-storage materials of methane-hydrogen mixtures, 9 SiH 4 (H 2 ) 2 , 10,11 Xe(H 2 ) 8 , 12 etc. Theoretically, HP hydrogen-storage materials with high hydrogen contents in Li-H 2 , 13 Na-H 2 , 14 Mg-H 2 , 15 and Ca-H 2 16 systems were designed, some of which appear to be potential high temperature superconductors.…”

Superconductivity at ∼100 K in dense SiH 4 (H 2 ) 2 predicted by first principles

“…However, it is noteworthy that at relatively low temperatures, a large kinetic barrier might exist and prevent SiH 4 ðH 2 Þ 2 from decomposition. Indeed, experiments (12,13) (25). An accurate evaluation of decomposition kinetic barrier is needed to give a precise conclusion, but remains a major challenge and beyond the scope of this study.…”

“…Remarkably, recent experiments have demonstrated that these hydrogen-rich full-shell group IV hydrides (CH 4 (12,13), where SiH 4 molecules form a peculiar tetragonal or a distorted face-centered cubic lattice with the orientationally disordered H 2 molecules occupying the interstitial sites. The validity of our structural model has been supported by the excellent mutual agreement between theoretical and experimental equation of states, Raman, and X-ray diffraction data (Fig.…”