Superconductivity at approximately 100 K in dense SiH4(H2)2 predicted by first principles

Prediction of Room-Temperature Superconductivity in Quasi-Atomic H2-Type Hydrides at High Pressure

Achieving superconductivity at room temperature (RT) is a holy grail in physics. Recent discoveries on high-Tc superconductivity in binary hydrides H3S and LaH10 at high pressure have directed the search for RT superconductors to compress hydrides with conventional electron-phonon mechanisms. Here, an exceptional family of superhydrides is predicated under high pressures, MH12 (M = Mg, Sc, Zr, Hf, Lu), all exhibiting RT superconductivity with calculated Tcs ranging from 313 to 398 K. In contrast to H3S and LaH10, the hydrogen sublattice in MH12 is arranged as quasi-atomic H2 units. This unique configuration is closely associated with high Tc, attributed to the high electronic density of states derived from H2 antibonding states at the Fermi level and the strong electron-phonon coupling related to the bending vibration of H2 and H-M-H. Notably, MgH12 and ScH12 remain dynamically stable even at pressure below 100 GPa. The findings offer crucial insights into achieving RT superconductivity and pave the way for innovative directions in experimental research.

Pressure-induced stability and superconductivity in LuH12 polyhydrides

The phase stability and superconductivity of lutetium polyhydrides under pressure were systematically explored via particle swarm optimization. Several lutetium hydrides, such as LuH, LuH₃, LuH₄, LuH₆, LuH₈, and LuH₁₂, were found to be dynamically and thermodynamically stable. Combined with the electronic properties, there are a large number of H-s states and low density of Lu-f states at the Fermi level, leading to superconductivity. The phonon spectrum and electron-phonon coupling interaction are considered to calculate the superconducting critical temperature (T_c) of stable lutetium hydrides at high pressure. The new predicted cubic LuH₁₂ has the highest T_c value of 187.2 K at 400 GPa in all the stable LuH_n compounds, which was estimated by directly solving the Eliashberg equation. The calculated results provide insights into the design of new superconducting hydrides under pressure.

Superconducting H7 chain in gallium hydrides at high pressure

Pressure-stabilized hydrides have potential as an outstanding reservoir for high-temperature (T_c) superconductors. We undertook a systematic study of crystal structures and superconducting properties of gallium hydrides using an advanced structure-search method together with first-principles calculations. We identified an unconventional stoichiometric GaH₇ gallium hydride that is thermodynamically stable at pressures above 247 GPa. Interestingly, the H atoms are clustered to form a unique H₇ chain intercalating the Ga framework. Further calculations show a high estimated T_c above 100 K at 200-300 GPa for GaH₇, closely related to the strong coupling between electrons of Ga and H atoms, and phonon vibrations of H₇ chains. Our work provides an example of exploration for diverse superconducting hydrogen motifs under high pressure, and may stimulate further experimental syntheses.

X-ray diffraction and equation of state of the C-S-H room-temperature superconductor

X-ray diffraction indicates that the structure of the recently discovered carbonaceous sulfur hydride (C-S-H) room-temperature superconductor is derived from previously established van der Waals compounds found in the H₂S-H₂ and CH₄-H₂ systems. Crystals of the superconducting phase were produced by a photochemical synthesis technique, leading to the superconducting critical temperature T_c of 288 K at 267 GPa. X-ray diffraction patterns measured from 124 to 178 GPa, within the pressure range of the superconducting phase, are consistent with an orthorhombic structure derived from the Al₂Cu-type determined for (H₂S)₂H₂ and (CH₄)₂H₂ that differs from those predicted and observed for the S-H system at these pressures. The formation and stability of the C-S-H compound can be understood in terms of the close similarity in effective volumes of the H₂S and CH₄ components, and denser carbon-bearing S-H phases may form at higher pressures. The results are crucial for understanding the very high superconducting T_c found in the C-S-H system at megabar pressures.

Strong correlation between electronic bonding network and critical temperature in hydrogen-based superconductors

By analyzing structural and electronic properties of more than a hundred predicted hydrogen-based superconductors, we determine that the capacity of creating an electronic bonding network between localized units is key to enhance the critical temperature in hydrogen-based superconductors. We define a magnitude named as the networking value, which correlates with the predicted critical temperature better than any other descriptor analyzed thus far. By classifying the studied compounds according to their bonding nature, we observe that such correlation is bonding-type independent, showing a broad scope and generality. Furthermore, combining the networking value with the hydrogen fraction in the system and the hydrogen contribution to the density of states at the Fermi level, we can predict the critical temperature of hydrogen-based compounds with an accuracy of about 60 K. Such correlation is useful to screen new superconducting compounds and offers a deeper understating of the chemical and physical properties of hydrogen-based superconductors, while setting clear paths for chemically engineering their critical temperatures.

Hydrogen Clathrate Structures in Uranium Hydrides at High Pressures

Room-temperature superconductivity has always been an area of intensive research. Recent findings of clathrate metal hydrides structures have opened up the doors for achieving room-temperature superconductivity in these materials. Here, we report first-principles calculations for stable H-rich clathrate structures of uranium hydrides at high pressures. The clathrate uranium hydrides contain H cages with stoichiometries of H24, H29, and H32, in which H atoms are bonded covalently to other H atoms, and U atoms occupy the centers of the cages. Especially, a UH10 clathrate structure containing H32 cages is predicted to have an estimated T c higher than 77 K at high pressures.

High-Pressure Phases and Properties of the Mg3Sb2 Compound

Pressure always plays an important role in influencing the structure configuration and electronic properties of materials. Here, combining density function theory and structure prediction algorithm, we systematically studied the Mg3Sb2 system from its phase transition to thermodynamic and electronic properties under high pressure. We find that two novel phases, namely Cm and C2/m, are stable under high pressure. Calculation results of phonon dispersions showed that both novel phases have no imaginary frequency, which indicates that the novel phases are thermodynamically stable. Due to the larger ionic radius of Sb compared to N, P, and As elements, the Mg3Sb2 compound shows a different electronic property at high pressure. The electronic calculations show that the novel phases of Cm and C2/m of Mg3Sb2 possess metallic behavior under high pressure. These results provide new insights for understanding the Mg3Sb2 compound.

Superconducting and Superhard Ice

Superconducting and superhard materials are essential for a myriad of scientific, biomedical and industrial applications. The contradiction between covalent bonds in superhard materials and metallic bonds in superconductors makes superconductivity and superhardness in the same material a very interesting and precious effect. Their abilities of zero-resistance and anti-pressure stem from the relationship between the crystal structure, chemical composition, and microstructure. The complexity of this interdependence limits researchers to conduct comprehensive experimental investigations, but can be supported by the theoretical calculations. Here, we report a general ab initio computational method to study three ice structures (Pmc2₁ , P2₁ and C2/m) and predict their phase transitions quantitatively at terapascal pressure. We predict that the ice structure will become a superhard material above 1.3 TPa (corresponding to P2₁ and C2/m), and turn into a superconductor above 5.0 TPa (corresponding to C2/m) with a critical temperature of 1.782 K. The proposed work benefits the applications of low temperature superconductors in high energy physics and fusion research and provides opportunities to advance the development of superconducting and superhard materials through computation.

Metallization and superconductivity in methane doped by beryllium at low pressure

As one of the simplest hydrocarbons, methane (CH₄) has great potential in the research of superconductors. However, the metallization of CH₄ has been an issue for a long time. Here, we report the structure, metallization, and superconductivity of CH₄ doped by Be at low pressures, based on first-principles calculations. The result shows that the thermodynamically stable BeCH₄ with P1[combining macron] space-group can transform into a metal at ambient pressure. This ternary hydride BeCH₄ exhibits a superconductivity of ∼6 K below 25.6 GPa. Interestingly, the superconducting critical temperature of BeCH₄ can reach ∼30 K at 80 GPa in the form of an a-P1 space-group phase. The charge transfer from Be to CH₄ molecules plays an important role in the superconductivity. Our results present a novel way to realize the metallization of methane at relative pressures and indicate that the doped methane is a potential candidate for seeking high temperature and low pressure superconductivity.

Stoichiometric evolutions of PH3 under high pressure: implication for high-T c superconducting hydrides

The superconductivity of hydrides under high pressure has attracted a great deal of attention since the recent observation of the superconducting transition at 203 K in strongly compressed H2S. It has been realized that the stoichiometry of hydrides might change under high pressure, which is crucial in understanding the superconducting mechanism. In this study, PH3 was studied to understand its superconducting transition and stoichiometry under high pressure using Raman, IR and X-ray diffraction measurements, as well as theoretical calculations. PH3 is stable below 11.7 GPa and then it starts to dehydrogenate through two dimerization processes at room temperature and pressures up to 25 GPa. Two resulting phosphorus hydrides, P2H4 and P4H6, were verified experimentally and can be recovered to ambient pressure. Under further compression above 35 GPa, the P4H6 directly decomposed into elemental phosphorus. Low temperature can greatly hinder polymerization/decomposition under high pressure and retains P4H6 up to at least 205 GPa. The superconductivity transition temperature of P4H6 is predicted to be 67 K at 200 GPa, which agrees with the reported result, suggesting that it might be responsible for superconductivity at higher pressures. Our results clearly show that P2H4 and P4H6 are the only stable P-H compounds between PH3 and elemental phosphorus, which is helpful for shedding light on the superconducting mechanism.

Strain-induced modulations of electronic structure and electron–phonon coupling in dense H3S

External stress is an effective tool to modulate the Fermi surface topology, logarithmic average frequency, and electron–phonon coupling parameter of dense H₃S and thus has a sensitive and considerable effect to the superconducting critical temperature.

Hydrogen Clathrate Structures in Rare Earth Hydrides at High Pressures: Possible Route to Room-Temperature Superconductivity

Room-temperature superconductivity has been a long-held dream and an area of intensive research. Recent experimental findings of superconductivity at 200 K in highly compressed hydrogen (H) sulfides have demonstrated the potential for achieving room-temperature superconductivity in compressed H-rich materials. We report first-principles structure searches for stable H-rich clathrate structures in rare earth hydrides at high pressures. The peculiarity of these structures lies in the emergence of unusual H cages with stoichiometries H_{24}, H_{29}, and H_{32}, in which H atoms are weakly covalently bonded to one another, with rare earth atoms occupying the centers of the cages. We have found that high-temperature superconductivity is closely associated with H clathrate structures, with large H-derived electronic densities of states at the Fermi level and strong electron-phonon coupling related to the stretching and rocking motions of H atoms within the cages. Strikingly, a yttrium (Y) H_{32} clathrate structure of stoichiometry YH_{10} is predicted to be a potential room-temperature superconductor with an estimated T_{c} of up to 303 K at 400 GPa, as derived by direct solution of the Eliashberg equation.

Emergence of superconductivity in doped H2O ice at high pressure

We investigate the possibility of achieving high-temperature superconductivity in hydrides under pressure by inducing metallization of otherwise insulating phases through doping, a path previously used to render standard semiconductors superconducting at ambient pressure. Following this idea, we study H2O, one of the most abundant and well-studied substances, we identify nitrogen as the most likely and promising substitution/dopant. We show that for realistic levels of doping of a few percent, the phase X of ice becomes superconducting with a critical temperature of about 60 K at 150 GPa. In view of the vast number of hydrides that are strongly covalent bonded, but that remain insulating up to rather large pressures, our results open a series of new possibilities in the quest for novel high-temperature superconductors.

Potential high-Tc superconducting lanthanum and yttrium hydrides at high pressure

A systematic structure search in the La-H and Y-H systems under pressure reveals some hydrogen-rich structures with intriguing electronic properties. For example, LaH₁₀ is found to adopt a sodalite-like face-centered cubic (fcc) structure, stable above 200 GPa, and LaH₈ a C2/m space group structure. Phonon calculations indicate both are dynamically stable; electron phonon calculations coupled to Bardeen-Cooper-Schrieffer (BCS) arguments indicate they might be high-T_c superconductors. In particular, the superconducting transition temperature T_c calculated for LaH₁₀ is 274-286 K at 210 GPa. Similar calculations for the Y-H system predict stability of the sodalite-like fcc YH₁₀ and a T_c above room temperature, reaching 305-326 K at 250 GPa. The study suggests that dense hydrides consisting of these and related hydrogen polyhedral networks may represent new classes of potential very high-temperature superconductors.

Prediction of superconducting iron-bismuth intermetallic compounds at high pressure

We report the discovery of novel iron-bismuth compounds, FeBi₂ and FeBi₃, at high-pressure.

The synthesis of materials in high-pressure experiments has recently attracted increasing attention, especially since the discovery of record breaking superconducting temperatures in the sulfur–hydrogen and other hydrogen-rich systems. Commonly, the initial precursor in a high pressure experiment contains constituent elements that are known to form compounds at ambient conditions, however the discovery of high-pressure phases in systems immiscible under ambient conditions poses an additional materials design challenge. We performed an extensive multi component ab initio structural search in the immiscible Fe–Bi system at high pressure and report on the surprising discovery of two stable compounds at pressures above ≈36 GPa, FeBi₂ and FeBi₃. According to our predictions, FeBi₂ is a metal at the border of magnetism with a conventional electron–phonon mediated superconducting transition temperature of T_c = 1.3 K at 40 GPa.

Structure and superconductivity of hydrides at high pressures

Hydrogen atoms can provide high phonon frequencies and strong electron–phonon coupling in hydrogen-rich materials, which are believed to be potential high-temperature superconductors at lower pressure than metallic hydrogen. Especially, recently both of theoretical and experimental reports on sulfur hydrides under pressure exhibiting superconductivity at temperatures as high as 200 K have further stimulated an intense search for room-temperature superconductors in hydrides. This review focuses on crystal structures, stabilities, pressure-induced transformations, metallization, and superconductivity of hydrogen-rich materials at high pressures.

Pressure-induced superconductivity in H2-containing hydride PbH4(H2)2

High pressure structure, stability, metallization, and superconductivity of PbH₄(H₂)₂, a H₂-containing compound combining one of the heaviest elements with the lightest element, are investigated by the first-principles calculations. The metallic character is found over the whole studied pressure range, although PbH₄(H₂)₂ is metastable and easily decompose at low pressure. The decomposition pressure point of 133 GPa is predicted above which PbH₄(H₂)₂ is stable both thermodynamically and dynamically with the C2/m symmetry. Interestedly, all hydrogen atoms pairwise couple into H₂ quasi-molecules and remain this style up to 400 GPa in the C2/m structure. At high-pressure, PbH₄(H₂)₂ tends to form the Pb-H₂ alloy. The superconductivity of T_c firstly rising and then falling is observed in the C2/m PbH₄(H₂)₂. The maximum of T_c is about 107 K at 230 GPa. The softening of intermediate-frequency phonon induced by more inserted H₂ molecules is the main origin of the high T_c. The results obtained represent a significant step toward the understanding of the high pressure behavior of metallic hydrogen and hydrogen-rich materials, which is helpful for obtaining the higher T_c.

Phase Diagram and High-Temperature Superconductivity of Compressed Selenium Hydrides

Recent discovery of high-temperature superconductivity (Tc = 190 K) in sulfur hydrides at megabar pressures breaks the traditional belief on the Tc limit of 40 K for conventional superconductors, and opens up the doors in searching new high-temperature superconductors in compounds made up of light elements. Selenium is a sister and isoelectronic element of sulfur, with a larger atomic core and a weaker electronegativity. Whether selenium hydrides share similar high-temperature superconductivity remains elusive, but it is a subject of considerable interest. First-principles swarm structure predictions are performed in an effort to seek for energetically stable and metallic selenium hydrides at high pressures. We find the phase diagram of selenium hydrides is rather different from its sulfur analogy, which is indicated by the emergence of new phases and the change of relative stabilities. Three stable and metallic species with stoichiometries of HSe2, HSe and H3Se are identified above ~120 GPa and they all exhibit superconductive behaviors, of which the hydrogen-rich HSe and H3Se phases show high Tc in the range of 40-110 K. Our simulations established the high-temperature superconductive nature of selenium hydrides and provided useful route for experimental verification.

Hydrogen segregation and its roles in structural stability and metallization: silane under pressure

We present results from first-principles calculations on silane (SiH4) under pressure. We find that a three dimensional P-3 structure becomes the most stable phase above 241 GPa. A prominent structural feature, which separates the P-3 structure from previously observed/predicted SiH4 structures, is that a fraction of hydrogen leaves the Si-H bonding environment and forms segregated H2 units. The H2 units are sparsely populated in the system and intercalated with a polymeric Si-H framework. Calculations of enthalpy of formation suggest that the P-3 structure is against the decomposition into Si-H binaries and/or the elemental crystals. Structural stability of the P-3 structure is attributed to the electron-deficient multicenter Si-H-Si interactions when neighboring silicon atoms are linked together through a common hydrogen atom. Within the multicenter bonds, electrons are delocalized and this leads to a metallic state, possibly also a superconducting state, for SiH4. An interesting outcome of the present study is that the enthalpy sum of SiH4 (P-3 structure) and Si (fcc structure) appears to be lower than the enthalpy of disilane (Si2H6) between 200 and 300 GPa (for all previously predicted crystalline forms of Si2H6), which calls for a revisit of the stability of Si2H6 under high pressure.

Perspective: crystal structure prediction at high pressures

Crystal structure prediction at high pressures unbiased by any prior known structure information has recently become a topic of considerable interest. We here present a short overview of recently developed structure prediction methods and propose current challenges for crystal structure prediction. We focus on first-principles crystal structure prediction at high pressures, paying particular attention to novel high pressure structures uncovered by efficient structure prediction methods. Finally, a brief perspective on the outstanding issues that remain to be solved and some directions for future structure prediction researches at high pressure are presented and discussed.

Pressure-stabilized superconductive yttrium hydrides

The search for high-temperature superconductors has been focused on compounds containing a large fraction of hydrogen, such as SiH₄(H₂)₂, CaH₆ and KH₆. Through a systematic investigation of yttrium hydrides at different hydrogen contents using an structure prediction method based on the particle swarm optimization algorithm, we have predicted two new yttrium hydrides (YH₄ andYH₆), which are stable above 110 GPa. Three types of hydrogen species with increased H contents were found, monatomic H in YH₃, monatomic H+molecular “H₂” in YH₄ and hexagonal “H₆” unit in YH₆. Interestingly, H atoms in YH₆ form sodalite-like cage sublattice with centered Y atom. Electron-phonon calculations revealed the superconductive potential of YH₄ and YH₆ with estimated transition temperatures (T_c) of 84–95 K and 251–264 K at 120 GPa, respectively. These values are higher than the predicted maximal T_c of 40 K in YH₃.

Enhancement of Tc in the atomic phase of iodine-doped hydrogen at high pressures

H₂ molecular units dissociate and form a novel atomic phase with R3̄m.

Stable compositions and structures in the Na–Bi system

Predicted phase diversity and interesting properties of the Na–Bi system in the range of 0–320 GPa from first-principles calculations.

A novel stable hydrogen-rich SnH8 under high pressure

By structure searching techniques, a novel metallic SnH₈ crystal with the space group of I4̄m2 is predicted.

High-pressure superconducting phase diagram of 6Li: isotope effects in dense lithium

We measured the superconducting transition temperature of (6)Li between 16 and 26 GPa, and report the lightest system to exhibit superconductivity to date. The superconducting phase diagram of (6)Li is compared with that of (7)Li through simultaneous measurement in a diamond anvil cell (DAC). Below 21 GPa, Li exhibits a direct (the superconducting coefficient, α, T(c) proportional M(-α), is positive), but unusually large isotope effect, whereas between 21 and 26 GPa, lithium shows an inverse superconducting isotope effect. The unusual dependence of the superconducting phase diagram of lithium on its atomic mass opens up the question of whether the lattice quantum dynamic effects dominate the low-temperature properties of dense lithium.

Pressure-induced metallization of dense (H₂S)₂H₂ with high-Tc superconductivity

The high pressure structures, metallization, and superconductivity of recently synthesized H₂-containing compounds (H₂S)₂H₂ are elucidated by ab initio calculations. The ordered crystal structure with P1 symmetry is determined, supported by the good agreement between theoretical and experimental X-ray diffraction data, equation of states, and Raman spectra. The Cccm structure is favorable with partial hydrogen bond symmetrization above 37 GPa. Upon further compression, H₂ molecules disappear and two intriguing metallic structures with R3m and Im-3m symmetries are reconstructive above 111 and 180 GPa, respectively. The predicted metallization pressure is 111 GPa, which is approximately one-third of the currently suggested metallization pressure of bulk molecular hydrogen. Application of the Allen-Dynes-modified McMillan equation for the Im-3m structure yields high T_c values of 191 K to 204 K at 200 GPa, which is among the highest values reported for H₂-rich van der Waals compounds and MH₃ type hydride thus far.

Superconductivity of lithium-doped hydrogen under high pressure

The high-pressure lattice dynamics and superconductivity of newly proposed lithium hydrides (LiH2, LiH6and LiH8) have been extensively studied using density functional theory. The application of the Allen–Dynes modified McMillan equation and electron–phonon coupling calculations show that LiH6and LiH8are superconductors with critical temperatures (Tc) of 38 K at 150 GPa for LiH6and 31 K at 100 GPa for LiH8, while LiH2is not a superconductor. TheTcof LiH6increases rapidly with pressure and reaches 82 K at 300 GPa due to enhancement of the electron–phonon coupling and the increased density of states at the Fermi level, while theTcof LiH8remains almost constant.

Vibrational and structural properties of tetramethyltin under pressure

The vibrational and structural properties of a hydrogen-rich group IVa hydride, Sn(CH(3))(4), have been investigated by combining Raman spectroscopy and synchrotron x-ray diffraction measurements at room temperature and at pressures up to 49.9 GPa. Both techniques allow the obtaining of complementary information on the high-pressure behaviors and yield consistent phase transitions at 0.9 GPa for the liquid to solid and 2.8, 10.4, 20.4, and 32.6 GPa for the solid to solid. The foregoing solid phases are identified to have the orthorhombic, tetragonal, monoclinic crystal structures with space groups of Pmmm for phase I, P4/mmm for phase II, P2/m for phase III, respectively. The phases IV and V coexist with phase III, resulting in complex analysis on the possible structures. These transitions suggest the variation in the inter- and intra-molecular bonding of this compound.

Compressed cesium polyhydrides: Cs+ sublattices and H3(-) three-connected nets

The cesium polyhydrides (CsH(n), n > 1) are predicted to become stable, with respect to decomposition into CsH and H2, at pressures as low as 2 GPa. The CsH3 stoichiometry is found to have the lowest enthalpy of formation from CsH and H2 between 30 and 200 GPa. Evolutionary algorithms predict five distinct, mechanically stable, nearly isoenthalpic CsH3 phases consisting of H3(–) molecules and Cs+ atoms. The H3(–) sublattices in two of these adopt a hexagonal three-connected net; in the other three the net is twisted, like the silicon sublattice in the α-ThSi2 structure. The former emerge as being metallic below 100 GPa in our screened hybrid density functional theory calculations, whereas the latter remain insulating up to pressures greater than 250 GPa. The Cs+ cations in the most-stable I4(1)/amd CsH3 phase adopt the positions of the Cs atoms in Cs-IV, and the H3(–) molecules are found in the (interstitial) regions which display a maximum in the electron density.

Superconductive sodalite-like clathrate calcium hydride at high pressures

Hydrogen-rich compounds hold promise as high-temperature superconductors under high pressures. Recent theoretical hydride structures on achieving high-pressure superconductivity are composed mainly of H(2) fragments. Through a systematic investigation of Ca hydrides with different hydrogen contents using particle-swam optimization structural search, we show that in the stoichiometry CaH(6) a body-centered cubic structure with hydrogen that forms unusual "sodalite" cages containing enclathrated Ca stabilizes above pressure 150 GPa. The stability of this structure is derived from the acceptance by two H(2) of electrons donated by Ca forming an "H(4)" unit as the building block in the construction of the three-dimensional sodalite cage. This unique structure has a partial occupation of the degenerated orbitals at the zone center. The resultant dynamic Jahn-Teller effect helps to enhance electron-phonon coupling and leads to superconductivity of CaH(6). A superconducting critical temperature (T(c)) of 220-235 K at 150 GPa obtained from the solution of the Eliashberg equations is the highest among all hydrides studied thus far.

High-pressure study of tetramethylsilane by Raman spectroscopy

High-pressure behavior of tetramethylsilane, one of the Group IVa hydrides, was investigated by Raman scattering measurements at pressures up to 142 GPa and room temperature. Our results revealed the phase transitions at 0.6, 9, and 16 GPa from both the mode frequency shifts with pressure and the changes of the full width half maxima of these modes. These transitions were suggested to result from the changes in the inter- and intra-molecular bonding of this material. We also observed two other possible phase transitions at 49-69 GPa and 96 GPa. No indication of metallization in tetramethylsilane was found with stepwise compression to 142 GPa.

Silane plus molecular hydrogen as a possible pathway to metallic hydrogen

The high-pressure behavior of silane, SiH(4), plus molecular hydrogen was investigated using a structural search method and ab initio molecular dynamics to predict the structures and examine the physical origin of the pressure-induced drop in hydrogen intramolecular vibrational (vibron) frequencies. A structural distortion is predicted at 15 GPa from a slightly strained fcc cell to a rhombohedral cell that involves a small volume change. The predicted equation of state and the pressure-induced drop in the hydrogen vibron frequencies reproduces well the experimental data (Strobel TA, Somayazulu M, Hemley RJ (2009) Phys Rev Lett 103:065701). The bond weakening in H(2) is induced by intermolecular interactions between the H(2) and SiH(4) molecules. A significant feature of the high-pressure structures of SiH(4)(H(2))(2) is the dynamical behavior of the H(2) molecules. It is found that H(2) molecules are rotating in this pressure range whereas the SiH(4) molecules remain rigid. The detailed nature of the interactions of molecular hydrogen with SiH(4) in SiH(4)(H(2))(2) is therefore strongly influenced by the dynamical behavior of the H(2) molecules in the high-pressure structure. The phase with the calculated structure is predicted to become metallic near 120 GPa, which is significantly lower than the currently suggested pressure for metallization of bulk molecular hydrogen.