Phonon Dispersion Analysis as an Indispensable Tool for Predictions of Solid State Polymorphism and Dynamic Metastability: Case of Compressed Silane

Abstract

This work is dedicated to Prof. Russell J. Hemley in recognition of his seminal contributions to high pressure sciencesDiamond anvil cell experiments suggest that upon compression above 26.5 GPa silane (SiH 4 ) forms a polymeric phase VI, whose crystal structure has not yet been solved. Here we present DFT calculations showing how phonon-guided optimization leads to a polymeric F dd2 structure which is the lowest-enthalpy polymorph of SiH 4 above 26.8 GPa, and which most probably can be identified as the experimentally observed polymeric phase. The new algorithm of predicting the lowest-energy structures enables simultaneous inspection of the potential energy surface of a given system, calculation of its vibrational properties, and assessment of chances for obtaining a metastable ambient-pressure structure via decompression. Our calculations indicate that at room temperature the differences in the vibrational and entropy terms contributing to the Gibbs free energy of different polymorphs of silane are negligible in comparison with corresponding differences in the zero-point energy corrections, in contrast to earlier suggestions. We also show that the F dd2 polymorph should be metastable upon decompression up to 5 GPa, which suggests the possibility of obtaining a polymeric ambient-pressure form of SiH 4 . Polymeric silane should exhibit facile thermal decomposition with evolution of molecular hydrogen and thus constitute an efficient (12.5 wt%) material for hydrogen storage.

Predicting crystal structures and properties of matter under extreme conditions via quantum mechanics: the pressure is on

“…3) led to a polymeric Fdd2 structure, which is the lowestenthalpy polymorph of SiH 4 above 26.8 GPa, and which most probably corresponds to the experimentally observed polymeric phase. 34 The phonon dispersion for the I% 42d polymorph calculated for P = 15 GPa indicates that this system is dynamically unstable. Two imaginary vibrations appear at the G-point (one of B 1 symmetry and wavenumber 143i cm À1 , the second of B 2 symmetry and wavenumber 945i cm À1 ).…”

“…Indeed, calculations indicate that at room temperature the differences in the vibrational and entropic contributions to the Gibbs free energy of different SiH 4 polymorphs are of the order of 10 meV and thus they are negligible in comparison with the corresponding differences in the zeropoint energy corrections, which reach up to 100 meV. 34 Another valuable result related to calculations of phonon spectra of different forms of silane is that polymerization may easily be detected by analyzing lattice oscillations. The highpressure Fdd2 form of SiH 4 (occurring above 27 GPa) is polymeric with the Si atoms exhibiting a 2 + 2 + 2 coordination with two terminal Si-H bonds of 1.47 Å and four bridging ones at 1.59 and 1.65 Å at 30 GPa.…”

Electronic Structure and Superconductivity of Compressed Metal Tetrahydrides

“…[1][2][3] Ever since Ashcroft predicted that the addition of a second element to hydrogen can decrease its metallization pressure via "chemical precompression" whilst retaining all of the beneficial properties required for superconductivity, [4] first-principles studies employing crystal structure prediction methods to explore the potential energy surfaces of hydrides have been used to search for the most promising candidates at pressures attainable in diamond anvil cells. Among the first class of compounds to be studied theoretically were the group 14 hydrides SiH 4 , [5][6][7][8] GeH 4 , [9,10] SnH 4 [11,12] and PbH 4 . [13] Within a decade, the preferred stoichiometries of most binary hydrides, their emergent structures, and propensity for superconductivity have been studied by theory.…”

Modeling Molecules under Pressure with Gaussian Potentials

“…[17][18][19][20][21][22][23][24][25][26] Moreover, by using periodic boundary conditions (PBC) in calculations, the multifaceted behavior of extended systems under high pressure can be predicted, including pressure-induced changes in crystal structures of diverse materials. [27][28][29][30][31][32][33][34] In many cases, the success of such calculations in terms of comparability with experiments is remarkable. 35 In Molecular Dynamics (MD) simulations, pressure can be applied to the investigated systems by manipulation of the box parameters, 36,37 allowing the simulation of biological, 38 organic 39 and inorganic [40][41][42] materials under pressure.…”