The Pressing Role of Theory in Studies of Compressed Matter

Abstract

Our chemical intuition, and indeed the periodic table, is based upon experience at 1 atm. However, in our universe, pressure spans an astounding 62 orders of magnitude (from interstellar space to the center of a neutron star). Chemistry is very different at high pressures, and because high‐pressure experiments can be very difficult or even impossible to carry out, first‐principles calculations are necessary to study matter at conditions of extreme pressure. In this chapter, we(i) describe computational techniques that can be employed to predict the structure of an extended system under pressure; (ii) describe how structure, chemical bonding, and electronic structure are affected by pressure; and (iii) provide examples of how theory has helped to interpret experimental data and provide predictions for experimental validation. We also comment on how well density functional theory does in predicting the structures and properties of compressed matter.

New Calcium Hydrides with Mixed Atomic and Molecular Hydrogen

“…High pressure can be used to synthesize compounds with novel stoichiometries and crystal structures that are not stable at atmospheric conditions, some with exotic electronic structures and properties. , Intense research has been devoted to high-pressure hydrides because of their potential as hydrogen-storage media or as high-temperature superconductors and because these compounds are likely to employ novel bonding strategies while adopting hitherto unobserved crystalline lattices …”

“…High pressure can be used to synthesize compounds with novel stoichiometries and crystal structures that are not stable at atmospheric conditions, some with exotic electronic structures and properties. 1,2 Intense research has been devoted to highpressure hydrides because of their potential as hydrogenstorage media 3 or as high-temperature superconductors 4 and because these compounds are likely to employ novel bonding strategies while adopting hitherto unobserved crystalline lattices. 5 The last decade has witnessed an explosion in the application of a priori crystal structure prediction techniques coupled with first-principles calculations toward the phase diagrams of high-hydrides as a function of stoichiometry and pressure.…”

Structural Diversity and Superconductivity in S-P-H Ternary Hydrides Under Pressure

“…78 Im3m H 3 S is a highly symmetric structure where each H-S distance measures 1.492 Å at 200 GPa because of bond symmetrization that occurs under increasing pressure. 94 At relatively low levels of S → P substitutions the doped structures retain the same basic lattice, with slight distortions that can be traced back to the atomic size and electronegativity differences between the two p-group elements. With the exception of 12.5% doping, the DOS at E F of all of the aforementioned cage-like structures was found to be smaller than that of the H 3 S parent.…”

Materials under high pressure: A chemical perspective

“…[10][11][12][13][14][15][16][17][18][19] As is evident from the strange elemental ratios in the aforementioned iconic superconductoring hydrides, pressure can have a profound effect on the compositions, structures, properties, and stabilities of solid phases. [9,[20][21][22][23][24][25][26][27][28][29][30][31][32] Experimentally observed compounds often could not have been predicted using the rules and bonding schemes that stem from our ambient-pressure-taught intuition. For these reasons, methods based on data mining [33] and chemical intuition [34], tend to break down when predicting solid phases under pressure.…”