[Paper Review] How do chemical properties of the atoms change under pressure

Xiao Dong, Artem R. Oganov+4 more|arXiv (Cornell University)|Mar 1, 2015

High-pressure geophysics and materialsEarth and Planetary Sciences1 references16 citations

TL;DR

This paper proposes pressure-dependent scales for electronegativity and chemical hardness, showing they evolve non-monotonically under high pressure: electronegativity increases then decreases, while chemical hardness generally declines but rebounds in active metals. The study reveals that s-d orbital transfer transforms elements like Ni into pseudo-noble gases and Fe/Co into strong electron acceptors, enabling predictive modeling of high-pressure chemical behavior.

ABSTRACT

Abundant evidence has shown the emergence of dramatic new chemical phenomena under pressure, including the formation of unexpected crystal structures and completely new counterintuitive compounds. In many cases, there is no convincing explanation for these phenomena and there are virtually no chemical rules or models capable of predicting or even rationalizing these phenomena. Here we consider two central chemical properties of atoms, electronegativity and chemical hardness, and determine them as a function of pressure up to 500 GPa. For elements without orbital transfer at high pressure, electronegativity first increases and then decreases, while chemical hardness monotonically decreases as pressure increases. For some active metals, the chemical hardness has a further increase at pressures of the order of tens-hundreds of gigapascals. Furthermore, we discover that orbital transfer, in particular s-d transfer, makes Ni a pseudo-noble-gas, Fe and Co strong electron acceptors, while Cu and Zn become active metals. We show the explicative and predictive power of our electronegativity and chemical hardness scales under pressure.

Motivation & Objective

• To develop predictive scales for electronegativity and chemical hardness under high pressure, extending traditional periodic trends.
• To explain the emergence of unexpected high-pressure compounds and crystal structures through changes in fundamental atomic properties.
• To investigate the role of orbital electron redistribution, particularly s-d transitions, in altering chemical behavior under extreme conditions.
• To establish a theoretical framework capable of rationalizing and predicting novel chemical phenomena under pressure.

Proposed method

• The study derives pressure-dependent electronegativity and chemical hardness using quantum mechanical principles and electronic structure calculations up to 500 GPa.
• It applies the Mulliken electronegativity and hardness definitions, modified to account for pressure-induced changes in atomic energy levels.
• Orbital energy shifts due to compression are calculated to assess s-d electron transfer and its impact on chemical character.
• The model evaluates how changes in ionization energy and electron affinity under pressure affect electronegativity and hardness.
• Theoretical predictions are validated by comparing trends with known high-pressure phase transitions and compound stability.

Experimental results

Research questions

• RQ1 How does electronegativity evolve with increasing pressure for elements without orbital transfer?
• RQ2 How does chemical hardness change under high pressure, and what causes deviations in active metals?
• RQ3 To what extent does s-d electron transfer alter the chemical character of transition metals under pressure?
• RQ4 Can the modified electronegativity and hardness scales predict the formation of unexpected high-pressure compounds?

Key findings

• Electronegativity increases initially with pressure and then decreases for elements without orbital transfer, deviating from classical periodic trends.
• Chemical hardness monotonically decreases with pressure for most elements, but shows a secondary rise in active metals at tens to hundreds of gigapascals.
• s-d orbital transfer transforms Ni into a pseudo-noble-gas configuration, stabilizing it under pressure.
• Fe and Co become strong electron acceptors due to s-d transition, explaining their tendency to form unusual compounds.
• Cu and Zn undergo a transition to active metal behavior under high pressure, contrary to their low reactivity at ambient conditions.
• The proposed electronegativity and hardness scales demonstrate strong explicative and predictive power for high-pressure chemical phenomena.