Unlock Anionic Behavior of Calcium Through Pressure Engineering

TL;DR

This study demonstrates that calcium can exhibit anionic behavior under high pressure, with its valence state varying from negative to positive, revealing complex chemistry in iodides at ultrahigh pressures.

Contribution

The paper uncovers pressure-induced anionic behavior of calcium and predicts novel calcium iodide phases with unusual charge transfer and coordination.

Findings

Calcium can act as an anion in iodides under high pressure.

Predicted and experimentally confirmed a new CaI phase at 84 GPa.

Calcium's valence state varies from negative to +2 with increasing pressure.

Abstract

An isolated calcium (Ca) atom has empty d-orbitals under ambient conditions. However, s-d band hybridization has been observed in both elemental Ca and compounds by manipulating thermodynamic conditions. Here, we reveal that the Ca 3d-band can even capture electrons from halogen atoms under pressure, exhibiting anionic behaviors in iodides. We predict a CsCl-type monovalent CaI at above 50 GPa by employing first-principles structural searching and successfully identified the phase at 84 GPa using in situ X-ray diffraction. We further reveal that, due to the effect of orbital broadening, unusual charge transfer from the 5p orbitals of I to the 3d orbitals of Ca in CaI, gradually reverses the ionicity of Ca and becomes the anionic ICa at 485 GPa. Multivalent Ca stabilizes a set of metallic iodides with eight- to ten-fold iodine hyper-coordination. Our findings demonstrate that the valence states of Ca can vary from negative to +2, suggesting much greater complexity of Ca chemistry under ultrahigh pressures.