Sodium: a charge-transfer insulator at high pressures

Abstract:

By means of first-principles methods we analyse the optical response of transparent dense sodium as a function of applied pressure. We discover an unusual kind of charge-transfer exciton that proceeds from the interstitial distribution of valence electrons repelled away from the ionic cores by the Coulomb interaction and the Pauli repulsion. The predicted absorption spectrum shows a strong anisotropy with light polarization that just at pressures above the metal-insulator transition manifests as sodium being optically transparent in one direction but reflective in the other. This result provides a key information for an experimental conclusive determination of the crystal structure of transparent sodium, a new unconventional inorganic electride.

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https://arxiv.org/pdf/1003.0540.pdf