High pressure physics of elemental solids
The profound changes in elements induced by extreme conditions are fundamental to a broad range of problems in physics. The group-I alkali metals have been the testing grounds for simple systems, which have been predicted to adopt low-symmetry structures upon compression. (ref 1, 2) The proposed instability of cubic phases of Li under compression and the prediction that the low-symmetry phases will take over at higher pressures (ref 2) were later confirmed experimentally for both Li and Na. (ref 3–7) The appearance of open and incommensurate structures has been explained in terms of Peierls distortions, (ref 2,8) s → p and s → d electronic transitions, (ref 3) Fermi surface–Brillouin zone interactions, ref (9,10) and more recently in terms of a combined effect of Couloumb repulsion, Pauli exclusion, and orbital orthogonality that results in an increase of valence electrons in interstitial regions. (ref 11) Theory also suggested that as the density rises both Li and Na will become increasingly less metallic, approaching a semiconducting phase; several candidate structures have been proposed at very high pressures, including oC8 and hP 4. (ref 12,13) Indeed, the experimental studies (ref 5,14) reported the existence of Raman activity, color change, and reflectivity decrease in Na, signaling profound electronic modifications. We have studied the evolution of the Raman spectrum of the high pressure phases of Li and Na at extreme pressures (up to 200 GPa). The measured low frequency and high resolution Raman spectra are in excellent agrement with the ab initio simulated ones providing insights into the experimentally observed solid-solid phase transitions and anomalous melting behavior of sodium and lithium (ref 15, 16).
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