State Key Laboratory of Superhard Materials Makes Breakthrough in Ultra High Pressure Research
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The academic community has always believed that high pressure can effectively shorten the atomic spacing of materials, leading to the valence band and conduction band broadening of the material, which in turn causes the valence band and the conduction band of the insulator (or semiconductor) to overlap, causing an insulator-metal phase change, or The degree of overlap of the valence band and the conduction band of the metal is further increased. Professor Ma Yuming and his collaborators of the superhard materials laboratory found that the metal sodium turned into a wide band gap insulator under high pressure. This discovery poses a challenge to the classical high pressure theory and brings an opportunity for the further development of the high pressure theory.
Professor Ma Yuming theoretically predicted that alkali metal sodium will be transformed into a wide band gap insulator under ultra high pressure. The insulator sodium has a simple and unique crystal structure - a c-axis highly compressed double hexagonal close-packed structure. This is the first time this structure has been found in elemental elements. Generally, alkali metals undergo charge transfer under high pressure. The s electrons of lithium are transferred to the p orbital, and the s electrons of potassium, strontium and barium are transferred to the d orbital, while the charge transfer of sodium is very special, and the s electrons are transferred to the p and d orbitals, and the hybridization of p and d electrons is formed. Sodium double hexagonal close-packed crystal structure. Surprisingly, there is a high overlap between the core electron clouds of the insulator sodium. All valence electrons of the sodium atom are repelled by the core electrons and are highly localized in the lattice gap. These are the values ​​of "freezing" in the gap. The electrons completely lose the characteristics of free electrons, turning the metal sodium into insulator sodium. The electronic properties of the insulator sodium are very similar to the recently discovered new electron compound Electride. The cationic sodium is at the lattice point of the lattice, and the highly localized valence electrons in the lattice gap act as anions. At this time, the core electrons have an important influence on the chemical bond between the atoms, resulting in an abnormal pressure-induced metal-insulator phase transition.
The colleague Professor Eremets of Germany used high-voltage in-situ optical measurement, high-pressure synchrotron radiation X-ray diffraction and high-pressure Raman experiments to confirm the metal-insulator phase transition predicted by Professor Ma Yuming's theory.
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