The State Key Laboratory of Superhard Materials, in collaboration with scientists such as the Max Planck Institute in Germany, made breakthrough progress in the research on the structural phase transition of alkali sodium under high pressure, and found that metallic sodium turned into a "transparent" wide band gap insulator at 2 million atmospheres. This result was published in the latest issue of Nature on March 12, and the State Key Laboratory of Superhard Materials is the first author and corresponding author unit.
The academic community has always believed that high voltage can effectively shorten the atomic distance of the material, causing the valence band and conduction band of the material to widen, which in turn causes the valence band and conduction band of the insulator (or semiconductor) to overlap, an insulator-metal phase transition, or The degree of overlap between the valence band and the conduction band of the metal is further increased. Professor Ma Yanming and his collaborators in the Superhard Materials Laboratory discovered that sodium metal turned into a wide band gap insulator under high voltage. This discovery challenges the classic high voltage theory and brings opportunities for the further development of high voltage theory.
Professor Ma Yanming theoretically predicted that alkali metal sodium will transform into a wide-bandgap insulator under ultra-high pressure. The insulator sodium has a simple and unique crystal structure-a double-hexagonal close-packed structure with highly compressed c axis. 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, the s electrons of potassium, rubidium and cesium are transferred to the d orbital, and the charge transfer of sodium is very special, but the s electrons are transferred to the p and d orbitals. The hybridization of the p and d electrons forms The double hexagonal close-packed crystal structure of sodium. Surprisingly: there is a high degree of overlap between the core electron clouds of the insulator sodium, all valence electrons of sodium atoms are repelled by the core electrons and are highly localized in the lattice gap, these val The electron completely loses the characteristics of free electron, so that the metallic sodium becomes an insulator sodium. The electronic properties of insulator sodium are very similar to the recently discovered new electronic compound Electride. The sodium cation is at the lattice point of the lattice, and the highly localized valence electrons in the lattice gap play the role of anions. At this time, the core electrons have an important influence on the chemical bonding between atoms, which leads to an abnormal pressure-induced metal-insulator phase transition.
The collaborator Professor Eremets's group in Germany confirmed the metal-insulator phase transition predicted by Professor Ma Yanming's theory using high-pressure in-situ optical measurement, high-pressure synchrotron radiation X-ray diffraction and high-pressure Raman experiments.
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