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Physicists predicted metallic silicon at atmospheric pressure

Ha-Jun Sung et al. / Phys. Rev. Lett.

Korean scientists predicted the existence of a silicon metal phase, which remains stable at atmospheric pressure and transforms into a superconducting state at a temperature of about 12 kelvins. To do this, they modeled the compression of a compound with the NaSi 6 formula to pressures of the order of 200,000 atmospheres, and then the load was removed and the sodium atoms from the resulting clathrate were removed. The article is published in  Physical Review Letters , the preprint of the paper is posted on the website arXiv.org.

Silicon is the second most common chemical element in the earth’s crust and acts as the main component of a huge number of electronic devices, from computer processors to solar cells. Under normal conditions – that is, at a pressure of about one atmosphere and a temperature of about zero degrees Celsius – silicon crystallizes into a stable diamond structure, and also exists in the form of numerous metastable forms , such as amorphous silicon, porous silicon, silicon nanowires, and so on. New metastable phases can be created by applying a large pressure to the sample, and then sharply reducing the load-for example, thus obtaining the rhombohedral phase of Si-XII or the hexagonal phase of Si-IV. Another way to create allotropicModifications of silicon involves the use of chemical precursors  – compounds from which it is possible to easily remove “interfering” atoms. It is especially convenient to use clathrates as precursors , in which the molecules of “guest matter” are trapped in the cells of the main crystal lattice. In particular, the allotropic modification of Fd3m-Si 136 is obtained by removing sodium atoms from the Na 24 Si 136 clathrate .

Despite the fact that at standard pressure silicon can exist in the form of a large number of metastable phases, they are all semiconductors, that is, they conduct electricity poorly. On the other hand, when the pressure in silicon is increased, metal phases may appear , but their crystal structure breaks down after the load is removed. For example, the β-Sn phase, which resembles its tin structure, ceases to exist at a pressure of less than 112 atmospheres. At the same time, metallic silicon, which does not break down under normal conditions, could be useful for creating new electronic devices.

A group of scientists led by Kai-Wei Chang of the Korea Institute of Advanced Technologies for the first time discovered a purely silicon structure that, at a pressure of the order of one atmosphere, not only conducts electricity well, but even transitions to a superconducting state. For this purpose, the researchers considered compounds with the general formula NaSi x , where x varied from 0.5 to 6, and their compression was modeled to a pressure of the order of two hundred thousand atmospheres. It turned out that the compound with the formula NaSi 6 under such conditions forms a stable clathrate structure of P6 / m-NaSi 6, which remains stable even after depressurization. If we replace sodium atoms with silicon atoms in this lattice, we obtain a simple hexagonal lattice that corresponds to the metallic phase of silicon existing at a pressure of the order of 132,000 atmospheres. Then scientists lowered the pressure and removed sodium atoms from the crystal lattice (in life, a process of thermal degassing at a temperature of about 600 Kelvin can be used). As a result, they obtained a new allotropic form of silicon P6 / m-Si 6 , which remained stable at atmospheric pressure.

Then scientists numerically calculated the electronic band structure ofsynthesized compounds using the molecular dynamics method , and found out that they should exhibit metallic properties. Moreover, physics investigated superconducting properties of the compounds by the perturbation theory density functional (density-functional perturbation theory). Within the framework of this theory, they estimated the electron-phonon coupling constant in each of the materials and found out at what temperature the Bose condensate of Cooper pairs would arise in them. It turned out that at zero pressure P6 / m-NaSi 6 passes into the superconducting state at a temperature of about 13.1 kelvin, and the allotropic form of silicon P6 / m-Si 6 – at a temperature of about 12.2 kelvin. With increasing pressure, it becomes more difficult for electrons to bind, and the critical temperature slowly begins to decrease. For example, at a pressure of about 150 thousand atmospheres it drops to 4 kelvins.

Finally, scientists investigated the stability of a new allotropic modification of silicon. To do this, they calculated the average energy of the compound, which was approximately 0.35 electron volts per atom. This means that P6 / m-Si 6 is a metastable compound that will sooner or later pass into a more stable diamond structure having the minimum possible energy. Nevertheless, at temperatures up to 400 Kelvin (slightly more than 100 degrees Celsius), P6 / m-Si 6  as a whole should remain stable long enough for it to be used in practice.

Many substances, which are ordinary dielectrics under ordinary conditions, transfer to a metallic state at high pressures. For example, noble gases near the nuclei of gas giants, where the pressure can reach hundreds of thousands of atmospheres, lose transparency and begin to conduct electricity . And hydrogen sulphide, compressed to a pressure of about 1.6 million atmospheres and cooled to a temperature of the order of -70 degrees Celsius, generally becomes a superconductor . Read more about how the properties of materials under extreme pressure conditions change, you can in our material “Journey to the Center of the Earth .  “

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