The researchers proposed the concept of “generalized pressure”

On May 5, 2022, the Lei Li team of the Institute of Atomic and Molecular Physics of Sichuan University published a paper in the international authoritative journal of physics, Physical Review B.

Inspired by Einstein’s equivalence principle of general relativity, the research team proposed the concept of “generalized pressure.”

The first author and second author of the paper are Tao Yu, a 20th-level master’s student of LeiLi Researcher, and Zhang Leilei, a 14th-grade master’s student, respectively, and Leili Researcher is the corresponding author. The Institute of Atomic and Molecular Physics of Sichuan University is the sole author unit.

In physics, pressure is a thermodynamic parameter that describes the magnitude of the pressure an object is subjected to per unit area. The effect of pressure on an object is usually manifested as volume contraction and density increase. Under high pressure conditions (more than 10,000 atmospheres), the space occupied by atoms is significantly compressed, the interactions between adjacent electrons are enhanced, and the material exhibits complex properties and states. The Equation of State (EOS) derived from the Stress-Strain relationship reflects the compressive properties of a substance at high pressure. In general, the equation of state only considers the effect of external physical loading on matter, and is usually measured experimentally by high-pressure diffraction experiments.

In 2004, condensed matter physics theorist N. W. Ashcroft proposed the concept of “Chemical precompression” for hydrogen metallization. He pointed out that hydrogen can form hydrides with IVa group elements, which produces a certain degree of chemical pre-pressure, thereby reducing the pressure required for hydrogen metallization. Subsequent high-pressure experiments proved the correctness of Ashcroft’s thinking, and a variety of high-temperature superconducting hydrides were discovered, such as H3S (100 GPa), LaH10 (150 GPa), CaH6 (172 GPa). The metallization pressure of these hydrides is much lower than the pressure required for solid metal hydrogen (>500 GPa).

The concept of “chemical pre-compression” derives from the idea of chemical pressure. In the 1990s, when studying the critical temperature of superconductivity of alloys, it was found that lattice contraction (or expansion) caused by chemical doping or ion substitution can produce an effect similar to physical pressure. Although, chemical pressure is an indispensable factor in the experimental design and mechanism analysis of hydride metallization and superconductivity studies. But to what extent is chemical pressure equivalent to physical pressure? What is the relationship between chemical and physical pressure? How is the chemical pressure scaled? These questions have not been answered reasonably.

In view of the above problems, the concept of “generalized pressure” proposed by the Lei Li team believes that all the thermodynamic factors that cause the same body strain are considered equivalent, and the generalized pressure is the synthesis of all the thermodynamic factors that can cause body strain. That is, as long as the degree of body strain caused is the same, the chemical and physical pressures are equivalent. Substances under high pressure have an inherent generalized pressure phase change point, and after being subjected to a certain chemical pressure, as long as a small physical pressure is applied, the phase transition can be triggered. This is why the metallization pressure of the hydride after chemical pre-pressing is lower than that of pure hydrogen systems.

Figure 1. The relationship between physical, chemical and generalized pressure

In addition, considering that the pressure is an amount of intensity, simple algebraic addition and subtraction cannot be performed between various pressures. The researchers established an equal relationship by wide-scale volume, and under given boundary conditions, derived the equation of state (EGP) of generalized pressure under the action of multiple thermodynamic factors:

In order to verify the correctness of EGP, they based on the ideal alloy model, taking the ternary semiconductor alloy AlxGa1-xN as the research object, and verified the correctness of EGP by predicting the high-pressure structural phase transition point of the alloy and the intrinsic material. The researchers also note that equations of state obtained based on the equivalence of bulk strains are only one form of describing generalized pressure. If other observable physical quantities subject to generalized pressure factors are used as equivalent parameters (such as electron state density, bond length, phonon frequency, etc.), the corresponding EGP can theoretically be derived. EGP allows prediction of high-voltage phase transitions other than structural phase transitions. The research results are of great significance for in-depth understanding of the correlation between physical pressure and chemical pressure, and open up a new horizon for high-pressure physics research. (Source: Science Network)

Related paper information:DOI: 10.1103/PhysRevB.105.174102

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