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12Complex Systems of Charged Particles and their Interactions with Electromagnetic Radiation 2018
TWIN TRANSITIONS TO THE CONDUCTING STATE IN SOLID AND FLUID
HYDROGEN UNDER PRESSURE Genri E. Norman2, Ilnur M. Saitov1,2
1National Research University Higher School of Economics, Russia 2Joint Institute for High Temperatures of RAS, Russia E-mail: [email protected]
An idea is introduced that ionization of H2 molecules takes place at the fluid-fluid and solid-solid phase transitions in hydrogen/deuterium with possible formation of molecular ions. Density functional theory with the VASP plane-wave code is used. Proton-proton pair correlation functions (PCF) g(r), conductivity and pressure are calculated.
The first maximum of the PCF rmaxi is equal to the interatomic distance 0.74A, in the H2 molecule. The first minimum rmin1 is close to the interatomic distances 1.06A, and 0.92A, in the molecular ions H2+ and H3+. Let g1(r) and g2(r) are PCF's which are the closest to the phase transition before and after it. Ag(r)= g2(r)-g1(r) is close to zero for r>2A. The function Ag(r) has a deep minimum at r=0.74A and a strongly pronounced maximum in the range of r from 0.92A to 1.06A. It means that the number of H2 decreases and ions H2+ and H3+ appears at the phase transition.
In the fluid hydrogen it turns out that the PCF's obtained can be modeled for r > 2A, by the soft sphere PCF's at number densities which are equal to the total density of H2, H2+ and H3+. The latter value remains close to the constant one at the phase transition. The repulsion "diameters" is close to the theoretical estimate.
The analysis of the PCFs allows to suggest a two-step mechanism of the transition both in fluid and solid hydrogen. The first stage is related to the partial ionization of H2 molecules at the phase transition with formation of the molecular ions H2+. The second stage is a reaction of H2 molecules and H2+ ions to form H3+ ions. The nature of the phase transition combines the ionization and the structure transformation. Strong ionization can be related to the Norman-Starostin plasma phase transition prediction. However, it differs from it by inherent structural changes. The transition in hydrogen at high pressures is an exceptional case.
The work is supported by grant of Russian Foundation for Basic Research 16-08-01218(A), grant of President of Russian Federation for young scientists MK-3231.2017.2 and program of the Presidium of the Russian Academy of Sciences No. 13 "Thermal physics of high energy densities".
