Abstract
The study of the physical properties of hydrogen sorbent metals is of interest in connection with the use of hydrogen as an energy carrier. Ruthenium is a good hydrogen sorbent. Of interest is the experimental construction of isotherms and isobars for the adsorbed hydrogen solubility on crystals during their transition from an isotropic to an anisotropic state, accompanied by ordering of hydrogen atoms, in order to reveal possibly the regularity of their behavior and, in particular, the possible detection of an extreme dependence of the atomic order on temperature and pressure, as predicted by theory. In this work, a theoretical calculation of the solubility isostere for adsorbed hydrogen on the (0001) face of an HCP crystal was performed. The calculation of the surface free energy was carried out on the basis of molecular kinetic concepts. The equilibrium equations made it possible to obtain relations that determine the P-T-c diagrams of the crystal surface state. Isotherms, isobars, and isosteres of the adsorbed hydrogen solubility are constructed, their analysis is carried out, and their possible course is elucidated. In this case, the energies of hydrogen atoms in the surface interstices were estimated using experimental isosteres for a ruthenium crystal. The obtained theoretical isosteres confirmed the possibility of the appearance of jumps and kinks in the graphs due to the phase transition of the crystal from an isotropic to an anisotropic state. This result is in full agreement with the experimental data on the solubility isosteres of adsorbed hydrogen on ruthenium. An experimental study of an adsorbed interstitial impurity solubility isosteres can make it possible to reveal in crystals of various structures phase transformations of the isotropic-anisotropic state type, or of the cubic-tetragonal structure type, or of the surface order-disorder type in a system of adsorbed hydrogen, nitrogen, and oxygen atoms.