Potential Room Temperature Superconductivity in Clathrate Lanthanide/Actinides Octadechydrides at Extreme Pressures

Atomic metallic hydrogen (AMH) hosting high-temperature superconductivity has long been considered a holy grail in condensed matter physics and attracted great interest, but attempts to produce AMH remain in intense exploration and debate. Meanwhile, hydrogen-rich compounds known as superhydrides offer a promising route toward creating AMH-like state and property, as showcased by the recent prediction and ensuing synthesis of LaH10 that hosts extraordinary superconducting critical temperatures (Tc) of 250-260 K at 170-190 GPa. Here we show via advanced crystal structure search a series of hydrogen-superrich clathrate compounds MH18 (M: rare-earth/actinide metals) comprising H36-cage networks, which are predicted to host Tc up to 329 K at 350 GPa. An in-depth examination of these extreme superhydrides offers key insights for elucidating and further exploring ultimate phonon-mediated superconductivity in a broad class of AMH-like materials.

Modeling Phase Behavior of Semi-Clathrate Hydrates of CO2, CH4, and N2 in Aqueous Solution of Tetra-n-butyl Ammonium Fluoride

Semi-clathrate hydrates are members of the class of clathrate compounds. In comparison with clathrate hydrates, where the networks are formed only by H2O molecules, the networks of semi-clathrate hydrates are formed by mixtures of H2O and quaternary ammonium salts (QASs). The addition of QASs to the solution enables to improve the formation of semi-clathrate hydrates at much milder conditions comparing to clathrate hydrates. In this work, we study the phase equilibria of semi-clathrate hydrates of CH4, CO2, and N2gas in an aqueous solution of tetra-n-butyl ammonium fluoride (TBAF). An extension of the Chen–Guo model is proposed as a thermodynamic model. The Peng–Robinson equation of state (PREOS) was applied to calculate the fugacity of the gas phase and in order to determine the water activity in the presence of TBAF, a correlation between the system temperature, the TBAF mass fraction, and the nature of the guest molecules has been used. These equations were solved simultaneously and through optimizing tuning parameters via the Nelder–Mead simplex algorithm. The results are compared to experimental data and good agreement is observed.

First-principles calculations for thermodynamic properties of type-I silicon clathrate intercalated by sodium atoms

The ground state properties of the silicon clathrate [Formula: see text] intercalated by alkali metal sodium atoms [Formula: see text] are investigated by first-principle methods. Birch–Murnaghan equation of state is fitted to two sets of the [Formula: see text]–[Formula: see text] data calculated by density functional theory based on the plane-wave basis set within both the local density approximation (LDA) and the generalized gradient approximation (GGA). Through quasi-harmonic Debye model, some thermodynamic properties comprise the heat capacity, the thermal expansion coefficient, Debye temperature and the Grüneisen parameter for this clathrate compounds [Formula: see text] are obtained, which agree well with experimental results. Comparing the calculated heat specific in two ways with experimental results, we find that it is more accurate to describe the “rattle” modes of gust Na atoms in the cages as Einstein oscillators. Moreover, the effects of high pressure on these thermodynamic properties are also investigated which will be very helpful for a synthesis of these clathrate compounds in experiments under high pressure and high temperature condition.