Rational Design of Superconducting Metal Hydrides via Chemical Pressure Tuning**

Abstract

The high critical superconducting temperatures (Tcs) of metal hydride phases with clathrate‐like hydrogen networks have generated great interest. Herein, we employ the Density Functional Theory‐Chemical Pressure (DFT‐CP) method to explain why certain electropositive elements adopt these structure types, whereas others distort the hydrogenic lattice, thereby decreasing the Tc. The progressive opening of the H24 polyhedra in MH6 phases is shown to arise from internal pressures exerted by large metal atoms, some of which may favor an even higher hydrogen content that loosens the metal atom coordination environments. The stability of the LaH10 and LaBH8 phases is tied to stuffing of their shared hydrogen network with either additional hydrogen or boron atoms. The predictive capabilities of DFT‐CP are finally applied to the Y−X−H system to identify possible ternary additions yielding a superconducting phase stable to low pressures.

Conventional High-Temperature Superconductivity in Metallic, Covalently Bonded, Binary-Guest C–B Clathrates

“…52 For those hydrides that did not correspond to local minima, DFT-CP was used to understand the structural distortions and chemical modifications that could be used to stabilize them. 52 In the CP schemes, shown for XB 3 C 3 in Figure 1, negative pressures, which are indicative of an atom too small for its coordination environment, are represented with black, and positive pressures�in white�indicate an atom too large for the cavity within which it resides. For the alkaline earth borocarbides, the metal atom is in all cases surrounded by negative CP, suggesting that the borocarbide cages are plenty large enough�and in some cases too large�to comfortably fit the electropositive atoms.…”

“…The X B 3 C 3 alkaline earth metal borocarbides (all nominally X 2+ ) offer a series for which, based on the emergence of dynamic stability when moving from Mg to Ba, atomic size is expected to play the main differentiating role. Recently, we have employed the DFT-CP method to elucidate the relationship between the metal atom size and the dynamic stability of related systems: Pm 3̅ n X B 3 Si 3 ( X = Na, K, Rb, Cs) and Im3̅ m X H 6 . For those hydrides that did not correspond to local minima, DFT-CP was used to understand the structural distortions and chemical modifications that could be used to stabilize them …”

“…Recently, we have employed the DFT-CP method to elucidate the relationship between the metal atom size and the dynamic stability of related systems: Pm 3̅ n X B 3 Si 3 ( X = Na, K, Rb, Cs) and Im3̅ m X H 6 . For those hydrides that did not correspond to local minima, DFT-CP was used to understand the structural distortions and chemical modifications that could be used to stabilize them …”

Hydride superconductor structures of LaH10, EuH9, and UH8 rationalized by electron counting and Jahn-Teller distortions in a covalent cluster model

“…hexagonal channel widening in SrH 6 compared to CaH 6 ). 22,23 Chemical insight also led to the discovery of Eu 8 H 46 , a high-pressure hydride isostructural to the "Zintl clathrates" (e.g. Ba 8 Si 46 ), because such large unit cells are too expensive to be included in routine CSP studies.…”

A Chemical-Pressure-Induced Phase Transition Controlled by Lone Electron Pair Activity

“…Chemical pressure is an efficient tool to tune properties and create functional materials. − The intentional introduction or substitution of chemical elements into a material is a fundamental process to modify its intrinsic structure and properties while changing its composition. − Usually, chemical pressure is mainly associated with structural effects provoked by the different sizes of the doping agents with respect to the host structure. These strains can recreate the response of the material under stress conditions facilitating the equivalence between physical or mechanical pressure and chemical pressure.…”