Superconductive sodalite-like clathrate calcium hydride at high pressures

Abstract

Hydrogen-rich compounds hold promise as high-temperature superconductors under high pressures. Recent theoretical hydride structures on achieving high-pressure superconductivity are composed mainly of H 2 fragments. Through a systematic investigation of Ca hydrides with different hydrogen contents using particleswam optimization structural search, we show that in the stoichiometry CaH 6 a body-centered cubic structure with hydrogen that forms unusual "sodalite" cages containing enclathrated Ca stabilizes above pressure 150 GPa. The stability of this structure is derived from the acceptance by two H 2 of electrons donated by Ca forming an "H 4 " unit as the building block in the construction of the three-dimensional sodalite cage. This unique structure has a partial occupation of the degenerated orbitals at the zone center. The resultant dynamic Jahn-Teller effect helps to enhance electronphonon coupling and leads to superconductivity of CaH 6 . A superconducting critical temperature (T c ) of 220-235 K at 150 GPa obtained from the solution of the Eliashberg equations is the highest among all hydrides studied thus far.calcium hydride | sodalite structure

Theoretical study of the ground-state structures and properties of niobium hydrides under pressure

“…The situation thus differs markedly from LiH n , 2 and also from CaH n 5 where for certain stoichiometries one clearly finds paired hydrogens. There is a definite connection to the CaH 6 structure at 150 GPa, 5 which features an extended H network, albeit different from the ones we find here. We should also mention that it is certain that paired hydrogens will be found even at low pressures, if we were to examine a higher n, n>6 in NbH n (as in the W/H systems for WH 8 9 ).…”

“…By comparison the networks of hydrogens, all the hydrogens in the high-pressure structure (I m-3m) of CaH 6 form a sodalite like clathrate and the H-H separations between H-H neighbors are all 1.24Å at a much lower pressure, 150 GPa. 5 The important difference between the CaH 6 cagelike structure and our NbH 6 equivalent is that our structure is not stabilized until much higher pressures (than the 150 GPa reported for CaH 6 ). The relative compressions are 2.06 for NbH 6 at 400 GPa and 1.85 for CaH 6 at 150 GPa.…”

“…Aside from the intrinsic interest in the metal-hydrogen bond and the electron density around hydrogen in solid-state hydrides, there is the known capacity of some metals (e.g., Y, Rh, Pd, Pt) and alloys (e.g., LaNi 5 , Mg 2 Ni, FeTi) to absorb relatively large amounts of hydrogen. This property has led to the real and potential use of metal hydrides as hydrogen storage materials.…”

“…28 The study then continues to the also available (at 1 atm) NbH 2 . From these we proceed to the as yet hypothetical NbH 3 , NbH 4 , NbH 5 , and NbH 6 stoichiometries, all studied over a range of pressures up to 500 GPa, with relative compressions reaching V /V 0 ∼2.2.…”

High-Energy Density and Superhard Nitrogen-Rich B-N Compounds

“…CALYPSO has been used to investigate a great variety of materials at high pressures [33][34][35][36][37][38]. Detailed information on the calculations is provided in the Supplemental Material [39].…”