Potential high- T _c superconducting lanthanum and yttrium hydrides at high pressure

Abstract

A systematic structure search in the La-H and Y-H systems under pressure reveals some hydrogen-rich structures with intriguing electronic properties. For example, LaH 10 is found to adopt a sodalite-like face-centered cubic (fcc) structure, stable above 200 GPa, and LaH 8 a C2/m space group structure. Phonon calculations indicate both are dynamically stable; electron phonon calculations coupled to Bardeen-Cooper-Schrieffer (BCS) arguments indicate they might be high-T c superconductors. In particular, the superconducting transition temperature T c calculated for LaH 10 is 274-286 K at 210 GPa. Similar calculations for the Y-H system predict stability of the sodalite-like fcc YH 10 and a T c above room temperature, reaching 305-326 K at 250 GPa. The study suggests that dense hydrides consisting of these and related hydrogen polyhedral networks may represent new classes of potential very hightemperature superconductors.high pressure | superconductivity | hydrides | structure search E xtending his original predictions of very high-temperature superconductivity of high-pressure metallic hydrogen (1), Ashcroft later proposed that hydrogen-rich materials containing main group elements might exhibit superconductivity at lower pressures, as the hydrogen in these structures may be considered "chemically precompressed" (2). These proposals, which were based on the Bardeen-Cooper-Schrieffer (BCS) (3) phononmediated theory of superconductivity, have motivated many theoretical and experimental efforts in the search for hightemperature superconductivity in hydrides at elevated pressures (4-10). Theory has predicted the stability of a variety of dense hydride structures for which BCS arguments give superconducting transition temperatures, T c s, that are very high (11)(12)(13)(14)(15)(16)(17)(18).In recent times, compression of hydrogen sulfides has provided a new incentive in hydride superconductivity, one in which theory played an important role. First, theoretical calculations predicted H 2 S to have a T c of ∼80 K at pressures above 100 GPa (19). Compression of H 2 S led to the striking discovery of a superconducting material with a T c of 203 K at 200 GPa (20). Moreover, the critical temperature exhibits a pronounced isotope shift consistent with BCS theory. It was proposed that the superconducting phase is not stoichiometric H 2 S but SH 3 , with a calculated T c of 194 K at 200 GPa including anharmonic effects (21,22). A subsequent experiment (23) suggested that the superconducting phase is cubic SH 3 , in agreement with a theoretical study that gave T c = 204 K within the harmonic approximation (24). Compression of another hydride, PH 3 , was reported to reach a T c of ∼100 K at high pressures (25). Subsequent theoretical calculations predicted possible structures and calculated T c s close to the experimental results (26-28). The experimental picture for these materials remains not entirely clear, and the synthesis of the superconducting phases, which has been reproduced for hydrogen sulfide, is path-dependent (23).Th...

Synthesis and Stability of Lanthanum Superhydrides

“…[8] Indeed, more recent theoretical studies predict the high-pressure stability of very hydrogen-rich compounds,s o-called superhydrides XH n with n > 5( for example,M gH 6 ,C aH 6 ,Y H 6 ), [7,9,10] with calculated superconducting transitions well above 200 K. Avery recent study has predicted that lanthanum can form as eries of superhydride phases under pressure,i ncluding fcc LaH 10 with La atoms surrounded by cages of thirty-two Hatoms and strong electron-phonon coupling that leads to as uperconducting transition temperature up to about 280 Ka t2 10 GPa. [11] The same structure is found for YH 10 at high pressures as well, also predicted in asubsequent theoretical study. [12] Thea ppropriate tuning of pressure and chemistry to create and characterize such materials,i ncluding superhydrides,h as been difficult to achieve experimentally in part because of the difficulty of synthesis of these materials at the required megabar (> 100 GPa) pressures.E xperiments reported to date have shown that these pressures can be used to dissociate hydrogen in IrH 3 ,AlH 3 ,F eH n (n = 1to5), and the high-temperature superconductor H 3 S. [13][14][15][16][17][18] In contrast, molecular H 2 or H 3 À is stabilized within compounds Xe(H 2 ) 8 and NaH 7 synthesized and stabilized to megabar pressures.…”

Superconducting Hydrogen Sulfide

“…An example for the former is presented in the metallic phases of solid hydrogen and SrH 10 , both predicted to possess a T c higher than 200 K. The puckered honeycomb layers of hydrogen has been recently identified in FeH 5 which is also predicted to be superconductive . The cage‐like 3D frameworks of hydrogen were predicted in the high‐pressure phases of CaH 6 and LaH 10 . In both structures, depending on the pressure, the nearest neighbour H⋅⋅⋅H contacts range between 1.0 to 1.3 Å indicating weak H−H bonds.…”

“…Many theoretical studies based on Goldhammer–Herzfeld criterion and Bardeen–Cooper–Schrieffer (BCS) theory support this suggestion. Among these studies, several Group IV hydrides, transition metal hydrides, alkali and alkaline earth hydrides, have been calculated to possess a high T c .…”

Structures and Superconducting Properties of Ultra‐Hydrogen‐Rich Selenium Hydride H6Se

“…The hydrides of tellurium and polonium, H 4 Te, H 5 Te, H 4 Po, and H 6 Po, which have heavier atomic masses and weaker electronegativities than sulfur and selenium hydrides, were predicted to have T c values of 104, 58, 47, and 5 K, respectively . In addition, other hydrogen‐rich compounds have been predicted to be superconductors, e.g., LiH 6 , KH 6 , MgH 6 , CaH 6 , PbH 4 (H 2 ) 2 , AsH 8 , SbH 4 , YH 6 , ScH 6 , YH 10 , and LaH 10 . In a very recent study, Geballe et al successfully synthesized the lanthanum superhydride LaH 10 at 170 GPa and 1000 K in an experiment, which had the same clathrate‐like structure that was predicted previously .…”

“…In addition, other hydrogen‐rich compounds have been predicted to be superconductors, e.g., LiH 6 , KH 6 , MgH 6 , CaH 6 , PbH 4 (H 2 ) 2 , AsH 8 , SbH 4 , YH 6 , ScH 6 , YH 10 , and LaH 10 . In a very recent study, Geballe et al successfully synthesized the lanthanum superhydride LaH 10 at 170 GPa and 1000 K in an experiment, which had the same clathrate‐like structure that was predicted previously . This reignited interest in searching for hydrogen‐rich compounds or the so‐called superhydrides (MH n with n > 5), considering that increasing the content of hydrogen promotes the probability of those materials behaving like the pure solid hydrogen .…”