Abstract
Motivated by the recent claim of hot superconductivity with critical temperatures up to 550 K in La + x hydrides, we investigate the high-pressure phase diagram of compounds that may have formed in the experiment, using first-principles calculations for evolutionary crystal structure prediction and superconductivity. Starting from the hypothesis that the observed Tc may be realized by successive heating upon a pre-formed LaH10 phase, we examine plausible ternaries of lanthanum, hydrogen and other elements present in the diamond anvil cell: boron, nitrogen, carbon, platinum, gallium, gold. We find that only boron and, to a lesser extent, gallium form metastable superhydride-like structures that can host high-Tc superconductivity, but the predicted Tc’s are incompatible with the experimental reports. Our results indicate that, while the claims of hot superconductivity should be reconsidered, it is very likely that unknown H-rich ternary or multinary phases containing lanthanum, hydrogen, and possibly boron or gallium may have formed under the reported experimental conditions, and that these may exhibit superconducting properties comparable, or even superior, to those of currently known hydrides.
Highlights
Since the discovery of high-temperature superconductivity (HTSC) in compressed sulfur hydride in 20141,1,2, the race for HTSC has dramatically accelerated, leading to a hydride rush fueled by ab initio predictions
Since Migdal–Eliashberg theory does not pose a hard limit to Tc, it is possible that multinary hydrides may exhibit superconductivity at sensibly higher Tcs than the known binaries; for example, Tc’s largely exceeding room temperature have been predicted in a Li–Mg–H alloy[14]
Our structural searches were carried out using evolutionary algorithms as implemented in the Universal Structure Predictor: Evolutionary Xtallography (USPEX) code[22,23]
Summary
Since the discovery of high-temperature superconductivity (HTSC) in compressed sulfur hydride in 20141,1,2, the race for HTSC has dramatically accelerated, leading to a hydride rush fueled by ab initio predictions.As of 2020, all binary hydrides have been computationally explored[3,4,5], and many have been synthesized[6,7,8,9,10,11,12]. In the summer of 2020, Grockowiak et al reported experimental evidence of superconductivity with onset temperatures growing from 294 to 550 K upon successive heating cycles of a mixture of lanthanum and ammonia borane at about 180 GPa15. This may have been an experimental observation of hot superconductivity in a multinary hydride; due to COVID restrictions, the authors were able to report only partial evidences, and did not provide information on the chemical composition and structure of the superconducting samples, which would be fundamental for reproducibility. Even if one is skeptical about the highest values of
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