Potential High-Tc Superconducting Lanthanum and Yttrium Hydrides at High Pressure
Total Page：16
File Type：pdf, Size：1020Kb
Potential high-Tc superconducting lanthanum and yttrium hydrides at high pressure Hanyu Liua, Ivan I. Naumova, Roald Hoffmannb, N. W. Ashcroftc, and Russell J. Hemleyd,e,1 aGeophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015; bDepartment of Chemistry and Chemical Biology, Cornell University, Ithaca, NY 14853; cLaboratory of Atomic and Solid State Physics, Cornell University, Ithaca, NY 14853; dDepartment of Civil and Environmental Engineering, The George Washington University, Washington, DC 20052; and eSchool of Applied and Engineering Physics, Cornell University, Ithaca, NY 14853 Contributed by Russell J. Hemley, May 5, 2017 (sent for review March 20, 2017; reviewed by Panchapakesan Ganesh, Jeffrey M. McMahon, and Dimitrios Papaconstantopoulos) A systematic structure search in the La–H and Y–H systems under predicted superconducting Tcs are MgH6 (271 K at 300 GPa) pressure reveals some hydrogen-rich structures with intriguing (29), CaH6 (235 K at 150 GPa) (17), and YH6 (264 K at electronic properties. For example, LaH10 is found to adopt a 120 GPa) (30). In searching for other high-Tc superconducting sodalite-like face-centered cubic (fcc) structure, stable above hydrides, here we investigated theoretically possible high-pressure 200 GPa, and LaH8 a C2/m space group structure. Phonon calcula- crystal structures of La–H and Y–H. We predict the existence of tions indicate both are dynamically stable; electron phonon calcu- new stable hydride phases of these elements, with remarkably – – lations coupled to Bardeen Cooper Schrieffer (BCS) arguments high Tcs at attainable pressures. The search for low-energy indicate they might be high-Tc superconductors. In particular, the crystalline structures of La–H was performed using parti- superconducting transition temperature Tc calculated for LaH10 is cle swarm optimization methodology implemented in the 274–286 K at 210 GPa. Similar calculations for the Y–H system pre- CALYPSO code (31, 32). This method has been applied dict stability of the sodalite-like fcc YH10 and a Tc above room successfully to a wide range of crystalline systems ranging from temperature, reaching 305–326 K at 250 GPa. The study suggests elemental solids to binary and ternary compounds (33–35) and that dense hydrides consisting of these and related hydrogen poly- has proven to be a powerful tool for predicting crystal structures – hedral networks may represent new classes of potential very high- at high pressures (36 39). Structure searches were performed in PHYSICS temperature superconductors. the pressure range of 150–300 GPa using models consisting of 1–4 formula units. In general, the structure search was termi- high pressure | superconductivity | hydrides | structure search nated after the generation of 1,500 structures. Structural opti- mizations, enthalpies, electronic structures, and phonons were xtending his original predictions of very high-temperature calculated using density-functional theory (DFT). Structure re- Esuperconductivity of high-pressure metallic hydrogen (1), laxations were performed using DFT using the Perdew–Burke– Ashcroft later proposed that hydrogen-rich materials containing Ernzerhof (40) generalized gradient approximation. Phonon – main group elements might exhibit superconductivity at lower dispersion and electron phonon coupling (EPC) calculations pressures, as the hydrogen in these structures may be considered were performed with density functional perturbation theory. “chemically precompressed” (2). These proposals, which were Ultrasoft pseudopotentials for La and H were used with a ki- × × based on the Bardeen–Cooper–Schrieffer (BCS) (3) phonon- netic energy cutoff of 80 Ry. A q mesh of 6 6 6andk mesh × × mediated theory of superconductivity, have motivated many of 24 24 24 for fcc-LaH10 structure in the first Brillouin zone theoretical and experimental efforts in the search for high- (BZ) was used in the EPC calculations. The superconductivity calculations were performed with the Quantum-ESPRESSO temperature superconductivity in hydrides at elevated pressures · (4–10). Theory has predicted the stability of a variety of dense package (17) hydride structures for which BCS arguments give superconducting Significance transition temperatures, Tcs, that are very high (11–18). In recent times, compression of hydrogen sulfides has pro- vided a new incentive in hydride superconductivity, one in which Theoretical predictions and subsequent experimental obser- theory played an important role. First, theoretical calculations vations of high-temperature superconductivity in dense predicted H2S to have a Tc of ∼80 K at pressures above 100 GPa hydrogen-rich compounds have reinvigorated the field of su- perconductivity. A systematic computational study of the hy- (19). Compression of H2S led to the striking discovery of a drides of lanthanum and yttrium over a wide composition superconducting material with a Tc of 203 K at 200 GPa (20). Moreover, the critical temperature exhibits a pronounced iso- range reveals hydrogen-rich structures with intriguing elec- tope shift consistent with BCS theory. It was proposed that the tronic properties under pressure. Electron–phonon coupling calculations predict the existence of new superconducting superconducting phase is not stoichiometric H2S but SH3, with a phases, some exhibiting superconductivity in the range of calculated Tc of 194 K at 200 GPa including anharmonic effects (21, 22). A subsequent experiment (23) suggested that the room temperature. Moreover, the calculated stabilities indicate superconducting phase is cubic SH , in agreement with a theo- the materials could be synthesized at pressures that are cur- 3 rently accessible in the laboratory. The results open the pros- retical study that gave Tc = 204 K within the harmonic approx- imation (24). Compression of another hydride, PH , was pect for the design, synthesis, and recovery of new high- 3 temperature superconductors with potential practical applications. reported to reach a Tc of ∼100 K at high pressures (25). Sub- sequent theoretical calculations predicted possible structures and Author contributions: H.L. and R.J.H. designed research; H.L., I.I.N., R.H., N.W.A., and calculated Tcs close to the experimental results (26–28). The R.J.H. analyzed data; and H.L., I.I.N., R.H., N.W.A., and R.J.H. wrote the paper. experimental picture for these materials remains not entirely Reviewers: P.G., Oak Ridge National Laboratory; J.M.M., Washington State University; and clear, and the synthesis of the superconducting phases, which has D.P., George Mason University. been reproduced for hydrogen sulfide, is path-dependent (23). The authors declare no conflict of interest. There is great experimental and theoretical interest in searching 1To whom correspondence should be addressed. Email: [email protected]. for related materials with both higher Tc and potentially broader This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. ranges of stability. To date, simple hydrides with the highest 1073/pnas.1704505114/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1704505114 PNAS Early Edition | 1of6 Downloaded by guest on October 2, 2021 Results community as AST (41). The La atoms sit at the 4b Wyckoff We began with structure searches at ambient pressure for LaH2. position (0, 0, 0), the H atoms at the 32f position (0.12, 0.38, The fcc structure is found to be stable at low pressure, in 0.12) and 8c position (0.25, 0.25, 0.75). The shortest H–H agreement with experiment. Thinking the high-pressure regime distance is 1.1 Å (250 GPa), which is close to H–Hdistancepre- would be most productive for new stoichiometries, we moved dicted for atomic metallic hydrogen near 500 GPa (1 Å) (1). In – directly to 150 and 300 GPa. We first performed structure pre- contrast, the H H distance in sodalite-type CaH6 is 1.24 Å at diction at 150 and 300 GPa for LaHx (x = 1–12). LaH2, LaH3, 150 GPa. The above structures of LaH encouraged us to explore the LaH4, LaH5, LaH8, and LaH10 are found to be stable at 150 GPa 10 Y–H system at similar pressures (Fig. S2). Previously, YH was (Fig. 1). Note that this figure is plotted with LaH2 as the 3 lanthanum-rich endpoint, because this composition has the most predicted to adopt an fcc structure of Y, with atomic H located negative enthalpy/atom at every pressure considered. Fig. 2 in the tetrahedral and octahedral interstitial sites. The com- shows the computed geometries for the most stable phases we pound was predicted to be superconducting with a maximum Tc μ = found at each stoichiometry, for n = 2–8. Interestingly, the stable is 40 K near 18 GPa ( * 0.1) (16). Another theoretical study suggested two energetically competing hydrogen-rich poly- phase predicted for LaH6 is not sodalite-type but an R-3m morphs YH and YH (30). At 120 GPa, both are predicted to be structure. The convex hull shows that LaH6 is not stable from 4 6 superconductors with maximum T of 95 K and 264 K for YH 150 to 300 GPa (Fig. S1); LaH5 in a P-1 structure is stable c 4 and YH , respectively. Our calculations indicate that YH is from 150 to 200 GPa. LaH2 adopts a C2/m structure with 6 6 stable in the sodalite structure up to 300 GPa (Fig. 4 and Fig. aH–Hdistanceof1.53Å,LaH3 adopts a Cmcm structure S2). As expected, YH adopts the same structure as LaH over with H–H distance of 1.42 Å, and LaH8 has a C2/m struc- 10 10 ture with H–H distance of 1.02 Å (all at 300 GPa). a range of pressures. We predict that YH10 is the energetically stable phase from 250 to 300 GPa, and is dynamically stable Most interestingly, we find that LaH10 adopts a sodalite-like structurewiththeLaatomsarrayedonanfccratherthana down to 220 GPa. bcc lattice (Fig. 3). To illustrate the difference between fcc Fig. 5 shows computed YHn geometries for the most stable phases found at each stoichiometry, for n = 2, 3, 4, 6, 8, and 12.