New Calcium Hydrides with Mixed Atomic and Molecular Hydrogen

Abstract

Two new polyhydrides of calcium have been synthesized at high pressures and high temperatures and characterized by Raman spectroscopy, infrared spectroscopy, and synchrotron X-ray diffraction. Above 20 GPa and 700 K, we synthesize a phase having a monoclinic (C2/m) structure with Ca 2 H 5 composition, which is characterized by a distinctive vibration at 3789 cm −1 at 25 GPa. The observed Raman spectrum is in close agreement with first-principles calculations of a Ca 2 H 5 structure characterized by a lattice containing a central layer of H 2 molecules oriented along the (100) direction. At higher pressures (e.g., 116 GPa and 1600 K), we synthesize another phase, which has the composition of CaH 4 and a denser body-centered tetragonal structure. This weakly metallic phase also contains molecular-like H 2 units, and its spectroscopic as well as diffraction signatures match closely with those predicted from first-principles calculations. This phase is observed to persist on decompression to 60 GPa at room temperature. The elongation of the H−H bond in these hydrides is a result of the Ca−H 2 interaction, analogous to what occurs in molecular compounds, where H 2 binds side-on to a d-element, such as in Kubas complex.

A Metastable CaSH3 Phase Composed of HS Honeycomb Sheets that is Superconducting Under Pressure

“…S13) weakens the H-H bond in these ternaries, in-line with the findings for CaH 4 . 42 The Bader charges illustrate that S becomes negatively charged because of electron transfer from Ca, and accordingly the interaction between these two atoms, as gauged by the -ICOHPs, is non-negligible, and even larger than in CaS at comparable pressures. The H-S bond strength is of a similar magnitude as the Ca-S interaction, and the markedly smaller H-Ca interaction is on par with results obtained previously for CaH 4 .…”

Electronic Structure and Superconductivity of Compressed Metal Tetrahydrides

“…The first successful synthesis of a material assuming this structure type was CaH 4 . [32] Shortly thereafter another group synthesized this same phase, [46] and I4=mmm symmetry CeH 4 , [47] ThH 4 , [48] YH 4 [49] and NdH 4 [41] compounds were made suggesting that other isotypic high-pressure hydrides may soon be discovered. A number of these phases are predicted to either be semiconductors or poor metals within their stability range (e. g. CaH 4 , [32] SrH 4 [34] ), to have relatively low T c values (e. g. LaH 4 , [19] ZrH 4 , [30] ThH 4 [44] and NdH 4 [41] ), or their superconducting properties have not yet been calculated (e. g. CeH 4 , PrH 4 , PmH 4 , SmH 4 , EuH 4 , GdH 4 , DyH 4 , HoH 4 , ErH 4 , TmH 4 , YbH 4 and LuH 4 [18] ).…”

“…[32] Shortly thereafter another group synthesized this same phase, [46] and I4=mmm symmetry CeH 4 , [47] ThH 4 , [48] YH 4 [49] and NdH 4 [41] compounds were made suggesting that other isotypic high-pressure hydrides may soon be discovered. A number of these phases are predicted to either be semiconductors or poor metals within their stability range (e. g. CaH 4 , [32] SrH 4 [34] ), to have relatively low T c values (e. g. LaH 4 , [19] ZrH 4 , [30] ThH 4 [44] and NdH 4 [41] ), or their superconducting properties have not yet been calculated (e. g. CeH 4 , PrH 4 , PmH 4 , SmH 4 , EuH 4 , GdH 4 , DyH 4 , HoH 4 , ErH 4 , TmH 4 , YbH 4 and LuH 4 [18] ). Only MgH 4 (81 K at 255 GPa), [30] ScH 4 (92 K at 120 GPa, [38] 67-81 K at 195 GPa, [36] and 98 K at 200 GPa [37] ), and YH 4 (84-95 K at 120 GPa [50] ) are predicted to have T c values higher than the boiling point of liquid nitrogen, whereas the T c of ZrH 4 (47 K at 230 GPa [30] ) and TbH 4 (41 K at 200 GPa [43] ) is predicted to be somewhat lower.…”

“…. Theoretical studies based upon a priori crystal structure prediction have found that MgH 4 , [30] CaH 4 , [31][32][33] SrH 4 , [34,35] ScH 4 , [18,[36][37][38] YH 4 , [18] ZrH 4 , [30] LaH 4 , [19] CeH 4 , [18,39,40] PrH 4 , [18] NdH 4 , [41] EuH 4 , [42] TbH 4 , [43] ThH 4 , [44] PuH 4 , [45] and all of the REH 4 [29] phases assuming this structure type are thermodynamically stable in their respective pressure ranges. Therefore, this I4=mmm structure may be a universal feature in the phase diagram of most of the compressed alkaline and rare earth, as well as a few transition metal and actinide tetrahydrides under pressure.…”

A First-Principles Exploration of NaxSy Binary Phases at 1 atm and Under Pressure

“…The inclusion of the ZPE corrections did not affect the identity of the thermodynamically stable stoichiometries at 100 and 150 GPa, but it added Na 2 S 2 and Na 2 S 3 to the 200 GPa hull, and pushed Na 2 S 4 away from it. Because phonon calculations (see the Supplementary Materials) confirmed that all of these phases were dynamically stable, and high-pressure experiments can lead to the formation of metastable species [86]; the structures of all of these phases are discussed below.…”