Na–Ni–H phase formation at high pressures and high temperatures: hydrido complexes [NiH5]3– versus the perovskite NaNiH3

The Na-Ni-H system was investigated by in situ synchrotron diffraction studies of reaction mixtures NaH-Ni-H-2 at around 5, 10, and 12 GPa. The existence of ternary hydrogen-rich hydrides with compositions Na3NiH5 and NaNiH3, where Ni attains the oxidation state II, is demonstrated. Upon heating at similar to 5 GPa, face-centered cubic (fcc) Na3NiH5 forms above 430 degrees C. Upon cooling, it undergoes a rapid and reversible phase transition at 330 degrees C to an orthorhombic (Cmcm) form. Upon pressure release, Na3NiH5 further transforms into its recoverable Pnma form whose structure was elucidated from synchrotron powder diffraction data, aided by first-principles density functional theory (DFT) calculations. Na3NiH5 features previously unknown square pyramidal 18- electron complexes NiH53-. In the high temperature fcc form, metal atoms are arranged as in the Heusler structure, and ab initio molecular dynamics simulations suggest that the complexes are dynamically disordered. The Heusler-type metal partial structure is essentially maintained in the low temperature Cmcm form, in which NiH53- complexes are ordered. It is considerably rearranged in the low pressure Pnma form. Experiments at 10 GPa showed an initial formation of fcc Na3NiH5 followed by the addition of the perovskite hydride NaNiH3, in which Ni(II) attains an octahedral environment by H atoms. NaNiH3 is recoverable at ambient pressures and represents the sole product of 12 GPa experiments. DFT calculations show that the decomposition of Na3NiH5 = NaNiH3 + 2 NaH is enthalpically favored at all pressures, suggesting that Na3NiH5 is metastable and its formation is kinetically favored. Ni-H bonding in metallic NaNiH3 is considered covalent, as in electron precise Na3NiH5, but delocalized in the polyanion [NiH3](-).Funding Agencies|Swedish Research Council (VR)Swedish Research Council [2019-05551]; Swedish Government Strategic Research Area in Materials Science on Advanced Functional Materials at at Linkoping University (Faculty Grant SFO-Mat-LiU) [200900971]; Carl Tryggers Stiftelse (CTS) [16:198, 17:206]</p

Hypervalent hydridosilicate in the Na-Si-H system

Hydrogenation reactions at gigapascal pressures can yield hydrogen-rich materials with properties relating to superconductivity, ion conductivity, and hydrogen storage. Here, we investigated the ternary Na-Si-H system by computational structure prediction and in situ synchrotron diffraction studies of reaction mixtures NaH-Si-H-2 at 5-10 GPa. Structure prediction indicated the existence of various hypervalent hydridosilicate phases with compositions NamSiH(4+m) (m = 1-3) at comparatively low pressures, 0-20 GPa. These ternary Na-Si-H phases share, as a common structural feature, octahedral SiH62- complexes which are condensed into chains for m = 1 and occur as isolated species for m = 2, 3. In situ studies demonstrated the formation of the double salt Na-3[SiH6]H (Na3SiH7, m = 3) containing both octahedral SiH62- moieties and hydridic H-. Upon formation at elevated temperatures (&gt;500 degrees C), Na3SiH7 attains a tetragonal structure (P4/mbm, Z = 2) which, during cooling, transforms to an orthorhombic polymorph (Pbam, Z = 4). Upon decompression, Pbam-Na3SiH7 was retained to approx. 4.5 GPa, below which a further transition into a yet unknown polymorph occurred. Na3SiH7 is a new representative of yet elusive hydridosilicate compounds. Its double salt nature and polymorphism are strongly reminiscent of fluorosilicates and germanates

Formation and Polymorphism of Semiconducting K2_2SiH6_6 and Strategy for Metallization

K$_2$SiH$_6$, crystallizing in the cubic K$_2$PtCl$_6$ structure type (Fm$\bar{3}$m), features unusual hypervalent SiH$_{6}$$^{2–}$ complexes. Here, the formation of K$_2$SiH$_6$ at high pressures is revisited by in situ synchrotron diffraction experiments, considering KSiH$_3$ as a precursor. At the investigated pressures, 8 and 13 GPa, K$_2$SiH$_6$ adopts the trigonal (NH$_4$)$_2$SiF$_6$ structure type (P$\bar{3}$m1) upon formation. The trigonal polymorph is stable up to 725 °C at 13 GPa. At room temperature, the transition into an ambient pressure recoverable cubic form occurs below 6.7 GPa. Theory suggests the existence of an additional, hexagonal, variant in the pressure interval 3–5 GPa. According to density functional theory band structure calculations, K$_2$SiH$_6$ is a semiconductor with a band gap around 2 eV. Nonbonding H-dominated states are situated below and Si–H anti-bonding states are located above the Fermi level. Enthalpically feasible and dynamically stable metallic variants of K$_2$SiH$_6$ may be obtained when substituting Si partially by Al or P, thus inducing p- and n-type metallicity, respectively. Yet, electron–phonon coupling appears weak, and calculated superconducting transition temperatures are <1 K

Unraveling Hidden Mg–Mn–H Phase Relations at High Pressures and Temperatures by <i>in Situ</i> Synchrotron Diffraction

The Mg–Mn–H system was investigated by <i>in situ</i> high pressure studies of reaction mixtures MgH₂–Mn–H₂. The formation conditions of two complex hydrides with composition Mg₃MnH₇ were established. Previously known hexagonal Mg₃MnH₇ (h-Mg₃MnH₇) formed at pressures 1.5–2 GPa and temperatures between 480 and 500 °C, whereas an orthorhombic form (o-Mg₃MnH₇) was obtained at pressures above 5 GPa and temperatures above 600 °C. The crystal structures of the polymorphs feature octahedral [Mn­(I)­H₆]^5– complexes and interstitial H^–. Interstitial H^– is located in trigonal bipyramidal and square pyramidal interstices formed by Mg²⁺ ions in h- and o-Mg₃MnH₇, respectively. The hexagonal form can be retained at ambient pressure, whereas the orthorhombic form upon decompression undergoes a distortion to monoclinic Mg₃MnH₇ (m-Mg₃MnH₇). The structure elucidation of o- and m-Mg₃MnH₇ was aided by first-principles density functional theory (DFT) calculations. Calculated enthalpy versus pressure relations predict m- and o-Mg₃MnH₇ to be more stable than h-Mg₃MnH₇ above 4.3 GPa. Phonon calculations revealed o-Mg₃MnH₇ to be dynamically unstable at pressures below 5 GPa, which explains its phase transition to m-Mg₃MnH₇ on decompression. The electronic structure of the quenchable polymorphs h- and m-Mg₃MnH₇ is very similar. The stable 18-electron complex [MnH₆]^5– is mirrored in the occupied states, and calculated band gaps are around 1.5 eV. The study underlines the significance of <i>in situ</i> investigations for mapping reaction conditions and understanding phase relations for hydrogen-rich complex transition metal hydrides

Unraveling Hidden Mg–Mn–H Phase Relations at High Pressures and Temperatures by <i>in Situ</i> Synchrotron Diffraction

The Mg–Mn–H system was investigated by <i>in situ</i> high pressure studies of reaction mixtures MgH₂–Mn–H₂. The formation conditions of two complex hydrides with composition Mg₃MnH₇ were established. Previously known hexagonal Mg₃MnH₇ (h-Mg₃MnH₇) formed at pressures 1.5–2 GPa and temperatures between 480 and 500 °C, whereas an orthorhombic form (o-Mg₃MnH₇) was obtained at pressures above 5 GPa and temperatures above 600 °C. The crystal structures of the polymorphs feature octahedral [Mn­(I)­H₆]^5– complexes and interstitial H^–. Interstitial H^– is located in trigonal bipyramidal and square pyramidal interstices formed by Mg²⁺ ions in h- and o-Mg₃MnH₇, respectively. The hexagonal form can be retained at ambient pressure, whereas the orthorhombic form upon decompression undergoes a distortion to monoclinic Mg₃MnH₇ (m-Mg₃MnH₇). The structure elucidation of o- and m-Mg₃MnH₇ was aided by first-principles density functional theory (DFT) calculations. Calculated enthalpy versus pressure relations predict m- and o-Mg₃MnH₇ to be more stable than h-Mg₃MnH₇ above 4.3 GPa. Phonon calculations revealed o-Mg₃MnH₇ to be dynamically unstable at pressures below 5 GPa, which explains its phase transition to m-Mg₃MnH₇ on decompression. The electronic structure of the quenchable polymorphs h- and m-Mg₃MnH₇ is very similar. The stable 18-electron complex [MnH₆]^5– is mirrored in the occupied states, and calculated band gaps are around 1.5 eV. The study underlines the significance of <i>in situ</i> investigations for mapping reaction conditions and understanding phase relations for hydrogen-rich complex transition metal hydrides

Ice-Rich Yedoma Permafrost: A Synthesis of Circum-Arctic Distribution and Thickness

Vast portions of Arctic and sub-Arctic Siberia, Alaska and the Yukon Territory are covered by ice-rich silts that are penetrated by large ice wedges, resulting from syngenetic sedimentation and freezing. Accompanied by wedge-ice growth, the sedimentation process was driven by cold continental climatic and environmental conditions in unglaciated regions during the late Pleistocene, inducing the accumulation of the unique Yedoma permafrost deposits up to 50 meter thick. Because of fast incorporation of organic material into permafrost during formation, Yedoma deposits include low-decomposed organic matter. Moreover, ice-rich permafrost deposits like Yedoma are especially prone to degradation triggered by climate changes or human activity. When Yedoma deposits degrade, large amounts of sequestered organic carbon as well as other nutrients are released and become part of active biogeochemical cycling. This could be of global significance for the climate warming, as increased permafrost thaw is likely to cause a positive feedback loop. Therefore, a detailed assessment of the Yedoma deposit volume is of importance to estimate its potential future climate response. Moreover, as a step beyond the objectives of this synthesis study, our coverage (see figure for the Yedoma domain) and thickness estimation will provide critical data to refine the Yedoma permafrost organic carbon inventory, which is assumed to have freeze-locked between 83±12 and 129±30 gigatonnes (Gt) of organic carbon. Hence, we here synthesize data on the circum-Arctic and sub-Arctic distribution and thickness of Yedoma permafrost (see figure for the Yedoma domain) in the framework of an Action Group funded by the International Permafrost Association (IPA). The quantification of the Yedoma coverage is conducted by the digitization of geomorphological and Quaternary geological maps. Further data on Yedoma thickness is contributed from boreholes and exposures reported in the scientific literature

Ice-Rich Yedoma Permafrost: A Synthesis of Northern Hemisphere Distribution and Thickness (IPA Action Group)

Vast portions of Arctic and sub-Arctic Siberia, Alaska and the Yukon Territory are covered by ice-rich silty to sandy deposits that are containing large ice wedges, resulting from syngenetic sedimentation and freezing. Accompanied by wedge-ice growth in polygonal landscapes, the sedimentation process was driven by cold continental climatic and environmental conditions in unglaciated regions during the late Pleistocene, inducing the accumulation of the unique Yedoma deposits up to >50 meters thick. Because of fast incorporation of organic material into syngenetic permafrost during its formation, Yedoma deposits include well-preserved organic matter. Ice-rich deposits like Yedoma are especially prone to degradation triggered by climate changes or human activity. When Yedoma deposits degrade, large amounts of sequestered organic carbon as well as other nutrients are released and become part of active biogeochemical cycling. This could be of global significance for future climate warming as increased permafrost thaw is likely to lead to a positive feedback through enhanced greenhouse gas fluxes. Therefore, a detailed assessment of the current Yedoma deposit coverage and its volume is of importance to estimate its potential response to future climate changes. We synthesized the map of the coverage (see figure) and thickness estimation, which will provide critical data needed for further research. In particular, this preliminary Yedoma map is a great step forward to understand the spatial heterogeneity of Yedoma deposits and its regional coverage. There will be further applications in the context of reconstructing paleo-environmental dynamics and past ecosystems like the mammoth-steppe-tundra, or ground ice distribution including future thermokarst vulnerability. Moreover, the map will be a crucial improvement of the data basis needed to refine the present-day Yedoma permafrost organic carbon inventory, which is assumed to be between 83±12 (Strauss et al., 2013) and 129±30 (Walter Anthony et al., 2014) gigatonnes (Gt) of organic carbon in perennially-frozen archives. Hence, here we synthesize data on the circum-Arctic and sub-Arctic distribution and thickness of Yedoma for compiling a preliminary circum-polar Yedoma map (see figure). For compiling this map, we used (1) maps of the previous Yedoma coverage estimates, (2) included the digitized areas from Grosse et al. (2013) as well as extracted areas of potential Yedoma distribution from additional surface geological and Quaternary geological maps (1.: 1:500,000: Q-51-V,G; P-51-A,B; P-52-A,B; Q-52-V,G; P-52-V,G; Q-51-A,B; R-51-V,G; R-52-V,G; R-52-A,B; 2.: 1:1,000,000: P-50-51; P-52-53; P-58-59; Q-42-43; Q-44-45; Q-50-51; Q-52-53; Q-54-55; Q-56-57; Q-58-59; Q-60; R-(40)-42; R-43-(45); R-(45)-47; R-48-(50); R-51; R-53-(55); R-(55)-57; R-58-(60); S-44-46; S-47-49; S-50-52; S-53-55; 3.: 1:2,500,000: Quaternary map of the territory of Russian Federation, 4.: Alaska Permafrost Map). The digitalization was done using GIS techniques (ArcGIS) and vectorization of raster Images (Adobe Photoshop and Illustrator). Data on Yedoma thickness are obtained from boreholes and exposures reported in the scientific literature. The map and database are still preliminary and will have to undergo a technical and scientific vetting and review process. In their current form, we included a range of attributes for Yedoma area polygons based on lithological and stratigraphical information from the original source maps as well as a confidence level for our classification of an area as Yedoma (3 stages: confirmed, likely, or uncertain). In its current version, our database includes more than 365 boreholes and exposures and more than 2000 digitized Yedoma areas. We expect that the database will continue to grow. In this preliminary stage, we estimate the Northern Hemisphere Yedoma deposit area to cover approximately 625,000 km². We estimate that 53% of the total Yedoma area today is located in the tundra zone, 47% in the taiga zone. Separated from west to east, 29% of the Yedoma area is found in North America and 71 % in North Asia. The latter include 9% in West Siberia, 11% in Central Siberia, 44% in East Siberia and 7% in Far East Russia. Adding the recent maximum Yedoma region (including all Yedoma uplands, thermokarst lakes and basins, and river valleys) of 1.4 million km² (see figure and Strauss et al. (2013)) and postulating that Yedoma occupied up to 80% of the adjacent formerly exposed and now flooded Beringia shelves (1.9 million km², down to 125 m below modern sea level, between 105°E – 128°W and >68°N), we assume that the Last Glacial Maximum Yedoma region likely covered more than 3 million km² of Beringia. Acknowledgements: This project is part of the Action Group “The Yedoma Region: A Synthesis of Circum-Arctic Distribution and Thickness” (funded by the International Permafrost Association (IPA) to J. Strauss) and is embedded into the Permafrost Carbon Network (working group Yedoma Carbon Stocks). We acknowledge the support by the European Research Council (Starting Grant #338335), the German Federal Ministry of Education and Research (Grant 01DM12011 and “CarboPerm” (03G0836A)), the Initiative and Networking Fund of the Helmholtz Association (#ERC-0013) and the German Federal Environment Agency (UBA, project UFOPLAN FKZ 3712 41 106). References Grosse, G., Robinson, J.E., Bryant, R., Taylor, M.D., Harper, W., DeMasi, A., Kyker-Snowman, E., Veremeeva, A., Schirrmeister, L. and Harden, J., 2013. Distribution of late Pleistocene ice-rich syngenetic permafrost of the Yedoma Suite in east and central Siberia, Russia. US Geological Survey Open File Report, 1078. U.S. Geological Survey Reston, Virginia, 37 pp. Strauss, J., Schirrmeister, L., Grosse, G., Wetterich, S., Ulrich, M., Herzschuh, U. and Hubberten, H.-W., 2013. The Deep Permafrost Carbon Pool of the Yedoma Region in Siberia and Alaska. Geophysical Research Letters, 40: 6165–6170, doi:10.1002/2013GL058088. Walter Anthony, K.M., Zimov, S.A., Grosse, G., Jones, M.C., Anthony, P.M., Chapin III, F.S., Finlay, J.C., Mack, M.C., Davydov, S., Frenzel, P. and Frolking, S., 2014. A shift of thermokarst lakes from carbon sources to sinks during the Holocene epoch. Nature, 511: 452–456, doi:10.1038/nature13560