High-pressure Phase Stability and Superconductivity of Pnictogen Hydrides and Chemical Trends for Compressed Hydrides

The recent breakthrough discovery of unprecedentedly high temperature superconductivity of 203 K in compressed sulfur hydrides has stimulated significant interest in finding new hydrogen-containing superconductors and elucidating the physical and chemical principles that govern these materials and their superconductivity. Here we report the prediction of high temperature superconductivity in the family of pnictogen hydrides using first principles calculations in combination with global optimization structure searching methods. The hitherto unknown high-pressure phase diagrams of binary hydrides formed by the pnictogens of phosphorus, arsenic and antimony are explored, stable structures are identified and their electronic, vibrational and superconducting properties are investigated. We predict that SbH_4 and AsH_8 are high-temperature superconductors at megabar pressures, with critical temperatures in excess of 100 K. The highly symmetrical hexagonal SbH_4 phase is predicted to be stabilized above about 150 GPa, which is readily achievable in diamond anvil cell experiments. We find that all phosphorus hydrides are metastable with respect to decomposition into the elements within the pressure range studied. Trends based on our results and data in the literature reveal a connection between the high-pressure behaviors and ambient-pressure chemical quantities which provides insight into understanding which elements may form hydrogen-rich high-temperature superconducting phases at high pressures.The authors thank Eva Zurek for sharing structure data for iodine hydride. The work at Jilin Univ. is supported by the funding of National Natural Science Foundation of China under Grant Nos. 11274136 and 11534003, 2012 Changjiang Scholar of Ministry of Education and the Postdoctoral Science Foundation of China under grant 2013M541283. L.Z. acknowledges funding support from the Recruitment Program of Global Youth Experts in China. Part of calculations was performed in the high performance computing center of Jilin Univ. R.J.N. acknowledges financial support from the Engineering and Physical Sciences Research Council (EPSRC) of the UK [EP/J017639/1]. R.J.N. and C.J.P. acknowledge use of the Archer facility of the U.K.’s national high-performance computing service (for which access was obtained via the UKCP consortium [EP/K013564/1]).This is the final version of the article. It first appeared from ACS via https://doi.org/10.1021/acs.chemmater.5b0463

Structures and Stability of Iron Halides at the Earth’s Mantle and Core Pressures: Implications for the Missing Halogen Paradox

The terrestrial abundance of heavy halogens Cl, Br, and I is depleted by approximately one order of magnitude relative to those predicted on the basis of their volatilities. One plausible explanation for this missing halogen paradox is their sequestration into the Earth’s core. Therefore, heavy halogens in the core may combine with the dominant element, Fe, to form iron halides that potentially exert important effects on the properties and dynamic evolution of the Earth’s inner core. In this study, stable iron halide phases have been predicted from first-principles structural searches at four pressures corresponding to those at the Earth’s mantle and core. At 360 GPa (corresponding to the inner core), the most stable iron chloride is CsCl-type FeCl, supporting the hypothesis that light-element impurities can stabilize the body-centered cubic Fe structure. At pressures of the Earth’s core, it is also observed that the chemical nature of iodine changes from an electron acceptor to an electron donor. This change results in an enhancement of the stability and the formation of a novel Fe₂I compound containing a Fe–I framework with linear Fe chains intercalated in the open channels. Thus, the role of pressure in determining the stoichiometry of stable high-pressure halides is demonstrated by our theoretical calculations. These findings suggest the possibility of thermodynamic stability of iron halides in the assemblage in the Earth’s inner core