Duck Young Kim

Crystal structure of the pressure-induced metallic phase of SiH4 from ab initio theory

Metallization of pure solid hydrogen is of great interest, not least because it could lead to high-temperature superconductivity, but it continues to be an elusive goal because of great experimental challenges. Hydrogen-rich materials, in particular, CH4, SiH4, and GeH4, provide an opportunity to study related phenomena at experimentally achievable pressures, and they too are expected to be high-temperature superconductors. Recently, the emergence of a metallic phase has been observed in silane for pressures just above 60 GPa. However, some uncertainty exists about the crystal structure of the discovered metallic phase. Here, we show by way of elimination, that a single structure that possesses all of the required characteristics of the experimentally observed metallic phase of silane from a pool of plausible candidates can be identified. Our density functional theory and GW calculations show that a structure with space group P4/nbm is metallic at pressures >60 GPa. Based on phonon calculations, we furthermore demonstrate that the P4/nbm structure is dynamically stable at >43 GPa and becomes the ground state at 97 GPa when zero-point energy contributions are considered. These findings could lead the way for further theoretical analysis of metallic phases of hydrogen-rich materials and stimulate experimental studies.

Rhodium dihydride (RhH2) with high volumetric hydrogen density

Materials with very high hydrogen density have attracted considerable interest due to a range of motivations, including the search for chemically precompressed metallic hydrogen and hydrogen storage applications. Using high-pressure synchrotron X-ray diffraction technique and theoretical calculations, we have discovered a new rhodium dihydride (RhH2) with high volumetric hydrogen density (163.7 g/L). Compressing rhodium in fluid hydrogen at ambient temperature, the fcc rhodium metal absorbs hydrogen and expands unit-cell volume by two discrete steps to form NaCl-typed fcc rhodium monohydride at 4 GPa and fluorite-typed fcc RhH2 at 8 GPa. RhH2 is the first dihydride discovered in the platinum group metals under high pressure. Our low-temperature experiments show that RhH2 is recoverable after releasing pressure cryogenically to 1 bar and is capable of retaining hydrogen up to 150 K for minutes and 77 K for an indefinite length of time.

Rhodium dihydride (RhH[subscript 2]) with high volumetric hydrogen density

Materials with very high hydrogen density have attracted considerable interest due to a range of motivations, including the search for chemically precompressed metallic hydrogen and hydrogen storage applications. Using high-pressure synchrotron X-ray diffraction technique and theoretical calculations, we have discovered a new rhodium dihydride (RhH2) with high volumetric hydrogen density (163.7 g/L). Compressing rhodium in fluid hydrogen at ambient temperature, the fcc rhodium metal absorbs hydrogen and expands unit-cell volume by two discrete steps to form NaCl-typed fcc rhodium monohydride at 4 GPa and fluorite-typed fcc RhH2 at 8 GPa. RhH2 is the first dihydride discovered in the platinum group metals under high pressure. Our low-temperature experiments show that RhH2 is recoverable after releasing pressure cryogenically to 1 bar and is capable of retaining hydrogen up to 150 K for minutes and 77 K for an indefinite length of time.

General trend for pressurized superconducting hydrogen-dense materials

The long-standing prediction that hydrogen can assume a metallic state under high pressure, combined with arguments put forward more recently that this state might even be superconducting up to high temperatures, continues to spur tremendous research activities toward the experimental realization of metallic hydrogen. These efforts have however so far been impeded by the enormous challenges associated with the exceedingly large required pressure. Hydrogen-dense materials, of the MH4 form (where M can be, e.g., Si, Ge, or Sn) or of the MH3 form (with M being, e.g., Al, Sc, Y, or La), allow for the rather exciting opportunity to carry out a proxy study of metallic hydrogen and associated high-temperature superconductivity at pressures within the reach of current techniques. At least one experimental report indicates that a superconducting state might have been observed already in SiH4, and several theoretical studies have predicted superconductivity in pressurized hydrogen-rich materials; however, no systematic dependence on the applied pressure has yet been identified so far. In the present work, we have used first-principles methods in an attempt to predict the superconducting critical temperature (Tc) as a function of pressure (P) for three metal-hydride systems of the MH3 form, namely ScH3, YH3, and LaH3. By comparing the obtained results, we are able to point out a general trend in the Tc-dependence on P. These gained insights presented here are likely to stimulate further theoretical studies of metallic phases of hydrogen-dense materials and should lead to new experimental investigations of their superconducting properties.

Dynamical stability of the hardest known oxide and the cubic solar material: TiO2

The authors have studied dynamical stability of different polymorphs of TiO2 using ab initio phonon calculations based on density functional theory in conjunction with force-constant method. Rutile TiO2 was found stable at ambient pressure, but unstable at high pressure. The calculated Raman frequency and phonon density of states (PDOS) of rutile TiO2 are in a good agreement with experiment. Concerning two cubic phases (solar materials), fluorite stabilized under pressure, whereas pyrite showed instability throughout the whole pressure range. Furthermore, the PDOS of cotunnite (the hardest known oxide) phase confirmed that it exists at high pressure and can be quenched down to a low pressure limit.

Synthesis of Mg2C: A Magnesium Methanide

Carbides, which have been intensively studied for more than half a century, still remain a major center of scientific and technological attention. A large number of new promising phases have been predicted to exhibit exceptional structural and electronic properties, as well as high-temperature superconductivity. In particular, magnesium compounds containing Mg C and C C bonds are quite fascinating from both fundamental science and synthesis perspectives. The properties of such compounds are determined by the nature of the chemical bonds present, allowing a variety of different materials to be suggested, such as ionic semiconductors, superhard sp and/or sp carbon networks intercalated with Mg, 9] and novel polymeric carbides. Furthermore, the intrinsic nature of Mg C chemical bonding is of great importance to polar organometallic compounds and to understanding the covalent/ionic nature of carbanions. The ambient-pressure chemistry of the Mg C system was studied quite thoroughly in the past. Magnesium forms an acetylide-type carbide, MgC2, [13] similar to all other alkalineearth metals. Mg also forms Mg2C3, [14] a derivative of propadiene (H2C=C=CH2), which is unique for the alkalineearth metals and is one of only a handful of examples that contain the rare [C=C=C] group. Herein, we present the formation of a third carbide of magnesium, namely Mg2C. This compound is stabilized at pressures above 15 GPa, but is fully recoverable to ambient conditions and contains the very unusual C methanide anion. 15] Both in situ and ex situ X-ray diffraction experiments revealed the formation of magnesium carbide, Mg2C, directly from a stoichiometric mixture of the elements at pressures between 15–30 GPa and temperatures of 1775–2275 K (Figure 1). Samples were recovered in powder form, which have a brown color, and Rietveld analysis indicates that the compound takes on the antifluorite structure (Li2O) in the cubic crystal system with space group Fm3̄m (No. 225) with lattice parameter a = 5.4480(4) . A comparison of local bonding environments (structural coordination) for Mg2C is presented in Figure 1b. Contrary to Mg2C3 and MgC2, Mg2C does not contain covalent C C bonds. According to our structural data, the ambient-pressure Mg C distance in Mg2C (2.36 ) is larger than the minimal Mg C distances in both Mg2C3 (2.21 ) and MgC2 (2.17 ) and smaller than the Mg C distance in Al2MgC2 (2.487), where Mg has octahedral coordination. Carbon within Mg2C is coordinated eightfold by magnesium, whereas carbon coordination within Mg2C3 and MgC2 is much more sophisticated. If the whole carbon anions are considered as structural units, Mg2C3 and Mg2C have the same coordination number 8, but in the first case they form a distorted and elongated dodecahedron, while in the second case the coordination polyhedron is a regular cube. In MgC2 the C2 dumbbell coordination number is 6 (elongated octahedron). Among the Group 2 elements, beryllium forms the only known methanide-type carbide. Be2C, as well as a second known methanide, Al4C3, are quite hard, low-compressibility compounds with a large degree of covalent bonding character (the ionic/covalent nature is described below). The minority phase synthesis of Li4C was reported previously, but minimal yields (0–10%) have precluded definitive characterization. Although never experimentally observed until now, the isostructural magnesium analogue of Be2C, namely Mg2C, was first suggested by Corkill and Cohen about twenty years Figure 1. a) X-ray diffraction data with MoKa radiation (*), Rietveld refinement (c), and difference (at bottom). Tick marks are shown for Mg2C (top) and MgO impurity (bottom). b) Carbon and magnesium coordination in Mg2C. c) NMR spectrum of Mg2 C (99% of isotope purity).

Synthesis of sodium polyhydrides at high pressures

The only known compound of sodium and hydrogen is archetypal ionic NaH. Application of high pressure is known to promote states with higher atomic coordination, but extensive searches for polyhydrides with unusual stoichiometry have had only limited success in spite of several theoretical predictions. Here we report the first observation of the formation of polyhydrides of Na (NaH3 and NaH7) above 40 GPa and 2,000 K. We combine synchrotron X-ray diffraction and Raman spectroscopy in a laser-heated diamond anvil cell and theoretical random structure searching, which both agree on the stable structures and compositions. Our results support the formation of multicenter bonding in a material with unusual stoichiometry. These results are applicable to the design of new energetic solids and high-temperature superconductors based on hydrogen-rich materials.

Synthesis of β‑Mg 2 C 3 : A Monoclinic High-Pressure Polymorph of Magnesium Sesquicarbide

A new monoclinic variation of Mg2C3 was synthesized from the elements under high-pressure (HP), high-temperature (HT) conditions. Formation of the new compound, which can be recovered to ambient conditions, was observed in situ using X-ray diffraction with synchrotron radiation. The structural solution was achieved by utilizing accurate theoretical results obtained from ab initio evolutionary structure prediction algorithm USPEX. Like the previously known orthorhombic Pnnm structure (α-Mg2C3), the new monoclinic C2/m structure (β-Mg2C3) contains linear C3(4-) chains that are isoelectronic with CO2. Unlike α-Mg2C3, which contains alternating layers of C3(4-) chains oriented in opposite directions, all C3(4-) chains within β-Mg2C3 are nearly aligned along the crystallographic c-axis. Hydrolysis of β-Mg2C3 yields C3H4, as detected by mass spectrometry, while Raman and NMR measurements show clear C═C stretching near 1200 cm(-1) and (13)C resonances confirming the presence of the rare allylenide anion.

Distortions and stabilization of simple-cubic calcium at high pressure and low temperature

Ca-III, the first superconducting calcium phase under pressure, was identified as simple-cubic (sc) by previous X-ray diffraction (XRD) experiments. In contrast, all previous theoretical calculations showed that sc had a higher enthalpy than many proposed structures and had an imaginary (unstable) phonon branch. By using our newly developed submicrometer high-pressure single-crystal XRD, cryogenic high-pressure XRD, and theoretical calculations, we demonstrate that Ca-III is neither exactly sc nor any of the lower-enthalpy phases, but sustains the sc-like, primitive unit by a rhombohedral distortion at 300 K and a monoclinic distortion below 30 K. This surprising discovery reveals a scenario that the high-pressure structure of calcium does not go to the zero-temperature global enthalpy minimum but is dictated by high-temperature anharmonicity and low-temperature metastability fine-tuned with phonon stability at the local minimum.

Mechanical stability of TiO2 polymorphs under pressure : ab initio calculations

First-principles calculations using plane-wave basis sets and ultrasoft pseudopotentials have been performed to study the mechanical stabilities of the rutile, pyrite, fluorite and cotunnite phases of titanium dioxide (TiO2). For these polymorphs, we have calculated the equilibrium volumes, equations of state, bulk moduli and selected elastic constants. Compared to the three phases rutile, pyrite and fluorite, the recently discovered cotunnite phase shows the highest c44 for all pressures considered. Cotunnite also shows the highest bulk modulus amongst the four studied phases at an ambient pressure of B0 = 272 GPa.