Session F17: Matter in Extreme Environments: Novel Materials

Hydrocarbons under pressure: phase diagrams and surprising new compounds in the C-H system

Understanding the high-pressure behavior of C-H system is of great importance due to its key role in organic, bio-, petroleum and planetary chemistry. We have performed a systematic investigation of the pressure-composition phase diagram of the C-H system at pressures up to 400 GPa using evolutionary structure prediction coupled with ab initio calculations and discovered that only saturated hydrocarbons are thermodynamically stable. Concluding our results, in C-H system only saturated hydrocarbons are thermodynamically stable. Several surprising stable methane-hydrogen co-crystals are predicted, some of them have potential energy storage purpose. We consider effects of temperature through the quasiharmonic approximation and report the pressure-temperature-composition phase diagram of the C-H system and pressure-temperature phase diagram of CH₄ (up to 2000 K).   

Evolutionary prediction and experimental synthesis of SiOS at high pressure

SiOS represents a ternary generalization of the well-known and highly important family of binary AB₂ compounds including CO₂, SiO₂, GeO₂, CS₂, SiS₂, etc. The presence of two different bond lengths suggests that its crystal structures may be different from those found in SiO₂ and SiS₂. We applied evolutionary search based on DFT ab initio calculations to determine crystal structures of SiOS for pressures up to 100 GPa. We predict the SiOS phase diagram at zero temperature and examined the structural, electronic and vibrational properties of the stable phases. At low pressure the stable phase is a tetrahedrally coordinated layered orthorhombic Cmc2₁ structure. This is predicted to transform at 14 GPa to an octahedrally coordinated layered monoclinic C2/m structure similar to the P-3m1 phase of SiS₂. The system remains insulating up to 100 GPa with band gap above 1.8 eV. Following the theoretical prediction we synthesized SiOS by laser heating elemental Si, O and S in the diamond anvil cell at pressure of 8 GPa. The observed XRD pattern is in very good agreement with the theoretical prediction for the Cmc2₁ structure.   

Structure identification at extreme conditions

We are investigating a series of low-crystallinity, amorphous uranium oxides relevant for long term storage of spent nuclear fuel. One compound has stoichiometry of ~UO_3.5 (a-UO₃), which appears to be x-ray amorphous. To understand the a-UO₃ structure, we couple density functional theory with a genetic algorithm structure prediction mechanism to identify stable structures of non-stoichiometric uranium oxides. The lowest energy predicted structure with stoichiometry UO_3.5 contains a peroxide bridge, compatible with a study of comparable material using neutron pair-distribution function techniques. We attempted to crystallize a-UO₃ using a diamond anvil cell and Raman spectroscopy up to 25 GPa and observed highly anharmonic responses to pressure, but no long range order. Computational studies on the low-energy structure from the genetic algorithm show commensurate responses to the dynamical observations, along with anomalous changes in local bonding character, suggesting an indirect pathway for material identification.
  

Pressure-induced New Oxidation States for Gold Element

The formalism of the oxidation state of atoms in compounds is a key concept in chemistry. Finding novel compounds containing elements with unusually high oxidation states allows a deeper understanding of chemical behavior of elements. On the other hand, high oxidation state compounds usually bring new types of bonds with interesting physical and chemical properties. Thus, the preparation of compounds with unusual oxidation states becomes an attractive topic in chemistry and condensed-matter physics. Gold (Au) is a well-known fascinating element, but still hides interesting surprises to be discovered, especially for oxidation state. Here, we propose that high pressure becomes a controllable method for preparing high negative oxidation state of Au through its reaction with lithium. Au acts as a 6p-element in dense lithium aurides. Moreover, we identify two hitherto unknown stoichiometric compounds, AuF₄ and AuF₆, exhibiting typical molecular crystal character, in which Au demonstrates +4 and +6 oxidation states. Our work represents a significant step forward in a more complete understanding of the oxidation states of Au.   

Site-selective Mott insulator-metal transition in Fe2O3 under ultra-high pressures

The insulator-metal transition, induced by pressure, composition or by other means, represents perhaps the most profound transformation of the chemical bond in materials. Combining density functional plus dynamical mean-field theory (DFT+DMFT) calculations with experiment, we demonstrate that upon compression of Fe₂O₃ a novel type of Mott insulator-metal transition occurs, which is characterized by site-selective delocalization of the electrons [1]. Within the P2₁/n crystal structure, which is stable in the pressure range 45-68 GPa, we observe equal abundances of ferric ions (Fe³⁺) and ions having delocalized electrons (Fe^M). Thereby the transition is characterized by delocalization/metallization of the 3d electrons on half the Fe sites, with a site-dependent collapse of local moments. Upon further compression above 75 GPa, we predict another phase transition, to a metal with a post-perovskite crystal structure and site-selective local moments [2]. Our results suggest that site-selective local moments in Fe₂O₃ persist up to ultra-high pressure of ~200-250 GPa, i.e. to pressure sufficiently above that at the core-mantle boundary.

[1] E. Greenberg, et al., Phys. Rev. X 8, 031059 (2018).
[2] I. Leonov, G. Kh. Rozenberg, I. A. Abrikosov (submitted)
  

First-Principles Modeling and Evolutionary-Algorithm Prediction of Superhard B-C and B-N systems

Superhard materials with a Vickers hardness larger than ~40 GPa have a wide range of industrial applications such as cutting tools and protective coatings. Superhard boron-carbon (B-C) and boron-nitride (B-N) composites are especially important because of their superior high-temperature performance as compared to diamond and their low reactivity with ferrous metals. Here we employ evolutionary algorithm to predict the crystal structures of superhard B-C and B-N composites. We also apply density functional theory to study mechanical properties, electronic structures, phonon and Raman spectra of different B-C and B-N systems, including boron-doped cubic diamond and icosahedral boron subnitrides. Comparison of our calculations and recent experimental data will be discussed as well.   

A First-Principles Study of Two High-Pressure Modifications of Fe3N

Most of the iron nitride compounds of the type Fe_xN are unstable at ambient pressure and temperature. However, Fe-rich nitrides can be synthesized in a high pressure confined environment. Using a large volume press and X-ray diffraction in combination with scanning electron microscopy techniques, two new high pressure phases of Fe₃N were observed with symmetries P6₃22 and P312 [1]. In this work, we perform a first-principles study based on density functional theory (DFT) within the general gradient approximation (GGA) and the local density approximation (LDA), and report on the structural, electronic, and phase stability properties as a function of pressure of these two iron nitride modifications. The exploration and understanding of high-energy density nitride materials is important as many of them exhibit remarkable technological applications due to their hardness, superconductivity or mechanical traits.

[1] Serghiou et al., High Press. Res. 35, 28-36, (2015)
* MNV is also at the Helmholtz Zentrum-Potsdam, Deutsches GeoForschungsZentrum (GFZ)   

Study of peptides under simultaneous application of high pressure and shear

There is a renewed interest in the role of non-hydrostatic mechanical stress, or shear stress, as a driver for reactions. Combining high pressure with mechanical shearing can significantly alter reaction mechanisms and lead to a decrease in the onset pressure of structural transformations. Such conditions can be achieved with a rotational diamond anvil cell (RDAC). We will discuss the study of simple amino acids and peptides subjected to high-pressure, high-shear conditions using an RDAC. Amino acids have been studied extensively under static compression due to their significance in the fields of chemistry and biology. We compare amino acid behavior under high shear to that in a conventional DAC. μ-Raman spectroscopy and X-ray diffraction are used to probe the phase transitions and amino acid condensation. Theoretical calculations are also performed using USPEX, metadynamics, and quantum molecular dynamics to predict novel phases and reaction barriers under compressive shear.   

Effect of structure and composition on the electronic excitation-induced amorphization of La2Ti2-xZrxO7 ceramics

An understanding of the response of ceramics (A₂B₂O₇) operating in extreme environments is of interest for a broad range of applications, including potential nuclear wasteforms and in-core electronics. Here, ab initio molecular dynamic simulations have been used to investigate the effect of structure and B-site (=Ti, Zr) cation composition of lanthanum-based oxides on electronic-excitation-induced amorphization. We find that the amorphous transition in monoclinic layered perovskite La₂Ti₂O₇ occurs for a lower degree of electronic excitation than for cubic pyrochlore La₂Zr₂O₇. While in each case the formation of O₂-like molecules drives the structure to an amorphous state, an analysis of the polyhedral connection network reveals that the rotation of TiO₆ octahedra in the monoclinic perovskite phase facilitates the formation of O₂-like molecules, while such octahedral rotation is not possible in the cubic pyrochlore phase. However, once the symmetry of the cubic structure is broken by substituting Ti for Zr, it becomes less resistant to amorphization. A compound made of 50% Ti and 50% Zr (La₂TiZrO₇) is found to be more resistant in the monoclinic perovskite than in the cubic pyrochlore phase, which may be related to the lower bandgap of the cubic phase.   

Structure and electrical properties of a single-crystal layered strontium manganese vanadate as a function of pressure

We investigate the structural and electrical properties of a strontium manganese vanadate layered material, SrMn₂(VO₄)₂(OH)(H₂O), as a function of hydrostatic pressure. The material crystalizes in the space group C2/m. Variable temperature conductivity measurements, performed on single crystals, indicate semiconducting behavior, supported by DFT calculations. The manganese vanadate layers, consisting of chains of edge sharing MnO₆ octahedra bridged by corner-sharing VO₄ tetrahedra, are connected in the third dimension by octahedral Sr ions. Single-crystal x-ray diffraction under pressure up to 4 GPa indicates a reversible compression of the interlayer distance. The electrical conductivity of a single crystal as a function of pressure decreases at moderate pressures; beyond 0.4 GPa, the material is highly insulating. Interestingly, this change is not reversible. The same irreversible change in conductivity is also observed for the Ca and Ba analogs. Probing structural details at the lower pressure range by Raman spectroscopy, we observe a softening mode around 0.8 GPa related to the Mn octahedra.
  

Pressure- induced metallization and Superconductivity in PdSe2

Transition Metal Dichalcognides (TMDs) continue to attract scientific interest due to their intriguing physical properties and potential technological applications. Here, we report the emergence of a pressure-induced dome-shaped superconductivity in PdSe₂ with a maximum critical temperature (T_c) of 13.1 K at 23 GPa, which is so far the highest T_c among TMDs family.
Lattice dynamics studies under pressure revealed a strong correlation between Se-Se dumbbells bonding and T_c. In addition, Ab initio calculations of the PdSe₂ electronic structure show the existence of topological non-trivial state with Dirac crossing and nodal lines in the vicinity of Fermi level.
PdSe₂ provides therefore an exciting example to study the relationship between superconductivity and topologically non-trivial states, which can lead to discovery of novel superconducting states. We have revisited other TMD compounds to search for new superconductors. The related compounds NiSe₂ and PdS₂ have also been studied in comparison with PdSe₂. These compounds are not only interesting from the fundamental research prespective, but also for technological application as the electronically-driven insulator-metal transition at relatively low pressures could be promising in electronics applications such as memory devices.   

High pressure synthesis of a new perovskite-type cuprate with doped Cu-O chains

The discovery of high-temperature superconductivity in cuprates has spurred the search for novel low-dimensional cuprates such as spin-ladder compounds. On the other hand, less attention has been paid for the three-dimensional counterpart, the perovskite-type cuprates ACuO₃, presumably because of the difficulty in high-pressure synthesis. In this study, we have succeeded in the high-pressure synthesis of a novel GdFeO₃-type perovskite cuprate PrCuO₃ with quasi-one-dimensional Cu-O chains showing nearly metallic conductivity. Synchrotron x-ray diffraction and x-ray absorption spectroscopy reveal that the oxidation state can be described as Pr^(4-δ)+Cu^(2+δ)+O₃. The first principles calculations suggest that the formation of the Cu-O chain is caused by the A-B site charge transfer and the cooperative Jahn-Teller distortion of nearly divalent Cu ions. This system offers a unique opportunity to explore novel quantum phases of correlated electrons as the simplest one-dimensional counterpart of the high-temperature superconducting cuprates.