Andreas Hermann

High pressure ices

H2O will be more resistant to metallization than previously thought. From computational evolutionary structure searches, we find a sequence of new stable and meta-stable structures for the ground state of ice in the 1–5 TPa (10 to 50 Mbar) regime, in the static approximation. The previously proposed Pbcm structure is superseded by a Pmc21 phase at p = 930 GPa, followed by a predicted transition to a P21 crystal structure at p = 1.3 TPa. This phase, featuring higher coordination at O and H, is stable over a wide pressure range, reaching 4.8 TPa. We analyze carefully the geometrical changes in the calculated structures, especially the buckling at the H in O-H-O motifs. All structures are insulating—chemistry burns a deep and (with pressure increase) lasting hole in the density of states near the highest occupied electronic levels of what might be component metallic lattices. Metallization of ice in our calculations occurs only near 4.8 TPa, where the metallic C2/m phase becomes most stable. In this regime, zero-point energies much larger than typical enthalpy differences suggest possible melting of the H sublattice, or even the entire crystal.

From Wade−Mingos to Zintl−Klemm at 100 GPa: Binary Compounds of Boron and Lithium

Structural diversity and a variety of bonding schemes emerge as characteristics of the Li-B phase diagram in this ground-state theoretical investigation. We studied stoichiometries ranging from LiB(15) to Li(5)B, over a pressure range from 1 atm to 300 GPa. At P = 1 atm, stability is found for the experimentally known LiB(0.8-1.0), LiB(3), and Li(3)B(14) phases. As the pressure rises, the latter two structures are no longer even metastable, while the LiB(0.8-1.0) structures change in geometry and narrow their range of off-stoichiometry, eventually coming at high pressure to a diamondoid NaTl-type LiB. This phase then dominates the convex hull of stability. Other phases emerge as stable points at some pressure: LiB(4), Li(3)B(2), Li(2)B, and Li(5)B. At the boron-rich end, one obtains structures expectedly containing polyhedral motifs, and geometries are governed by Wade-Mingos electron counts; LiB(4) has a BaAl(4) structure. In the center and on the lithium-rich side of the phase diagram, Zintl-phase considerations, i.e., bonding between B(n-) entities, give us insight into the structures-tetrahedral B(-) networks in LiB; B pairs to isolated bonds in Li(5)B.

Insights into the geometries, electronic and magnetic properties of neutral and charged palladium clusters.

We performed an unbiased structure search for low-lying energetic minima of neutral and charged palladium PdnQ (n = 2–20, Q = 0, + 1 and –1) clusters using CALYPSO method in combination with density functional theory (DFT) calculations. The main candidates for the lowest energy neutral, cationic and anionic clusters are identified, and several new candidate structures for the cationic and anionic ground states are obtained. It is found that the ground state structures of small palladium clusters are more sensitive to the charge states. For the medium size Pdn0/+/– (n = 16–20) clusters, a fcc-like growth behavior is found. The structural transition from bilayer-like structures to cage-like structures is likely to occur at n = 14 for the neutral and cationic clusters. In contrast, for the anionic counterparts, the structural transition occurs at Pd13–. The photoelectron spectra (PES) of palladium clusters are simulated based on the time-dependent density functional theory (TD-DFT) method and compared with the experimental data. The good agreement between the experimental PES and simulated spectra provides us unequivocal structural information to fully solve the global minimum structures, allowing for new molecular insights into the chemical interactions in the Pd cages.

Xenon Suboxides Stable under Pressure

We present results from first-principles calculations on solid xenon-oxygen compounds under pressure. We find that the xenon suboxide Xe3O2 is the first compound to become more stable than the elements, at around P = 75 GPa. Other, even more xenon-rich compounds follow at higher pressures, while no region of enthalpic stability is found for the monoxide XeO. We establish the spectroscopic fingerprints of a variety of structural candidates for a recently synthesized xenon-oxygen compound at atmospheric pressure and, on the basis of the proposed stoichiometry XeO2, suggest an orthorhombic structure that comprises extended sheets of square-planar-coordinated xenon atoms connected through bent Xe-O-Xe linkages.

Complete basis set limit second-order Møller-Plesset calculations for the fcc lattices of neon, argon, krypton, and xenon.

Complete basis set (CBS) limit calculations using second-order Møller-Plesset (MP2) theory for electron correlation within a many-body expansion of the interaction potential up to third order are carried out for the fcc lattices of Ne, Ar, Kr, and Xe. Lattice constants and cohesive energies from recent localized MP2 solid-state calculations by Halo et al. [Chem. Phys. Lett. 467, 294 (2009)] are in reasonable agreement with our CBS limit results. A detailed analysis reveals that MP2 severely underestimates long-range three-body effects, thus the Axilrod-Teller term is incorrectly described causing bond contractions for all rare gas solids considered. Further, any deviations in the MP2 lattice constant, cohesive energy, and bulk modulus can be traced back to inaccuracies in the binding energy and equilibrium distance of the rare gas dimer. Without inclusion of phonon dispersion, MP2 prefers the hcp over the fcc crystal structure for all rare gas solids considered.

The Unusual Solid-State Structure of Mercury Oxide: Relativistic Density Functional Calculations for the Group 12 Oxides ZnO, CdO, and HgO †

The solid-state structure of mercury oxide and its low-pressure modifications are known to significantly differ from those found for the corresponding zinc and cadmium compounds, that is, one changes from a simple hexagonal wurtzite or cubic rock salt structure found in zinc oxide and cadmium oxide to unusual chainlike montroydite and cinnabar structures in mercury oxide. Here, we present relativistic and nonrelativistic density functional studies which demonstrate that this marked structural difference is caused by relativistic effects. For HgO, the simple rock salt structure is only accessible at higher pressures. Relativistic effects reduce the cohesive energy by 2.2 eV per HgO unit and decrease the density of the crystal by 14% due to a change in the crystal symmetry. Band structure and density of states calculations also reveal large changes in the electronic structure due to relativistic effects, and we argue that the unusual yellow to red color of HgO is a relativistic effect as well.

AuO: Evolving from Dis- to Comproportionation and Back Again

The structural, electronic, and dynamic properties of hypothetical gold(II) oxide (AuO) are studied theoretically, at atmospheric and elevated pressures, with the use of hybrid density functional theory. At p = 1 atm, hypothetical AuO (metastable with respect to the elements) is predicted to crystallize in a new structure type, unique among the late-transition-metal monoxides, with disproportionation of the Au ions to Au(I/III) and featuring aurophilic interactions. Under pressure, familiar structure types are stabilized: a semiconducting AgO-type structure at ∼2.5 GPa and, with a further increase of the pressure up to ∼80 GPa, an AuSO4-type structure containing Au2 pairs. Finally, above 105 GPa, distorted NaCl- and CsCl-type Au(II)O structures dominate, and metallization is predicted at 329 GPa.

Binary compounds of boron and beryllium: a rich structural arena with space for predictions.

We explore ground-state structures and stoichiometries of the Be-B system in the static limit, with Be atom concentrations of 20 % or greater, and from P = 1 atm up to 320 GPa. At P = 1 atm, predictions are offered for several known compounds, the structures of which have not yet been determined experimentally. Specifically, at 1 atm, we predict a structure of R3m symmetry for the compound Be2B3, seen experimentally at high temperatures, which contains interesting BeBBBBe rods; and for the compound BeB4 we calculate metastability with respect to the elements with a structure similar to MgB4, which is quickly replaced as the pressure is elevated by a Cmcm structure that features 6- and 4-membered rings in B cages, with Be interstitials. For another high-temperature compound, Be2B, we confirm the CaF2 structure, but find a competitive and actually slightly more stable ground-state structure of C2/m symmetry that features B2 pairs. In the case of BeB2, a material for which the stoichiometry has been the subject of debate, we have a clear prediction of a stable F43m structure at P=1 atm. It has a diamondoid structure that is based on cubic (lower P) or hexagonal (higher P) diamond networks of B, but with Be in the interstices. This Zintl structure is a semiconductor at low and intermediate pressures. At higher pressures, BeB2 dominates the phase diagram. In general, the Zintl-Klemm concept of effective electron transfer from the more electropositive ion and bond formation among the resulting anions has proven useful in analyzing the structural preferences of many compositions in the Be-B system at P=1 atm and at elevated pressures. An unusual feature of this binary system is that the 1:1 BeB stoichiometry never appears to reach stability in the static limit, although it comes close, as does Be17B12. Also stable at high pressures are stoichiometries BeB3, BeB4, and Be5B2.

Prediction of Stable Ruthenium Silicides from First-Principles Calculations: Stoichiometries, Crystal Structures, and Physical Properties

We present results of an unbiased structure search for stable ruthenium silicide compounds with various stoichiometries, using a recently developed technique that combines particle swarm optimization algorithms with first-principles calculations. Two experimentally observed structures of ruthenium silicides, RuSi (space group P2(1)3) and Ru2Si3 (space group Pbcn), are successfully reproduced under ambient pressure conditions. In addition, a stable RuSi2 compound with β-FeSi2 structure type (space group Cmca) was found. The calculations of the formation enthalpy, elastic constants, and phonon dispersions demonstrate the Cmca-RuSi2 compound is energetically, mechanically, and dynamically stable. The analysis of electronic band structures and densities of state reveals that the Cmca-RuSi2 phase is a semiconductor with a direct band gap of 0.480 eV and is stabilized by strong covalent bonding between Ru and neighboring Si atoms. On the basis of the Mulliken overlap population analysis, the Vickers hardness of the Cmca structure RuSi2 is estimated to be 28.0 GPa, indicating its ultra-incompressible nature.

Short range order at the amorphous TiO2–water interface probed by silicic acid adsorption and interfacial oligomerization: An ATR-IR and 29Si MAS-NMR study

Adsorption and oligomerization of H(4)SiO(4) at the amorphous TiO(2)-aqueous interface were studied using in situ Attenuated Total Reflectance Infrared (ATR-IR) and ex situ solid state (29)Si nuclear magnetic resonance (NMR). The ATR-IR spectra indicate that a monomeric silicate species is present at low silicate surface concentration (Γ(Si)). Above a threshold Γ(Si) linear silicate oligomers are formed and these oligomers dominate the surface at high Γ(Si). Interestingly the ATR-IR spectra of H(4)SiO(4) on the TiO(2) surface are very similar to those previously observed on the poorly ordered iron oxide phase ferrihydrite. The (29)Si NMR spectrum of silicate on the TiO(2) surface shows the presence of Si in three states with chemical shifts corresponding to isolated monomers (Q(0)), the ends of linear oligomers (Q(1)) and the middle of linear oligomers (Q(2)). The ratio of the area of the Q(1) and Q(2) peaks was ≈2:1 which is consistent with the proposed formation of linear silicate trimers by insertion of a solution H(4)SiO(4) between adjacent suitably orientated adsorbed silicate monomers. A structural interpretation indicates that the observed interfacial silicate oligomerization behavior is a general phenomenon whereby bidentate silicate monomers on oxide surfaces are disposed towards forming linear oligomers by condensation reactions involving their two terminal Si-OH groups. The high surface curvature of nanometer sized spheres inhibits the formation of interfacial silicates with a higher degree of polymerization.