Sandro Jahn

Microscopic structure of water at elevated pressures and temperatures

We report on the microscopic structure of water at sub- and supercritical conditions studied using X-ray Raman spectroscopy, ab initio molecular dynamics simulations, and density functional theory. Systematic changes in the X-ray Raman spectra with increasing pressure and temperature are observed. Throughout the studied thermodynamic range, the experimental spectra can be interpreted with a structural model obtained from the molecular dynamics simulations. A spatial statistical analysis using Ripley’s K-function shows that this model is homogeneous on the nanometer length scale. According to the simulations, distortions of the hydrogen-bond network increase dramatically when temperature and pressure increase to the supercritical regime. In particular, the average number of hydrogen bonds per molecule decreases to ≈0.6 at 600 °C and p = 134 MPa.

Including many-body effects in models for ionic liquids

Realistic modeling of ionic systems necessitates taking explicitly account of many-body effects. In molecular dynamics simulations, it is possible to introduce explicitly these effects through the use of additional degrees of freedom. Here, we present two models: The first one only includes dipole polarization effect, while the second also accounts for quadrupole polarization as well as the effects of compression and deformation of an ion by its immediate coordination environment. All the parameters involved in these models are extracted from first-principles density functional theory calculations. This step is routinely done through an extended force-matching procedure, which has proven to be very successful for molten oxides and molten fluorides. Recent developments based on the use of localized orbitals can be used to complement the force-matching procedure by allowing for the direct calculations of several parameters such as the individual polarizabilities.

Ab initio prediction of equilibrium boron isotope fractionation between minerals and aqueous fluids at high P and T

Abstract Over the last decade experimental studies have shown a large B isotope fractionation between materials carrying boron incorporated in trigonally and tetrahedrally coordinated sites, but the mechanisms responsible for producing the observed isotopic signatures are poorly known. In order to understand the boron isotope fractionation processes and to obtain a better interpretation of the experimental data and isotopic signatures observed in natural samples, we use first principles calculations based on density functional theory in conjunction with ab initio molecular dynamics and a new pseudofrequency analysis method to investigate the B isotope fractionation between B-bearing minerals (such as tourmaline and micas) and aqueous fluids containing H 3 BO 3 and H 4 BO 4 - species. We confirm the experimental finding that the isotope fractionation is mainly driven by the coordination of the fractionating boron atoms and have found in addition that the strength of the produced isotopic signature is strongly correlated with the B O bond length. We also demonstrate the ability of our computational scheme to predict the isotopic signatures of fluids at extreme pressures by showing the consistency of computed pressure-dependent β factors with the measured pressure shifts of the B O vibrational frequencies of H 3 BO 3 and H 4 BO 4 - in aqueous fluid. The comparison of the predicted with measured fractionation factors between boromuscovite and neutral fluid confirms the existence of the admixture of tetrahedral boron species in neutral fluid at high P and T found experimentally, which also explains the inconsistency between the various measurements on the tourmaline–mica system reported in the literature. Our investigation shows that the calculated equilibrium isotope fractionation factors have an accuracy comparable to the experiments and give unique and valuable insight into the processes governing the isotope fractionation mechanisms on the atomic scale.

Condensed phase ionic polarizabilities from plane wave density functional theory calculations

A method is presented to allow the calculation of the dipole polarizabilities of ions and molecules in a condensed-phase coordination environment. These values will be useful for understanding the optical properties of materials and for developing simulation potentials which incorporate polarization effects. The reported values are derived from plane wave density functional theory calculations, though the method itself will apply to first-principles calculations on periodic systems more generally. After reporting results of test calculations on atoms to validate the procedure, values for the polarizabilities of the oxide ion and various cations in a range of materials are reported and compared with experimental information as well as previous theoretical results.

Prediction of equilibrium Li isotope fractionation between minerals and aqueous solutions at high P and T: An efficient ab initio approach

Abstract The mass-dependent equilibrium stable isotope fractionation between different materials is an important geochemical process. Here we present an efficient method to compute the isotope fractionation between complex minerals and fluids at high pressure, P , and temperature, T , representative for the Earth’s crust and mantle. The method is tested by computation of the equilibrium fractionation of lithium isotopes between aqueous fluids and various Li bearing minerals such as staurolite, spodumene and mica. We are able to correctly predict the direction of the isotope fractionation as observed in the experiments. On the quantitative level the computed fractionation factors agree within 1.0‰ with the experimental values indicating predictive power of ab initio methods. We show that with ab initio methods we are able to investigate the underlying mechanisms driving the equilibrium isotope fractionation process, such as coordination of the fractionating elements, their bond strengths to the neighboring atoms, compression of fluids and thermal expansion of solids. This gives valuable insight into the processes governing the isotope fractionation mechanisms on the atomic scale. The method is applicable to any state and does not require different treatment of crystals and fluids.

Phase transitions and equation of state of forsterite to 90 GPa from single-crystal X-ray diffraction and molecular modeling

Abstract Forsterite, Mg2SiO4, the magnesian end-member of the olivine system, is the archetypal example of an orthosilicate structure. We have conducted synchrotron-based single-crystal X‑ray diffraction experiments to 90 GPa on synthetic end-member forsterite to study its equation of state and phase transitions. Upon room-temperature compression, the forsterite structure is observed to 48 GPa. By fitting a third-order Birch-Murnaghan equation of state to our compression data, we obtain the zeropressure isothermal bulk modulus, K0T = 130.0(9) GPa and its pressure derivative, K′0T = 4.12(7) for a fixed room-pressure volume, V0 = 290.1(1) Å3, in good agreement with earlier work. At 50 GPa, a phase transition to a new structure (forsterite II) occurs, followed by a second transition to forsterite III at 58 GPa. Forsterite III undergoes no additional phase transitions until at least 90 GPa. There is an ∾4.8% volume reduction between forsterite and forsterite II, and a further ∾4.2% volume reduction between forsterite II and III. On decompression forsterite III remains until as low as 12 GPa, but becomes amorphous at ambient conditions. Using our X-ray diffraction data together with an evolutionary crystal structure prediction algorithm and metadynamics simulations, we find that forsterite II has triclinic space group P1 and forsterite III has orthorhombic space group Cmc21. Both high-pressure phases are metastable. Metadynamics simulations show a stepwise phase transition sequence from 4-coordinated Si in forsterite to mixed tetrahedral and octahedral Si (as in forsterite II), and then fully sixfold-coordinated Si (as in forsterite III), occurring by displacement in [001](100). The forsterite III structure is a member of the family of post-spinel structures adopted by compositions such as CaFe2O4 and CaTi2O4.

Speciation in aqueous MgSO(4) fluids at high pressures and high temperatures from ab initio molecular dynamics and Raman spectroscopy.

Ab initio molecular dynamics simulations and in situ Raman spectroscopy are used to study the speciation in two molal aqueous MgSO4 solutions at high pressures, P, and temperatures, T. While at ambient conditions the fluid is dominated by dissociated SO42−(aq) ions and solvent-separated ion pairs, ion association strongly increases with increasing temperature and pressure along a 1.33 g/cm3 isochore. At T = 450 °C and P = 1.4GPa, the ν1(SO42−) Raman band is well described by three Gaussian + Lorentzian components of about equal intensity with peaks at about 980, 995, and 1005 cm−1. Analysis of the simulations, however, indicates the coexistence of more than three species, including dissociated SO42−(aq) ions, and contact and triple ion pairs as well as larger complexes. In addition, the sulfate groups may be bonded to Mg as monodentate or bidentate ligands. The frequencies of the associated species seem to depend mainly on the type and number of Mg−SO4 bonds. We interpret the two rather broad high-frequency Raman components as a single “Mg−SO4 contact” component with variable frequency distribution. As a consequence, the ν1(SO42−) Raman band provides only information on the molecular environment of the sulfate group; i.e., individual species cannot be resolved. At fluid densities less than about 1.2 g/cm3 and temperatures above 400 °C, the formation of HSO4−(aq)-containing species is observed in both simulations and experiments, which may be accompanied by a change in pH and electrical conductivity.

Li-isotope fractionation between silicates and fluids: Pressure dependence and influence of the bonding environment

Isotope fractionation experiments and molecular simulations were performed to determine the relation of Li-isotope fractionation between silicates and fluids and the corresponding cation coordination environments. The effect of pressure-induced changes of Li hydration in aqueous fluids on solid-fluid Li-isotope fractionation was studied by performing experiments in the system spodumene-fluid at three different pressures of 1, 4 and 8 GPa at temperatures ranging from 500 to 625 °C. 7 Li preferentially partitioned into the fluid in all three experiments. The Li-isotope fractionation of experiments at 1 and 4 GPa does not show a significant P dependence in comparison to previously published data at 2 GPa. At 8 GPa the Li-isotope fractionation is slightly decreased compared to the low-pressure data. In addition, the fractionation of lithium isotopes between Li-bearing amphibole and fluid was determined experimentally at 700 °C and 2 GPa, which resulted in a Δ 7 Li (Li-amph-fluid) of −1.7 ‰. Our experiments are complemented by ab initio molecular dynamics simulations of Li-bearing aqueous fluids aimed to determine structural properties at high P and T . Despite the increase in Li coordination from 4.0 to 5.4 with pressure at isothermal conditions, the mean Li-O distance of the fluid is almost unchanged between 1 and 8 GPa at 727 °C. This might explain the insignificant effect of pressure over a large P range observed experimentally. The new experimental results indicate a partial inapplicability of the coordination-principle on isotope fractionation. Therefore, we additionally analyzed the relation of isotope fractionation and Li-O bond length and applied the bond valence model. Using the available structural data of solids and fluids, in a first approximation, the bond valence model seems to be more appropriate to relate the local atomic structure to isotope fractionation than the simple coordination-dependent principle.

High-pressure phase transitions in MgSiO3 orthoenstatite studied by atomistic computer simulation

Abstract Molecular dynamics simulations and first-principles electronic structure calculations are used to study the structural behavior of orthoenstatite, MgSiO3, at high pressures. The calculations suggest two possible high-pressure polymorphs of orthoenstatite, one with P21ca and the other with Pbca symmetry. Both polymorphs are structurally related to orthoenstatite. Molecular dynamics simulations reveal the displacive nature of the phase transitions between the three phases. Electronic structure calculations predict a phase transition from orthoenstatite to the metastable P21ca structure at 9 GPa, which may explain the anomalies in elastic and vibrational properties observed experimentally. A second metastable transition from the P21ca to the high-pressure Pbca structure may be observable above 20 GPa.

The construction and application of a fully flexible computer simulation model for lithium oxide

An aspherical ion model (AIM) is constructed for lithium oxide, Li2O. The model incorporates both many-body polarization and short-range ion distortion effects. A procedure for extracting the required model parameters by fitting to results from a series of electronic structure calculations is described. The model is tested with respect to both static and dynamic properties. The experimentally observed Cauchy violation in the elastic constants and phonon frequencies are well reproduced as is the onset temperature for superionic behaviour in the Li+ sublattice. The system is shown to display a peak in the heat capacity as a function of temperature. The correlated and uncorrelated ion dynamics are studied and the origin of the respective solid- and liquid-state Haven ratios is rationalized.