Mikko Hakala

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.

Native defects and self-diffusion in GaSb

The native defects in GaSb have been studied with first-principles total-energy calculations. We report the structures and the formation energies of the stable defects and estimate the defect concentrations under different growth conditions. The most important native defect is the GaSb antisite, which acts as an acceptor. The other important defects are the acceptor-type Ga vacancy and the donor-type Ga interstitial. The Sb vacancies and interstitials are found to have much higher formation energies. A metastable state is observed for the SbGa antisite. The significantly larger concentrations of the Ga vacancies and interstitials compared to the corresponding Sb defects is in accordance with the asymmetric self-diffusion behavior in GaSb. The data supports the next-nearest-neighbor model for the self-diffusion, in which the migration occurs independently in the different sublattices. Self-diffusion is dominated by moving Ga atoms.

Role of non-hydrogen-bonded molecules in the oxygen K-edge spectrum of ice.

We report the oxygen K-edge spectra of ices Ih, VI, VII, and VIII measured with X-ray Raman scattering. The pre-edge and main-edge contributions increase strongly with density, even though the hydrogen bond arrangements are very similar in these phases. While the near-edge spectral features in water and ice have often been linked to hydrogen bonding, we show that the spectral changes in the phases studied here can be quantitatively related to structural changes in the second coordination shell. Density-functional theory calculations reproduce the experimental results and support the conclusion. Our results suggest that non-hydrogen-bonded neighbors can have a significant effect also in the liquid water spectrum. We discuss the implications of the results for the actively debated interpretation of the liquid water spectrum in terms of local structure.

Structure of Liquid Linear Alcohols

The properties of linear alcohols in the liquid phase are studied by molecular dynamics simulations. We analyze the effects of the use of bond length constraints on the simulation density, self-diffusion constant, and hydrogen-bonding characteristics of the alcohol series. We find that the densities are well-reproduced in each of the cases but that the constraints have clear effects on the value of the diffusion constant and hydrogen-bonding properties, which is probably caused by the use of a gas-phase reference value in the OH bond length constraint. Although finite size effects are found to be present in the hydrogen bond networks, the networks are determined to be composed of chain-type structures that are well-converged. The results indicate that liquid alcohols consist of hydrogen-bonded chains of molecules. This finding can likely be tested experimentally with inelastic X-ray techniques at modern synchrotron radiation sources.

Modeling the momentum distributions of annihilating electron-positron pairs in solids

Measuring the Doppler broadening of the positron annihilation radiation or the angular correlation between the two annihilation gamma quanta reflects the momentum distribution of electrons seen by positrons in the material. Vacancy-type defects in solids localize positrons and the measured spectra are sensitive to the detailed chemical and geometric environments of the defects. However, the measured information is indirect and when using it in defect identification comparisons with theoretically predicted spectra is indispensable. In this article we present a computational scheme for calculating momentum distributions of electron-positron pairs annihilating in solids. Valence electron states and their interaction with ion cores are described using the all-electron projector augmented-wave method, and atomic orbitals are used to describe the core states. We apply our numerical scheme to selected systems and compare three different enhancement electron-positron correlation schemes previously used in the calculation of momentum distributions of annihilating electron-positron pairs within the density-functional theory. We show that the use of a state-dependent enhancement scheme leads to better results than a position-dependent enhancement factor in the case of ratios of Doppler spectra between different systems. Further, we demonstrate the applicability of our scheme for studying vacancy-type defects in metals and semiconductors. Especially we study the effect of forces due to a positron localized at a vacancytype defect on the ionic relaxations. DOI: 10.1103/PhysRevB.73.035103

ERKALE—A flexible program package for X‐ray properties of atoms and molecules

ERKALE is a novel software program for computing X‐ray properties, such as ground‐state electron momentum densities, Compton profiles, and core and valence electron excitation spectra of atoms and molecules. The program operates at Hartree–Fock or density‐functional level of theory and supports Gaussian basis sets of arbitrary angular momentum and a wide variety of exchange‐correlation functionals. ERKALE includes modern convergence accelerators such as Broyden and ADIIS and it is suitable for general use, as calculations with thousands of basis functions can routinely be performed on desktop computers. Furthermore, ERKALE is written in an object oriented manner, making the code easy to understand and to extend to new properties while being ideal also for teaching purposes.

Correlation of hydrogen bond lengths and angles in liquid water based on Compton scattering

The temperature-dependent hydrogen-bond geometry in liquid water is studied by x-ray Compton scattering using synchrotron radiation combined with density functional theory analysis. Systematic changes, related to the weakening of hydrogen bonding, are observed in the shape of the Compton profile upon increasing the temperature. Using model calculations and published distribution functions of hydrogen-bond geometries obtained from a NMR study we find a significant correlation between the hydrogen-bond length and angle. This imposes a new constraint on the possible local structure distributions in liquid water. In particular, the angular distortions of the short hydrogen bonds are significantly restricted.

Saturation behavior in X-ray Raman scattering spectra of aqueous LiCl.

We report a study on the hydrogen-bond network of water in aqueous LiCl solutions using X-ray Raman scattering (XRS) spectroscopy. A wide concentration range of 0-17 mol/kg was covered. We find that the XRS spectral features change systematically at low concentrations and saturate at 11 mol/kg. This behavior suggests a gradual destruction in the hydrogen-bond network until the saturation concentration. The surprisingly large concentration required for the saturation supports an interpretation in which the ions affect the structure of water only within their first hydration shell. The study is complemented by density-functional-theory calculations and molecular dynamics simulations.

Comparison of chain versus sheet crystal structures for the cyanides M C N ( M = Cu Au ) and dicarbides M C 2 ( M = Be Ba , Zn Hg )

The cyanides

Universal signature of hydrogen bonding in the oxygen K-edge spectrum of alcohols.

M\mathrm{C}\mathrm{N}