S. K. Sikka

Bio: S. K. Sikka is an academic researcher from Government of India. The author has contributed to research in topics: Phase (matter) & Phase transition. The author has an hindex of 28, co-authored 169 publications receiving 3684 citations. Previous affiliations of S. K. Sikka include Bhabha Atomic Research Centre.

α-Glycine under high pressures: a Raman scattering study

Abstract: High-pressure behaviour of α-glycine has been investigated up to ∼23 GPa using Raman scattering technique. The experimental results show slope change in the CO2 bending, NH3 torsional and NH3 rocking modes around 3 GPa and are interpreted in terms of change in the nature of an N–H⋯O–C intra-layer hydrogen bond at this pressure. Several other spectral features seem to arise from pressure-induced variations in the inter-molecular coupling.

The relationship between liquid, supercooled and glassy water

Abstract: That water can exist in two distinct ‘glassy’ forms — low- and high-density amorphous ice — may provide the key to understanding some of the puzzling characteristics of cold and supercooled water, of which the glassy solids are more-viscous counterparts. Recent experimental and theoretical studies of both liquid and glassy water are now starting to offer the prospect of a coherent picture of the unusual properties of this ubiquitous substance.

The Open Quantum Materials Database (OQMD): assessing the accuracy of DFT formation energies

Abstract: The Open Quantum Materials Database (OQMD) is a high-throughput database currently consisting of nearly 300,000 density functional theory (DFT) total energy calculations of compounds from the Inorganic Crystal Structure Database (ICSD) and decorations of commonly occurring crystal structures. To maximise the impact of these data, the entire database is being made available, without restrictions, at www.oqmd.org/download . In this paper, we outline the structure and contents of the database, and then use it to evaluate the accuracy of the calculations therein by comparing DFT predictions with experimental measurements for the stability of all elemental ground-state structures and 1,670 experimental formation energies of compounds. This represents the largest comparison between DFT and experimental formation energies to date. The apparent mean absolute error between experimental measurements and our calculations is 0.096 eV/atom. In order to estimate how much error to attribute to the DFT calculations, we also examine deviation between different experimental measurements themselves where multiple sources are available, and find a surprisingly large mean absolute error of 0.082 eV/atom. Hence, we suggest that a significant fraction of the error between DFT and experimental formation energies may be attributed to experimental uncertainties. Finally, we evaluate the stability of compounds in the OQMD (including compounds obtained from the ICSD as well as hypothetical structures), which allows us to predict the existence of ~3,200 new compounds that have not been experimentally characterised and uncover trends in material discovery, based on historical data available within the ICSD. Researchers in the USA and Germany introduce a database of over 300,000 calculations detailing the electronic structure and stability of inorganic materials. Chris Wolverton and co-workers from Northwestern University and the Leibniz Institute for Information Infrastructure describe the structure of the Open Quantum Materials Database—a catalog storing information about the electronic properties of a significant fraction of the known crystalline solids determined using density functional theory calculations. Density functional theory is a powerful computational technique that uses quantum mechanics to determine the lowest energy state of the electrons travelling through a lattice of atoms. The researchers verified the accuracy of the calculations by comparing them to experimental results on 1,670 crystals. The database is freely available to scientists, enabling them to design and predict the properties of as yet unrealised materials.

Universal features of the equation of state of solids

Abstract: A study of the energetics of solids leads to the conclusion that the equation of state for all classes of solids in compression can be expressed in terms of a universal function. The form of this universal function is determined by scaling experimental compression data for measured isotherms of a wide variety of solids. The equation of state is thus known (in the absence of phase transitions), if zero-pressure volume and isothermal compression and its pressure derivative are known. The discovery described in this paper has two immediate consequences: first, despite the well known differences in the microscopic energetics of the various classes of solids, there is a single equation of state for all classes in compression; and second, a new method is provided for analyzing measured isotherms and extrapolating high-pressure data from low-pressure (e.g. acoustic) data.

A new quotidian equation of state (QEOS) for hot dense matter

Abstract: The quotidian equation of state (QEOS) is a general‐purpose equation of state model for use in hydrodynamic simulation of high‐pressure phenomena. Electronic properties are obtained from a modified Thomas–Fermi statistical model, while ion thermal motion is described by a multiphase equation of state combining Debye, Gruneisen, Lindemann, and fluid‐scaling laws. The theory gives smooth and usable predictions for ionization state, pressure, energy, entropy, and Helmholtz free energy. When necessary, the results may be modified by a temperature‐dependent pressure multiplier which greatly extends the class of materials that can be treated with reasonable accuracy. In this paper a comprehensive evaluation of the resulting thermodynamic data is given including comparison with other theories and shock‐wave data.

High-pressure phases of group-IV, III–V, and II–VI compounds

Abstract: Advances in the accuracy and efficiency of first-principles electronic structure calculations have allowed detailed studies of the energetics of materials under high pressures. At the same time, improvements in the resolution of powder x-ray diffraction experiments and more sophisticated methods of data analysis have revealed the existence of many new and unexpected high-pressure phases. The most complete set of theoretical and experimental data obtained to date is for the group-IVA elements and the group-IIIA--VA and IIB--VIA compounds. Here the authors review the currently known structures and high-pressure behavior of these materials and the theoretical work that has been done on them. The capabilities of modern first-principles methods are illustrated by a full comparison with the experimental data.