Atomic Spectra Database

Most metallic elements have a crystal structure that is either body-centered cubic (bcc), face-centered close packed, or hexagonal close packed. If the bcc lattice is the thermodynamically most stable structure, the close-packed structures usually are dynamically unstable, i.e., have elastic constants violating the Born stability conditions or, more generally, have phonons with imaginary frequencies. Conversely, the bcc lattice tends to be dynamically unstable if the equilibrium structure is close packed. This striking regularity essentially went unnoticed until ab initio total-energy calculations in the 1990s became accurate enough to model dynamical properties of solids in hypothetical lattice structures. After a review of stability criteria, thermodynamic functions in the vicinity of an instability, Bain paths, and how instabilities may arise or disappear when pressure, temperature, and/or chemical composition is varied are discussed. The role of dynamical instabilities in the ideal strength of solids and in metallurgical phase diagrams is then considered, and comments are made on amorphization, melting, and low-dimensional systems. The review concludes with extensive references to theoretical work on the stability properties of metallic elements.

https://perssongroup.lbl.gov/papers/rmp2012-instabilities.pdf

Lattice instabilities in metallic elements

Hydrogen dissociation and diffusion on transition metal (= Ti, Zr, V, Fe, Ru, Co, Rh, Ni, Pd, Cu, Ag)-doped Mg(0001) surfaces

The kinetics of hydrogen absorption by magnesium bulk is affected by two main activated processes: the dissociation of the H$_2$ molecule and the diffusion of atomic H into the bulk. In order to have fast absorption kinetics both activated processed need to have a low barrier. Here we report a systematic ab-initio density functional theory investigation of H$_2$ dissociation and subsequent atomic H diffusion on TM(=Ti,V,Zr,Fe,Ru,Co,Rh,Ni,Pd,Cu,Ag)-doped Mg(0001) surfaces. The calculations show that doping the surface with TM's on the left of the periodic table eliminates the barrier for the dissociation of the molecule, but the H atoms bind very strongly to the TM, therefore hindering diffusion. Conversely, TM's on the right of the periodic table don't bind H, however, they do not reduce the barrier to dissociate H$_2$ significantly. Our results show that Fe, Ni and Rh, and to some extent Co and Pd, are all exceptions, combining low activation barriers for both processes, with Ni being the best possible choice.

Hydrogen Dissociation and Diffusion on Transition Metal(=Ti,Zr,V,Fe,Ru,Co,Rh,Ni,Pd,Cu,Ag)-doped Mg(0001) Surfaces

Since invention of the diamond anvil cell technique in the late 1950s for studying materials at extreme conditions, the maximum static pressure generated so far at room temperature was reported to be about 400 GPa. Here we show that use of micro-semi-balls made of nanodiamond as second-stage anvils in conventional diamond anvil cells drastically extends the achievable pressure range in static compression experiments to above 600 GPa. Micro-anvils (10–50 μm in diameter) of superhard nanodiamond (with a grain size below ∼50 nm) were synthesized in a large volume press using a newly developed technique. In our pilot experiments on rhenium and gold we have studied the equation of state of rhenium at pressures up to 640 GPa and demonstrated the feasibility and crucial necessity of the in situ ultra high-pressure measurements for accurate determination of material properties at extreme conditions. The study of materials at high pressure has been limited by the conditions achievable using single-crystal diamond anvils. The use of anvils that incorporate a second stage consisting of two hemispherical nanocrystalline diamond micro-balls, extends the range of static pressures that can be generated in the lab.

/pdf/implementation-of-micro-ball-nanodiamond-anvils-for-high-j991riu9sc.pdf

Implementation of micro-ball nanodiamond anvils for high-pressure studies above 6 Mbar

We have investigated the electronic properties, phonon dispersion relations, elastic constants, structural phase transitions, and pressure-volume equations-of-state of thorium (Th) and uranium (U) mononitrides (ThN and UN) under pressure (0\ensuremath{-}100 GPa) using pseudopotential density functional theoretical methods. The generalized gradient approximation (GGA) is found to describe the ground-state and high-pressure experimental data much better than the local density approximation (LDA) for both compounds. ThN shows acoustic mode phonon softening along the \ensuremath{\Gamma}\ensuremath{-}X direction of the Brillouin zone in the NaCl phase under pressure, followed by a transition to a CsCl structure at 72.5 GPa. Detailed electronic structure analysis revealed an electronic topological transition under pressure that could be responsible for acoustic mode phonon softening and structural phase transition. Unlike ThN, UN shows a structural phase transition from an NaCl to R-3m structure at a much smaller pressure (18 GPa), and the calculated ${C}_{44}$ shear elastic constant decreases with pressure and becomes negative at 15 GPa. A Peierls-like distortion, due to $f$ states, is found responsible for elastic instability and structural phase transition. Our results are in reasonably good agreement with available experimental data. We have also tested the effect of on-site Coulomb interactions on a few ground-state properties and on the phase-transition behavior of UN.

First-principles investigation of electronic, vibrational, elastic, and structural properties of ThN and UN up to 100 GPa

Ab initio investigations of native and protonic point defects in Mg2SiO4 polymorphs under high pressure

The study of hydrogen bonds near symmetrization limit at high pressures is of importance to understand proton dynamics in complex bio-geological processes. We report here the evidence of hydrogen bond symmetrization in the simplest amino acid-carboxylic acid complex, glycinium oxalate, at moderate pressures of 8 GPa using in-situ infrared and Raman spectroscopic investigations combined with first-principles simulations. The dynamic proton sharing between semioxalate units results in covalent-like infinite oxalate chains. At pressures above 12 GPa, the glycine units systematically reorient with pressure to form hydrogen-bonded supramolecular assemblies held together by these chains.

Hydrogen Bond Symmetrization in Glycinium Oxalate under Pressure

Quantitative elemental analysis of nickel alloys using calibration-based laser-induced breakdown spectroscopy

We present the Hugoniots of Al, Ta, Mo and W in their solid as well as liquid phases. The liquid phase calculations are carried out on the basis of the corrected rigid spheres (CRIS) model. The 0 K isotherm of the solid phases, which are the necessary inputs for our computations, have been obtained by full potential first principles electronic structure calculations with generalized gradient approximation (GGA) for the exchange–correlation terms. The melting curve as a function of pressure was obtained according to the recently published model based on dislocation mediated melting, and also compared with that using Lindemann criterion. Though the adiabatic pressure–volume curve is affected little by melting, the pressure–temperature curve shows substantial change.

https://iopscience.iop.org/article/10.1088/0953-8984/16/28/004/pdf

Ashok K. Verma

Papers

First-principles investigation of electronic, vibrational, elastic, and structural properties of ThN and UN up to 100 GPa

Ab initio investigations of native and protonic point defects in Mg2SiO4 polymorphs under high pressure

Hydrogen Bond Symmetrization in Glycinium Oxalate under Pressure

Quantitative elemental analysis of nickel alloys using calibration-based laser-induced breakdown spectroscopy

Theoretical solid and liquid state shock Hugoniots of Al, Ta, Mo and W