36 research outputs found
Stability and Mechanical Properties of w1-X Mox b4.2 (X=0.0-1.0) From First Principles
Heavy transition-metal tetraborides (e.g., tungsten tetraboride, molybdenum tetraboride, and molybdenum-doped tungsten tetraboride) exhibit superior mechanical properties, but solving their complex crystal structures has been a long-standing challenge. Recent experimental x-ray and neutron diffraction measurements combined with first-principles structural searches have identified a complex structure model for tungsten tetraboride that contains a boron trimer as an unusual structural unit with a stoichiometry of 1:4.2. In this paper, we expand the study to binary MoB4.2 and ternary W1-xMoxB4.2 (x=0.0-1.0) compounds to assess their thermodynamic stability and mechanical properties using a tailor-designed crystal structure search method in conjunction with first-principles energetic calculations. Our results reveal that an orthorhombic MoB4.2 structure in Cmcm symmetry matches well the experimental x-ray diffraction patterns. For the synthesized ternary Mo-doped tungsten tetraborides, a series of W1-xMoxB4.2 structures are theoretically designed using a random substitution approach by replacing the W to Mo atoms in the Cmcm binary crystal structure. This approach leads to the discovery of several W1-xMoxB4.2 structures that are energetically superior and stable against decomposition into binary WB4.2 and MoB4.2. The structural and mechanical properties of these low-energy W1-xMoxB4.2 structures largely follow the Vegard\u27s law. Under changing composition parameter x=0.0-1.0, the superior mechanical properties of W1-xMoxB4.2 stay in a narrow range. This unusual phenomenon stems from the strong covalent network with directional bonding configurations formed by boron atoms to resist elastic deformation. The findings offer insights into the fundamental structural and physical properties of ternary W1-xMoxB4.2 in relation to the binary WB4.2/MoB4.2 compounds, which open a promising avenue for further rational optimization of the functional performance of transition-metal borides that can be synthesized under favorable experimental conditions for wide applications
Imposing Correct Jellium Response Is Key to Predict the Density Response by Orbital-Free DFT
Orbital-free density functional theory (OF-DFT) constitutes a computationally
highly effective tool for modeling electronic structures of systems ranging
from room-temperature materials to warm dense matter. Its accuracy critically
depends on the employed kinetic energy (KE) density functional, which has to be
supplied as an external input. In this work we consider several nonlocal and
Laplacian-level KE functionals and use an external harmonic perturbation to
compute the static density response at T=0 K in the linear and beyond linear
response regimes. We test for the satisfaction of exact conditions in the limit
of uniform densities and for how approximate KE functionals reproduce the
density response of realistic materials (e.g., Al and Si) against the Kohn-Sham
DFT reference which employs the exact KE. The results illustrate that several
functionals violate exact conditions in the UEG limit. We find a strong
correlation between the accuracy of the KE functionals in the UEG limit and in
the strongly inhomogeneous case. This empirically demonstrates the importance
of imposing the limit of UEG response for uniform densities and validates the
use of the Lindhard function in the formulation of kernels for nonlocal
functionals. This conclusion is substantiated by additional calculations for
bulk Aluminum (Al) with a face-centered cubic (fcc) lattice and Silicon (Si)
with an fcc lattice, body-centered cubic (bcc) lattice, and semiconducting
crystal diamond (cd) state. The analysis of fcc Al, and fcc as well as bcc Si
data follows closely the conclusions drawn for the UEG, allowing us to extend
our conclusions to realistic systems that are subject to density
inhomogeneities induced by ions
Bound state breaking and the importance of thermal exchange-correlation effects in warm dense hydrogen
Hydrogen at extreme temperatures and pressures is ubiquitous throughout our
universe and naturally occurs in a variety of astrophysical objects. In
addition, it is of key relevance for cutting-edge technological applications,
with inertial confinement fusion research being a prime example. In the present
work, we present exact \emph{ab initio} path integral Monte Carlo (PIMC)
results for the electronic density of warm dense hydrogen along a line of
constant degeneracy across a broad range of densities. Using the well-known
concept of reduced density gradients, we develop a new framework to identify
the breaking of bound states due to pressure ionization in bulk hydrogen.
Moreover, we use our PIMC results as a reference to rigorously assess the
accuracy of a variety of exchange--correlation (XC) functionals in density
functional theory calculations for different density regions. Here a key
finding is the importance of thermal XC effects for the accurate description of
density gradients in high-energy density systems. Our exact PIMC test set is
freely available online and can be used to guide the development of new
methodologies for the simulation of warm dense matter and beyond
Pressure-stabilized divalent ozonide CaO3 and its impact on Earth's oxygen cycles.
High pressure can drastically alter chemical bonding and produce exotic compounds that defy conventional wisdom. Especially significant are compounds pertaining to oxygen cycles inside Earth, which hold key to understanding major geological events that impact the environment essential to life on Earth. Here we report the discovery of pressure-stabilized divalent ozonide CaO3 crystal that exhibits intriguing bonding and oxidation states with profound geological implications. Our computational study identifies a crystalline phase of CaO3 by reaction of CaO and O2 at high pressure and high temperature conditions; ensuing experiments synthesize this rare compound under compression in a diamond anvil cell with laser heating. High-pressure x-ray diffraction data show that CaO3 crystal forms at 35 GPa and persists down to 20 GPa on decompression. Analysis of charge states reveals a formal oxidation state of -2 for ozone anions in CaO3. These findings unravel the ozonide chemistry at high pressure and offer insights for elucidating prominent seismic anomalies and oxygen cycles in Earth's interior. We further predict multiple reactions producing CaO3 by geologically abundant mineral precursors at various depths in Earth's mantle
Robust estimation of bacterial cell count from optical density
Optical density (OD) is widely used to estimate the density of cells in liquid culture, but cannot be compared between instruments without a standardized calibration protocol and is challenging to relate to actual cell count. We address this with an interlaboratory study comparing three simple, low-cost, and highly accessible OD calibration protocols across 244 laboratories, applied to eight strains of constitutive GFP-expressing E. coli. Based on our results, we recommend calibrating OD to estimated cell count using serial dilution of silica microspheres, which produces highly precise calibration (95.5% of residuals <1.2-fold), is easily assessed for quality control, also assesses instrument effective linear range, and can be combined with fluorescence calibration to obtain units of Molecules of Equivalent Fluorescein (MEFL) per cell, allowing direct comparison and data fusion with flow cytometry measurements: in our study, fluorescence per cell measurements showed only a 1.07-fold mean difference between plate reader and flow cytometry data
Accelerating Structural Optimization through Fingerprinting Space Integration on the Potential Energy Surface
Structural
optimization has been a crucial component in computational
materials research, and structure predictions have relied heavily
on this technique, in particular. In this study, we introduce a novel
method that enhances the efficiency of local optimization by integrating
extra fingerprint space into the optimization process. Our approach
utilizes a mixed energy concept in the hyper potential energy surface
(PES), combining real energy and a newly introduced fingerprint energy
derived from the symmetry of the local atomic environment. This method
strategically guides the optimization process toward high-symmetry,
low-energy structures by leveraging the intrinsic symmetry of the
atomic configurations. The effectiveness of our approach was demonstrated
through structural optimizations of silicon, silicon carbide, and
Lennard-Jones cluster systems. Our results show that the fingerprint
space biasing technique significantly enhances the performance and
probability of discovering energetically favorable, high-symmetry
structures as compared to conventional optimizations. The proposed
method is anticipated to streamline the search for new materials and
facilitate the discovery of novel energetically favorable configurations