scholarly journals Structure variations within RSi2 and R 2Si3 silicides. Part II. Structure driving factors

2020 ◽  
Vol 76 (3) ◽  
pp. 378-410
Author(s):  
M. Nentwich ◽  
M. Zschornak ◽  
M. Sonntag ◽  
R. Gumeniuk ◽  
S. Gemming ◽  
...  
Keyword(s):  
Density Functional ◽  
Alkaline Earth ◽  
Density Functional Theory Calculations ◽  
Structure Type ◽  
Lattice Parameters ◽  
Driving Factors ◽  
Molecular Orbital Theory ◽  
Orbital Theory ◽  
Functional Theory ◽  
Bader Analysis

To gain an overview of the various structure reports on RSi2 and R 2 TSi3 compounds (R is a member of the Sc group, an alkaline earth, lanthanide or actinide metal, T is a transition metal), compositions, lattice parameters a and c, ratios c/a, formula units per unit cell, and structure types are summarized in extensive tables and the variations of these properties when varying the R or T elements are analyzed. Following the structural systematization given in Part I, Part II focuses on revealing the driving factors for certain structure types, in particular, the electronic structure. Here, concepts of different complexity are presented, including molecular orbital theory, the principle of hard and soft acids and bases, and a Bader analysis based on Density Functional Theory calculations for representatives of the reported structure types. The potential Si/T ordering in different structures is discussed. Additionally, the influences from intrinsic and extrinsic properties (e.g. elemental size and electronics as well as lattice parameters and structure type) are investigated on each other using correlation plots. Thermal treatment is identified as an important factor for the ordering of Si/T atoms.

Ab intio Investigations of Lattice Parameters in ZnMgO Alloys

2009 ◽  
Vol 1201 ◽  
Author(s):  
Markus Heinemann ◽  
Marcel Giar ◽  
Christian Heiliger
Keyword(s):  
Density Functional ◽  
Local Density ◽  
Density Functional Theory Calculations ◽  
Lattice Parameters ◽  
Generalized Gradient ◽  
Functional Theory ◽  
Lattice Constants ◽  
Exchange Correlation ◽  
Vegard’S Law ◽  
Constant Unit

AbstractWe perform density functional theory calculations to determine equilibrium lattice parameters of wurtzite Zn1-xMgxO alloys for Mg concentrations x ranging from 0 to 31.25 %. We use the local density approximation (LDA) as well as the generalized gradient approximation (GGA) for the exchange correlation functional. For the lattice constants a and c we find a deviation from Vegard's law and a constant unit cell volume independent of the Mg concentration.

scholarly journals Crossover from metal to insulator in dense lithium-rich compound CLi4

2016 ◽  
Vol 113 (9) ◽  
pp. 2366-2369 ◽  
Author(s):  
Xilian Jin ◽  
Xiao-Jia Chen ◽  
Tian Cui ◽  
Ho-kwang Mao ◽  
Huadi Zhang ◽  
...  
Keyword(s):  
Phase Transition ◽  
Density Functional Theory ◽  
Fermi Surface ◽  
Density Functional ◽  
Alkali Metals ◽  
Alkaline Earth ◽  
Density Functional Theory Calculations ◽  
Alkaline Earth Metals ◽  
Functional Theory ◽  
Surface Filling

At room environment, all materials can be classified as insulators or metals or in-between semiconductors, by judging whether they are capable of conducting the flow of electrons. One can expect an insulator to convert into a metal and to remain in this state upon further compression, i.e., pressure-induced metallization. Some exceptions were reported recently in elementary metals such as all of the alkali metals and heavy alkaline earth metals (Ca, Sr, and Ba). Here we show that a compound of CLi4 becomes progressively less conductive and eventually insulating upon compression based on ab initio density-functional theory calculations. An unusual path with pressure is found for the phase transition from metal to semimetal, to semiconductor, and eventually to insulator. The Fermi surface filling parameter is used to describe such an antimetallization process.

scholarly journals Structure variations within RSi2 and R 2 TSi3 silicides. Part I. Structure overview

2020 ◽  
Vol 76 (2) ◽  
pp. 177-200 ◽  
Author(s):  
M. Nentwich ◽  
M. Zschornak ◽  
M. Sonntag ◽  
R. Gumeniuk ◽  
S. Gemming ◽  
...  
Keyword(s):  
Density Functional ◽  
Alkaline Earth ◽  
Structural Parameters ◽  
Earth Metal ◽  
Structure Type ◽  
Underlying Structure ◽  
Alkaline Earth Metal ◽  
Formula Unit ◽  
Functional Theory ◽  
Subgroup Scheme

Here, structural parameters of various structure reports on RSi2 and R 2 TSi3 compounds [where R is an alkaline earth metal, a rare earth metal (i.e. an element of the Sc group or a lathanide), or an actinide and T is a transition metal] are summarized. The parameters comprising composition, lattice parameters a and c, ratio c/a, formula unit per unit cell and structure type are tabulated. The relationships between the underlying structure types are presented within a group–subgroup scheme (Bärnighausen diagram). Additionally, unexpectedly missing compounds within the R 2 TSi3 compounds were examined with density functional theory and compounds that are promising candidates for synthesis are listed. Furthermore, a correlation was detected between the orthorhombic AlB2-like lattices of, for example, Ca2AgSi3 and the divalence of R and the monovalence of T. Finally, a potential tetragonal structure with ordered Si/T sites is proposed.

scholarly journals Materials with the CrVO4 Structure Type as Candidate Superprotonic Conductors

2019 ◽  
Author(s):  
Pandu Wisesa ◽  
Chenyang Li ◽  
chuhong wang ◽  
Tim Mueller
Keyword(s):  
Density Functional ◽  
Density Functional Theory Calculations ◽  
Structure Type ◽  
Migration Energy ◽  
Functional Theory ◽  
Proton Conducting ◽  
Conducting Oxides ◽  
Proton Diffusion ◽  
High Proton ◽  
Proton Migration

<p>To accelerate the search for novel proton-conducting oxides, we have performed a computational screen of the proton migration energy in 41 different commonly-occurring oxide structure types. The results of this screen, which are supported by a comprehensive set of density functional theory calculations, indicate that known materials with the CrVO<sub>4</sub> structure type have an average migration energy for proton diffusion of less than 0.2 eV, with several known materials having calculated migration energies below 0.1 eV. These results indicate that materials with the CrVO<sub>4</sub> structure type may exhibit very high proton conductivity that surpasses that of leading proton-conducting oxides. We present the results of our screen as well as diffusion dimensionality analysis and thermodynamic stability analysis for materials with the CrVO<sub>4</sub> structure.</p>

Activation of Cellulose with Alkaline Earth Metals

2021 ◽  
Author(s):  
Gregory Facas ◽  
Vineet Maliekkal ◽  
Matthew Neurock ◽  
Paul Dauenhauer
Keyword(s):  
Density Functional ◽  
Alkaline Earth ◽  
Earth Metal ◽  
Density Functional Theory Calculations ◽  
Alkaline Earth Metal ◽  
Magnesium Ions ◽  
Functional Theory ◽  
Naturally Occurring ◽  
Alkaline Earth Metal Ions ◽  
And Control

Alkaline earth metal ions accelerate the breaking of cellulose bonds and control the distribution of products in the pyrolysis of lignocellulose to biofuels and chemicals. Here, the activation of cellulose via magnesium ions was measured over a range of temperatures from 370 to 430 ⁰C for 20 to 2000 milliseconds and compared with activation of cellulose via calcium, another naturally-occurring alkaline earth metal in lignocellulose materials. The experimental approach of pulse heated analysis of solid/surface reactions (PHASR) showed that magnesium significantly catalyzes cellulose activation with a second order rate dependence on the catalyst concentration. An experimental barrier of 45.6 ± 2.1 kcal mol-1 and a pre-factor of 1.18 x 1016 (mmol Mg2+ / g CD)-2 * s-1 was obtained for the activation of α-cyclodextrin (CD), a cellulose surrogate, for catalyst concentrations of 0.1 to 0.5 mmol Mg+2 per gram of CD. First principles density functional theory calculations showed that magnesium ions play a dual role in catalyzing the reaction by breaking the hydrogen bonds with hydroxymethyl groups and destabilizing the reacting cellulose chain, thus making it more active. The calculated barrier of 47 kcal mol-1 is in agreement with the experimentally measured barriers and similar to that for calcium ion catalysts (~50 kcal mol-1).

scholarly journals Chemical trends in the optical properties of rocksalt nanoparticles

2015 ◽  
Vol 17 (43) ◽  
pp. 28892-28900 ◽  
Author(s):  
Milena C. C. Wobbe ◽  
Martijn A. Zwijnenburg
Keyword(s):  
Density Functional Theory ◽  
Density Functional ◽  
Alkaline Earth ◽  
Density Functional Theory Calculations ◽  
Time Dependent ◽  
Functional Theory ◽  
Metal Chalcogenide ◽  
Pbs Nanoparticles ◽  
Chemical Trends ◽  
Dependent Density

The nature and magnitude of the optical gaps of rocksalt alkaline earth (MgO, CaO, SrO, MgS, MgSe) and transition metal chalcogenide (CdO, PbS) nanoparticles are studied using time-dependent density functional theory calculations.

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