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3 pages, 177 KiB

Open AccessEditorial

First-Principles Prediction of Structures and Properties in Crystals

by Andreas Hermann and Dominik Kurzydłowski

Crystals 2019, 9(9), 463; https://doi.org/10.3390/cryst9090463 - 4 Sep 2019

Cited by 2 | Viewed by 2772

Abstract

The term “first-principles calculations” is a synonym for the numerical determination of the electronic structure of atoms, molecules, clusters, or materials from ‘first principles’, i [...] Full article

(This article belongs to the Special Issue First-Principles Prediction of Structures and Properties in Crystals)

Research

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17 pages, 3730 KiB

Open AccessArticle

A First-Principles Exploration of Na_xS_y Binary Phases at 1 atm and Under Pressure

by Nisha Geng, Tiange Bi, Niloofar Zarifi, Yan Yan and Eva Zurek

Crystals 2019, 9(9), 441; https://doi.org/10.3390/cryst9090441 - 24 Aug 2019

Cited by 10 | Viewed by 4098

Abstract

Interest in Na-S compounds stems from their use in battery materials at 1 atm, as well as the potential for superconductivity under pressure. Evolutionary structure searches coupled with Density Functional Theory calculations were employed to predict stable and low-lying metastable phases of sodium [...] Read more.

Interest in Na-S compounds stems from their use in battery materials at 1 atm, as well as the potential for superconductivity under pressure. Evolutionary structure searches coupled with Density Functional Theory calculations were employed to predict stable and low-lying metastable phases of sodium poor and sodium rich sulfides at 1 atm and within 100–200 GPa. At ambient pressures, four new stable or metastable phases with unbranched sulfur motifs were predicted: Na₂S₃ with

C 2 / c

and Imm2 symmetry,

C 2

-Na₂S₅ and

C 2

-Na₂S₈. Van der Waals interactions were shown to affect the energy ordering of various polymorphs. At high pressure, several novel phases that contained a wide variety of zero-, one-, and two-dimensional sulfur motifs were predicted, and their electronic structures and bonding were analyzed. At 200 GPa,

P 4 / m m m

-Na₂S₈ was predicted to become superconducting below 15.5 K, which is close to results previously obtained for the

β

-Po phase of elemental sulfur. The structures of the most stable M₃S and M₄S, M = Na, phases differed from those previously reported for compounds with M = H, Li, K. Full article

(This article belongs to the Special Issue First-Principles Prediction of Structures and Properties in Crystals)

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19 pages, 1336 KiB

Open AccessArticle

Simultaneous Prediction of the Magnetic and Crystal Structure of Materials Using a Genetic Algorithm

by Edward J. Higgins, Phil J. Hasnip and Matt I.J. Probert

Crystals 2019, 9(9), 439; https://doi.org/10.3390/cryst9090439 - 23 Aug 2019

Cited by 9 | Viewed by 3317

Abstract

We introduce a number of extensions and enhancements to a genetic algorithm for crystal structure prediction, to make it suitable to study magnetic systems. The coupling between magnetic properties and crystal structure means that it is essential to take a holistic approach, and [...] Read more.

We introduce a number of extensions and enhancements to a genetic algorithm for crystal structure prediction, to make it suitable to study magnetic systems. The coupling between magnetic properties and crystal structure means that it is essential to take a holistic approach, and we present for the first time, a genetic algorithm that performs a simultaneous global optimisation of both magnetic structure and crystal structure. We first illustrate the power of this approach on a novel test system—the magnetic Lennard–Jones potential—which we define. Then we study the complex interface structures found at the junction of a Heusler alloy and a semiconductor substrate as found in a proposed spintronic device and show the impact of the magnetic interface structure on the device performance. Full article

(This article belongs to the Special Issue First-Principles Prediction of Structures and Properties in Crystals)

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19 pages, 4545 KiB

Open AccessArticle

Quest for Compounds at the Verge of Charge Transfer Instabilities: The Case of Silver(II) Chloride ^†

by Mariana Derzsi, Adam Grzelak, Paweł Kondratiuk, Kamil Tokár and Wojciech Grochala

Crystals 2019, 9(8), 423; https://doi.org/10.3390/cryst9080423 - 15 Aug 2019

Cited by 8 | Viewed by 6927

Abstract

Electron-transfer processes constitute one important limiting factor governing stability of solids. One classical case is that of CuI₂, which has never been prepared at ambient pressure conditions due to feasibility of charge transfer between metal and nonmetal (CuI₂ → CuI [...] Read more.

Electron-transfer processes constitute one important limiting factor governing stability of solids. One classical case is that of CuI₂, which has never been prepared at ambient pressure conditions due to feasibility of charge transfer between metal and nonmetal (CuI₂ → CuI + ½ I₂). Sometimes, redox instabilities involve two metal centers, e.g., AgO is not an oxide of divalent silver but rather silver(I) dioxoargentate(III), Ag(I)[Ag(III)O₂]. Here, we look at the particularly interesting case of a hypothetical AgCl₂ where both types of redox instabilities operate simultaneously. Since standard redox potential of the Ag(II)/Ag(I) redox pair reaches some 2 V versus Normal Hydrogen Electrode (NHE), it might be expected that Ag(II) would oxidize Cl⁻ anion with great ease (standard redox potential of the ½ Cl₂/Cl⁻ pair is + 1.36 V versus Normal Hydrogen Electrode). However, ionic Ag(II)Cl₂ benefits from long-distance electrostatic stabilization to a much larger degree than Ag(I)Cl + ½ Cl₂, which affects relative stability. Moreover, Ag(II) may disproportionate in its chloride, just like it does in an oxide; this is what AuCl₂ does, its formula corresponding in fact to Au(I)[Au(III)Cl₄]. Formation of polychloride substructure, as for organic derivatives of Cl₃⁻ anion, is yet another possibility. All that creates a very complicated potential energy surface with a few chemically distinct minima i.e., diverse polymorphic forms present. Here, results of our theoretical study for AgCl₂ will be presented including outcome of evolutionary algorithm structure prediction method, and the chemical identity of the most stable form will be uncovered together with its presumed magnetic properties. Contrary to previous rough estimates suggesting substantial instability of AgCl₂, we find that AgCl₂ is only slightly metastable (by 52 meV per formula unit) with respect to the known AgCl and ½ Cl₂, stable with respect to elements, and simultaneously dynamically (i.e., phonon) stable. Thus, our results point out to conceivable existence of AgCl₂ which should be targeted via non-equilibrium approaches. Full article

(This article belongs to the Special Issue First-Principles Prediction of Structures and Properties in Crystals)

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15 pages, 5484 KiB

Open AccessArticle

Insight into the Optoelectronic and Thermoelectric Properties of Mn Doped ZnTe from First Principles Calculation

by Wilayat Khan, Sikander Azam, Inam Ullah, Malika Rani, Ayesha Younus, Muhammad Irfan, Paweł Czaja and Iwan V. Kityk

Crystals 2019, 9(5), 247; https://doi.org/10.3390/cryst9050247 - 13 May 2019

Cited by 8 | Viewed by 3682

Abstract

Using DFT band structure simulations together with semi-classical Boltzmann transport kinetics equations, we have explored the optoelectronic and transport features of Mn_xZn_1−xTe (x = 8% and 16%) crystals. Optimization of the doping and related technological processes it is extremely [...] Read more.

Using DFT band structure simulations together with semi-classical Boltzmann transport kinetics equations, we have explored the optoelectronic and transport features of Mn_xZn_1−xTe (x = 8% and 16%) crystals. Optimization of the doping and related technological processes it is extremely important for optimization of the technological parameters. The Generalized Gradient Approximation is applied to compute the corresponding band structure parameters. We have applied the Generalized Gradient Approximation Plus U (GGA+U). We have demonstrated that Mn_xZn_1−xTe (x = 8% and 16%) is a direct type band semiconductor with principal energy gap values equal to 2.20 and 2.0 eV for x = 8% and 16%, respectively. The energy gap demonstrates significant decrease with increasing Mn content. Additionally, the origin of the corresponding bands is explored from the electronic density of states. The optical dispersion functions are calculated from the spectra of dielectric function. The theoretical simulations performed unambiguously showed that the titled materials are simultaneously promising optoelectronic and thermoelectric devices. The theoretical simulations performed showed ways for amendment of their transport properties by replacement of particular ions. Full article

(This article belongs to the Special Issue First-Principles Prediction of Structures and Properties in Crystals)

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15 pages, 1497 KiB

Open AccessEditor’s ChoiceArticle

Van der Waals Density Functional Theory vdW-DFq for Semihard Materials

by Qing Peng, Guangyu Wang, Gui-Rong Liu and Suvranu De

Crystals 2019, 9(5), 243; https://doi.org/10.3390/cryst9050243 - 8 May 2019

Cited by 22 | Viewed by 5441

Abstract

There are a large number of materials with mild stiffness, which are not as soft as tissues and not as strong as metals. These semihard materials include energetic materials, molecular crystals, layered materials, and van der Waals crystals. The integrity and mechanical stability [...] Read more.

There are a large number of materials with mild stiffness, which are not as soft as tissues and not as strong as metals. These semihard materials include energetic materials, molecular crystals, layered materials, and van der Waals crystals. The integrity and mechanical stability are mainly determined by the interactions between instantaneously induced dipoles, the so called London dispersion force or van der Waals force. It is challenging to accurately model the structural and mechanical properties of these semihard materials in the frame of density functional theory where the non-local correlation functionals are not well known. Here, we propose a van der Waals density functional named vdW-DFq to accurately model the density and geometry of semihard materials. Using

β

-cyclotetramethylene tetranitramine as a prototype, we adjust the enhancement factor of the exchange energy functional with generalized gradient approximations. We find this method to be simple and robust over a wide tuning range when calibrating the functional on-demand with experimental data. With a calibrated value

q = 1.05

, the proposed vdW-DFq method shows good performance in predicting the geometries of 11 common energetic material molecular crystals and three typical layered van der Waals crystals. This success could be attributed to the similar electronic charge density gradients, suggesting a wide use in modeling semihard materials. This method could be useful in developing non-empirical density functional theories for semihard and soft materials. Full article

(This article belongs to the Special Issue First-Principles Prediction of Structures and Properties in Crystals)

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15 pages, 2900 KiB

Open AccessArticle

Insight into Physical and Thermodynamic Properties of X₃Ir (X = Ti, V, Cr, Nb and Mo) Compounds Influenced by Refractory Elements: A First-Principles Calculation

by Dong Chen, Jiwei Geng, Yi Wu, Mingliang Wang and Cunjuan Xia

Crystals 2019, 9(2), 104; https://doi.org/10.3390/cryst9020104 - 18 Feb 2019

Cited by 3 | Viewed by 3781

Abstract

The effects of refractory metals on physical and thermodynamic properties of X₃Ir (X = Ti, V, Cr, Nb and Mo) compounds were investigated using local density approximation (LDA) and generalized gradient approximation (GGA) methods within the first-principles calculations based on density [...] Read more.

The effects of refractory metals on physical and thermodynamic properties of X₃Ir (X = Ti, V, Cr, Nb and Mo) compounds were investigated using local density approximation (LDA) and generalized gradient approximation (GGA) methods within the first-principles calculations based on density functional theory. The optimized lattice parameters were both in good compliance with the experimental parameters. The GGA method could achieve an improved structural optimization compared to the LDA method, and thus was utilized to predict the elastic, thermodynamic and electronic properties of X₃Ir (X = Ti, V, Cr, Nb and Mo) compounds. The calculated mechanical properties (i.e., elastic constants, elastic moduli and elastic anisotropic behaviors) were rationalized and discussed in these intermetallics. For instance, the derived bulk moduli exhibited the sequence of Ti₃Ir < Nb₃Ir < V₃Ir < Cr₃Ir < Mo₃Ir. This behavior was discussed in terms of the volume of unit cell and electron density. Furthermore, Debye temperatures were derived and were found to show good consistency with the experimental values, indicating the precision of our calculations. Finally, the electronic structures were analyzed to explain the ductile essences in the iridium compounds. Full article

(This article belongs to the Special Issue First-Principles Prediction of Structures and Properties in Crystals)

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15 pages, 1986 KiB

Open AccessArticle

First-Principles Assessment of the Structure and Stability of 15 Intrinsic Point Defects in Zinc-Blende Indium Arsenide

by Qing Peng, Nanjun Chen, Danhong Huang, Eric R. Heller, David A. Cardimona and Fei Gao

Crystals 2019, 9(1), 48; https://doi.org/10.3390/cryst9010048 - 17 Jan 2019

Cited by 5 | Viewed by 6798

Abstract

Point defects are inevitable, at least due to thermodynamics, and essential for engineering semiconductors. Herein, we investigate the formation and electronic structures of fifteen different kinds of intrinsic point defects of zinc blende indium arsenide (zb-InAs ) using first-principles calculations. For [...] Read more.

Point defects are inevitable, at least due to thermodynamics, and essential for engineering semiconductors. Herein, we investigate the formation and electronic structures of fifteen different kinds of intrinsic point defects of zinc blende indium arsenide (zb-InAs ) using first-principles calculations. For As-rich environment, substitutional point defects are the primary intrinsic point defects in zb-InAs until the n-type doping region with Fermi level above 0.32 eV is reached, where the dominant intrinsic point defects are changed to In vacancies. For In-rich environment, In tetrahedral interstitial has the lowest formation energy till n-type doped region with Fermi level 0.24 eV where substitutional point defects In

_{A s}

take over. The dumbbell interstitials prefer

< 110 >

configurations. For tetrahedral interstitials, In atoms prefer 4-As tetrahedral site for both As-rich and In-rich environments until the Fermi level goes above 0.26 eV in n-type doped region, where In atoms acquire the same formation energy at both tetrahedral sites and the same charge state. This implies a fast diffusion along the

t - T - t

path among the tetrahedral sites for In atoms. The In vacancies V

_{I n}

decrease quickly and monotonically with increasing Fermi level and has a

q = - 3 e

charge state at the same time. The most popular vacancy-type defect is V

_{I n}

in an As-rich environment, but switches to V

_{A s}

in an In-rich environment at light p-doped region when Fermi level below 0.2 eV. This study sheds light on the relative stabilities of these intrinsic point defects, their concentrations and possible diffusions, which is expected useful in defect-engineering zb-InAs based semiconductors, as well as the material design for radiation-tolerant electronics. Full article

(This article belongs to the Special Issue First-Principles Prediction of Structures and Properties in Crystals)

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