Materials

Research

13 pages, 7668 KiB

Open AccessArticle

First-Principles Calculate the Stability, Mechanical Properties and Electronic Structure of Carbide MC, M₂C and M₆C in M50NiL Steel

by Xi Yong, Xiating Liu, Maosheng Yang and Xiaolong Zhou

Materials 2024, 17(14), 3498; https://doi.org/10.3390/ma17143498 - 15 Jul 2024

Viewed by 786

Abstract

In this paper, the stability, mechanical properties and electronic structure of carbides in steel were calculated using the first-principles method based on the density functional theory (DFT). Firstly, the MC, M₂C, M₆C (M = Cr, Mo, V, Fe) carbides [...] Read more.

In this paper, the stability, mechanical properties and electronic structure of carbides in steel were calculated using the first-principles method based on the density functional theory (DFT). Firstly, the MC, M₂C, M₆C (M = Cr, Mo, V, Fe) carbides models were established. Then, different interphases’ lattice constants, formation enthalpy, binding energy and elastic modulus were calculated. The stability, hardness, ductility and anisotropy of each phase were finally analyzed. The results show that these phases are stable, and the stability is closely related to the electron loss ability of its metal elements. The stronger the electron loss ability of its metal elements, the more stable the formed phase. As for MC carbides, MoC has the largest bulk modulus and hardness. As for M₂C carbides, the Poisson’s ratio of Cr₂C is the smallest, and all phases except for Cr₂C show toughness and ductility. The anisotropy of M₆C carbides is relatively poor. Full article

(This article belongs to the Special Issue Density Functional Theory and Its Applications in Materials Science: A Critical Comparison between Theoretical Modelling of Crystals and Experiments)

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20 pages, 5656 KiB

Open AccessArticle

Electronic, Optical, and Vibrational Properties of an AgAlS₂ Crystal in a High-Pressure Phase

by Myron Ya. Rudysh, Anatolii O. Fedorchuk, Mikhail G. Brik, Jurij Grechenkov, Dmitry Bocharov, Sergei Piskunov, Anatoli I. Popov and Michal Piasecki

Materials 2023, 16(21), 7017; https://doi.org/10.3390/ma16217017 - 2 Nov 2023

Cited by 1 | Viewed by 1204

Abstract

The aim of this study is to comprehensively examine the structural composition and properties of the AgAlS₂ crystal during its high-pressure phase. This analysis delves into the second coordination environment of the crystal structure and elucidates the distinct transformations it undergoes during [...] Read more.

The aim of this study is to comprehensively examine the structural composition and properties of the AgAlS₂ crystal during its high-pressure phase. This analysis delves into the second coordination environment of the crystal structure and elucidates the distinct transformations it undergoes during the phase transition. The band energy structure was calculated, and the origin of electronic levels was clarified. It is shown that the crystal becomes non-stratified during the phase transition. This study also determined the values of the crystal’s carrier effective masses, underscoring its spatial anisotropy. It was found that the calculated optical functions are similar to the crystal in the chalcopyrite structure, and their differences are shown. Further, this study involved the calculation of the crystal’s phonon spectrum, revealing the spectrum’s transformation during the phase transition. The vibrational frequencies were also obtained, with a symmetrical classification of vibrational modes. Finally, this study derived the infrared and Raman spectra of the AgAlS₂ crystal, thereby providing a comprehensive picture of the crystal during its high-pressure phase. Full article

(This article belongs to the Special Issue Density Functional Theory and Its Applications in Materials Science: A Critical Comparison between Theoretical Modelling of Crystals and Experiments)

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15 pages, 1270 KiB

Open AccessArticle

Modeling of the Lattice Dynamics in Strontium Titanate Films of Various Thicknesses: Raman Scattering Studies

by Veera Krasnenko, Alexander Platonenko, Aleksandr Liivand, Leonid L. Rusevich, Yuri A. Mastrikov, Guntars Zvejnieks, Maksim Sokolov and Eugene A. Kotomin

Materials 2023, 16(18), 6207; https://doi.org/10.3390/ma16186207 - 14 Sep 2023

Cited by 1 | Viewed by 1314

Abstract

While the bulk strontium titanate (STO) crystal characteristics are relatively well known, ultrathin perovskites’ nanostructure, chemical composition, and crystallinity are quite complex and challenging to understand in detail. In our study, the DFT methods were used for modelling the Raman spectra of the [...] Read more.

While the bulk strontium titanate (STO) crystal characteristics are relatively well known, ultrathin perovskites’ nanostructure, chemical composition, and crystallinity are quite complex and challenging to understand in detail. In our study, the DFT methods were used for modelling the Raman spectra of the STO bulk (space group I4/mcm) and 5–21-layer thin films (layer group p4/mbm) in tetragonal phase with different thicknesses ranging from ~0.8 to 3.9 nm. Our calculations revealed features in the Raman spectra of the films that were absent in the bulk spectra. Out of the seven Raman-active modes associated with bulk STO, the frequencies of five modes (2E_g, A_1g, B_2g, and B_1g) decreased as the film thickness increased, while the low-frequency B_2g and higher-frequency E_g modes frequencies increased. The modes in the films exhibited vibrations with different amplitudes in the central or surface parts of the films compared to the bulk, resulting in frequency shifts. Some peaks related to bulk vibrations were too weak (compared to the new modes related to films) to distinguish in the Raman spectra. However, as the film thickness increased, the Raman modes approached the frequencies of the bulk, and their intensities became higher, making them more noticeable in the Raman spectrum. Our results could help to explain inconsistencies in the experimental data for thin STO films, providing insights into the behavior of Raman modes and their relationship with film thickness. Full article

(This article belongs to the Special Issue Density Functional Theory and Its Applications in Materials Science: A Critical Comparison between Theoretical Modelling of Crystals and Experiments)

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20 pages, 3751 KiB

Open AccessEditor’s ChoiceArticle

Molecular Mechanism of the Piezoelectric Response in the β-Phase PVDF Crystals Interpreted by Periodic Boundary Conditions DFT Calculations

by Gianluca Serra, Alessia Arrigoni, Mirella Del Zoppo, Chiara Castiglioni and Matteo Tommasini

Materials 2023, 16(17), 6004; https://doi.org/10.3390/ma16176004 - 31 Aug 2023

Cited by 3 | Viewed by 1535

Abstract

A theoretical approach based on Periodic Boundary Conditions (PBC) and a Linear Combination of Atomic Orbitals (LCAO) in the framework of the density functional theory (DFT) is used to investigate the molecular mechanism that rules the piezoelectric behavior of poly(vinylidene fluoride) (PVDF) polymer [...] Read more.

A theoretical approach based on Periodic Boundary Conditions (PBC) and a Linear Combination of Atomic Orbitals (LCAO) in the framework of the density functional theory (DFT) is used to investigate the molecular mechanism that rules the piezoelectric behavior of poly(vinylidene fluoride) (PVDF) polymer in the crystalline β-phase. We present several computational tests highlighting the peculiar electrostatic potential energy landscape the polymer chains feel when they change their orientation by a rigid rotation in the lattice cell. We demonstrate that a rotation of the permanent dipole through chain rotation has a rather low energy cost and leads to a lattice relaxation. This justifies the macroscopic strain observed when the material is subjected to an electric field. Moreover, we investigate the effect on the molecular geometry of the expansion of the lattice parameters in the (a, b) plane, proving that the rotation of the dipole can take place spontaneously under mechanical deformation. By band deconvolution of the IR and Raman spectra of a PVDF film with a high content of β-phase, we provide the experimental phonon wavenumbers and relative band intensities, which we compare against the predictions from DFT calculations. This analysis shows the reliability of the LCAO approach, as implemented in the CRYSTAL software, for calculating the vibrational spectra. Finally, we investigate how the IR/Raman spectra evolve as a function of inter-chain distance, moving towards the isolated chain limit and to the limit of a single crystal slab. The results show the relevance of the inter-molecular interactions on the vibrational dynamics and on the electro-optical features ruling the intensity pattern of the vibrational spectra. Full article

(This article belongs to the Special Issue Density Functional Theory and Its Applications in Materials Science: A Critical Comparison between Theoretical Modelling of Crystals and Experiments)

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Graphical abstract

11 pages, 660 KiB

Open AccessArticle

On the Symmetry, Electronic Properties, and Possible Metallic States in NASICON-Structured A₄V₂(PO₄)₃ (A = Li, Na, K) Phosphates

by Denis Gryaznov and Linas Vilčiauskas

Materials 2023, 16(12), 4361; https://doi.org/10.3390/ma16124361 - 13 Jun 2023

Viewed by 1153

Abstract

In this work, the electronic structure and properties of NASICON-structured A₄V₂(PO₄)₃, where A = Li, Na, K were studied using hybrid density functional theory calculations. The symmetries were analyzed using a group theoretical approach, and [...] Read more.

In this work, the electronic structure and properties of NASICON-structured A₄V₂(PO₄)₃, where A = Li, Na, K were studied using hybrid density functional theory calculations. The symmetries were analyzed using a group theoretical approach, and the band structures were examined by the atom and orbital projected density of states analyses. Li₄V₂(PO₄)₃ and Na₄V₂(PO₄)₃ adopted monoclinic structures with the

C 2

space group and averaged vanadium oxidation states of V

^{+ 2.5}

in the ground state, whereas K₄V₂(PO₄)₃ adopted a monoclinic structure with the

C 2

space group and mixed vanadium oxidation states V

^{+ 2}

/V

^{+ 3}

in the ground state. The mixed oxidation state is the least stable state in Na₄V₂(PO₄)₃ and Li₄V₂(PO₄)₃. Symmetry increases in Li₄V₂(PO₄)₃ and Na₄V₂(PO₄)₃ led to the appearance of a metallic state that was independent of the vanadium oxidation states (except for the averaged oxidation state

R 32

Na₄V₂(PO₄)₃). On the other hand, K₄V₂(PO₄)₃ retained a small band gap in all studied configurations. These results might provide valuable guidance for crystallography and electronic structure investigations for this important class of materials. Full article

(This article belongs to the Special Issue Density Functional Theory and Its Applications in Materials Science: A Critical Comparison between Theoretical Modelling of Crystals and Experiments)

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12 pages, 6641 KiB

Open AccessArticle

The Electronic Structures and Energies of the Lowest Excited States of the N_s⁰, N_s⁺, N_s⁻ and N_s-H Defects in Diamond

by Alexander Platonenko, William C. Mackrodt and Roberto Dovesi

Materials 2023, 16(5), 1979; https://doi.org/10.3390/ma16051979 - 28 Feb 2023

Cited by 1 | Viewed by 1512

Abstract

This paper reports the energies and charge and spin distributions of the mono-substituted N defects, N⁰_s, N⁺_s, N⁻_s and N_s-H in diamonds from direct Δ-SCF calculations based on Gaussian orbitals within the B3LYP [...] Read more.

This paper reports the energies and charge and spin distributions of the mono-substituted N defects, N⁰_s, N⁺_s, N⁻_s and N_s-H in diamonds from direct Δ-SCF calculations based on Gaussian orbitals within the B3LYP function. These predict that (i) N_s⁰, N_s⁺ and N_s⁻ all absorb in the region of the strong optical absorption at 270 nm (4.59 eV) reported by Khan et al., with the individual contributions dependent on the experimental conditions; (ii) N_s-H, or some other impurity, is responsible for the weak optical peak at 360 nm (3.44 eV); and that N_s⁺ is the source of the 520 nm (2.38 eV) absorption. All excitations below the absorption edge of the diamond host are predicted to be excitonic, with substantial re-distributions of charge and spin. The present calculations support the suggestion by Jones et al. that N_s⁺ contributes to, and in the absence of N_s⁰ is responsible for, the 4.59 eV optical absorption in N-doped diamonds. The semi-conductivity of the N-doped diamond is predicted to rise from a spin-flip thermal excitation of a CN hybrid orbital of the donor band resulting from multiple in-elastic phonon scattering. Calculations of the self-trapped exciton in the vicinity of N_s⁰ indicate that it is essentially a local defect consisting of an N and four nn C atoms, and that beyond these the host lattice is essential a pristine diamond as predicted by Ferrari et al. from the calculated EPR hyperfine constants. Full article

(This article belongs to the Special Issue Density Functional Theory and Its Applications in Materials Science: A Critical Comparison between Theoretical Modelling of Crystals and Experiments)

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26 pages, 1407 KiB

Open AccessArticle

The d Orbital Multi Pattern Occupancy in a Partially Filled d Shell: The KFeF₃ Perovskite as a Test Case

by Fabien Pascale, Sami Mustapha, Philippe D’Arco and Roberto Dovesi

Materials 2023, 16(4), 1532; https://doi.org/10.3390/ma16041532 - 12 Feb 2023

Cited by 6 | Viewed by 1460

Abstract

The occupancy of the d shell in KFeF

_{3}

is

t_{2 g}^{4}

e

_{g}^{2}

, with five

α

and one

β

electrons. The Jahn–Teller lift of degeneracy in the

t_{2 g}

sub-shell produces a tetragonal relaxation of the unit [...] Read more.

The occupancy of the d shell in KFeF

_{3}

is

t_{2 g}^{4}

e

_{g}^{2}

, with five

α

and one

β

electrons. The Jahn–Teller lift of degeneracy in the

t_{2 g}

sub-shell produces a tetragonal relaxation of the unit cell (4.09 vs. 4.22 Å, B3LYP result) not observed experimentally. In order to understand the origin of this apparent contradiction, we explored, with a 2 × 2 × 2 supercell (40 atoms per cell), all possible local structures in which contiguous Fe atoms have a different occupancy of the

t_{2 g}

orbitals with the minority spin electron. A total of 6561 configurations (with occupancies from (8,0,0) to (3,2,2) of the 3

t_{2 g}

orbitals of the 8 Fe atoms) have been explored, with energies in many cases lower (by up to 1550

μ

E

_{h}

per 2 Fe atoms) than the one of the fully ordered case, both for the ferromagnetic and the anti-ferromagnetic solutions. The results confirm that the orientation of the

β

d electron of Fe influences the electrostatics (more efficient relative orientation of the Fe quadrupoles of the d shell) of the system, but not the magnetic interactions. Three hybrid functionals, B3LYP, PBE0, and HSE06, provide very similar results. Full article

(This article belongs to the Special Issue Density Functional Theory and Its Applications in Materials Science: A Critical Comparison between Theoretical Modelling of Crystals and Experiments)

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17 pages, 1588 KiB

Open AccessArticle

Difference in Structure and Electronic Properties of Oxygen Vacancies in α-Quartz and α-Cristobalite Phases of SiO₂

by Katherine L. Milton, Thomas R. Durrant, Teofilo Cobos Freire and Alexander L. Shluger

Materials 2023, 16(4), 1382; https://doi.org/10.3390/ma16041382 - 7 Feb 2023

Cited by 5 | Viewed by 2513

Abstract

α

-cristobalite (

α

-C) is a polymorph of silica, mainly found in space exploration and geochemistry research. Due to similar densities,

α

-C is often used as a proxy for amorphous SiO

_{2}

, particularly in computer simulations of SiO

_{2}

surfaces [...] Read more.

α

-cristobalite (

α

-C) is a polymorph of silica, mainly found in space exploration and geochemistry research. Due to similar densities,

α

-C is often used as a proxy for amorphous SiO

_{2}

, particularly in computer simulations of SiO

_{2}

surfaces and interfaces. However, little is known about the properties of

α

-C and its basic oxygen defects. Using density functional theory (DFT) simulations we provide a comprehensive report on the properties of perfect structure and oxygen vacancies in

α

-C. The calculated properties of

α

-C are compared with those of the better-characterized

α

-quartz (

α

-Q). Our results demonstrated that the positively charged O vacancy in

α

-C is most stable in the dimer configuration, in contrast to

α

-Q, which favors the puckered configuration. A back-projected configuration was also predicted in both polymorphs. We calculated the optical transition energies and isotropic hyperfine constants for O vacancies in both

α

-Q and

α

-C, and compared our findings with the results of previous studies and experiments. This work, thus, offers one of the first in-depth investigations of the properties of oxygen vacancies in

α

-C. Full article

(This article belongs to the Special Issue Density Functional Theory and Its Applications in Materials Science: A Critical Comparison between Theoretical Modelling of Crystals and Experiments)

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