Crystal Structure Evolution of Fluorine under High Pressure

Fluorinated compounds in the last decade were applied as photo-thermo-refractive glasses, high-stress lubricants, and pharmaceutical drugs due to their good mechanical properties and biocompatibility. Although fluorinated materials are largely employed, the possibility of predicting new structures was limited by the impossibility to use density functional theory (DFT) to describe interatomic and intermolecular interactions correctly. This is seen linearly to increase with fluorine concentration. In crystal structure prediction, modern algorithms are usually combined with first-principles methods employed for global solution, which sometimes fail to describe systems as in the case of strongly correlated materials. Fluorine is one of the tricky elements, which is characterized by relativistic effects and no overlap between the DFT exchange hole and the exact exchange hole. Thus, no relativistic exchange–correlation functional was seen to adequately describe fluorine. In this work, we have found an excellent compromise to investigate fluorinated materials using a combination of SCAN (exchange) and rVV10 (correlation) functionals. This was found through the fundamental study of α- and β-fluorine phases, showing α-fluorine as the most stable structure at temperatures lower than 35 K and 0 GPa with respect to β-fluorine. Further, we have computed crystal structure evolution under pressure looking for new stable fluorine allotropes using the USPEX evolutionary algorithm coupled with the SCAN-rVV10 exchange–correlation functional discovering two phase transitions: one from C2/c (i.e., α-fluorine) to Cmca at ∼5.5 GPa and from Cmca to the P4̅21c phase at 220 GPa; all these structures possess metallic behavior. The achievements of this work lie far beyond the thermodynamic of fluorine crystals under pressure. It will give the right instrument to understand the chemical behavior of fluorinated materials under pressure with consequent great speed up to the crystal structure prediction of fluorinated and fluorine-based materials

Crystal Structure Evolution of Fluorine under High Pressure

Fluorinated compounds in the last decade were applied as photothermo-refractive glasses, high-stress lubricants, and pharmaceutical drugs due to their good mechanical properties and biocompatibility. Although fluorinated materials are largely employed, the possibility of predicting new structures was limited by the impossibility to use density functional theory (DFT) to describe interatomic and intermolecular interactions correctly. This is seen linearly to increase with fluorine concentration. In crystal structure prediction, modern algorithms are usually combined with first-principles methods employed for global solution, which sometimes fail to describe systems as in the case of strongly correlated materials. Fluorine is one of the tricky elements, which is characterized by relativistic effects and no overlap between the DFT exchange hole and the exact exchange hole. Thus, no relativistic exchange−correlation functional was seen to adequately describe fluorine. In this work, we have found an excellent compromise to investigate fluorinated materials using a combination of SCAN (exchange) and rVV10 (correlation) functionals. This was found through the fundamental study of α- and β-fluorine phases, showing α-fluorine as the most stable structure at temperatures lower than 35 K and 0 GPa with respect to β-fluorine. Further, we have computed crystal structure evolution under pressure looking for new stable fluorine allotropes using the USPEX evolutionary algorithm coupled with the SCAN-rVV10 exchange−correlation functional discovering two phase transitions: one from C2/c (i.e., α-fluorine) to Cmca at ∼5.5 GPa and from Cmca to the P4̅21c phase at 220 GPa; all these structures possess metallic behavior. The achievements of this work lie far beyond the thermodynamic of fluorine crystals under pressure. It will give the right instrument to understand the chemical behavior of fluorinated materials under pressure with consequent great speed up to the crystal structure prediction of fluorinated and fluorine-based materials

Generating and grading 34 Optimised Norm-Conserving Vanderbilt Pseudopotentials for Actinides and Super Heavy Elements in the PseudoDojo

In the last decades, material discovery has been a very active research field driven by the necessity of new materials for different applications. This has also included materials incorporating heavy elements, beyond the stable isotopes of lead. Most of actinides exhibit unique properties that make them useful in various applications. Further, new heavy elements, taking the name of super-heavy elements, have been synthesized, filling previously empty space of Mendeleev periodic table. Their chemical bonding behaviour, of academic interest at present, would also benefit of state-of-the-art modelling approaches. In particular, in order to perform first-principles calculations with planewave basis sets, one needs corresponding pseudopotentials. In this work, we present a series of fully-relativistic optimised norm-conserving Vanderbilt pseudopotentials (ONCVPs) for thirty-four actinides and super-heavy elements. The scalar relativistic version of these ONCVPs is tested by comparing equations of states for crystals, obtained with \textsc{abinit} 9.6, with those obtained by all-electron zeroth-order regular approximation (ZORA) calculations performed with the Amsterdam Modelling Suite BAND code. $\Delta$-Gauge and $\Delta_1$-Gauge indicators are used to validate these pseudopotentials. This work is a contribution to the PseudoDojo project, in which pseudopotentials for the whole periodic table are developed and systematically tested. The fully-relativistic pseudopotential files (i.e. including spin-orbit coupling) are available on the PseudoDojo web-interface pseudo-dojo.org under the name NC FR (ONCVPSP) v4.x. Pseudopotentials are made available in psp8 and UPF2 formats, both convenient for \textsc{abinit}, the latter being also suitable for Quantum ESPRESSO

Constrained Density Functional Theory: A Potential-Based Self-Consistency Approach

Chemical reactions, charge transfer reactions, and magnetic materials are notoriously difficult to describe within Kohn−Sham density functional theory, which is strictly a groundstate technique. However, over the last few decades, an approximate method known as constrained density functional theory (cDFT) has been developed to model low-lying excitations linked to charge transfer or spin fluctuations. Nevertheless, despite becoming very popular due to its versatility, low computational cost, and availability in numerous software applications, none of the previous cDFT implementations is strictly similar to the corresponding ground-state self-consistent density functional theory: the target value of constraints (e.g., local magnetization) is not treated equivalently with atomic positions or lattice parameters. In the present work, by considering a potential-based formulation of the self-consistency problem, the cDFT is recast in the same framework as Kohn−Sham DFT: a new functional of the potential that includes the constraints is proposed, where the constraints, the atomic positions, or the lattice parameters are treated all alike, while all other ingredients of the usual potentialbased DFT algorithms are unchanged, thanks to the formulation of the adequate residual. Tests of this approach for the case of spin constraints (collinear and noncollinear) and charge constraints are performed. Expressions for the derivatives with respect to constraints (e.g., the spin torque) for the atomic forces and the stress tensor in cDFT are provided. The latter allows one to study striction effects as a function of the angle between spins. We apply this formalism to body-centered cubic iron and first reproduce the well-known magnetization amplitude as a function of the angle between local magnetizations. We also study stress as a function of such an angle. Then, the local collinear magnetization and the local atomic charge are varied together. Since the atomic spin magnetizations, local atomic charges, atomic positions, and lattice parameters are treated on an equal footing, this formalism is an ideal starting point for the generation of model Hamiltonians and machine-learning potentials, computation of second or third derivatives of the energy as delivered from density-functional perturbation theory, or for second-principles approaches

Hardness Descriptor Derived from Symbolic Regression

Hard and superhard materials play a vital role in numerous industrial applications necessary for sustainable development. However, discovering new materials with high hardness is challenging due to the complexity of this multiscale property and its and its intricate relationship with the atomic properties of the material. Here, we introduce a low-dimensional physical descriptor for Vickers hardness derived from a symbolic-regression artificial intelligence approach to data analysis. This descriptor is a mathematical combination of materials' properties that can be evaluated much more easily than hardness itself through the atomistic simulations, therefore suitable for a high-throughput screening. The artificial intelligence model was developed and trained using the experimental hardness values and high-throughput screening was performed on 635 compounds, including binary, ternary, and quaternary transition-metal borides, carbides, nitrides, carbonitrides, carboborides, and boronitrides to identify the optimal superhard material. The proposed descriptor is a physically interpretable analytic formula that provides insight into the multiscale relationship between atomic structure (micro) and hardness (macro). We discovered that hardness is proportional to the Voigt-averaged bulk modulus and inversely proportional to the Poisson's ratio and Reuss-averaged shear modulus. Results of high-throughput search suggest the enhancement of material hardness through mixing with harder, yet metastable structures (e.g., metastable VN, TaN, ReN$_2$, Cr$_3$N$_4$, and ZrB$_6$, all of them exhibit high hardness)

Electronic Properties of Functionalized Diamanes for Field-Emission Displays

Ultrathin diamond films, or diamanes, are promising quasi-2D materials that are characterized by high stiffness, extreme wear resistance, high thermal conductivity, and chemical stability. Surface functionalization of multilayer graphene with different stackings of layers could be an interesting opportunity to induce proper electronic properties into diamanes. Combination of these electronic properties together with extraordinary mechanical ones will lead to their applications as field-emission displays substituting original devices with light-emitting diodes or organic light-emitting diodes. In the present study, we focus on the electronic properties of fluorinated and hydrogenated diamanes with (111), (110), (0001), (101̅0), and (2̅110) crystallographic orientations of surfaces of various thicknesses by using first-principles calculations and Bader analysis of electron density. We see that fluorine induces an occupied surface electronic state, while hydrogen modifies the occupied bulk state and also induces unoccupied surface states. Furthermore, a lower number of layers is necessary for hydrogenated diamanes to achieve the convergence of the work function in comparison with fluorinated diamanes, with the exception of fluorinated (110) and (2̅110) films that achieve rapid convergence and have the same behavior as other hydrogenated surfaces. This induces a modification of the work function with an increase of the number of layers that makes hydrogenated (2̅110) diamanes the most suitable surface for field-emission displays, better than the fluorinated counterparts. In addition, a quasi-quantitative descriptor of surface dipole moment based on the Tantardini−Oganov electronegativity scale is introduced as the average of bond dipole moments between the surface atoms. This new fundamental descriptor is capable of predicting a priori the bond dipole moment and may be considered as a new useful feature for crystal structure prediction based on artificial intelligence

Translucency and Color Stability of a Simplified Shade Nanohybrid Composite after Ultrasonic Scaling and Air-Powder Polishing

We aimed to assess the influence of professional dental prophylaxis on the translucency and color stability of a novel simplified shade nanohybrid composite material. Sixty composite disks (5 mm in diameter and 2 mm thick) of light (n = 30) and dark (n = 30) shades were prepared. The specimens were randomly divided into the following three groups (n = 10) according to the prophylaxis procedure used: ultrasonic scaling, air-powder polishing with sodium bicarbonate, and controls. The specimens were submitted to translucency and color analysis based on the CIELab system. Two measurements were performed before and after 48-h storage in coffee. Translucency values of untreated light and dark specimens were 9.15 ± 0.38 and 5.28 ± 1.10, respectively. Airpowder polishing decreased the translucency of the light composite specimens. Storage in coffee resulted in color changes (∆E) ranging between 2.69 and 12.05 and a mean translucency decrease ranging between −0.88 and −6.91. The samples in the light group tended to exhibit greater staining; the treatment method had no effect on ∆E. It can be concluded that light-shade composite restorations are more prone to translucency and color changes resulting from air-powder polishing and contact with staining media. However, further research using other composites and powders is required

Sr-Doped Molecular Hydrogen: Synthesis and Properties of SrH22_{22}

Recently, several research groups announced reaching the point of metallization of hydrogen above 400 GPa. Following the mainstream of extensive investigations of compressed polyhydrides, in this work we demonstrate that small (4 atom %) doping of molecular hydrogen by strontium leads to a dramatic reduction in the metallization pressure to about 200 GPa. Studying the high-pressure chemistry of the Sr-H system at 56-180 GPa, we observed the formation of several previously unknown compounds: C2/m-Sr$_3$H$_{13}$, pseudocubic SrH$_6$, SrH$_9$ with cubic F-43m Sr sublattice, and pseudotetragonal P1-SrH$_{22}$, the metal hydride with the highest hydrogen content discovered so far. Unlike Ca and Y, strontium forms molecular semiconducting polyhydrides, whereas calcium and yttrium polyhydrides are high-Tc superconductors with an atomic H sublattice. The latter phase, SrH$_{22}$ or Sr$_{0.04}$H$_{0.96}$, may be considered as a convenient model of the consistent bandgap closure and metallization of hydrogen. Using the impedance measurements in diamond anvil cells at 300-440 K, we estimated the direct bandgap of the Pm-3n-like compound P1-SrH$_6$ to be 0.44-0.51 eV at 150 GPa, and its metallization pressure to be 220 GPa. Together with the machine learning interatomic potentials, the impedance spectroscopy allowed us to estimate the diffusion coefficients of hydrogen D$_H$ = 1.0-2.8 E-10 m$^2$/s in SrH$_6$ and 1.2-2.1 E-9 m$^2$/s in P1-SrH$_{22}$ at 500-600 K.Comment: Supporting information was compressed and reduced by 2 times to 36 figure

Band gap bowing and spectral width of Ga(1−x)InxN alloys for modelling light emitting diodes

Ga(1−)InN alloys, widely employed to produce light-emitting diodes, exhibit a bowing of the band gap as a function of concentration , and a luminescence spectral width which differs from the expected value of 1.8 kT. Through first-principles calculations, based on many-body perturbation theory and density-functional theory with a meta-GGA exchange–correlation functional, we explore jointly these effects, in an exhaustive set of Ga(1−)InN supercells with 16 atoms. We disentangle the bowing due to the average volume change with the one due local atomic configuration and local relaxation. The first one account for about 40% of the bowing, despite that fact that the change of volume with respect to concentration is nearly linear (Vegard’s law). The computed bowing parameter is 1.39 eV. The experimental broadening between 3 kT and 8 kT, not examined theoretically until now, is well accounted by local atomic configuration changes and lifting of the degeneracy of the top of the valence band