Some good new news: The book on zeolites and porous frameworks for which I wrote a chapter on modeling with Bartlomiej Szyja has become open access and can be found here.

# Tag: DFT

Sep 23

## A combined experimental and theoretical investigation of the Al-Melamine reactive milling system: a mechanistic study towards AlN-based ceramics

Authors: | Seyyed Amin Rounaghi, Danny E.P. Vanpoucke, Hossein Eshghi, Sergio Scudino, Elaheh Esmaeili, Steffen Oswald and Jürgen Eckert |

Journal: | J. Alloys Compd. 729, 240-248 (2017) |

doi: | 10.1016/j.jallcom.2017.09.168 |

IF(2017): | 3.779 |

export: | bibtex |

pdf: | <J.Alloys Compd.> |

Graphical Abstract: Evolution of the end products as function of Al and N content during ball-milling synthesis of AlN. |

## Abstract

A versatile ball milling process was employed for the synthesis of hexagonal aluminum nitride (h-AlN) through the reaction of metallic aluminum with melamine. A combined experimental and theoretical study was carried out to evaluate the synthesized products. Milling intermediates and products were fully characterized via various techniques including XRD, FTIR, XPS, Raman and TEM. Moreover, a Boltzmann distribution model was proposed to investigate the effect of milling energy and reactant ratios on the thermodynamic stability and the proportion of different milling products. According to the results, the reaction mechanism and milling products were significantly influenced by the reactant ratio. The optimized condition for AlN synthesis was found to be at Al/M molar ratio of 6, where the final products were consisted of nanostructured AlN with average crystallite size of 11 nm and non-crystalline heterogeneous carbon.

Apr 28

## Mechanochemical synthesis of nanostructured metal nitrides, carbonitrides and carbon nitride: A combined theoretical and experimental study

Authors: | Seyyed Amin Rounaghi, Danny E.P. Vanpoucke, Hossein Eshghi, Sergio Scudino, Elaheh Esmaeili, Steffen Oswald and Jürgen Eckert |

Journal: | Phys. Chem. Chem. Phys. 19, 12414-12424 (2017) |

doi: | 10.1039/C7CP00998D |

IF(2017): | 3.906 |

export: | bibtex |

pdf: | <Phys.Chem.Chem.Phys.> |

## Abstract

Nowadays, the development of highly efficient routes for the low cost synthesis of nitrides is greatly growing. Mechanochemical synthesis is one of those promising techniques which is conventionally employed for the synthesis of nitrides by long term milling of metallic elements under pressurized N_{2} or NH_{3} atmosphere (A. Calka and J. I. Nikolov, Nanostruct. Mater., 1995, 6, 409-412). In the present study, we describe a versatile, room-temperature and low cost mechanochemical process for the synthesis of nanostructured metal nitrides (MNs), carbonitrides (MCNs) and carbon nitride (CNx). Based on this technique, melamine as a solid nitrogen-containing organic compound (SNCOC) is ball milled with four different metal powders (Al, Ti, Cr and V) to produce nanostructured AlN, TiC_{x}N_{1-x}, CrC_{x}N_{1-x}, and VC_{x}N_{1-x} (x~0.05). Both theoretical and experimental techniques are implemented to determine the reaction intermediates, products, by-products and finally, the mechanism underling this synthetic route. According to the results, melamine is polymerized in the presence of metallic elements at intermediate stages of the milling process, leading to the formation of a carbon nitride network. The CN_{x} phase subsequently reacts with the metallic precursors to form MN, MCN or even MCN-CN_{x} nano-composites depending on the defect formation energy and thermodynamic stability of the corresponding metal nitride, carbide and C/N co-doped structures.

Nov 16

## Modern art in research.

Although it looks a bit like a modern piece of art, it is one more attempt at trying to find an optimum combination of parameters.

I’m currently trying to find “the best choice” for U and J for a DFT+U based project… DFT??? Density Functional Theory. This is an approximate method which is used in computational materials science to calculate the quantum mechanical behavior of electrons in matter. Instead of solving the Schrödinger equation, known from any quantum mechanic course, one solves the Hohenberg-Kohn-Sham equations. In these equations it are not the electrons which play a central role (which they do in the Schrödinger equations) but the electron density. Hohenberg, Kohn and Sham were able to show that their equations give the exact same results as the Schrödinger equations. There is, however, one small caveat: you need to have an “exact” exchange-correlation functional (a functional is just a function of a function). Unfortunately there is no known analytic form for this functional, so one needs to use approximated functionals. As you probably guessed, with these approximate functionals the solution of the Hohenberg-Kohn-Sham equations is no longer an exact solution.

For some molecules or solids the error is much larger than average due to the error in the exchange-correlation functional. These systems are therefore called “strongly-correlated” systems. Over the years, several ways have been devised to solve this problem in DFT. One of them is called DFT+U. It entails adding additional coulomb interactions (Hubbard-U-potential) between the “strongly interacting electrons”. However this additional interaction depends on the system at hand, so one always needs to fit this parameter against one of more properties one is interested in. The law of conservation of misery, however, makes sure that improving one property goes hand in hand with a deterioration of another property.

Since actual DFT+U has two independent parameters (U and J, though for many systems they can be dependent reducing to a single parameter) I had quite some fun running calculations for a 21×21 grid of possible pairs. Afterward, collecting the data I wanted to use for fitting purposes took my script about 2h! 😯 Unfortunately the 10 properties of interest I wanted to fit give optimum (U,J)-pair all over the grid. In the picture above, you see my most recent attempt at trying to deal with them. It shows for the entire grid how many of the 10 properties are reasonably well fit.There are two regions which fit 6 properties; One around (U,J)=(5,10) and another around (U,J)=(8.5,17.5). There will be more work before this gives a satisfactory result, the show will go on.

Sep 25

## Mechanochemical route to the synthesis of nanostructured Aluminium nitride

Authors: | S. A. Rounaghi, H. Eshghi, S. Scudino, A. Vyalikh, D. E. P. Vanpoucke, W. Gruner, S. Oswald, A. R. Kiani Rashid, M. Samadi Khoshkhoo, U. Scheler and J. Eckert |

Journal: | Scientific Reports 6, 33375 (2016) |

doi: | 10.1038/srep33375 |

IF(2016): | 4.259 |

export: | bibtex |

pdf: | <Sci.Rep.> (open access) |

## Abstract

Hexagonal Aluminium nitride (h-AlN) is an important wide-bandgap semiconductor material which is conventionally fabricated by high temperature carbothermal reduction of alumina under toxic ammonia atmosphere. Here we report a simple, low cost and potentially scalable mechanochemical procedure for the green synthesis of nanostructured h-AlN from a powder mixture of Aluminium and melamine precursors. A combination of experimental and theoretical techniques has been employed to provide comprehensive mechanistic insights on the reactivity of melamine, solid state metalorganic interactions and the structural transformation of Al to h-AlN under non-equilibrium ball milling conditions. The results reveal that melamine is adsorbed through the amine groups on the Aluminium surface due to the long-range van der Waals forces. The high energy provided by milling leads to the deammoniation of melamine at the initial stages followed by the polymerization and formation of a carbon nitride network, by the decomposition of the amine groups and, finally, by the subsequent diffusion of nitrogen into the Aluminium structure to form h-AlN

Feb 11

##
First-Principles Study of Antisite Defect Configurations in ZnGa_{2}O_{4}:Cr Persistent Phosphors

Authors: | Arthur De Vos, Kurt Lejaeghere, Danny E. P. Vanpoucke, Jonas J. Joos, Philippe F. Smet, and Karen Hemelsoet |

Journal: | Inorg. Chem. 55(5), 2402-2412 (2016) |

doi: | 10.1021/acs.inorgchem.5b02805 |

IF(2016): | 4.857 |

export: | bibtex |

pdf: | <Inorg.Chem> |

Graphical Abstract: First-principles simulations on zinc gallate solid phosphors (ZGO) containing a chromium dopant and antisite defects (left) rationalize the attractive interactions between the various elements. A large number of antisite pair configurations is investigated and compared with isolated antisite defects. Defect energies point out the stability of the antisite defects in ZGO. Local structural distortions are reported, and charge transfer mechanisms are analyzed based on theoretical density of states (right) and Hirshfeld-I charges. |

## Abstract

Zinc gallate doped with chromium is a recently developed near-infrared emitting persistent phosphor, which is now extensively studied for in vivo bioimaging and security applications. The precise mechanism of this persistent luminescence relies on defects, in particular, on antisite defects and antisite pairs. A theoretical model combining the solid host, the dopant, and/or antisite defects is constructed to elucidate the mutual interactions in these complex materials. Energies of formation as well as dopant, and defect energies are calculated through density-functional theory simulations of large periodic supercells. The calculations support the chromium substitution on the slightly distorted octahedrally coordinated gallium site, and additional energy levels are introduced in the band gap of the host. Antisite pairs are found to be energetically favored over isolated antisites due to significant charge compensation as shown by calculated Hirshfeld-I charges. Significant structural distortions are found around all antisite defects. The local Cr surrounding is mainly distorted due to a ZnGa antisite. The stability analysis reveals that the distance between both antisites dominates the overall stability picture of the material containing the Cr dopant and an antisite pair. The findings are further rationalized using calculated densities of states and Hirshfeld-I charges.

Feb 04

## Virtual Winterschool 2016: Computational Solid State Physics & Chemistry

In just an hour, I’ll be presenting my talk at the virtual winterschool 2016. In an attempt to tempt fate as much as possible I will try to give/run real-time examples on our HPC in Gent, however at this moment no nodes are available yet to do so. Let’s keep our fingers crossed and see if it all works out.

### Abstract

Modern materials research has evolved to the point where it is now common practice to manipulate materials at nanometer scale or even at the atomic scale (e.g. Intel’s skylake architecture with 14nm features, atomic layer deposition and surface structure manipulations with an STM-tip). At these scales, quantum mechanical effects become ever more relevant, making their prediction important for the field of materials science.

In this session, we will discuss how advanced quantum mechanical calculations can be performed for solids and indicate some differences with standard quantum chemical approaches. We will touch upon the relevant concepts for performing such calculations (plane-wave basis-sets, pseudo-potentials, periodic boundary conditions,…) and show how the basic calculations are performed with the VASP-code. You will familiarize yourself with the required input files and we will discuss several of the most important output-files and the data they contain.

At the end of this session you should be able to set up a single-point calculation, a structure optimization, a density of states and band structure calculation.

### Additional Files/Info

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