Tag: Materials Science

Holiday-Conference roller coaster

Visit to Stockholm. The knight at the Medeltidsmuseet (top left), brown bear in Skansen (top right), visiting the Royal palace (bottom left) and local entertainment in the old city center (bottom right).

Visit to Stockholm. The knight at the Medeltidsmuseet (top left), brown bear in Skansen (top right), visiting the Royal palace (bottom left) and local entertainment in the old city center (bottom right).

Summertime is a time of rest for most people. For our little academic family, last summer was a bit of a roller coaster; alternating holidays with hard work which had been postponed too much. The last vestige of my start of a new chapter (moving the remaining stuff from the apartment to our house) was finally bested. Now the conference roller coaster has started with Sylvia’s plenary lecture on conceptual spaces in Stockholm.

As neither of us ever visited Sweden before, we decided to turn it into a semi-family-holiday as well. Our 4-year-old son enjoyed his first ever plane flight (he wasn’t really convinced something impressive was going on). And while Sylvia was of to the conference, the two of us went to explore Stockholm: Finding the knight in the Medeltidsmuseet (at the left in the back of this beautiful museum 🙂 ) and searching for the king and queen at their palace (they weren’t there 🙁 ). Or visiting one of the oldest open-air musea; Skansen (similar to Bokrijk in Belgium) where we saw old professions at work (making cheese for example) and native Scandinavian farm and wild animals (from peacocks to brown bears).

Next weekend starts the next episode of the conference roller-coaster with me hosting a 2-day colloquium on porous frameworks together with Bartek Szyja and Ionut Tranca at the CMD-26 conference in Groningen. We have a nicely packed colloquium with about 20 presentations (8 invited and 12 contributed) covering the whole realm of porous materials from zeolites to COFs and MOFs. The program of the colloquium can be downloaded below:Program Porous Frameworks Colloquium

I have a Question: about thermal expansion

“I have a question”(ik heb een vraag). This is the name of a Belgian (Flemisch) website aimed at bringing Flemisch scientists and the general public together through scientific or science related questions. The basic idea is rather simple. Someone has a scientific question and poses it on this website, and a scientist will provide an answer. It is an excellent opportunity for the latter to hone his/her own science communication skills (and do some outreach) and for the former to get an good answer to his/her question.

All questions and answers are collected in a searchable database, which currently contains about fifteen thousand questions answered by a (growing) group of nearly one thousand scientists. This is rather impressive for a region of about 6.5 Million people. I recently joined the group of scientists providing answers.

An interesting materials-related question was posed by Denis (my translation of his question and context):

What is the relation between the density of a material and its thermal expansion?

I was wondering if there exists a relation between the density of a material and the thermal expansion (at the same temperature)? In general, gasses expand more than solids, so can I extend this to the following: Materials with a small density will expand more because the particles are separated more and thus experience a small cohesive force. If this statement is true, then this would imply that a volume of alcohol should expand more than the same volume of air, which I think is puzzling. Can you explain this to me?

Answer (a bit more expanded than the Dutch one):

Unfortunately there exists no simple relation between the density of a material and its thermal expansion coefficient.

Let us first correct something in the example given: the density of alcohol (or ethanol) is 46.07 g/mol (methanol would be 32.04 g/mol) which is significantly more than the density of air which is 28.96 g/mol. So following the suggested assumption, air should expand more. If we look at liquids, it is better to compare ethanol (0.789 g/cm3) to compare water (1 g/cm3) as liquid air (0.87 g/cm3) needs to be cooled below  -196 °C (77K). The thermal expansion coefficients of wtare and ethanol are 207×10-6/°C and 750×10-6/°C, respectively. So in this case, we see that alcohol will expand more than water (at 20°C). Supporting Denis’ statement.

Unfortunately, these are just two simple materials at a very specific temperature for which this statement is true. In reality, there are many interesting aspects complicating life. A few things to keep in mind are:

  • A gas (in contrast to a liquid or solid) has no own boundary. So if you do not put it in any type of a container, then it will just keep expanding. The change in volume observed when a gas is heated is due to an increase in pressure (the higher kinetic energy of the gas molecules makes them bounce harder of the walls of your container, which can make a piston move or a balloon grow). In a liquid or a solid on the other hand, the expansion is rather a stretching of the material itself.
  • Furthermore, the density does not play a role at all, in case of the expansion of an ideal gas, since p*V=n*R*T. From this it follows that 1 mole of H2 gas, at 20°C and a pressure of 1 atmosphere, has the exact same volume as 1 mole of O2 gas, at 20°C and a pressure of 1 atmosphere, even though the latter has a density which is 16 times higher.
  • There are quite a lot of materials which show a negative thermal expansion in a certain temperature region (i.e. they shrink when you increase the temperature). One well-known example is water. The density of liquid water at 0 °C is lower than that of water at 4 °C. This is the reason why there remains some liquid water at the bottom of a pond when it is frozen over.
  • There are also materials which show “breathing” behavior (this are reversible volume changes in solids which made the originators of the term think of human breathing: inhaling expands our lungs and chest, while exhaling contracts it again.) One specific class of these materials are breathing Metal-Organic Frameworks (MOFs). Some of these look like wine-racks (see figure here) which can open and close due to temperature variations. These volume variations can be 50% or more! 😯

The way a material expands due to temperature variations is a rather complex combination of different aspects. It depends on how thermal vibrations (or phonons) propagate through the material, but also on the possible presence of phase-transitions. In some materials there are even phase-transitions between solid phases with a different crystal structure. These, just like solid/liquid phase transitions can lead to very sudden jumps in volume during heating or cooling. These different crystal phases can also have very different physical properties. During the middle-ages, tin pest was a large source of worries for organ-builders. At a temperature below 13°C β-tin is more stable α-tin, which is what was used in organ pipes. However, the high activation energy prevents the phase-transformation from α-tin to β-tin to happen too readily. At temperatures of -30 °C and lower this barrier is more easily overcome.This phase-transition gives rise to a volume reduction of 27%. In addition, β-tin is also a brittle material, which easily disintegrates. During the middle ages this lead to the rapid deterioration and collapse of organ-pipes in church organs during strong winters. It is also said to have caused the buttons of the clothing of Napoleon’s troops to disintegrate during his Russian campaign. As a result, the troops’ clothing fell apart during the cold Russian winter, letting many of them freeze to death.



Call for Abstracts: Condensed Matter Science in Porous Frameworks: On Zeolites, Metal- and Covalent-Organic Frameworks

Flyer for the Colloquium on Porous Frameworks at the CMD26Together with Ionut Tranca (TU Eindhoven, The Netherlands) and Bartłomiej Szyja (Wrocław University of Technology, Poland) I am organizing a colloquium “Condensed Matter Science in Porous Frameworks: On Zeolites, Metal- and Covalent-Organic Frameworks” which will take place during the 26th biannual Conference & Exhibition CMD26 – Condensed Matter in Groningen (September 4th – 9th, 2016). During our colloquium, we hope to bring together experimental and theoretical researchers working in the field of porous frameworks, providing them the opportunity to present and discuss their latest work and discoveries.

Zeolites, Metal-Organic Frameworks, and Covalent-Organic Frameworks are an interesting class of hybrid materials. They are situated at the boundary of research fields, with properties akin to both molecules and solids. In addition, their porosity puts them at the boundary between surfaces and bulk materials, while their modular nature provides a wealthy playground for materials design.

We invite you to submit your abstract for oral or poster contributions to our colloquium. Poster contributions participate in a Best Poster Prize competition.

The deadline for abstract submission is April 30th, 2016.

The extended deadline for abstract submission is May 14th, 2016.


CMD26 – Condensed Matter in Groningen is an international conference, organized by the Condensed Matter Division of the European Physical Society, covering all aspects of condensed matter physics, including soft condensed matter, biophysics, materials science, quantum physics and quantum simulators, low temperature physics, quantum fluids, strongly correlated materials, semiconductor physics, magnetism, surface and interface physics, electronic, optical and structural properties of materials. The scientific programme will consist of a series of plenary and semi-plenary talks and Mini-colloquia. Within each Mini-colloquium, there will be invited lectures, oral contributions and posters.


Feel free to distribute this call for abstracts and our flyer and we hope to see you in Groningen!


SBDD XXI logoToday was the first day of the three-day long diamond conference at the university of Hasselt. And although this sounds as-if it is a mere small-scale local conference, it is actually one of the two main international conferences in the field. The Surface and Bulk Defects in Diamond (SBDD) workshop grew in twenty years from a small event with only a few dozen participants to the current event with over 200 participants. As such, it is the place to be, for one as me, who is dipping into a new field of materials.

One thing that already became quite clear today, is the fact that there are many opportunities in this field for the computational materials scientist, as the large majority of the researchers are experimentalists. Of the >120 posters presented, I have only discovered about 5 theoretical ones. Having had very nice chats with their presenters I already learned a lot of what I will have to keep in mind when studying diamond. But so far, I have not come across any issues that are impossible to resolve, which is good news :-).

Helium flash: the beginning of a new chapter.

During the past two and a half years, part of being a delocalized physicist has meant for me that I had to work at one end of the country while my girlfriend and son lived at the other. Today this situation drastically changed, as I moved with my FWO-postdoctoral project from my alma mater to the University of Hasselt, where I started in the Wide Band Gap Materials group of Prof. Ken Haenen.

My delocalization will now take the form of Metal-Organic Frameworks on the one side and Diamond based materials on the other. As the sole computational solid state physicist in an otherwise entirely experimental group (and even institute) I seem to have returned to a well known configuration (At Ghent university I was initially the house-theoretician of the SCRiPTS group). Also the idea of performing calculations on diamond brings back memories, since this allotrope of carbon lives two levels above the germanium on which Pt nanowires grow. All-in-all I look forward to an exciting time. But first things first: getting my HPC credentials and data safely transported from the one end of the country to the other.

First-Principles Study of Antisite Defect Configurations in ZnGa2O4: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: (left) Ball-and-stick model of zinc gallate (right) density of states of Cr doped zinc gallate.
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.


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.

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.


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

Computational Materials Science: Where Theory Meets Experiments

Authors: Danny E. P. Vanpoucke,
Journal: Developments in Strategic Ceramic Materials:
Ceramic Engineering and Science Proceedings 36(8), 323-334 (2016)
(ICACC 2015 conference proceeding)
Editors: Waltraud M. Kriven, Jingyang Wang, Dongming Zhu,Thomas Fischer, Soshu Kirihara
ISBN: 978-1-119-21173-0
webpage: Wiley-VCH
export: bibtex
pdf: <preprint> 


In contemporary materials research, we are able to create and manipulate materials at ever smaller scales: the growth of wires with nanoscale dimensions and the deposition of layers with a thickness of only a few atoms are just two examples that have become common practice. At this small scale, quantum mechanical effects become important, and this is where computational materials research comes into play. Using clever approximations, it is possible to simulate systems with a scale relevant for experiments. The resulting theoretical models provide fundamental insights in the underlying physics and chemistry, essential for advancing modern materials research. As a result, the use of computational experiments is rapidly becoming an important tool in materials research both for predictive modeling of new materials and for gaining fundamental insights in the behavior of existing materials. Computer and lab experiments have complementary limitations and strengths; only by combining them can the deepest fundamental secrets of a material be revealed.

In this paper, we discuss the application of computational materials science for nanowires on semiconductor surfaces, ceramic materials and flexible metal-organic frameworks, and how direct comparison can advance insight in the structure and properties of these materials.

Doping of CeO2 as a Tunable Buffer Layer for Coated Superconductors: A DFT Study of Mechanical and Electronic Properties

Authors: Danny E. P. Vanpoucke,
Journal: Developments in Strategic Ceramic Materials:
Ceramic Engineering and Science Proceedings 36(8), 169-177 (2016)
(ICACC 2015 conference proceeding)
Editors: Waltraud M. Kriven, Jingyang Wang, Dongming Zhu,Thomas Fischer, Soshu Kirihara
ISBN: 978-1-119-21173-0
webpage: Wiley-VCH
export: bibtex
pdf: <preprint> 


In layered ceramic superconductor architectures, CeO2 buffer layers are known to form micro cracks during the fabrication process. To prevent this crack formation, doping of the CeO2 layer has been suggested. In this theoretical study, the influence of dopants (both tetravalent and aliovalent) on the mechanical and structural properties of CeO2 is investigated by means of density functional theory. Group IVa and IVb dopants show clearly distinct stability, with the former favouring interface and surface doping, while the latter favour uniform bulk doping. This behaviour is linked to the dopant electronic structure. The presence of charge compensating vacancies is shown to complicate the mechanical and structural picture for aliovalent dopants. We find that the vacancies often counteract the dopant modifications of the host material. In contrast, all dopants show an inverse relation between the bulk modulus and thermal expansion coefficient, independent of their valency and the presence of oxygen vacancies. Based on the study of these idealized systems, new dopants are suggested for applications.

A Flexible Photoactive Titanium Metal-Organic Framework Based on a [TiIV33-O)(O)2(COO)6] Cluster

Authors: Bart Bueken, Frederik Vermoortele, Danny E. P. Vanpoucke, Helge Reinsch, Chih-Chin Tsou, Pieterjan Valvekens, Trees De Baerdemaeker, Rob Ameloot, Christine E. A. Kirschhock, Veronique Van Speybroeck, James M. Mayer and Dirk De Vos
Journal: Angew. Chem. Int. Ed. 54(47), 13912-13917 (2015)
doi: 10.1002/anie.201505512
IF(2015): 11.705
export: bibtex
pdf: <Angew.Chem.Int.Ed.> 


The synthesis of titanium-carboxylate metal-organic frameworks (MOFs) is hampered by the high reactivity of the commonly employed alkoxide precursors. Here, we present an innovative approach to Ti-based MOFs using titanocene dichloride to synthesize COK-69, the first breathing Ti-MOF built up of trans-1,4- cyclohexanedicarboxylate linkers and an unprecedented [TiIV33-O)(O)2(COO)6] cluster. The photoactive properties of COK-69 were investigated in-depth by proton-coupled electron transfer experiments, which revealed that up to one TiIV per cluster can be photoreduced to TiIII, while preserving the structural integrity of the framework. From molecular modeling, the electronic structure of COK-69 was determined and a band gap of 3.77 eV was found.