HEA workshop, dedicated to the 65th anniversary of Prof. Janez Dolinšek
13:00
Ante Bilušić, Julian Ledieu, Magdalena Wencka: opening
13:15
Sheng Guo Eutectic High-Entropy Alloys & Refractory High-Entropy Alloys: Opportunities and Challenges
The alloying concept of high entropy alloys (HEAs) opens a vast unexplored compositional space potentially leading to numerous new materials and new applications, but also brings new challenges on how to design these alloys properly. In this lecture, I will show opportunities and challenges that are brought by two particular examples of HEAs, refractory HEAs and eutectic HEAs. Refractory HEAs are promising candidates for ultra-high temperature applications, but how to satisfy demanding material requirements to simultaneously obtain decent room-temperature ductility, high-temperature strength and oxidation resistance pose a formidable scientific challenge. Eutectic HEAs, recently becoming prototype dual-phase materials, show a wide spectrum of mechanical properties when they are subject to thermomechanical treatments, while their good castability and decent mechanical properties and corrosion resistance render great application potential where directly cast materials are in need.
Session 1: Structure and surfaces
Chair person: Sheng Guo
14:00
Anton Meden Phase Identification and Quantification in High Entropy Alloys Using X-Ray Powder Diffraction
Ideally, high entropy alloys (HEA) are single phase with randomly mixed atoms of the constituent elements at crystallographic sites of a (simple) crystal structure. In such cases, X-ray powder diffraction (XRPD) is an efficient tool to: 1. prove the phase purity, 2. determine the structure type, 3. determine the exact parameters of the unit cell, 4. estimate the size and strain of the coherently scattering domains.
Often HEAs are (complex) multiphase mixtures, and in such cases the XRPD is an essential tool for phase identification and quantification. For the identified phases, points 2 and 3 above also apply, point 4 is only possible for predominant phases.
The reliability of the results is greatly increased when XRPD methods are used together with scanning electron microscopy techniques (SEM) (BSE imaging and EDS grain analysis).
The above is illustrated with examples (as in Fig. 1).
Fig. 1: Rietveld plot of ScHfNbTaZr showing the contributions of the three phases to the measured diffraction pattern and their weight fractions.
14:30
Marc Armbrűster HEAs in Catalysis - An Overview
High-entropy alloys are an interesting new class of materials and their physical and chemical properties are momentarily explored. For heterogeneous catalysis the randomly occupied crystallographic sites are principally offering a nearly endless variety of potentially active sites. This opens the possibility that there are active sites available for every heterogeneously catalysed reaction resulting in “general“ catalysts.
This resulted in tests of HEAs in a number of catalytic reactions. Publications in the field will be presented and critically analysed concerning the phases present and evaluation of the catalytic properties. In addition, a brief outlook on the potential of HEAs in heterogeneous catalysis will be presented.
15:00
Andreja Jelen Multi-phase structure of AlCoFeNiCux (x = 0.6 - 3.0) HEAs
Tailoring of new materials is a playground for many scientists. Searching for a material that possesses a combination of excellent magnetic softness and vanishing magnetostriction has led us to investigate the ferromagnetic high-entropy alloy (HEA) system AlCoFeNiCux (x = 0.6 – 3.0). The alloys develop multi-phase (up to three phases) composite microstructure that is further nanostructured on the 10-nm scale. One of them (x = 3) is presented at the micro-level in Fig.1 by SEM EDS elemental maps.
Fig. 1: The SEM EDS elemental maps of the x = 3.0 alloy, which is according to XRD a two-phase fcc(L)-fcc(S) composite.
15:20
Coffee break (birthday cake & more…)
Session 2: Physical properties
Chair person: Peter Gille
16:20
Jože Luzar AlCoFeNiCux (x = 0.6 – 3.0) zero-magnetostriction magnetically soft high-entropy alloys
This contribution [1] will present our efforts in further advancing soft ferromagnetic high-entropy alloys towards practical magnetic applications via a study of the AlCoFeNiCux (x = 0.6 – 3.0) system. AlCoFeNiCu2.0 proved to have superior magnetostriction and magnetic softness properties. It shows precisely zero magnetostriction, λs = 0, reasonably low coercivity Hc ≈ 650 Am–1 and substantial saturation magnetic polarization of Js ≈ 0.55 T. Near zero magnetostriction was also observed for x = 2.5 and 3.0, so that AlCoFeNiCux HEAs for x = 2.0 – 3.0 are all relevant for applications as supersilent (inaudible to a human ear) materials. In the alloys, three phases develop on the microscale and are further nanostructured on the 10-nm scale. It is considered that the ideal zero magnetostriction in the AlCoFeNiCu2.0 alloy is a consequence of the three-phase microstructure, in which the magnetostrictions of different signs in the three phases exactly compensate each other. The magnetic softness of the HEAs is attributed to the mechanism of exchange-averaging of magnetic anisotropy.
Reference:
[1] J. Luzar, P. Priputen, S. Vrtnik, et al., Adv. Mater. Interfaces. Available online prior to inclusion in an issue: https://doi.org/10.1002/admi.202201535
16:40
Primož Koželj Al0.5TiZrPdCuNi in HEA vs metallic-glass form: How important is the crystallinity of HEAs for electronic transport?
High-entropy alloys can be viewed as materials that are half way between ordered crystals and amorphous substances – they exhibit a topologically ordered lattice but on the other hand also an immense amorphous-type chemical disorder. This contribution will present our efforts [1] at determining the importance of the crystal lattice vs the importance of the disorder on the electronic transport in HEAs through a comparative study of a 6 component Al0.5TiZrPdCuNi synthesized in either high-entropy alloy (HEA) or metallic glass (MG) form.
Both forms exhibit a large, negative-temperature-coefficient resistivity, positive thermopower, positive Hall coefficient and small thermal conductivity. A spectral conductivity model within the Kubo-Greenwood formalism allowed us to analyze both modifications on equal footing and reproduce their electronic transport data. The contribution of the phonons to the transport coefficients in both forms is small, so the temperature dependence originates from the electronic properties as described by the spectral conductivity model.
The similarities between the HEA and MG forms of Al0.5TiZrPdCuNi indicate that the immense chemical disorder and not the crystal lattice is the main driving force behind electronic transport in HEAs.
Round table discussion and closing
Chair person: Magdalena Wencka
17:00
Trends in development of high-entropy alloys
17:20
Janez Dolinšek, Julian Ledieu: closing
Tuesday, November 22, 2022
time
lecturer, title and abstract
8:50
opening
Chair person: Ana Smontara, Zagreb
9:00
Emil Babić, Zagreb Compositionally complex alloys: some problems and prospects
Despite a huge expansion of research on compositionally complex alloys, CCA (such as the entropy alloys, HEA and the corresponding glassy alloys) their comprehension is still limited, which is detrimental both for the design of CCAs and for their application. The broad compositional range in CCAs enables simple tuning of their properties by varying the contents of their constituent elements. Accordingly, the study of transition from CCA to conventional alloy, CA composed from the same constituents is important, both for understanding the formation of solid solutions in CCAs and for proper evaluation of their potential with respect to that of CAs. However, this transition has so far been studied systematically in only two types of alloys: isopleths of the Cantor alloy and glassy TiZrNbCuNi/Co alloys with variable Co, Ni or Cu content (Materials 14 (2021) 5824 and ref. therein). In both alloy systems the variation of a given property with composition depends sharply on the selected principal component and can be either monotonic (like that in the ideal solid solution) or non-monotonic. When the variation is non-monotonic, the maximum value of the selected property is usually outside of HEA composition range.
9:30
Farid Labib, Tokyo Emergence of long-range magnetic order from spin-glass state by tuning electron density in Ga-based 1/1 quasicrystal approximant
This study reports the first observation of ferromagnetic (FM) order in the non-Au-based approximant crystals (ACs) using a novel approach whereby a total electron-per-atom (e/a) ratio of the spin-glass Ga50Pd36Gd14 1/1 AC is lowered by simultaneously substituting certain ratios of a tri-valent Ga and a zero-valent Pd by a mono-valent Au. The emergence of FM order by this method was confirmed via magnetic susceptibility, magnetization, and specific heat measurements. The findings of this study open up vast opportunities in developing more long-range magnetic orders from ACs, quasicrystals, and even other RKKY compounds with spin-glass behavior.
9:50
Mario Novak, Zagreb Nodal-line driven anomalous susceptibility in ZrSiS
We shall present a unique approach to test the signature of the nodal-line physics by thermodynamic methods. By measuring magnetic susceptibility in ZrSiS we found an intriguing temperature-driven crossover from dia- to paramagnetic behavior. We show that the anomalous behavior represents a real thermodynamic signature of the underlying nodal-line physics through the means o chemical pressure (isovalent substitution of Zr for Hf), quantum oscillations, and theoretical model ng. The anomalous part of the susceptibility is orbital by nature, and it arises due to the vicinity of the Fermi level to a degeneracy point created by the crossing of two nodal lines.
Furthermore, an unexpected Lifshitz topological transition at the degeneracy point is revealed by tuning the Ferm level. The present findings in ZrSiS give a new and attractive starting point for various nodal-lin physics-related phenomena to be tested by thermodynamic methods in other related materials.
10:10
Neveen Singh Dhami, Zagreb Pressure evolution of electronic and crystal structures of EuTGe3 (T = Co, Rh, Ir)
Intermetallic EuTX3 (T: transition metal, X= Si/Ge) forms a non-centrosymmetric BaNiSn3-type structure and has been attracting considerable attention due to complex magnetic structures and unique pressure phase diagram that potentially accommodates superconductivity under pressure [1, 2]. At ambient pressure, EuTGe3 series host divalent Eu ions (4f7, J=7/2) and exhibit antiferromagnetic ordering at low temperature (< 15 K), while the ordering direction depends on the selection of T [1]. We studied the pressure evolution of the electronic and crystal structures of EuTGe3 (T= Co, Rh, Ir) by high-resolution x-ray absorption spectroscopy and powder x-ray diffraction. Our study unveiled that the pressure gradient of the Eu valence change varies depending on T. By applying pressure, a continuous contraction of the lattice volume was observed in both EuCoGe3 and EuRhGe3. EuIrGe3 implied possible structural transition above 34 GPa.
This work is in part supported by the scientific project "Pressure- and Temperature-driven Phase transitions in Strongly Correlated Electron Systems" (PaT PiSCES, HrZZ UIP-2019-04-2154).
Holger Schwarz, Chemnitz Growth of crystalline CoCrFeNi high-entropy alloy thin films by magnetron sputtering
Multicomponent alloys of at least four elements with near equimolar percentage were first reported and investigated by Cantor et al.[1] and Yeh et al.[2] in 2004 and are nowadays often referred to as High-Entropy Alloys (HEAs). This composition is expected to support the formation of single-phase solid solutions, which lead to extensive research on the mechanical properties of HEAs in the past decades, but surface physics are barely investigated so far. We demonstrate the formation of crystalline thin films of CoCrFeNi via magnetron sputtering from homemade targets on MgO (100) and Al2O3 (0001) single crystal substrates. Following a proper UHV surface treatment, the structural and electronic behaviour is accessible by means of low energy electron diffraction and angle resolved photoelectron spectroscopy.
Priyanka Reddy, Zagreb Murunskite: a bridge between cuprates and pnictides
Despite exceptional scientific efforts over several decades, there is almost no universal agreement about the superconducting state of cuprate compounds. A constructive way to improve understanding would be to synthesize and investigate a new system, which displays superior crystal chemical flexibility and tunability of the valence of the transition metal ions. One could then manipulate its various electronic, metallic, and mechanical properties. Here we study Murunskite, which interpolates between cuprates and pnictides[1]. In this presentation, I will present the successful growth and characterization of the first-ever high-quality Murunskite single crystals. These crystals show semiconducting behaviour in resistivity and optical transmittance, and antiferromagnetic ordering around 100 K. Spectroscopy (XPS) and Density Functional Theory (DFT) calculations concur that the sulfur 3p orbitals are partially open, making them accessible for charge manipulation, which is a prerequisite for superconductivity in analogous layered structures. Furthermore, DFT indicates that the valence band is more cuprate-like, while the conduction band is more pnictide-like. We also managed to electron-dope the parent compound, whose transport and optical conductivity measurements will be discussed in the presentation.
Josipa Šćurla, Split High throughput in-vivo toxicity test of organo-metallic photovoltaic perovskites via micro fluidics on the C. elegans
The organo-metallic photovoltaic (PV) perovskite (CH3NH3PbI3) has reached more than 25% conversion efficiency (in single junction configuration [1]), which suggests its imminent commercialization. Before public usage its health hazard needs to be adressed because of the lead in the formula. We have examined in-vivo toxicity of two types of PV perovskites; CH3NH3PbI3 and its possibly less toxic sister compound CH3NH3SnI3. Assessment was conducted on common model for soil toxicity, nematode C. Elegans. Novel high-troughput microfluidic platfom was used, which frees toxicity experiments from tedious manual work and allows automated systematic approach. Main intoxication path and effect of perovskites on worms development and progeny was addressed, which is very important for design of health restrictions for this type of solar cells.
Željana Bonačić Lošić, Split Collective plasmon modes of Dirac electrons in composite systems
We study the coupling between plasmon modes due to the long-range Coulomb electron–electron interaction for a composite system consisting of a two-dimensional system of Dirac electrons placed in three-dimensional Dirac semimetal [1]. The mixing of the collective plasmon modes is obtained as the two-dimensional acoustic plasmon mode dispersion crosses the bulk plasmon mode. We show that the coupled plasmon modes become more separated with decreasing spacing between the two subsystems as the coupling increases due to the enhanced Coulomb interaction between electrons from different subsystems. We also investigate the effects of the Dirac cone tilts and anisotropies on coupled plasmon modes in both subsystems and find that they increase the coupled plasmon mode energies and renormalize their spectral weights [2]. The anisotropic effects of tilts are more significant for the larger separations of the subsystems when the Coulomb coupling is weaker.
Marc de Boissieu, Grenoble Thermal conductivity and lattice dynamics of aperiodic crystals
The lattice thermal conductivity of many aperiodic crystals displays a ‘glass like behavior’ [1], with a relatively small value of the lattice thermal conductivity at ambient temperature and an almost independent temperature dependence in the range 20 to 300 K. The Umklapp peak observed in simple structure is largely suppressed. This is true for instance for the icosahedral quasicrystal i-AlPdMn [2], but also for the Rb2ZnCl4 phase that displays an incommensurately modulated phase between 190 and 300 K [3]. The detailed understanding of this behavior is still a matter of debate.
In this presentation we propose to apply a simple modified Debye model of a phonon lattice gaz, as developed for Ge based clathrates [4]. It is based on two assumptions: (i) the acoustic regime for which heat is carried by phonon is limited and characterized by a cut-off energy; (ii) higher energy excitations do not carry heat since they are almost dispersion less (see also [5]). Using this simple model it is possible to calculate an average phonon lifetime which will be compared to phonon measurements in different aperiodic crystals. Implication and contribution of phason modes will be also discussed [6].
Silke Bühler Paschen, Vienna Electronic topology driven by strong correlations
Complexity has many faces, one of them being strongly correlated ternary intermetallic compounds that are further enriched by nontrivial electronic topology. I will provide an overview of the recent activities of the Quantum Materials group at TU Wien in this emerging field. They range from investigations of the first Weyl-Kondo semimetal Ce3Bi4Pd3 [1] and its relatives, to the exploration of topological phases and topological phase transitions by tuning [2], and the search for new topological phases by exploiting crystal symmetries [3] and quantum fluctuations [4].
The work in Vienna was supported by the Austrian Science Fund (FWF projects I4047, P29279, and I5868-FOR 5249-QUAST), the European Microkelvin Platform (H2020 project 824109), and the ERC (Adv. Grant 101055088—CorMeTop).
15:00
Gaku Eguchi, Vienna Specific heat of topological semimetals and insulators across the correlation spectrum
The past decade has seen a wealth of investigations on materials that are considered as topological semimetals and insulators. In the by far best studied regime of noninteracting or weakly interacting topological materials, key characterization tools are ARPES and quantum oscillation experiments together with density functional theory. In the recently evidenced strong correlation regime, by contrast, these tools fail and new ones have been put forward, with specific heat measurements being one of them [1-4].
In this presentation we report a systematic study of low-temperature specific heat in topological materials across the correlation spectrum. We will discuss which information specific heat can provide in the weakly interacting regime. The ground states of these materials are discussed, taking also recent studies of their charge transport properties, ARPES, and density functional theory calculations into account.
Shovan Dan, Wrocław Electrical transport in half-Heusler compound TmPdSb
In recent years, rare-earth-based half-Heusler (HH) pnictides have gained tremendous attention due to a range of non-trivial physical properties that have opened the door to various applications, such as in spintronics, green energy harvesting or quantum computing. Although many HH phases have been investigated in depth, some are still poorly characterized.
In this work, we studied the structural, magnetic, and electrical transport properties of TmPdSb single crystals, synthesized from Bi flux. The compound was found to have a MgAgAs-type structure, typical for HH materials, and to exhibit a Curie-Weiss paramagnetism down to 2 K due to the magnetic moments carried on Tm3+ ions. Its electrical conductivity, measured along the [001] direction, shows a semiconducting character at high temperatures and a metallic behavior below about 50 K. This finding is consistent with the variation in carrier (electron) concentration, derived from the Hall effect data. Below 10 K, the magnetic field-dependent conductivity in TmPdSb is governed by a weak antilocalization effect.
This work was supported by the National Science Centre (Poland) under research grant 2021/40/Q/ST5/00066.
15:40
Karan Singh, Wrocław Anomalous electrical transport in EuZn2Sb2
Manipulation of charge transport by spin degrees of freedom has attracted a great interest in topological materials because of their exotic fundamental physics and potential applications. In this context, the compounds EuCd2As2 and EuZn2As2 gained special attention [1,2].
In this work, we studied the structural, thermodynamic and electrical transport properties of high-quality single crystals of EuZn2Sb2. Alike the arsenides, the compound has a trigonal CaAl2Si2-type crystal structure, and orders antiferromagnetically (AFM) at low temperatures due to Eu2+ ions. The AFM structure is A type with the magnetic moments confined in the hexagonal ab plane. In external in-plane magnetic field, a spin flop transition occurs. Most interestingly, in the canted AFM state, the transverse magnetoresistance shows a distinct hump and the Hall resistivity becomes highly non-linear. These unusual transport features likely arise due to the non-collinear spin structure with finite vector spin chirality.
This work was supported by the National Science Centre (Poland) under research grant 2021/41/B/ST3/01141.
Petar Popčević, Zagreb Complexities of 2H-NbS2 intercalations
2H-NbS2 is superconducting, quasi 2D system, which can host different ions and molecules in van der Waals gaps between metallic layers. When intercalated with transition metal (TM) ions system becomes magnetic. The coexistence of metallic and magnetic degrees of freedom coupled with reduced dimensionality and anticipated frustration renders these compounds battlefield of different interactions resulting in different magnetically ordered ground states.
We have studied Ni and Co [1,2,3] intercalations. The degree of crystal order plays a vital role in obtaining correct conclusions. Thus, we started with synthesis and detailed characterization and managed to correlate physical properties with structural complexities. Finally, using angle-resolved photoelectron spectroscopy (ARPES), we identified limitations of the DFT calculations casting new light on these compounds.
This work is in part supported by the scientific project "Intercalated transition metal dichalcogenides" (HrZZ IP-2020-02-9666)
Mitja Krnel, Dresden Superconductivity in crystallographically disordered LaHg6.4
Although typically, the lack of translational symmetry prohibits the appearance of superconductivity, the coexistence of the latter with structural disorder has been observed in some materials [1-3]. However the influence of structural disorder on superconductivity is not yet fully understood. Hg-based materials often have complex crystallographic arrangements making them candidates for interesting physical properties. A detailed examination of crystallographic and physical properties of LaHg6.4 [4] reveals that this material has a transition to a superconducting state at Tc = 2.4 K while showing crystallographic disorder in one dimension. We were able to determine in detail the structure type of Lanthanum mercuride (space group Cmcm, a = 9.779(2) Å, b = 28.891(4) Å, c = 5.0012(8) Å, Z = 8), which has remained out of reach for nearly 50 years. In this crystal structure, strong disorder is present in the channels that propagate along the [001] direction. By using a combination of specific synthesis and characterization techniques, we were able to avoid the problems associated with the low formation temperature and chemical reactivity of this substance, thus making it possible to study the physical properties of LaHg6.4.
Diana Kirschbaum, Vienna Effects of hydrostatic pressure on the Weyl-Kondo semimetal candidate CeRu4Sn6
The interplay of nontrivial electronic topology and strong correlations can lead to the realization of entirely new quantum phases, and thus is of great current interest. A prime example is the newly discovered Weyl-Kondo semimetal phase realized in Ce3Bi4Pd3 [1-3]. There, a cubic-in-temperature contribution to the electronic specific heat [1] and a giant spontaneous Hall response [3] are attributed to the presence of Weyl nodes in the immediate vicinity of the Fermi energy. Another candidate material theoretically predicted to host Weyl points near EF is the tetragonal, noncentrosymmetric Kondo semimetal CeRu4Sn6 [4]. The recent discovery that CeRu4Sn6 is quantum critical without tuning [5], makes it a unique platform to study the possible role of quantum criticality in the formation mechanism of Weyl-Kondo semimetals. Here we probe the phase diagram of CeRu4Sn6 by means of electrical transport and specific heat measurements under hydrostatic pressure and discuss evidence for Weyl-Kondo physics emerging at finite pressure.
Yuri Grin, Dresden Chemical bonding and structural complexity
A common strategy to describe CMA from a geometric point of view is the grouping of atoms into local atomic arrange-ments (crystallographic clusters), which makes the crystal structure easier to visualize, to perceive and understand the geometric organisation; it reduces the degree of complexity and allows to relate the CMA to simpler crystal structures. This is the nested-polyhedra units approach, which was introduced first for the description of the γ–brass-derived structures. The choice of a crystallographic cluster is considered as reasonable, if its shells are as spherical as possible and are quite separate with respect to the distances between the shells and with respect to the centre of the nested polyhedrons. On the other hand, the so defined crystallographic clusters are not in agreement with the chemical definition of such an entity, as the chemical definition entails atomic interactions within the chemical clusters that are stronger than the interaction of the clusters with neighbouring aggregates, would be preferable, thus raising the question about the stabilization mechanisms for these structural. Quantum chemical investigations of chemical bonding in CMA are naturally hindered by the huge size of the unit cells and inherent disorder. In particular, the role of the charge transfer in the stabilization was studied on example of the CMA Mg29-xPt4+y [1] in comparison with Be21Pt5 [2], and their chemically analogous but non-complex Be5Pt [3].
Marc Armbrüster, Chemnitz Catalytic and material complexity – In-Pt/In2O3 in methanol steam reforming
Changing our energy supply towards a renewable and sustainable energy scenario requires storage and transport of large amounts of renewable energy. Due to their high chemical storage capacity for hydrogen, small molecules like CH4, NH3 or methanol are promising candidates. Methanol is liquid at normal conditions and possesses a high volumetric and gravimetric energy density.[1] To release the hydrogen on demand, methanol steam reforming can be applied (CH3OH + H2O › 3 H2 + CO2). Since the identification of ZnPd/ZnO as highly selective catalyst[2], it could be shown that high selectivity is achieved only if ZnPd and ZnO are present[3]. While the necessity could be shown, it is still unclear how the phases contribute to the catalytic cycle.
Recent material development led to the discovery of the very active and selective aerogel-based material In-Pt/ In2O3[4]. To clarify active phases, the oxide-metal interplay and the role of the oxidic species in the catalysis numerous in situ and operando methods as well as isotope-labelling of the reactants was conducted. This resulted in the detection of a Mars-van-Krevelen mechanism, the involvement of oxygen vacancies in the catalytic cycle as well as a delicate interplay between the oxygen potential of the atmosphere and the phases present in the catalyst. In addition, the underlying reason for deactivation of the materials could be revealed, opening the possibility for further development of materials, thus contributing to the energy turnaround.
Oytun Tiryaki, Chemnitz Size-dependent methanol steam reforming investigated on unsupported ZnPd nanoparticles
ZnPd-based catalysts for methanol steam reforming (MSR) have superior stability without being pyrophoric and possess comparable CO2 selectivity (about 99%) to commercial Cu-based catalysts[1-3]. As of now, supported ZnPd/ZnO systems have addressed the impact of ZnPd-particle size on the catalytic characteristics in MSR. However, no conclusive evidence was obtained regarding how the particle size affects the catalytic properties of ZnPd/ZnO in MSR[2,4,5]. To simplify the materials, unsupported ZnPd with different crystallite sizes have been successfully synthesized via electroless plating without the use of surfactants. These materials enable to address the size influence on the catalytic properties in methanol steam reforming without the influence of the supporting ZnO. Characterization after catalysis revealed that the smaller starting particles showed more severe sintering which turned the sample into the larger sized particles, which might be the reason for the inconclusive results described in literature.
Fatma Aras, Dresden Chemical behaviour of Mo2TMB2 (TM: Fe, Co, Ni) upon oxygen evolution reaction (OER)
Oxygen evolution reaction (OER) is the slowest step of water electrolysis, demanding an active electrocatalyst, that should be also stable under strong oxidative conditions of OER. With the help of (partially) ordered crystal structures and well-defined electronic states of the elements in these compounds, intermetallic compounds possess an interesting class of materials for electrocatalysis. [1] The binary compounds in Mo-Ni and Mo-B systems were extensively studied as electrocatalysts for the hydrogen evolution reaction (HER). [2-4] In present study, intermetallic compounds Mo2TMB2 (TM: Fe, Co, Ni) were extensively investigated under conditions of OER in alkaline media.
While Mo2CoB2 and Mo2NiB2 crystallizes with W2CoB2 structure type (Immm, a = 7.087(2) Å, b = 4.584(3) Å, c= 3.164(5) Å for Mo2CoB2 and a = 7.075(5) Å, b = 4.557(5) Å, c = 3.179(5) Å for Mo2NiB2), Mo2FeB2 possesses U3Si2-type of structure (P4/mbm, a = 5.807(4) Å, c = 3.142(3) Å). These compounds were synthesized via arc melting of initial materials, followed by homogenization annealing and manufacturing of the electrodes using spark plasma sintering (SPS). Chemical behaviour of Mo2TMB2 under OER conditions was studied using electrochemical techniques as well as comprehensive characterization methods.
Hem Raj Sharma, Liverpool Surface properties of Ga3Ni2 and In3Ni2 intermetallic catalysts
We present the recent studies of surface structures and chemical properties of Ga3Ni2 and In3Ni2 intermetallic catalysts by various surface science techniques, namely, x-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), and scanning tunneling microscopy (STM).
XPS reveals that all three high symmetry surfaces; (100), (001), and (2-10); of Ga3Ni2 prepared by the usual method of sputtering and annealing demonstrate segregation of Ga to the surface. However, the surfaces are still highly ordered as revealed by LEED and STM. The Ga3Ni2(100) and Ga3Ni2(010) surfaces are bulk truncated, while the Ga3Ni2(2-10) surface shows a c-(2×2) reconstruction. Exposure of O2 to the Ga3Ni2(001) surface selectively oxidises Ga but Ni core levels are not affected.
The In3Ni2(100) and In3Ni2(001) surfaces are also bulk truncated but the In3Ni2(2-10) surface demonstrates a (2×1) reconstruction. The In3Ni2 (001) surface yields a c-(2√3×4) rectangular superstructure with three domains upon exposure to H2. The superstructure is removed and the (1×1) structure is recovered after annealing the surface.
11:20
Alexis Front, Nancy Structural stability of In-Pd intermetallic nanoalloys
At the nanoscale, materials exhibit unique properties that differ greatly from those of the bulk state. In this context, atomic arrangments of nanoalloys must be perfectly characterized in order to master and tune physical-chemistry properties. Transition metal based nanoalloys have been extensively investigated both experimentally and theoretically [1]. However the search of sustainable materials imposes to go beyond. Intermetallic nanoalloys may be a key. In-Pd nanoalloys have been synthesized mainly for their unique catalytic properties [2-3]. The main bottleneck in the development of intermetallic nanoalloys is the lack of structural characterization.
Fig. 1: Stable structures of ordered intermetallic In-Pd nanoalloys designed with a radius of 25 Ang. within a Wulff shape.
We have adopted a multiscale approach using DFT calculations and atomistic modelling to investigate structural stability as a function of sizes and compositions of In-Pd nanoalloys. Equilibrum crystalline shapes have been determined with the Wulff theorem and compare to other structures. The most stable nanoparticles are those chemically ordered showing a Wulff shape (Fig. 1). Disordered nanoparticles are less stable and become amorphous when the concentration of In increases.
Thiago Trevizam Dorini, Nancy Complex ultrathin oxide structures revealed by evolutionary computations : InxOy/PdIn(001)
The formation of complex oxide structures is possible when two-dimensional (2D) oxide materials are grown on intermetallic single crystal substrates [1]. Modelling such phases using Density Functional Theory (DFT) computations is a challenging task. It has been shown for recently discovered ultrathin quasicrystal structures that methods of tiling, using particular atomic decorations for the tiles, can be used to develop such models [2]. Crystal structures of two-dimensional materials may also be predicted with great accuracy by evolutionary algorithms [3].
In this work, we concentrate on ultrathin complex indium oxide films produced on a PdIn(001) single crystal substrate. The atomic structures of these ultrathin oxide films are determined using a combination of DFT and evolutionary calculations. In the end, 114 models are chosen based on their formation energies, displaying a (1×3), (1×5), or (1×7) supercell. Their stability is examined using surface energy calculations with two thermodynamic models to take into consideration the growing conditions and electronic structure calculations.
In conclusion, based on evolutionary simulations, this work reveals plausible configurations for ultrathin oxide films produced on PdIn(001), generating a database with all structural and electronic information for the best structures, which will serve as a valuable road map for future experimental evaluation.
Acknowledgements: E. G. acknowledge financial support through the COMETE project (COnception in silico de Matériaux pour l’EnvironnemenT et l’Énergie) co-funded by the European Union under the program FEDER-FSE Lorraine et Massif des Vosges 2014-2020.
12:00
Oscar Shedwick, Livrepool Surface composition & oxidation studies of Ga3Ni2 intermetallic alloy catalyst
Ga3Ni2 has been reported as a potential heterogeneous catalyst in the hydrogenation of CO into methanol at atmospheric pressure [1]. Understanding the surface of a catalyst is paramount to get insight to the chemical processes [2].
To understand the optimal surface preparation conditions, X-ray photoelectron spectroscopy (XPS) is used to monitor the changes in the surface composition after sputtering and annealing at different temperatures from 250oC – 510oC. All high symmetry surfaces (100), (001) and (2-10) were studied by XPS.
For all three surfaces sputtering preferentially removed gallium, the heavier of the two elements, from the sample. After annealing the surface to temperatures >400oC gallium segregates from the bulk to the surface. The segregation was observed by XPS as an increase in the percentage of gallium on the surface from the expected 60% gallium to a maximum of 87.6%.
Angular resolved XPS from the surfaces confirms that the increase in gallium atomic concentration observer is on the top layers of all three surfaces. Despite this gallium segregation on the surface, surfaces are highly ordered as clear LEED patterns were observed and STM also shows large flat terraces with atomic resolution.
After exposing the Ga3Ni2 (0 0 1) surface to over 1000 Langmuir of oxygen, XPS showed that the Ga core levels have shifted to a higher binding energy. This confirms the presence of a gallium oxide. However, the nickel peaks remain unchanged in core level energy and shape for any exposure up to 1000 Langmuir.
References:
[1] Wencka, M., Kovac, J., Dasireddy, V.D., Likozar, B., Jelen, A., Vrtnik, S., Gille, P., Kim, H.J. and Dolinšek, J., 2018. Journal of Analytical Science and Technology, 9(1), pp.1-10
[2] Urban, K., 2010. Complex metallic alloys: fundamentals and applications. John Wiley & Sons
12:40
Lunch
Chair person: Ronan McGrath, Liverpool
14:30
Marek Mihalkovič, Bratislava Coherent interface between diamond and clathrate structures
Diamond and clathrate structures can form coherent, sp3-bonded interface stable against annealing at temperatures approaching melting point of the constituents, and bearing over 15% tensile deformation in ab-initio model simulations. Bonding between the two structures occurs via common transitional layer known as 3×3 dimer-stacking fault (DS) reconstruction on the diamond side, matching a layer with identical geometry found in clathrate type II or III. The uniform-chemistry interface ∼10% mismatch can be eliminated by tuning composition of one or both constituent structures. Appropriate diamond-structure support can be used to grow new metastable clathrate compositions.
Fig: diamond/clathrate InN/NaSi interface. In atoms are shown blue, N green, clathrate Si yellow and Na pink-orange.
15:00
Émilie Gaudry, Nancy Revealing the epitaxial interface between Al13Fe4 and Al5Fe2
Interfaces are known to play an important role in a broad range of scientific and technological fields. Their design is crucial to improve the performances of heterostructures. In metallurgy, aluminized steel combines the low weight of aluminium with the high strength of steel. But the distinct thermal and physical properties of Al and Fe metals make it challenging to join. Buffer layers made of Al-Fe intermetallic compounds generally forms at the interface. Their nature depends on the chemical potentials, the nucleation conditions and the mobilities of the elements.
On the basis of surface X-ray diffraction (SXRD), combined with calculations based on Density Functional Theory (DFT), we derive in this work a consistent model at the atomic scale for the complex Fe4Al13 // Fe2Al5 interface (Fig. 1). Calculations of adhesion and constrain energies for several structural models identify the lattice mismatch and the composition of the interfacial plane as a main factor for the stability. The matching of the [001] direction of Fe2Al5 with the [010] direction of Fe4Al13 supports a mechanism of easy Al diffusion in the Al-Fe system, to explain the formation of the complex Fe4Al13 and Fe2Al5 phases at the interface.
Fig 1: Structure of the Giant intertfacial model. The Al5Fe2 phase is depicted by cyan (Al) yellow (Fe), Al13Fe4 phase using red (Al) and green (Fe) spheres.
15:20
Wilfried Bajoun Mbajoun, Nancy Structural investigation of the Ho-Au-Si (100) approximant surface
RE-Au-Si (RE=Gd,Tb,Ho) Tsai-type 1/1 approximants structures are built from a specific cluster type where the central disordered tetrahedron can be partially or totally replaced by a RE atom leading to alteration of magnetic and thermoelectric properties[1].
Unlike for the bulk, the surface atomic structure and properties of RE-Au-Si systems remain unexplored. Several key questions are awaiting answers including the influence of the cluster center decoration on the surface plane selection and on the structural stability of surface layers.
To this end, we report the characterization of the (100) surface of the Ho1.04Au4.85Si1.324 Tsai-type 1/1 approximant using both experimental techniques and Density Functional Theory (DFT)-based methods. Low energy electron diffraction (LEED) pattern, and high-resolution scanning tunneling microscopy (STM) images show a (2×1) surface reconstruction and selection of a specific bulk plane as surface layer. To determine the latter, DFT calculations have been performed on several surface models. Finally, the calculated bulk density of states dominated by Au 5d states show great similarities with ultra-violet photoelectron spectroscopy measurements.
Vincent Fournée, Nancy Martensitic phase transition in epitaxial Ni–Mn–Ga magnetic shape memory thin films: a surface science perspective
Ferromagnetic shape memory (FSM) Heusler compounds are a class of “smart materials”, i.e. materials with multifunctional properties that can be activated through an external stimulus [1]. The prototype of FSM Heuslers is the Ni2MnGa intermetallic, because it shows the largest magnetic shape memory effect obtained so far. The key point at the heart of these externally driven physical changes and multifunctionality is a reversible martensitic transformation combined with a strong magnetostructural coupling. Here we report first results on the surface of an epitaxial Ni-Mn-Ga thin film grown on MgO(001) by RF sputtering technique at elevated temperature (623K) [2]. We use scanning tunneling microscopy (STM) imaging and low-energy electron diffraction (LEED) to study the structural and microstructural changes occuring across the phase transition between the high temperature austenite with L21 structure and the low temperature martensite with seven-fold modulated monoclinic structure (Fig.1).
Fig 1: LEED patterns and STM images of the austenite (a,b) and the martensite (c,d) phases.References:
[1] A.A. Cherechukin et al., Phys. Lett. A 291 (2001) 175
[2] M. Takhsha Ghahfarokhi et al., Acta Mat. 187 (2020) 135
16:00
Coffee break
Chair person: Eteri Svanidze, Dresden
16:30
Janusz Tobola, Kraków Entropy engineering in materials for conversion energy (thermoelectrics & ion batteries)
The efficiency of materials converting various forms of energy due to thermoelectric or electrochemical effects is partly related to peculiar electronic properties driving transport and electrochemical behaviors. The KKR-CPA method is implemented for electronic structure calculations to account for disorder in thermoelectric and ion-battery systems. The ab initio results combined with modeling of electron transport and electrochemical properties in selected thermoelectrics [1,2] and Li-/Na-ion batteries [3] are presented. Noteworthy, unusual electronic structure features appearing in these materials, namely band convergence, entropy induced band engineering (thermoelectrics) and specific character of charge/discharge curves correlated with electromotive force (batteries), directly determining their performance. Finally, recent studies of high-entropy oxides for the Na-ion cathode is discussed [4].
Monika Lužnik, Vienna Size matters: a study on transport properties of type-I clathrate nanowires
Sustainable solutions to our energy crisis are needed more urgently than ever. While not being the answer to all of our problems, thermoelectric materials play an important role in regaining some of the otherwise lost waste heat. Their conversion efficiency is defined by the dimensionless figure of merit ZT = σS2T/κ, with the electrical conductivity σ, the thermopower S, the temperature T and the thermal conductivity κ. Therefore, a good thermoelecttric material should have a high σ and S, but also a low κ. Here we present a size study on type-I clathrates, in which the phonon Kondo effect is held responsible for a flattening of the acoustic phonon modes in a sizable range of momentum space, thereby lowering the phonon thermal conductivity [1]. To scrutinize this picture, we use well-studied single crystals [2, 3] and shape them into nanowires with different diameters using a focused ion beam (FIB) technique. Because in the above picture only long-wavelength phonons carry heat in these materials, the dependence of κ on the diameter should be noticeable. Additionally, we compare the electrical resistivities of the nanowires to get a full picture of how size affects the transport properties of type-I clathrate nanowires.
Laura Agnarelli, Dresden Structural complexity in the apparently simple crystal structure of Be2Ru
During the study of the Be-Ru binary system, the crystal structure of Be2Ru was reinvestigated. Differently from what is reported in literature, Be2Ru crystallizes with hexagonal Fe2P-type structure. The crystal structure can be described as constituted by a substructure of ruthenium atoms arranged within planar layers in the form of condensed six-membered rings in the ab plane. The latter are separated by a layer formed by Be atoms found at half of the c axis’ length of the unit cell, in a ABAB sequence. Analysis of the collected single-crystal data revealed a crystal structure more complicated than expected. In fact, an additional position very close in distance to the Ru1 atom can be recognised. The possibility of a stacking fault in the ab plane was advanced and a TEM investigation was carried out, which confirmed such hypothesis. The calculated electronic density of states (DOS), revealed that, contrary to typical intermetallic compounds, Be2Ru shows a pseudo gap in the vicinity of the Fermi level. The temperature dependence of the electrical resistivity of Be2Ru, shows metallic behaviour in agreement with the non-zero DOS at the Fermi level.
17:30
Concluding remarks
19:30
Conference dinner
Thursday, November 24, 2022
time
event
8:15
Science Board Meeting
10:15
Break
10:30 - 12:00
Governing Board Meeting and General Assembly
13:00 - 17:00
Katarzyna Gliszczyńska
Innovative Researcher: How to design a marketable bestseller PhD thesis
During our workshop we will use a design-driven innovation approach to touch issues to learn, experience, practice, ask, answer and discuss what follows:
How may we use design thinking approach in order to sell before creating/researching?
How may we learn from the design thinking mindset how to commercialize scientific results better?
How may we benefit from the design thinking process in scientific research and academic publication?
How may we use design thinking tools to understand the stakeholders in the market better?
You will be led by two Innovation Consultants and Design Thinking Facilitators: Katarzyna Gliszczyńska, who has a successful track of business and consulting projects as well as training design thinking facilitators and Magdalena Wencka, PhD, who has a background in physics and is experienced in academic research projects. Katarzyna will be happy to share some success stories as well as failures and lessons learned from the projects accomplished with and for individuals, small start-ups and big corporations. The cherry on top: a story of an academic who started to use design thinking and turned insights from his PhD research into supermarkets best-selling book present in numerous bookstores.
