M. Konkel and M. Wencka:
Workshop Equality and diversity in material science
A workshop on "Intercultural cooperation in project team for researchers" is a part of "Equality and diversity in material science" ECMetAC RAD activity. The workshop content will touch issues of awareness of cultural influence on the way how trust is built within scientific teams, understanding cultural differences and similarities in cooperation in project teams and naming best practices on how to inclusively cooperate in project teams in spite of cultural differences. All the issues, increasing knowledge about D&I (Diversity and Inclusiveness) in teams of researchers make us all more comfortable at a field of international community of scientists. Our workshop is open to all members of our network at the internet way and personally for C-MAC Days 2021 participants.
17:00
J. Ledieu, Nancy: Welcome note
Chair person: Peter Gille, Munich
17:15
H. Schlich, MaTecK GmbH: Metal single crystal growth: Examples and challenges in the development of special crystals and their applications
Crystals are one of the major pillars in the development and achievement of our modern technologies. This is most obvious for modern optics and the semiconductor industry. But also, the growth of metal single crystals is a key factor in basic research as metallic crystals are used to investigate physical and chemical effects for better understanding.
This presentation will show the main growth methods for metals and their alloys and their pros and cons will be discussed. Furthermore, the growth activities of the MaTecK company will be presented. Several examples will point out that crystal growth is a highly interdisciplinary subject that requires expertise in physics, chemistry, materials science and engineering. In detail, the development of crystals for neutron monochromators (based on Cu and Fe), for superalloys (based on Ni), for electrical propulsion of satellites (based on La) and for other metal single crystals and their alloys will be discussed.
17:45
M. Armbrűster, Chemnitz: Hunting ideal functional materials for applications in corrosive environments
In many cases functional materials for chemical and physical applications have to withstand corrosive conditions during use. This comprises e.g. sensorics or catalysis. Especially the latter requires breaking and forming chemical bonds on the surface - of the material as well as the reactants and procucts. For an ideal catalysts, the material changes are fully reversible during a catalytic cycle, thus keeping the material functional on the long-term.
The required stability can be ensure by our recently developed chemical potential guided development (CPGD) which combines the information on the chemical potential of the elements within intermetallic compounds and the one of the corrosive environment. With this starting point, stable intermetallic platform materials[1] can be selected where each platform offers different electronic and structural properties.[2] The last step of the approach is to fine-tune electronic and geometric properties of the best platform materials by isostructural substitution with elements from the same period or the same group, respectively.[3,4]
Our approach enables the quick and directed development of functional materials in heterogeneous (electro)catalysis and beyond, circumventing laborous trial-and-error battles, which are often used in catalyst development.
References
[1] L. Rőβner, et al. ACS Appl. Mater. Inter. 13, 2021, 23616.
[2] R. Zerdoumi, et al. Z.Anorg.Allg. Chem. 10.1002/zaac.202100171.
[3] O. Matselko, et al. J. Phys. Chem. C 122, 2018, 21891.
[4] R. Zerdoumi, et al. ACS Appl. Energy Mater., accepted.
Tuesday, December 7, 2021
time
lecturer, title and abstract
Chair person: Ana Smontara, Zagreb
9:00
S. Penner, Innsbruck: Steering the Catalytic Properties of Intermetallic Compounds and Alloys in Reforming Reactions by Controlled in Situ Decomposition and Self-Activation
Based on the increasing importance of intermetallic compounds and alloys in heterogeneous catalysis, we explore the possibilities of using selected intermetallic compounds and alloy structures and phases as catalyst precursors to prepare highly active and CO2-selective methanol steam reforming (MSR) as well as dry reforming of methane (DRM) catalyst entities by controlled in situ decomposition and self-activation. The exemplary discussed examples (Cu51Zr14, CuZn, Pd2Zr, GaPd2, Cu2In, ZnPd, and InPd) show both the advantages and pitfalls of this approach and how the concept can be generalized to encompass a wider set of intermetallic compounds and alloy structures. Despite the common feature of all systems being the more or less pronounced decomposition of the intermetallic compound surface and bulk structure and the in situ formation of much more complex catalyst entities, differences arise due to the oxidation propensity and general thermodynamic stability of the chosen intermetallic compound/alloy and their constituents. The metastability and intrinsic reactivity of the evolving oxide polymorph introduced upon decomposition and the surface and bulk reactivity of carbon, governed by the nature of the metal/intermetallic compound-oxide interfacial sites, are of equal importance. Structural and chemical rearrangements, dictating the catalytic performance of the resulting entity, are present in the form of a complete destruction of the intermetallic compound bulk structure (Cu51Zr14) and the formation of an metal/oxide (Cu51Zr14, InPd) or intermetallic compound/oxide (ZnPd, Cu2In, CuZn) interface or the intertranformation of intermetallic compounds with varying composition (Pd2Zr) before the formation of Pd/ZrO2. In this talk, the prerequisites to obtain a leading theme for pronounced CO2 selectivity and high activity will be reviewed. Special focus will be put on raising awareness of the intrinsic properties of the discussed catalyst systems that need to be controlled to obtain catalytically prospective materials. The use of model systems to bridge the material's gap in catalysis will also be highlighted for selected examples.
9:30
M. Majewicz, Wrocław: Possible Lifshitz point in the magnetic phase diagram of UNi2Si2 single crystals
The ternary silicide UNi2Si2 shows complex magnetic properties. In zero magnetic field, it undergoes a series of subsequent magnetic phase transitions from paramagnetic (PM) to incommensurate spin-density-wave (ICSDW) to antiferromagnetic (AF), and eventually ferrimagnetic (UAF) states. The external magnetic field causes metamagnetic transitions and stabilizes the UAF state. In high magnetic fields, the PM, ICSDW and UAF phases meet at a single point that may bear features of bicritical Lifshitz point (LP), which is a critical behaviour strikingly different from any other.
With the main aim to verify that LP hypothesis, we have undertaken comprehensive reinvestigation on high-quality single crystals of UNi2Si2. Our magnetization, electrical resistivity and heat capacity measurements confirmed the existence of multiple magnetic phases, and the tendency to mearging the PM-ICLSW and ICLSW-UAF phase boundaries. However, we failed to observe LP up to 14 T, the highest magnetic field accessible in our laboratory. Therefore, further study of the compound in even stronger magnetic fields is indispensable.
This work was supported by the National Science Centre of Poland grant no. 2018/31/D/ST3/03295.
9:50
J. Juraszek, Wrocław: Destruction of a sign-changing order parameter by artificial atomic defects in multiband superconductor PrOs4Sb12
Chiral superconductivity is a long-sought unconventional state of matter that spontaneously breaks time-reversal symmetry through the development of Cooper pairing with finite angular momentum. Chiral superconductivity is a type of topological state which provides a natural platform for realizing Majorana edge modes being central to various proposals for quantum computation. However, despite intensive theoretical studies and huge experimental efforts, no material has been proven definitively to be a chiral superconductor.
The heavy-fermion and multiband superconductor PrOs4Sb12, for which a μSR study [1] and polar Kerr effect measurements [2] showed evidence of broken time-reversal symmetry spontaneously developing below the critical temperature Tc ≃ 1.85 K, is a leading candidate to display chiral superconductivity. Based on measurements of the temperature dependence of the lower critical field Hc1(T), we have recently proposed a multiband and multisymmetric scenario, in which a superconducting condensate is composed of a sign-changing smaller gap and a large isotropic s-wave gap [3].
To develop a detailed understanding of multicomponent superconductivity in PrOs4Sb12, we have extended measurements of Hc1(T) down to temperatures as low as 7 mK utilizing a 2DEG Hall magnetometry. We observe a sudden increase in Hc1(T) deep in a superconducting state, indicative of a rare case of two nearly decoupled bands. Furthermore, a non-saturating and concave behaviour of Hc1(T) below about 0.45 K clearly points at a sign-changing symmetry of the smaller gap. Equally remarkable is a high sensitivity of this characteristic to electron irradiation. Indeed, a concentration of artificial atomic defects as small as a few 0.1% results in both a saturation of Hc1(T) at T<0.15 K and a strong suppression of the anomalous enhancement below ≃ 0.25Tc, consistent with a destruction of an unconventional order parameter due to the smaller gap. In addition to this, theoretical description of possible symmetries of the smaller gap as well as results of a comparative study on the two-band isotropic s-wave homologue LaRu4As12 will be discussed in the context of a putative chiral spin-triplet pairing state in PrOs4Sb12 [4].
References:
[1] Y. Aoki et al., Time-Reversal Symmetry-Breaking Superconductivity in Heavy-Fermion PrOs4Sb12 Detected by Muon-Spin Relaxation. Phys. Rev. Lett. 91, 067003 (2003).
[2] E. M. Levenson-Falk et al., Polar Kerr Effect from Time-Reversal Symmetry Breaking in the Heavy-Fermion Superconductor PrOs4Sb12. Phys. Rev. Lett. 120, 187004 (2018).
[3] J. Juraszek et al., Symmetry of Order Parameters in Multiband Superconductors LaRu4As12 and PrOs4Sb12 Probed by Local Magnetization Measurements. Phys. Rev. Lett. 124, 027001 (2020).
[4] V. Kozii et al., Three-dimensional Majorana fermions in chiral superconductors. Sci. Adv. 2, e1601835 (2016).
10:10
I. Mandal, Krakow: Correlated Insulators in Twisted Bilayer Graphene
Recently, moire superlattices in 2d van der Waals heterostructures have been found to exhibit properties analogous to the high-Tc cuprates, namely, Mott insulating states and unconventional superconductivity. Examples include magic-angle twisted bilayer graphene and transition metal dichalcogenides. These being (chemically) simpler / cleaner systems compared to the transition metal oxide superconductors, have emerged as new experimental systems for studying strongly correlated phases, allowing higher degrees of experimental control. Experiments on twisted graphene bilayers, where the top layer is rotated with respect to the one below, have displayed insulating behaviorwhen the moiré bands are partially filled. I will elaborate on our recent calculations to find the static charge configurations in these phases, and to estimate the excitation gaps.
10:30
Break
Chair person: Silke Bűhler-Paschen, Vienna
11:00
P. Popčević, Zagreb: Interaction of magnetic and electronic subsystems in Co intercalated 2H-NbS2 seen by different probes
Co1/3NbS2 is a quasi 2D system where magnetic Co ions are positioned on triangular lattice in van der Waals gaps in-between metallic NbS2 layers. The coexistence of metallic and magnetic degrees of freedom coupled with reduced dimensionality and anticipated frustration render this compound battlefield of different interactions resulting in peculiar antiferromagnetic (AF) ground state below 28K. So far, this is the only system from its family where complete suppression of AF order under relatively low hydrostatic pressure of 1.7 GPa was reported [1]. Another curiosity of this system is enhancement of the c-axis electrical resistivity observed in AF ordered state, indicating spin-valve like mechanism [2]. Last but not the least, angle-resolved photoelectron spectroscopy (ARPES) of Co1/3NbS2 demonstrates limitations of the rigid-band model, which was traditionally used to describe materials from the same family. The most intriguing departure corresponds to appearance of a shallow electronic band at Fermi level with enhanced effective mass [3]. This band cannot be identified with any DFT calculated feature, and it is possibly related to the resonance caused by strong electron correlations.
References:
[1] N. Barisic et al. Phys. Rev. B, 84 (2011) 075157
[2] P. Popcevic et al. arXiv: 2003.08127
[3] P. Popcevic, Y. Utsumi et al. submitted.
11:20
S. Sekh, Krakow: Circular dichroism as a probe for band topology in chiral semimetals
Topological semimetals, characterized by a singular or line degeneracy in the 3D Brillouin zone (BZ), have shown several interesting transport properties over the years due to their band topology. Recent discovery of 'multifold semimetals' such as RhSi, CoSi shows multiple degeneracies in the BZ is also possible where two or more bands meet with non-zero Chern number. Such multifold degenerate points lead to higher monopole charges as they harbour higher-pseudospin excitations with no counterpart in high-energy physics. However, detecting the topological invariant experimentally can be a daunting task. In our work, we demonstrate that analyzing the optical response of a circular drive is an effective way to detect the topology of the lowest-energy Bloch band, as it can be connected to a frequency-dependent probing function. This response depends on the excitation rates induced on the filled band by the left- and right-circular drive, because of the geometrical properties of the Bloch bands. Our calculation reveals that the response is quantized for rotationally-invariant Hamiltonians when the frequency of the drive is above a critical value. We demonstrate this numerically by considering three kinds of semimetals with pseudospin values of 1/2, 1, and 3/2, respectively. We also investigate the role of tilt and anisotropy and find that although tilt does not have any effect on the response, the presence of anisotropy can drastically hamper the quantization. Our scheme thus provides an important methodology for designing future experiments to detect the topology of band structures.
11:40
J.-M. Hűbner, Dresden and Lund: The Borosilicide Rb8B8Si38 with a Clathrate-I type structure: high-pressure synthesis, cage adaption and properties
Clathrate-I borosilicides M8-xBySi46-y (M = K, Rb, Cs) with alkali metals form under high-pressure, high-temperature conditions [1-4]. In contrast to clathrate-I silicides featuring larger substitution atoms like transition metals, boron substitution affects the 16i position located on the body diagonal of the unit cell. Boron substitution leads to a pronounced contraction of the lattice compared to the binary clathrate-I silicides. Remarkably, the lattice parameter of Rb8B8Si38 in space group Pm3n (a = 9.9583(1) Å) is strikingly similar to that of K7B7Si39 [1], in which half of the potassium positions on the body diagonal are empty. The alternative description of the clathrate-I structure as sodalite filled by dodecahedral cages explains the framework adaption to the larger rubidium atoms. Rb8B8Si38 shows a high thermal and chemical stability. The compound is an electronically balanced Zintl phase according to the formula (Rb+)8(B-)8(Si0)38 consistent with its diamagnetic and semiconducting behavior.
References:
[1] Jung, W. et al. Angew. Chem., Int. Ed. 2007, 46, 6725- 6728.
[2] Jung, W. et al. Dalton Trans. 2021, 50, 1274-1282
[3] Hűbner, J.-M. et al. Inorg. Chem. 2021, 60, 4, 2160-2167.
[4] Hűbner, J.-M. et al. Z. Anorg. Allgem. Chem. 2021, 647 (2-3), 119-125
12:00
Lunch
Chair person: Magdalena Wencka, Poznan and Ljubljana
13:30
N. Kanas, Novi Sad: Heterostructuring - an effective way to boost zT of CaMnO3-based composites
The effect of micro- and hetero-structuring on the thermoelectric properties of CaMnO3/CaMn2O4 composites is presented. Single phase compositions with rock salt structure were synthesized in reducing atmosphere and subsequently densified by spark plasma sintering in vacuum. Annealing in air at 1340 oC between 1 and 24 hours activated redox exsolution which resulted in a variation in microstructure of materials with 10 and 15 vol% CaMn2O4, respectively. The nature of the CaMnO3-?/CaMn2O4 interface was analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) followed by a theoretical approach based on density functional theory (DFT). The bulk materials were characterized with respect to electrical conductivity (σ), thermal conductivity (κ) and Seebeck coefficient (S) at temperatures between 300 and 900 oC in air. Besides the electrical conductivity of 73 S cm-1 at 900 oC obtained with CMO15%24h, the highest σ and lowest κ were obtained for composites with 10 vol% CaMn2O4 (CMO10%8h), reaching 49 S cm-1 at 900 oC and 0.56 W m-1K-1 at 700 oC, respectively. However, the highest zT was obtained for samples with 15 vol% CaMn2O4 (CMO15%8h) reaching 0.11 at 800 and 900 oC, due to the enhanced power factor above 700 oC.
13:50
M. Uland, Chemnitz: Chemical potential as guide in material development
Over the past decades, a wealth of data on the physical and chemical properties of a vast suite of functional materials has been collected - however, less progress has been made in con-necting the catalytic properties of the material to those of its con-stituent elements. Understanding these relationships could allow for the prediction of a material's properties prior to synthesis. The elucidation of these connections is of particular relevance in the matter of catalyst stability, where there is a growing desire to pre-dict a given material's stability under reaction conditions without wasting time and resources on its development and testing.[1] A possible solution is directed material development as guided by a carefully chosen descriptor - a material property that correlates to a material's stability[2] - and, in this vein, we propose using chem-ical potential as such a descriptor. Chemical potential can be de-fined most simply as a measure of a material's capacity for change under a given set of conditions and thus can be used to predict a material's stability under reaction conditions.[3] In addition to encapsulating a whole host of other thermodynamic fac-tors[4], chemical potential is intrinsic to a material and independ-ent of the reaction being investigated[5], offering a clear advantage over other proposed descriptors such as the adsorption energies of key reaction intermediates[6].
References:
[1] G.S. Burkhanov et al. Russ. Chem. Rev. 2019, 78, 569.
[2] Z. W. Seh et al. Science. 2017, 355, 1.
[3] P.W. Atkins. "Physikali-sche Chemie" 2. Aufl., Wiley-VCH, Weinheim, 1996.
[4] R. Krie-gel et al. ChemPhysChem. 2020, 21, 977.
[5] A. I. Rusanov. Russ. Chem. Rev. 2016. 1.
[6] T. Bligaard et al. J. Cat. 2004, 224, 206.
14:10
I. Antonyshyn, Dresden: Isostructural M2Pt {M = Al, Ga, In, Sn}: structure - chemical properties - electrocatalysis
Isostructural M2Pt {M = Al, Ga, In, Sn} crystallize with anti-CaF2 type of crystal structure [1] and possess strongly polar covalent interactions between M and Pt atoms. The variation of the constituent component M leads to the different shift of the Pt 4f core levels towards higher binding energies (XPS data), increasing with decreasing of the atomic radii of M. In addition to charge transfer effects, XPS shifts are influenced by the final-state screening due to the conduction electrons. As a result, the correlation of XPS shifts with number of Pt 5d electrons was drawn based on ELI-D analysis.
The M2Pt specimens were used as oxygen evolution reaction (OER) electrocatalyst materials in water splitting reaction [2]. The OER activity follows the trend In2Pt > Ga2Pt > Al2Pt, governed by nature of M and their leaching rates upon conditions of OER. Intermetallic compounds M2Pt act only as catalyst precursors for formation of catalytically active MxPt1-x phase in near-surface region. The remnant M2Pt plays a role of current collector and ptovide the integrity of the bulk electrode. The compound Sn2Pt is inactive for OER due to the formation of passivating layer of SnO2 on the surface and hindrance of electron transfer through it. In additon to comprehensive experimental characterization of materials before and after OER, the first-principle calculations were performed to shed a light onto the change of Pt electronic state during the leaching.
References:
[1] E. Zintl, A. Harder, W. Haucke, Z. Phys. Chem. B 35 (1937) 354.
[2] A.M. Barrios Jiménez, A. Ormeci, U. Burkhardt, S.G. Altendorf, F. Kaiser, I. Veremchuk, G. Auffermann, Yu. Grin, I. Antonyshyn, Sustainable Energy & Fuels (2021), DOI: 10.1039/d1se01190a.
14:30
O. Shedwick, Liverpool: Analysis of the surface structure of a Ga3Ni2 binary alloy using LEED & STM
Intermetalic compounds have been of interest in the field of catalysis for many years[1]. From their reaction specifity to being made from more inexpenisve metals intermetallic compounds offer many new options in particular as relpacments for rare earth catalyts. Ga3Ni2 is a binary alloy that is used in the reduction of CO2 into CO and methanol at atmospheric pressures[2].
As Ga3Ni2 is used as a heterogeneous catalyst for the reduction of CO2 understanding the surface of catalyst is important [3]. Using the surface probing techniques of LEED, Low Energy Electron Diffraction a process that uses the diffractions of elections to probe the surface of the sample, and STM, Scanning Tunnelling Microscopy an experimental method used to image the surface in the sample by measuring the change in voltage as a tip moves across the surface. The (2, -1,0) surface of Ga3Ni2 has undergone both LEED and STM then the results were compared with the model structure of the bulk crystal.
The preliminary results seem to indicate that 1x1 reconstruction is being seen primarily in the LEED data with the reconstruction also been seen in the FFT of the large terraces of the Ga3Ni2 surface.
References:
[1] Marc Armbruster, Robert Schlogl, and Yuri Grin. Intermetallic compounds in heterogeneous catalysis-a quickly developing field. Science and Technology of Advanced Materials, 2014.
[2] Magdalena Wencka, Mirtha Pillaca, and Peter Gille. Single crystal growth of ga3ni2 by the czochralski method. Journal of Crystal Growth, 449:114-118, 2016.
[3] Magdalena Wencka, Janez Kovac, Venkata DBC Dasireddy, Blaz Likozar, Andreja Jelen, Stanislav Vrtnik, Peter Gille, Hae Jin Kim, and Janez Dolinsek. The effect of surface oxidation on the catalytic properties of Ga3Ni2 intermetallic compound for carbon dioxide reduction. Journal of Analytical Science and Technology, 9(1):1-10, 2018.
14:50
Break
Chair person: Janez Dolinšek, Ljubljana
15:20
A. Jelen, Ljubljana: Structure and microstructure of Sc-Hf-Nb-Ta-Ti-Zr refractory high-entropy alloys
We have investigated scandium (Sc)-containing Sc-Hf-Nb-Ta-Ti-Zr system of refractory high-entropy alloys (HEAs), with all elements in equiatomic concentrations.
Unlike the Hf-Nb-Ta-Ti-Zr parent HEAs, which possess a single-phase body-centered cubic (bcc) structure and quite homogeneous microstructure, the addition of Sc produces a two-phase structure in the Sc-Hf-Nb-Ta-Ti-Zr alloys, with one phase being body-centered cubic (bcc) and the other hexagonal close-packed (hcp). The hcp phase absorbs practically all Sc, whereas the bcc phase is identical to the bcc phase of the Hf-Nb-Ta-Ti-Zr parent system. Upon Sc addition, the microstructure becomes very inhomogeneous. Large bcc dendrites of sever 10 - 100 μm dimension are homogeneous in the central parts (Fig. 1 a), but become a fine dispersion of sub-micron precipitates of the bcc and hcp phases close to the edges. The interdendritic regions are also similar fine dispersion of the two phases, with no larger (µm-size) hcp regions (Fig. 1 b).
Fig. 1: BSE SEM images of a ScHfTaTi high-entropy alloy; a) low mag. image showing hcp + bcc (dark) and bcc (light) phases, with white rectangle indicating the enlarged area in b), showing fine dispersion of the two phases
15:40
A. Gačnik, Ljubljana: Superconductivity of Sc-Hf-Nb-Ta-Ti-Zr refractory high-entropy alloys
In an attempt to enhance the physical-mechanical properties of the Hf-Nb-Ta-Ti-Zr refractory HEAs, we have employed additional element scandium. Its addition caused the formation of a highly inhomogeneous two-phase (bcc and hcp) structure. The hcp phase has absorbed practically all Sc, leaving the elemental composition of the remaining bcc phase identical to the bcc phase of the Hf-Nb-Ta-Ti-Zr parent system.
Heat capacity, magnetic susceptibility, and electrical resistivity measurements revealed similar superconducting properties to the bcc Hf-Nb-Ta-Ti-Zr parent alloys. The superconducting phase is a type II superconductor and BCS-like. The enormous chemical and structural disorder classify it as the Anderson "dirty" superconductor [1]. The main effect of Sc addition is to suppress superconductivity of the hcp phase fraction and to introduce higher chemical disorder in the superconducting bcc phase.
References:
[1] P. W. Anderson, Theory of dirty superconductors, J. Phys. Chem. Solids, 1959, 11, 26.
16:00
P. Koželj, Ljubljana: Magnetism in the magnetically concentrated, randomly disordered and frustrated CoCrFeMnNi HEA
In addition to the obvious complication of several different kinds of atoms, magnetism in high entropy alloys is also influenced by the micro- and nanostructure formed during sample synthesis and annealing. This contribution [1] will deal with a high-quality non-equiatomic CoCrFeMnNi synthesized via the Czochralski method, where these two difficulties have been removed via proper thermal treatment so that the sample is homogenous on the microscale (SEM elemental mapping) and without short-range ordering or chemical inhomogeneity on the nanoscale (APT, HAADS STEM and EELS).
The appeal of our HEA as an unusual magnetic system is that it is a severely concentrated and multielemental magnetic system (since all 5 elements are magnetic) which is a completely random solid solution with frustrated FM and AFM interactions. We have observed that the system undergoes a transition to a spin glass state at Tf ≅ 20 K. The broken ergodicity below Tf in CoCrFeMnNi was demonstrated via ZFC-FC magnetization splitting, the shift of the cusp in AC susceptibility measurements, the ultra-slow decay of thermoremanent magnetization and the thermal memory effect.
References:
[1] P. Kozelj, et al., J. Magn. Magn. Mater. 523, 167579 (2021).
16:20
T. Seyller, Chemnitz: Deposition of CrCoFeNi thin films by magnetron sputtering
We report on the deposition of CrCoFeNi high entropy alloy (HEA) films by magnetron sputtering, which is a widely used technique for thin film deposition. In a first set of experiments, two type of sputter targets were produced by spark plasma sintering: a so-called alloy target from a homogeneous, equimolar CrCoFeNi HEA powder and a so-called blend target from an equimolar mixture of pure metal powders. Thin films were deposited by magnetron sputtering on Si(001) and stainless steel substrates. A characterization of the thin films using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), X-ray fluorescence spectroscopy (XRF) and scanning electron microscopy (SEM) revealed negligible differences in composition and structure of the polycrystalline, cubic (fcc) HEA films prepared from the two different targets. This suggests that HEA films can be grown from considerably cheaper blend targets.
In a second set of experiments, thin films were grown on MgO(001) and Al2O3(0001) substrates using the alloy target. This resulted in (001) and (111) oriented, cubic CrCoFeNi films with very well ordered surfaces as demonstrated by low-energy electron diffraction. This opens a route towards future studies of the surface properties of HEAs without the need for growing bulk single crystals.
C. Goyhenex, Strasbourg: Atomistic approach of the o-Al13Co4 quasicrystalline approximant: point defects and surfaces
The Al13Co4 intermetallic compound has attracted the attention of many research groups due to its unusual physical and chemical properties. A few examples include its strongly anisotropic magnetic and electronic transport behavior [1], its interesting catalytic performances towards hydrogenation reactions [2], as well as its unexpected wetting properties [3]. To understand these fascinating features, the determination of structure-properties relationships is crucial. This task is however challenging, since defects play a non negligible role in this compound [1,4]. Indeed accurate and reliable modelling of Al13Co4 where structural complexity and local disorder are at play, requires large computational cells. The same holds for studies involving surfaces. In this talk, we will present a tight-binding atomistic approach based on the use of an N-body potential parameterized for the o-Al13Co4 quasicrystalline approximant. We will show its ability to model this complex compound in the presence of atomic vacancies or surfaces thanks to the use of classical molecular dynamics simulations [5]. The importance of stress relaxation in vacancy formation will be highlighted through the presentation of a mapping of the local pressures in the bulk compound. Thanks to the many body character of the potential, the surfaces could be investigated which was not done before in atomistic studies of this complex phase. The results are compared to previous studies involving both DFT calculations and experiments [6]. Finally, preliminary investigations of the wetting of lead on the o-Al13Co4(100) surface, using classical molecular dynamics, will be also presented.
References :
[1] J. Dolinsek et al., Anisotropic magnetic and transport properties of orthorhombic Al13Co4, Phys. Rev. B 79 (2009) 184201.
[2] L. Piccolo et al., Catalytic Properties of Al13TM4Complex Intermetallics: Influence of the Transition Metal and the SurfaceOrientation on Butadiene Hydrogenation, Sci. Tech. Adv. Mater. 20 (2019) 557-567.
[3] K. Anand et al., Non-Wetting Behavior of Al-Co Quasicrystalline Approximants Owing to their Unique Electronic Structures, ACS Appl. Mater. Interfaces 12 (2020) 15793-15801.
[4] P.-F. Lory et al., Impact of Structural Complexity and Disorder on Lattice Dynamics and Thermal Conductivity in the o-Al13Co4 phase, Phys. Rev. B 102 (2020) 024303.
[5] O. Bindech et al., A tight-binding atomistic approach for point defects and surfaces applied to the o-Al13Co4 quasicrystalline approximant, Comput. Mater. Sci. 200 (2021) 110826.
[6] H. Shin et al., Structure of the OrthorhombicAl13Co4( 100 ) Surface Using LEED , STM and ab initio Studies, Phys.Rev. B 84 (2011) 085411 ; P. Scheid et al. Bonding Network and Stability of Clusters : the Case Study of Al13TM4Pseudo-Tenfold Surfaces, Acta Cryst. A75 (2019) 314-324.
9:30
O. Tiryaki, Chemnitz: Investigating well-defined ZnPd/ZnO catalysts in methanol steam reforming
ZnPd/ZnO catalysts are promising methanol steam reforming (MSR) catalysts, because they can produce lower CO containing hydrogen feeds for fuel cell applications, as compared to Cu-based catalysts. In addition, they have proven to have higher thermal stabilities. [1] Highly active and selective materials are Zn-rich compositions of ZnPd[2] (wide homogeneity range, Fig. 1), which determine the surface composition and thus also the oxi-disability of the zinc on the surface. Due to the complexity of the supported materials, it is expedient to first examine the changes to unsupported Zn-Pd samples and then to increase the complexity on the material side. The unsupported ZnPd compounds need to be examined operando regarding their dynamic material changes in a reactive atmosphere. This contribution will give preliminary insights into sample preparation and characterization in the pris-tine state.
References:
[1] H. Purnama et al., Appl. Catal. A Gen. 259, 2004, 83.
[2] M. Friedrich et al., J. Catal, 285, 2012, 41.
[3] M. Armbrüster at al., Catal. Rev.: Sci. Eng. 55, 2013, 289.
Figure 1: Pd-Zn phase diagram.[3]
9:50
M. Kotsch, Dresden: Bell-Like [Ga5] clusters in Sr3Li5Ga5
The unique bell-like [Ga5] cluster was first discovered by Fedorchuk et. al. in the compound Eu3Li5+xGa5-x (x = 0.15).[1] The presence of f electrons in combination with the dual valency of europium however hindered the application of quantum chemical calculations in order to gain further insight into the bonding situation of this cluster. This problem however, can be circumvented by replacing europium with an alkaline-earth metal, in analogy to Ba3LiGa5.[2] In the present case this was achieved through the utilization of strontium, resulting in the isostructural compound Sr3Li5Ga5 (space group R3m, a 9.6040(5) Å, c = 22.061(1) Å).
Identically to Eu3Li5+xGa5-x the compound Sr3Li5Ga5 can formally be counted as [Sr2+]3[Li1+]5[(1b)Ga4-]1[(3b)Ga2-]3[(4b)Ga1-]1 according to the Zintl concept, whereas the combined analysis of electron density (ED) and electron localizability indicator (ELI-D) revealed a much more complex situation. In particular, it was found that the Ga-Ga contacts forming the triangular base of the bell-like cluster have no interaction with each other, which leads to a different Zintl count, ([(1b)Ga4-]4[(4b)Ga1-]1) requiring more electrons than provided. However if the partially bonding character of the lone pairs is considered,[3] it results in an electronically balanced scheme: [(1b)Ga2.5-]4[(4b)Ga1-]1, In combination with the [Sr6] polycation found in the structure, this indicates a reorganization tendency towards a Wade-like cluster anion with less electrons necessary for its stabilization.
References:
A. Fedorchuk, Yu. Prots, W. Schnelle, Yu. Grin, Eur. J. Inorg. Chem. 2011, 26, 3904-3908.
M. Kotsch, Yu. Prots, A. Ormeci, M. Bobnar, F. R. Wagner, A. Senyshyn, Yu. Grin, Eur. J. Inorg. Chem. 2020, 2020, 2842-2849
A. Fedorchuck, Yu. Grin in Handbook of the Physics and Chemistry of Rare Earths, North Holland, 2018, pp. 81-143.
10:10
L. Rőβner, Chemnitz: Electrochemical investigations on intermetallic compounds
To bridge the gap of sample morphologies between solid-state chemistry (unsupported bulk samples, thin-films, powders) and applied electrocatalysis (carbon or oxide supported nano-particles), a unified approach for testing well characterized un-supported materials is developed. The approach allows to evaluate the intrinsic catalytic properties of numerous samples in a short period of time.
The approach is based on using intermetallic compoud s as platform materials, providing a reproducible sample surface. This can be used as a reference-state for further electrocatalytic- and physical characterization techniques (Fig. 1). The approach has been successfully applied for the hydrogen evolution reaction (HER) on intermetallic Mo-Ni compounds [1] as well as an high entropy alloy composed of Al, Fe, Co, Cu and Ni. Additionally, the methanol oxidation reaction (MOR) was investigated on intermetallic compounds within Pd2(Ga/In/Sn/Zn) (CoSi2 type of crystal structure) [2,3] and Pt(Sn/Bi/Pb) (NiAs type of crystal structure).
References:
[1] L. Rößner, H. Schwarz, I. Veremchuk, R. Zerdoumi, T. Seyller, M. Armbrüster, ACS Appl. Mater. Interfaces 2021, 13, 23616-23626.
[2] R. Zerdoumi, L. Rößner, M. Armbrüster, J. Electrochem. Soc. 2019, 166, F1079-F1087.
[3] R. Zerdoumi, M. Armbrüster, Z. Anorg. Allg. Chem. 2021, 0562, zaac.202100171.
Figure 1: Exemplary analysis strate-gy utilizing the reproducible reference state.
10:30
Break
Chair person: Yuri Grin, Dresden
11:00
Elise Pachoud, Grenoble: Crystal growth at the Néel Institute
Abstract - TBA
11:20
J. Smietanska, Krakow: Looking at the modulated structure of pathogenesis-related protein (Hyp-1) complex with ANS within the higher dimensional superspace approach
Newly discovered, and still uncommon, modulated crystal structure in organic systems require a deeper investigation. No exact and detailed solution of such systems has not been done up-to-date. One possibility is to use an approximation of commensurate modulation which enables constructing a supercell, extending to the case, where translational symmetry (periodicity) is recovered, and simplify the analysis [1]. An assumption of commensurateness of the modulation is, however, questionable and rather unverifiable.
The goal of our studies was to use a novel, original statistical method of structural modeling which enables a refinement based on the average unit cell with (commensurate or incommensurate) modulation without unclear assumption of commensurateness and supercell approach. The main concept of the statistical method is to express structure in terms of the statistical distribution of atomic positions concerning the periodic reference lattice with lattice constant related to characteristic length-scale present in the structure. The average unit cell, defined as a probability distribution, constructed for periodic crystal is the same as the unit cell. The statistical approach was successfully used for the description of not only periodic crystals or quasicrystals, as well as it can be expanded on modulated structures as well as aperiodic structures with singular continuous components in the diffraction pattern [2].
Our model system is a pathogenesis-related protein (Hyp-1) complex with fluorescent probe 8-anilino-1-naphthalene sulfonate (ANS), which is a unique example of a macromolecular system with the modulated crystal structure. Previous studies have shown that Hyp-1/ANS complexes are tetartohedral twinned and crystallized in an asymmetric unit cell containing a repetitive motif of four protein molecules arranged with 7-fold noncrystallographic repetition along the c axis of the C2 space group. Assumption of commensurate structure modulation demanded description of structure in the highly expanded unit cell with 28 unique protein molecules inside [3]. The Hyp-1/ANS structure was solved by molecular replacement and refined using maximum-likelihood targets with reliability factors Rwork/Rfree of 22.3/27.8%, respectively.
Our approach involved re-integration of raw data, development of the original software in Matlab environment and multidimensional analysis used to build the structure model and perform the refinement for significant improvement of results. The problem of incorporating disorder in the form of phonons into structural analysis was also carried out traditionally by Debye-Waller factor.
References:
[1] Sliwiak, J., Dauter, Z., McCoy, A., Jaskolski, M. & Read, R.J. (2014). Acta Cryst. D70, 471-480.
[2] Wolny, J., Buganski, I., Kuczera, P. & Strzalka, R. (2016). J. Appl. Crystallogr. 49, 2106-2115.
[3] Sliwiak, J., Dauter, Z., Kowiel, M., McCoy, A., Read, R.J & Jaskolski, M. (2015). Acta Cryst. D71, 829-843.
11:40
L. Agnarelli, Dresden: Mg3Pt2: Anionic Chains in a Eu3Ga2-Type Structure
During the investigation of the Mg-Pt system, a new intermetallic compound was identified, Mg3Pt2. The title compound can be obtained through a direct reaction between the components or using the spark-plasma synthesis with MgH2 and PtCl2 as reagents. Mg3Pt2 crystallizes in the monoclinic space group C2/c with a = 7.2096(3) Å, b = 7.1912(4) Å, c = 6.8977(3) Å, and β=106.072(3) o and is isotypic to Eu3Ga2. The striking feature of the crystal structure is anionic parallel chains of Pt atoms, with alternating Pt-Pt distances, stabilized by a complex multicentre interaction involving Pt and Mg species (Figure 1).
Figure 1. Parallel Pt metal chains with Mg atoms acting as bridges between two parallel chains. d1 (blackline), shorter distances between the Pt atoms in the chain; d2 (dashed black line), longer distance between the Pt atoms in the chain.
12:00
Lunch
Chair person: Johannes Roth, Stuttgart
13:30
R. Cardoso-Gil, Dresden: Ga3Ir: a simple but complex material
The intermetallic compounds with the composition T(8)Tr3 (T = group 8 transition metals and Tr: gallium or indium; crystal structure FeGa3-type, space group P42/mnm) are semiconductors. On the other hand, isostructural T(9)Ga3 compounds with T = group 9 transition metals should show metallic behavior. However, theoretical calculations predicted a narrow band-gap of ßF;0.15-0.07 eV for IrGa3.
ht-Ga3Ir is a high-temperature phase crystalizing also in the FeGa3 type of structure. Pure ht-Ga3Ir was synthetized from the elements by quenching from 1173 K and characterized by powder and single-crystal XRD, metallography, WDXS and thermal analysis. The thermoelectric characterization of ht-Ga3Ir was performed and the temperature-dependent electrical resistivity measurements evidenced a semiconductor behavior, with a band-gap of 30 meV. From the single-crystal X-ray diffraction data, a model for the real crystal structure of ht-Ga3Ir is derived by the split-position approach and confirmed by atomic-resolution TEM study [1].
The thermal analysis of ht-Ga3Ir shows its peritectic formation at 974 oC and peritectoid decomposition at 799 oC. Additionally, a yet unknown reaction at 530 oC was observed and interpreted as the peritectoid formation of the low temperature phase lt-Ga3Ir. The crystal structure of lt-Ga3Ir (sample annealed and quenched from 400 oC) was solved using 3D-ED electron diffraction data (space group Pnma, crystal structure Fe3C-type), optimized by quantum-mechanical techniques and refined from the X-ray powder diffraction data [2].
References:
[1] R. Cardoso-Gil, I. Zelenina, Q. E. Stahl, M. Bobnar, P. Kozelj, M. Krnel, U. Burkhardt, I. Veremchuk, P. Simon, W. Carrillo-Cabrera,M. Boström, Yu. Grin, ACS Mater. Au 2021, published online, DOI:10.1021acsmaterialsau.1c00025.
[2] R. Cardoso-Gil, W. Carrillo-Cabrera, F. R. Wagner, Yu. Grin, Z. Anorg. Allg. Chem. 2021, published online, DOI:10.1002/zaac.202100247.
13:50
E. Gaudry, Nancy: Two-dimensional metal structures revealed by evolutionary computations: Pb/Al13Co4(100) as a case study
We have combined extensive Density Functional Theory calculations with an evolutionary algorithm to investigate possible structural models for two-dimensional (2D) Pb films supported on the Al13Co4(100) quasicrystal approximant. The minimization of the total energy with the constraint of maximizing the atomic density in the layer leads to 2D atomic arrangement with pentagonal motifs, reflecting the symmetry of the substrate. Our findings show that the 2D Pb structure can be interpreted as a stable structure, with 16 Pb atoms in the surface cell, in good agreement with the measured coverage and scanning tunnelling microscopy images. Alternatively, a metastable 15-atom 2D film also fits with the experimental observations. This study opens a route towards the prediction of supported complex 2D films [1].
References:
[1] F. Brix and E. Gaudry , Two-dimensional metal structures revealed by evolutionary computations: Pb/Al13Co4(100) as a case study, submitted
Fig.1 : Adsorption energies, structures and simulated STM (scanning tunneling microscopy) images for 2D Pb-films with different atomic densities on Al13Co4(100).
14:10
I. Buganski, Krakow: The atomic model of the Tsai-type quasicrystal with AKNt rhombohedral units
The atomic structure model of CdYb [1] quasicrystal became a universal standard for all icosahedral quasicrystals with Tsai clusters. The model is praised for its geometrical simplicity and chemical order. Atoms cluster in simple shapes like icosahedron, triacontehedron, etc. Clusters are linked via rhombic faces of triacontahedron along 2-fold axis (b-linkage) and interpenetrate along 3-fold axis (c-lnkage). The unaviodable gaps between clusters are filled with rhombohedral units decorated according to the simple-decoration scheme.
Recently, a structural model of ZnMgTm was developed [2]. The model was founded on different principia than the CdYb model. The unique decoration of rhombohedra in Amman-Kramer-Neri tiling (AKNt) was found and exploited for strucutral study. When the model was interpreted for atomic clusters, the linkage along 5-fold axis was found. Not only that. The additional linkage discards the neccessity of having gap-filling atoms. Even though i-ZnMgTm is a Bergman quasicrystal, it brings questions regarding CdYb model. Can the Tsai quasicrystal be solved in the same manner? In this work, the atomic strucutre of CdYb quasicrystal is solved using the AKNt and the strucutral features arising as a consequence of the model are discussed.
References:
[1] Takakuram, H., et al., Nature Mat. 6 (2007), 58-63
[2] Buganski, I., et al., Acta Cryst. A76 (2020), 180-196
14:30
N. Saadi, Liverpool : Compression behaviours of a 3D-printed quasicrystal
For many years lattice type structures have been researched for their mechanical properties with the goal of producing lightweight materials that possess high strength and high stiffness. Additive manufacturing has been used as a way to rapidly prototype these mostly periodic or amorphous structures for mechanical testing. In this study, the mechanical properties of macro-scale aperiodic structures in compression are investigated.
3D models are manufactured using Stereolithography (Form 3, Formlabs, USA) a 3D-printing technology that uses a laser to cure a photosensitive resin layer-by-layer producing a part directly from a CAD model. Quasi-static compression tests are conducted on a 3D printed Tsai-type approximant model to study the mechanical properties of this aperiodic structure and in later stages, assess the energy absorption.
Using Finite Element Analysis (FEA), computer simulations of the lattice under stress are created to compare the theoretical compression behaviours of the aperiodic structure to the 3D-printed experimental findings. With control of the relative density, 3D printed quasicrystal structures could be compared to their periodic counterparts with respect to properties such as specific stiffness, strength and energy absorption.
Figure 1 Contour plot of a deformed Tsai-type QC in Abaqus CAE.
14:50
Break
Chair person: Marc de Boissieu, Grenoble
15:20
E. Svanidze, Dresden: Compounds with mercury - complexity two ways
Mercury, in both elemental form and as part of compounds and amalgams has played an important role in solid-state chemistry and condensed matter physics. However, much care must be taken both during synthesis as well as during characterization of this peculiar element and its compounds - from toxicity concerns to high chemical reactivity - these systems pose several experimental challenges. In this talk I will showcase that, nonetheless, mercury-based materials offer unique crystallographic motifs and, as a result, peculiar physical properties. In particular, I will concentrate on the uranium-mercury binary compounds which have so far been under investigated. By looking at the evolution of crystallographic complexity across the series that contains four binary compounds, I will discuss how atomic arrangement particularities affect the resultant ground states. I will also highlight some of the experimental advances we have been developing in order to synthesize and characterize uranium-mercury compounds, as they pose several handling difficulties as a result of extreme air-and moisture-sensitivity.
15:40
C. Ruano M, Nancy: 2D OQC approximants in reduced SrTiO3 grown on Pt(111)/Al2O3(0001)
We report an all-thin film stacing approach to investigate the SrTiO3/Pt(111)/Al2O3(00001) system, where three different oxide quasicrystal (OQC) approximants were observed by means of low energy electron diffraction and scanning tunneling microscope. High-resolution images show bright protrusions located at the node of the different tilings constructed with the same basic square, triangle and 30° rhombus tiles possessing a common edge length of about 6.7 Å.
Depending on the preparation conditions, we identified a giant square-like approximant, consisting in 72 elements; a large hexagonal approximant, consisting in 29 elements; and the ?-approximant, consisting in only 5 elements.
The structural and electronic properties of the phases were investigated by means of density functional theory (DFT). Simulated microscopy images were also obtained from a model identifying the bright protrusions as Sr atoms, providing an excellent agreement with the experimental observations. Charge transfer is found to occurs at the interface, from most electropositive elements (Sr, Ti) to most electronegative ones (Pt and O), influencing the rumpling and the adhesion energy of the oxide films
Figure1. Comparison between the experimental (orange) and calculated (gray) STM images for the square (left), the hexagonal (middle) and the ?-approximant (right). The ideal unit cells decorated with square, triangle and rhombus tiling elements are highlighted as well.
16:00
C.M.N. Kumar, Zagreb: Structural investigation of super-ionic Cu2-xSe
Materials that exhibit the super-ionic behavior present an extremely promising perspective for replacing liquid electrolytes in batteries with solid-state ion conductors and could revolutionize super-capacitors, fuel cells, and solid-state batteries [1]. Fast ion transport in many of such materials is supported by a disordered, 'liquid-like' sub-lattice of cations mobile within a rigid anionic sub-lattice. Transition metal chalcogenide (TMC) Cu2-xSe has attracted much attention recently due to its super-ionic behavior [2]. In Cu2-xSe with liquid-like copper lattice within the FCC selenium lattice, there is an indication of the super-ionic phase surviving down to room temperatures (and even below) making it an extremely promising material for technological applications. The phase diagram of Cu2-xSe has been intensively investigated previously, but data differ across literature [3, 4]. Here we present results of our synchrotron x-ray diffraction studies which show direct correlation between structure and ionic behavior of Cu2-xSe in a wide temperature range.
Reeferences:
[1] Hu L. et al, Nanomaterials 10, 302 (2020).
[2] Liu, H. et al., Nature Materials 11. 422-425 (2012).
[3] Glazov, V. M. et al., Inorganic Materials 36, 641-652 (2000).
[4] Tonejc, A. et al., J. Matter. Sci. 15, 3090-3094 (1980).
16:20
G. de Laitre, Grenoble: Dynamical properties of the incommensurately modulated Rb2ZnCl4 phase
Aperiodic crystals are long-range ordered crystals that lack periodicity. Although their sturcure is well described, their dynamics are more debated. In particular, phason modes have been experimentally observed in few incommensurately modulated phase.
The Rb2ZnCl4 phase displays several transitions. Above Ti=303K the HT phase is described as a crystal structure of space group Pmcn where ZnCl4 tetrahedrons have disordered orientations. From Ti = 303 K, down to TC = 195 K, orientations of these tetrahedrons gets incommensurately modulated along the c axis with an increasing anharmonicity. Below TC, the modulation gets locked-in, the c cell parameter is then tripled. We probed the dynamics of this material through inelastic neutron scattering for T between 140 K and 350 K.
At 140 K TA phonons associated to the satellite reflections are indistinguishable from others. However their intensity decreases abnormaly with increasing T, while a growing quasi-elastic signal localized around satellites positions expands. This signal remains localized in the HT phase despite the prevailing disorder. We discuss its relation with the modulation and its phasonic nature.
16:40
E. Pospišilova , Bratislava: Stable thin clathrate layers
Energies per atom and surface energies of pure Sn, Ge and Si clathrate free-standing nanofilms as a function of coverage (number of atoms per unit area of the slab) are evaluated in wide range of slab thicknesses from 1 to 10 ML (units of (111)-diamond monolayers). The stability of clathrate nanofilms is assessed by comparison with metalic beta and gamma phase slabs of Sn as well as with several distinct reconstructions of (111)-terminated diamond slabs including 3x3, 7x7 DAS (Dimer + Adatom + Stacking Fault) reconstructions which are the principal competitors to Ge and Si clathrates. In the case of Sn and Ge, the stability region of the clathrate thin films with reconstructued surface is clearly recognizable.
