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Semiconducting SnSe, an analog of black phosphorus, recently attracted great scientific interest due to a disputed report of a large thermoelectric figure of merit, which has not been reproduced subsequently. Here we concentrate on the low-temperature ground state. To gain a better understanding of the system, we present magneto-transport properties in high-quality single crystals of as-grown, lightly doped SnSe down to liquid helium temperatures. We show that SnSe behaves as a p-type doped semiconductor in the vicinity of a metal-insulator transition. Electronic transport at the lowest temperatures is dominated by the hopping mechanism. Negative magnetoresistance at low fields is well described by antilocalization, while positive magnetoresistance at higher fields is consistent with the shrinkage of localized impurity wavefunctions. At higher temperatures, a dilute metallic regime is realized where elusive T2 and B2 resistivity dependence is observed, posing a challenge to theoretical comprehension of the underlying physical mechanism.

The crystal structure of the complex intermetallic phase o-Al13Co4 was first investigated in 1994, but some open questions still remained. The new refinement of the crystal structure using high-resolution X-ray diffraction data resulted in a much more complex model including a large number of split positions (space group Pnm21, a = 8.1590(6) Å, b = 12.349(1) Å, c = 14.453(1) Å). The model is interpreted in terms of local violation of the translational symmetry, which is confirmed by HRTEM and HRSTEM investigations. Chemical bonding between two partial structures in o-Al13Co4 - three-dimensional framework and linear Co–Al–Co groups in cages of the latter – along with the stacking faults (local twinning) are discussed as possible reasons for such an extended disorder. Previous article in issue

Complex intermetallic compounds such as transition metal (TM) aluminides are promising alternatives to expensive Pd-based catalysts, in particular for the semi-hydrogenation of alkynes or alkadienes. Here, we compare the gas-phase butadiene hydrogenation performances of o-Al13Co4(100), m-Al13Fe4(010) and m-Al13Ru4(010) surfaces, whose bulk terminated structural models exhibit similar cluster-like arrangements. Moreover, the effect of the surface orientation is assessed through a comparison between o-Al13Co4(100) and o-Al13Co4(010). As a result, the following room-temperature activity order is determined: Al13Co4(100) < Al13Co4(010) < Al13Ru4(010) < Al13Fe4(010). Moreover, Al13Co4(010) is found to be the most active surface at 110°C, and even more selective to butene (100%) than previously investigated Al13Fe4(010). DFT calculations show that the activity and selectivity results can be rationalized through the determination of butadiene and butene adsorption energies; in contrast, hydrogen adsorption energies do not scale with the catalytic activities. Moreover, the calculation of projected densities of states provides an insight into the Al13TM4 surface electronic structure. Isolating the TM active centers within the Al matrix induces a narrowing of the TM d-band, which leads to the high catalytic performances of Al13TM4 compounds., Graphical abstract

The atomic structure of the Al13Ru4(010) approximant surface, investigated for annealing temperatures ranging from 873 K to 1143 K, can be described by pentagonal motifs and vacancies. It exhibits an atypical surface reconstruction which manifests itself in real space by well-separated stripes running about 10○ off the [001] direction. Their mutual ordering is drastically improving at high annealing temperature. This is simultaneously accompanied by a filling of the pentagonal hollow sites present in the stripes with atoms or groups of atoms. We provide evidence that these unidirectional features are of true surface origin. The bulk electronic density of states calculated within the density functional theory framework matches the experimental valence band spectra. In addition to step edges, molecular adsorption reveals that stripes are initially the most reactive sites, leading to anisotropic island growth.

Thermoelectric materials as energy harvesters are a hot spot in research community due to their ability to directly convert waste heat into electrical energy. The efficiency of thermoelectric materials is determined by the dimensionless figure of merit ZT. For high thermoelectric conversion efficiency large Seebeck coefficient, large electrical conductivity and low thermal conductivity are required. Unfortunately, last two requirements are often hard to realize simultaneously in the same material because charge carriers usually carry heat efficiently as well. Thus systems with relatively low charge carrier concentration are usually good thermoelectrics. In those systems phonons are dominant heat conducting channel and their contribution can be further reduced through material architecturing. But, it was recently reported that SnSe in single crystalline form, exhibits high thermoelectric figure of merit (2.6 at 973 K) mainly due to its intrinsic extremely low thermal conductivity [1]. This makes SnSe one of the most promising thermoelectric materials. Although physical properties of SnSe above room temperature are well investigated, there are very few studies of low-temperature single-crystalline phase [2]. Motivated to determine the possible origins of these remarkable properties, we measured thermoelectric properties of SnSe down to low temperatures. Here we present investigation of anisotropic thermoelectric transport properties of SnSe at low temperatures, along the three crystallographic directions. We find that the thermal conductivity features a pronounced umklapp maximum near 12K. Our results indicate that the lattice thermal conductivity of single-crystalline SnSe at room temperature is higher than this reported in the literature [1]. We further discuss possible reasons for the differences observed between our results and those previously obtained by Zhao et al. [1]. Wei et al. have reported that the lower densities of the samples measured by Zhao et al. (5.4 g/cm3) may be correlated with the lower thermal conductivity values of their samples (our sample density is close to theoretical density of 6.18 g/cm3) [3]. This implies that their reported thermal conductivities are not intrinsic to high quality SnSe single crystal. Possible routes for optimizing low- temperature thermoelectric efficiency will be discussed. [1] Zhao, L. D., Lo, S. H., Zhang, Y., Sun, H., Tan, G., Uher, C., Wolverton, C., Dravid, V. P., Kanatzidis, M. G. (2014). Nature 508, 373. [2] Ibrahim, D., Vaney, J.-B., Sassi, S., Candolfi, C., Ohorodniichuk, V., Levinsky, P., Semprimoschnig, C., Dauscher A., Lenoir, B. (2017). Appl. Phys. Lett. 110, 032103. [3] Wei, P.C., Bhattacharya, S., He, J., Neeleshwar, S., Podila, R., Chen, Y.Y., Rao, A. M. (2016). Nature 539, E1-E2.

Magneto-transport properties of single crystal SnSe thermoelectric

International audience; The intermetallic compound InPd (CsCl type of crystal structure with a broad compositional range) is considered as a candidate catalyst for the steam reforming of methanol. Single crystals of this phase have been grown to study the structure of its three low-index surfaces under ultra-high vacuum conditions, using low energy electron diffraction (LEED), X-ray photoemission spectroscopy (XPS), and scanning tunneling microscopy (STM). During surface preparation, preferential sputtering leads to a depletion of In within the top few layers for all three surfaces. The near-surface regions remain slightly Pd-rich until annealing to ∼580 K. A transition occurs between 580 and 660 K where In segregates towards the surface and the near-surface regions become slightly In-rich above ∼660 K. This transition is accompanied by a sharpening of LEED patterns and formation of flat step-terrace morphology, as observed by STM. Several superstructures have been identified for the different surfaces associated with this process. Annealing to higher temperatures (≥750 K) leads to faceting via thermal etching as shown for the (110) surface, with a bulk In composition close to the In-rich limit of the existence domain of the cubic phase. The Pd-rich InPd(111) is found to be consistent with a Pd-terminated bulk truncation model as shown by dynamical LEED analysis while, after annealing at higher temperature, the In-rich InPd(111) is consistent with an In-terminated bulk truncation, in agreement with density functional theory (DFT) calculations of the relative surface energies. More complex surface structures are observed for the (100) surface. Additionally, individual grains of a polycrystalline sample are characterized by micro-spot XPS and LEED as well as low-energy electron microscopy. Results from both individual grains and “global” measurements are interpreted based on comparison to our single crystals findings, DFT calculations and previous literature.

Oxidation of the Al 9 Co 2 (0 0 1) and Al 13 Co 4 (1 0 0) surfaces has been performed in a wide range of temperatures at 2 × 10 −8 or 1 × 10 −7 mbar oxygen pressure. Only Al O bonding is observed. The oxidation kinetics are controlled by the quantity of oxygen sticking on the surface. Oxidation results from a competition between several effects: formation of an oxide film, dissolution of the film, oxygen desorption and oxygen dissolution into the bulk. For temperatures lower than 710 K for the Al 9 Co 2 (0 0 1) surface and 925 K for the Al 13 Co 4 (1 0 0) surface, a ∼5 A thick oxide film is formed which does not show any long-range order and desorbs upon annealing. When oxidation is performed at higher temperatures, oxygen diffusion into the bulk is observed. A poorly ordered oxide film having a sixton structure is formed on the Al 9 Co 2 (0 0 1) surface when oxidation is performed at 775 K, which is dissolved when annealing at higher temperatures. On the Al 13 Co 4 (1 0 0) surface, only a weak streaky polar circle is observed following annealing at 925 K the film formed at room temperature, which corresponds to an hexagonal network of O atoms into small ultrathin oxide layers domains. The oxidation behaviour of the Al 9 Co 2 (0 0 1) and Al 13 Co 4 (1 0 0) surfaces has been ascribed to the strong covalent character of bonds present in these Al Co phases, which prevents aluminium diffusion. C 2 H n molecules ( n = 2, 4, 6) do not adsorb on the Al 13 Co 4 (1 0 0) surface in the experimental conditions used in this study, thus suggesting that this surface might not be the active one in the semi-hydrogenation of acetylene.

Low temperature transport properties of single crystal SnSe thermoelectric

The intermetallic phase InPd is a candidate material for the use as a catalyst in the methanol steam reforming process. To study the connection between the catalytic properties of the surface and the structural and electronic properties of the bulk, we have grown single crystals of the InPd phase by the Czochralski method and determined their electronic, thermal, magnetic and hydrogen-absorption properties. By growing crystals from a high-temperature solution, we could crystallize a slightly offstoichiometric In-rich composition In1.04Pd0.96, which contained a significant amount of constitutional defects in the lattice (Pd vacancies on the Pd sublattice) to retain the CsCl-type structure. The strongly inhomogeneously broadened 115 In NMR spectrum and the high residual (T / 0) electrical resistivity confirmed the presence of constitutional defects. Single crystals of InPd do not absorb hydrogen, as requested for a good hydrogenation catalyst material. Calculated electronic density of states (DOS) shows large contribution of Pd(d) states at the Fermi level. Application of the electron localizability indicator reveals ionic and multi-centre InePd interactions stabilizing the crystal structure. The electrical and thermal conductivities of InPd show metallic character, whereas the thermoelectric power and the Hall coefficient both show positive sign, revealing that InPd is a predominant hole-type conductor. The calculated electronic DOS at the Fermi energy is in a good agreement with the experimental value determined from the low-temperature specific heat. Magnetic measurements have shown that InPd is a diamagnet. All results are compared to the chemically related intermetallic compound GaPd. The active esite-isolation concept for increased catalytic selectivity is discussed in relation to the InPd and GaPd structures.

International audience; Isothermal oxidation of Al65Cr27Fe8 and Al80Cr15Fe5 was studied in the 600-1080 degrees C range. Formation of transient alumina layers is obtained up to 900 degrees C. On Al65Cr27Fe8 transient to alpha-phase transformations occur when performing oxidation at 1000 degrees C, together with the possible appearance of (Al0.9Cr0.1)(2)O-3. At 1080 degrees C, direct formation of alpha-alumina is obtained. On Al80Cr15Fe5, spallation of the oxide layer during the cooling stage is observed following oxidation at 800 and 900 degrees C, revealing thermal etching of the underneath alloy surface. At 1050 degrees C the alpha-Al2O3 scale is directly formed but plastic deformation and recrystallization of the underneath alloy into several intermetallic phases is observed.

International audience; Atomic oxygen adsorption on the pure aluminium terminated Al9Co2 (001) surface is studied by first principle calculations coupled with STM measurements. Relative adsorption energy of oxygen atoms have been calculated on different surface sites along with the associated STM images. The local electronic structure of the most favourable adsorption site is described. The preferential adsorption site is identified as a "bridge" type site between the cluster entities exposed at the (001) surface termination. The Al-O bonding between the adsorbate and the substrate presents a covalent character, with s − p hybridization occuring between the states of the adsorbed oxygen atom and the aluminium atoms of the surface. The simulated STM image of the preferential adsorption site is in agreement with experimental observations. This work shows that oxygen adsorption generates important atomic relaxations of the topmost surface layer and that sub-surface cobalt atoms strongly influence the values of the adsorption energies. The calculated Al-O distances are in agreement with those reported in Al2O and Al2O3 oxides and for oxygen adsorption on Al(111).

Proucavan je Hallov efekt u monokristalima monoklinskog Y-Al-Ni-Co [1], ortorompskog o-Al13Co4 [2] i monoklinskih m-Al13Fe4 i m-Al13(Fe, Ni)4 [3] aproksimanata dekagonalnih kvazikristala za sve kombinacije smjerova elektricne struje i magnetskog polja duž glavnih kristalnih osi u temperaturnom intervalu od 90 do 370 K. U sva cetiri spomenuta intermetalna spoja, koji pripadaju Al13TM4 (TM = prijelazni metal) klasi aproksimanata dekagonalnih (d) kvazikristala, Hallov koeficijent RH pokazuje dobro definiranu anizotropiju. RH je pozitivan (supljinskog karaktera) ili nula za magnetsko polje paralelno s ravninom koja odgovara kvaziperiodicnoj ravnini kod dekagonalnih kvazikristala, a negativan (elektronskog karaktera) ili nula za magnetsko polje okomito na tu ravninu. Jedini izuzetak je RH kod Al13Fe4, za polje usmjereno okomito na ravninu, gdje RH mijenja predznak od pozitivnog u negativni s porastom temperature. Rezultati anizotropije za RH korelirani su s anizotropijom RH u d-Al-Ni-Co [4] kvazikristalu i dan je kratak pregled teorijskih rezultata. Navedena istraživanja provode se u okviru European Integrated Centre for the Development of New Metallic Alloys and Compounds (C-MAC) i MZOS projekata broj 035-0352826-2848 i 286-0000000-3212. [1] A. Smontara, et al., Phys. Rev. B 78 (2008) 104204. [2] J. Dolinsek, et al., Phys. Rev. B 79 (2009) 184201. [3] P. Popcevic, et al., Phys. Rev. B 81 (2010) 184203. [4] P. Popcevic, et al., rad u pripremi ; D. Cmrk et al., poster na ovom ZS.

We present a detailed study of the (001) surface of the Al9Co2 crystalusing both experimental methods and first-principles calculations basedon the density functional theory. Experimentally, the (1 x 1) low-energyelectron diffraction pattern combined with the composition determinedusing X-ray photoemission spectroscopy is consistent with a bulkterminated surface. In addition, scanning tunneling microscopy (STM)measurements highlights the presence of only one type of surfacetermination. Combining experimental results with electronic structurecalculations provides many arguments to correlate the observed surfacetermination with the dense pure aluminum layer present in the Al9Co2bulk structure: (i) calculations show that this termination presents alower surface energy compared to another conceivable terminationobtained also from bulk truncation, (ii) step height measurements areconsistent with calculated interlayer spacings, (iii) calculatedbroadened densities of states and simulated STM images are qualitativelyin agreement with the experimental photoelectron spectra and theexperimental STM images.

The electrical (σ) and thermal conductivity (λ) of Al76Co22Ni2 decagonal approximant has been measured along three orthogonal directions a *, b and cof the Al76Co22Ni2 unit cell, where (a, c) monoclinic atomic planes are stacked along the perpendicular b direction. Electrical conductivity is relatively high in all crystalline directions, appearing in the order σb >> σc > σa* . Thermal conductivity appears in the same order λb > λc > λa*, so that b is the most conducting direction for both electricity and heat. The results are compared to the reported anisotropic electrical and thermal conductivity of the d-Al–Ni–Co decagonal quasicrystal.

We have investigated anisotropic electrical resistivity and thermoelectric power of the ypsilon-phase Al–Ni–Co (Y–Al–Ni–Co) decagonal approximant with composition Al76Co22Ni2. The crystalline-direction-dependent measurements were performed along three orthogonal directions a*, b and c of the Y–Al–Ni–Co unit cell, where (a, c) monoclinic atomic planes are stacked along the perpendicular b direction. Anisotropic electrical resistivity is low in all crystalline directions, appearing in the order ρ a* > ρ c > ρ b and showing positive temperature coefficient typical of electron-phonon scattering mechanism. Thermopower shows electron-phonon enhancement effect. Anisotropic bare thermopower (in the absence of electron-phonon interactions) was extracted, appearing in the same order as the resistivity, ΙS a* bare/TΙ > Ι S c bare/T Ι > Ι S b bare/T Ι.

International audience; We present a study of the anisotropic electrical, magnetic and thermal transport properties of a single-crystalline Al80Cr15Fe5 approximant to the decagonal quasicrystal. Temperature-dependent electrical resistivity along the b and c crystalline directions shows nonmetallic behavior with a broad maximum, whereas it shows metallic positive temperature coefficient along the a direction perpendicular to the (b,c) atomic planes. The resistivity can be treated with the model of slow charge carriers, where the increased electron-phonon scattering upon raising the temperature induces transition from dominant Boltzmann (metallic) to dominant non-Boltzmann (insulatinglike) regime. Magnetic susceptibility and thermal conductivity show small anisotropy as well.

Zeitschrift für Kristallographie, 223 (11-12), ISSN:1433-7266, ISSN:2194-4946, ISSN:0044-2968, ISSN:2196-7105

We have investigated anisotropic physical properties (magnetic susceptibility, electrical resistivity, thermoelectric power, Hall coefficient, and thermal conductivity) of Y-Al-Ni-Co decagonal approximant with composition ${\text{Al}}_{76}{\text{Co}}_{22}{\text{Ni}}_{2}$. The crystalline-direction-dependent measurements were performed along three orthogonal directions ${a}^{\ensuremath{\ast}}$, $b$, and $c$ of the Y-Al-Ni-Co unit cell, where $(a,c)$ monoclinic atomic planes are stacked along the perpendicular $b$ direction. Anisotropic magnetic susceptibility of conduction electrons is paramagnetic for the field lying within the $(a,c)$ atomic planes and diamagnetic for the field along the $b$ direction. Anisotropic electrical resistivity is low in all crystalline directions, appearing in the order ${\ensuremath{\rho}}_{{a}^{\ensuremath{\ast}}}g{\ensuremath{\rho}}_{c}⪢{\ensuremath{\rho}}_{b}$. Thermopower shows electron-phonon enhancement effect. Anisotropic bare thermopower (in the absence of electron-phonon interactions) was extracted, appearing in the same order as the resistivity, $|{S}_{{a}^{\ensuremath{\ast}}}^{\text{bare}}/T|g|{S}_{c}^{\text{bare}}/T|g|{S}_{b}^{\text{bare}}/T|$. Anisotropic thermal conductivity appears in the order ${\ensuremath{\kappa}}^{b}g{\ensuremath{\kappa}}^{c}g{\ensuremath{\kappa}}^{{a}^{\ensuremath{\ast}}}$, so that $b$ is the most conducting direction for both electricity and heat. Hall coefficient ${R}_{H}$ exhibits pronounced anisotropy, where the magnetic field in a given crystalline direction yields the same ${R}_{H}$ for the current along the other two crystalline directions in the perpendicular plane. These anisotropies are analyzed in terms of the anisotropic structure of the Y-Al-Ni-Co phase and the ab initio calculated anisotropic Fermi surface. The results are compared with the literature-reported anisotropy of the physical properties of the $d$-Al-Ni-Co decagonal quasicrystal.

International audience; Large single crystals of the decagonal AlCoNi phase were reproducibly grown by the Czochralski method from Al-rich melts. By using native seeds, it was possible to grow crystals with well-defined orientations, e.g. along the 10-fold periodic axis or parallel to one of the 2-fold axes. Quasicrystals, that had been pulled in periodic or quasiperiodic directions, were characterized by X-ray transmission topography at the synchrotron source (ESRF) and at laboratory sources using the Lang technique. Because of the metallurgical homogeneity of the grown quasicrystals, lattice parameter differences did not contribute to the topographic contrast, instead, orientation contrast dominated. A column-like substructure along the periodic direction could be revealed. From this it is concluded, that crystal growth along a two-fold direction might be favourable with respect to minimizing defect spreading.

We have investigated the surfaces of decagonal AlNiCo single quasicrystals in UHV by Reflection High Energy Electron Diffraction (RHEED), Atomic Force Microscopy (AFM) and Auger Electron Spectroscopy (AES). The surfaces were prepared by Ar + -sputtering and annealing. We observe well-defined RHEED patterns directly after sputtering at room temperature, revealing a ten-fold symmetry with respect to the azimuthal angle of the incident beam. Two patterns occur, alternating every 18°. This symmetry behaviour is probably caused by a bcc overlayer consisting of multiple domains in different orientations. The RHEED patterns show a strong temperature dependence. A spontaneous phase transition into a quasiperiodic structure is observed upon annealing to about 350 °C. RHEED patterns of a flat, quasicrystalline surface of good structural quality are observed after sputtering at 400 °C. As indicated by AES, the influence of oxygen on the structural behaviour seems to be important.

Low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM) results are used to study the pseudo-6-fold nature of the (100) surface of the orthorhombic quasicrystal approximant Al4(Cr, Fe). LEED patterns are also presented from the pseudo-10-fold (010) surface of this material. In each case the results are compared with the known bulk structure of this complex metallic alloy.