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The structure of the glassy GexTe1−x system, with x = 0.17, 0.21, 0.28, 0.30, and 0.45, is studied using the small-angle neutron scattering (SANS) technique. The very-low-momentum-transfer region of the diffractogram exhibits distinct behaviour depending on the germanium content. A similar conclusion is drawn from the analysis of the first diffraction peaks observed at higher angles. This system exhibits three composition regions with distinct behaviours: a first zone of low Ge content (up to about 20–25 at.%), a third zone richer in Ge (from about 30 at.% and above), and a second transitional zone between them. These changes are reflected in the parameters that govern Porod’s region, as well as in the region where the first diffraction peaks appear, corroborating previous observations made using other experimental and simulation techniques. Our study provides experimental evidence that could open up new possibilities for conducting simulations using neutron data. The results presented here show that increasing Ge content leads to a strengthening of the intermediate-range order at the expense of a weakening of the short-range order.

A semi-automated workflow relying on atomic-scale modelling is introduced to explore and understand the yet-unsolved structure of the crystalline AsTe3 material, recently obtained from crystallization of the parent AsTe3 glass, which shows promising properties for thermoelectric applications. The seemingly complex crystal structure of AsTe3 is investigated with density functional theory, from the stand point of As/Te disorder, in a structural template derived from elemental-Te (Teel), following experimental findings from combined X-ray total scattering and diffraction. Our workflow includes a combinatorial structure generation step followed by successive structure selection and relaxation steps with progressively-increasing accuracy levels and a multi-criterion evaluation procedure. A small set of high quality models with common structural features emerge, all consisting of intergrowth domains typically below 1 nm in thickness and with the local compositions and structures of Teel and α-As2Te3, but with different thicknesses and relative arrangements, which points to the presence of such defects in crystalline AsTe3. While predictions of the electronic bandgaps are in excellent agreement with the experimental value for our best models, we find that some of these defects may be associated with the locally-increased density of states around the Fermi level, potentially contributing to the overall electronic conductivity, which, along with intrinsic structural complexity, is among the key features of the reported outstanding thermoelectric properties of this compound. [ABSTRACT FROM AUTHOR]

The structure of the glassy GexTe1−x system, with x = 0.17, 0.21, 0.28, 0.30, and 0.45, is studied using the small-angle neutron scattering (SANS) technique. The very-low-momentum-transfer region of the diffractogram exhibits distinct behaviour depending on the germanium content. A similar conclusion is drawn from the analysis of the first diffraction peaks observed at higher angles. This system exhibits three composition regions with distinct behaviours: a first zone of low Ge content (up to about 20–25 at.%), a third zone richer in Ge (from about 30 at.% and above), and a second transitional zone between them. These changes are reflected in the parameters that govern Porod's region, as well as in the region where the first diffraction peaks appear, corroborating previous observations made using other experimental and simulation techniques. Our study provides experimental evidence that could open up new possibilities for conducting simulations using neutron data. The results presented here show that increasing Ge content leads to a strengthening of the intermediate-range order at the expense of a weakening of the short-range order. [ABSTRACT FROM AUTHOR]

Thiosilicate and selenosilicate glasses are the sulfide and selenide counterparts of the well-known silicate glasses. This paper reviews the main investigations carried out to shed light on the structural, thermal and electrical characteristics of these glasses, showing their particularities compared to their oxide counterparts, such as a more complex structure with the presence of both, corner- and edge-sharing tetrahedra and much higher ion conductivity due to the larger polarizability of the chalcogen and the more covalent nature of the bonds, but also, their similarities with the presence of both, bridging and non-bridging S or Se or the existence of a mixed alkali effect when two mobile $\mathrm{Li}^{+}$ and $\mathrm{Na}^{+}$ cations coexist in the glassy network. Structural consequences of competition between $\mathrm{SiS}_{2}$ and another former, $\mathrm{GeS}_{2}$, $\mathrm{PS}_{5/2}$ or $\mathrm{BS}_{3/2}$ are also discussed.

The structure of evaporated amorphous GexSbxTe100−2x (x = 6, 9, 13) alloys was investigated by neutron diffraction, X‐ray diffraction, and extended X‐ray absorption spectroscopy at the Ge, Sb, and Te K‐edges. Large‐scale structural models were generated by fitting the experimental datasets (five for each composition) simultaneously in the framework of the reverse Monte Carlo simulation technique. It was found that the alloys are chemically ordered (Ge and Sb have predominantly Te neighbors) and within the experimental uncertainty, each component satisfies the 8 – N rule. A comparison with the pair correlation functions of melt‐quenched Ge20Te80 revealed that the first minimum of gTeTe(r) is shallower in the ternary alloys than in Ge20Te80. On the other hand, the separation of the first and second coordination environments of Ge atoms is stronger in the Ge–Sb–Te alloys investigated. [ABSTRACT FROM AUTHOR]

Peak Force Quantitative Nano-Mechanical property mapping (PF-QNM) was used to explore the nanomechanical properties of binary GeySe100−y glasses and phase-separated Agx(Ge0.25Se0.75)100−x glasses. The indentation modulus decreases when the selenium content increases in the binary glasses and when the silver content increases in phase-separated ternary glasses as a consequence of the increasing flexibility of the network. At nanoscale level, PF-QNM measurements highlighted a higher indentation modulus for the Ag-rich phase than for the Ag-poor phase, together with a decrease of both local indentation moduli with the increase of silver content in the glass. A model to gain a further insight in the mean coordination numbers and compositions of the Ag-rich and Ag-poor phases, based upon the assumption of a decoupled effect of the variations in Ag and in Ge on the mechanical properties, is described in detail. In the framework of the model, the mean coordination numbers for the Ag-rich phase and Ag-poor phase keep on decreasing when the total silver content in the glasses increases. The Ge concentration differs in Ag-rich and Ag-poor phases. When the silver content in the glass increases, the Ge concentration decreases in both phases. The result, in turn, indicates that an increase in the conductivity of the Ag-rich phase when the total silver content increases, as shown previously by C-AFM, would not be only explained by an increase in silver content but also by a softening of the network and consequently by an increase of charge carrier mobility.

The transition from a semiconductor to a fast-ion conductor with increasing silver content along the Agx(Ge0.25Se0.75)(100−x) tie line (0≤x≤25) was investigated on multiple length scales by employing a combination of electric force microscopy, X-ray diffraction, and neutron diffraction. The microscopy results show separation into silver-rich and silver-poor phases, where the Ag-rich phase percolates at the onset of fast-ion conductivity. The method of neutron diffraction with Ag isotope substitution was applied to the x=5 and x=25 compositions, and the results indicate an evolution in structure of the Ag-rich phase with change of composition. The Ag–Se nearest-neighbours are distributed about a distance of 2.64(1) Å, and the Ag–Se coordination number increases from 2.6(3) at x=5 to 3.3(2) at x=25. For x=25, the measured Ag–Ag partial pair-distribution function gives 1.9(2) Ag–Ag nearest-neighbours at a distance of 3.02(2) Å. The results show breakage of Se–Se homopolar bonds as silver is added to the Ge0.25Se0.75 base glass, and the limit of glass-formation at x≃28 coincides with an elimination of these bonds. A model is proposed for tracking the breakage of Se–Se homopolar bonds as silver is added to the base glass.

Bi2Te3-based compounds are a well-known class of outstanding thermoelectric materials. β-As2Te3, another member of this family, exhibits promising thermoelectric properties around 400 K when appropriately doped. Herein, we investigate the high-temperature thermoelectric properties of the β-As2−xBixTe3 solid solution. Powder X-ray diffraction and scanning electron microscopy experiments showed that a solid solution only exists up to x = 0.035. We found that substituting Bi for As has a beneficial influence on the thermopower, which, combined with extremely low thermal conductivity values, results in a maximum ZT value of 0.7 at 423 K for x = 0.017 perpendicular to the pressing direction.

Platinum is the most employed electrocatalyst for the reactions taking place in energy converters, such as the oxygen reduction reaction in proton exchange membrane fuel cells, despite being a very low abundant element in the earth’s crust and thus extremely expensive. The search for more active electrocatalysts with ultra-low Pt loading is thus a very active field of investigation. Here, surface-limited redox replacement (SLRR) that utilizes the monolayer-limited nature of underpotential deposition (UPD) was used to prepare ultrathin deposits of Pt, using Te as sacrificial metal. Cyclic voltammetry and anodic potentiodynamic scanning experiments have been performed to determine the optimal deposition conditions. Physicochemical and electrochemical characterization of the deposited Pt was carried out. The deposit comprises a series of contiguous Pt islands that form along the grain interfaces of the Au substrate. The electrochemical surface area (ECSA) of the Pt deposit obtained after 5 replacements, estimated to be 18 m2/g, is in agreement with the ECSA of extended surface catalysts on flat surfaces.

We report a joint experimental and theoretical high-pressure study of the structural and vibrational properties of tetradymite-like (R3¯m) β-As2Te3. Two samples have been characterized by angle-dispersive synchrotron powder X-ray diffraction and Raman scattering measurements under hydrostatic conditions with the help of ab initio calculations. One sample was synthesized at high pressure and high-temperature conditions with a Paris-Edinburg cell and the other by the melt-quenching technique. Both β-As2Te3 samples show the same properties and exhibit two isostructural phase transitions of order higher than 2, i.e. of electronic origin, near 2.0(2) and 6.0(5) GPa that are compatible with the changes predicted by recent electronic band structure calculations. The first isostructural phase transition can be attributed to the topological quantum phase transition from a trivial insulator to a topological insulator, passing through a 3D Dirac topological semimetal. This topological transition, specific to β-As2Te3, is not observed in isostructural Te-based sesquichalcogenides α-Sb2Te3 and α-Bi2Te3 that are topological insulators at room conditions. The second isostructural phase transition is likely related to an insulator-metal transition. Additionally, we have observed two partially reversible first-order phase transitions in β-As2Te3 above 10 and 17 GPa. We have found a high anharmonic behavior of the two Raman-active modes with the lowest frequencies in β-As2Te3 that explains the already reported ultra-low lattice thermal conductivity of β-As2Te3. Moreover, we have studied the similarities of β-As2Te3 with α-Sb2Te3 and α-Bi2Te3 (two of the best thermoelectric materials), thus providing insights into the origin of the ultra-low lattice thermal conductivity values in these compounds related to unconventional chemical bonds present in these isostructural materials. [ABSTRACT FROM AUTHOR]