Switching of the topologically trivial and non-trivial quantum phase transitions in compressed 1T-TiTe2: Experiments and Theory

We report the structural, vibrational and electrical transport properties up to 16 GPa of the 1T-TiTe2, a prominent layered 2D system, which is predicted to show a series of topologically trivial - nontrivial transitions under hydrostatic compression. We clearly show signatures of two iso-structural transition at 2 GPa and 4 GPa obtained from the minima in c/a ratio concomitant with the phonon linewidth anomalies of Eg and A1g modes at around the same pressures, providing strong indication of unusual electron-phonon coupling associated to these transitions. Resistivity presents nonlinear behavior over similar pressure ranges providing a strong indication of the electronic origin of these pressure driven isostructural transitions. Our data thus provide clear evidences of topological changes at A and L point of the Brillouin zone predicted to be present in the compressed 1T-TiTe2. Between 4 GPa and 8 GPa, the c/a ratio shows a plateau suggesting a transformation from an anisotropic 2D layer to a quasi 3D crystal network. First principles calculations suggest that the 2D to quasi 3D evolution without any structural phase transitions is mainly due to the increased interlayer Te-Te interactions (bridging) via the charge density overlap. In addition to the pressure dependent isostructural phase transitions, our data also evidences the occurrence of a first order structural phase transition from the trigonal (P-3m1) phase at higher pressures. We estimate the start of this structural phase transition to be 8 GPa and the symmetric of the new high-pressure phase to be monoclinic (C2/m).Comment: 22 pages, 11 Figures, 2 Table

Structural, vibrational, and electrical properties of 1T-TiTe2 under hydrostatic pressure: Experiments and theory

We report the structural, vibrational, and electrical transport properties up to ∼16 GPa of 1T -TiTe2, a prominent layered 2D system. We clearly show signatures of two isostructural transitions at ∼2 GPa and ∼4 GPa obtained from the minima in c/a ratio concomitant with the phonon linewidth anomalies of Eg and A1g modes around the same pressures, providing a strong indication of unusual electron-phonon coupling associated with these transitions. Resistance measurements present nonlinear behavior over similar pressure ranges shedding light on the electronic origin of these pressure-driven isostructural transitions. These multiple indirect signatures of an electronic transition at ∼2 GPa and ∼4 GPa are discussed in connection with the recent theoretical proposal for 1T -TiTe2 and also the possibility of an electronic topological transition from our electronic Fermi surface calculations. Between 4 GPa and ∼8 GPa, the c/a ratio shows a plateau suggesting a transformation from an anisotropic 2D layer to a quasi-3D crystal network. First-principles calculations suggest that the 2D to quasi-3D evolution without any structural phase transitions is mainly due to the increased interlayer Te-Te interactions (bridging) via the charge density overlap. In addition, we observed a first-order structural phase transition from the trigonal (P3¯m1) to monoclinic (C2/m) phase at higher pressure regions. We estimate the start of this structural phase transition to be ∼8 GPa and also the coexistence of two phases [trigonal (P3¯m1) and monoclinic (C2/m)] was observed from ∼8 GPa to ∼16 GPa

The sustainable materials roadmap

Over the past 150 years, our ability to produce and transform engineered materials has been responsible for our current high standards of living, especially in developed economies. However, we must carefully think of the effects our addiction to creating and using materials at this fast rate will have on the future generations. The way we currently make and use materials detrimentally affects the planet Earth, creating many severe environmental problems. It affects the next generations by putting in danger the future of the economy, energy, and climate. We are at the point where something must drastically change, and it must change now. We must create more sustainable materials alternatives using natural raw materials and inspiration from nature while making sure not to deplete important resources, i.e. in competition with the food chain supply. We must use less materials, eliminate the use of toxic materials and create a circular materials economy where reuse and recycle are priorities. We must develop sustainable methods for materials recycling and encourage design for disassembly. We must look across the whole materials life cycle from raw resources till end of life and apply thorough life cycle assessments (LCAs) based on reliable and relevant data to quantify sustainability. We need to seriously start thinking of where our future materials will come from and how could we track them, given that we are confronted with resource scarcity and geographical constrains. This is particularly important for the development of new and sustainable energy technologies, key to our transition to net zero. Currently 'critical materials' are central components of sustainable energy systems because they are the best performing. A few examples include the permanent magnets based on rare earth metals (Dy, Nd, Pr) used in wind turbines, Li and Co in Li-ion batteries, Pt and Ir in fuel cells and electrolysers, Si in solar cells just to mention a few. These materials are classified as 'critical' by the European Union and Department of Energy. Except in sustainable energy, materials are also key components in packaging, construction, and textile industry along with many other industrial sectors. This roadmap authored by prominent researchers working across disciplines in the very important field of sustainable materials is intended to highlight the outstanding issues that must be addressed and provide an insight into the pathways towards solving them adopted by the sustainable materials community. In compiling this roadmap, we hope to aid the development of the wider sustainable materials research community, providing a guide for academia, industry, government, and funding agencies in this critically important and rapidly developing research space which is key to future sustainability.journal articl

“Breaking the O═O Bond”: Deciphering the Role of Each Element in Highly Durable CoPd2Se2CoPd_{2}Se_{2} toward Oxygen Reduction Reaction

In-depth insight into oxygen reduction reaction (ORR) electrocatalyst with high figures of merit (activity, stability, and selectivity) is highly crucial to rationally design electrocatalyst with a potential to replace state-of-the-art Pt/C. This work reports the synthesis of CoPd$_2$Se$_2$ nanoparticles that show remarkably high stability of 50000 electrochemical cycles toward ORR. Morphology of the particles is characterized using SEM and TEM microscopy techniques and simulated using Bravais–Friedel–Donay–Harker (BFDH) morphology calculation method. A deconvoluted approach was used to understand the role of each element in the compound. DFT calculation was performed to have an in-depth analysis of the active site. Co and Pd provided an active site for the O$_2$ adsorption and Pd dissociates the O═O bond. The back-donation of substantial electron density to the π* antibonding orbital of the molecule expedites the 4e- reduction of O$_2$ throughout the entire potential range. During the electrochemical stability test, Se forms a protective layer and prevents the active Co and Pd sites from OH poisoning

Are we underrating rare earths as an electrocatalyst? The effect of their substitution in palladium nanoparticles enhances the activity towards ethanol oxidation reaction

Since the advent of catalysis, transition metal-based materials have continually been exploited as efficient electrocatalysts, whereas rare-earths (REs) have been neglected due to their misleading name, i.e. rare-earth, and cost. In fact, most REs are abundant and less expensive than the most explored transition metals. In view of this, we attempted to study the chemical effects of small amounts of RE (10%) substitution in the Pd lattice (REPd) for comparison with transition metal-substituted Pd (TMPd) towards the ethanol oxidation reaction (EOR). The electrochemical activities of REPd (RE = Eu and Yb) towards EOR were found to show many fold increases in both specific and mass activities as compared to those of TMPd (TM = Cr and Ni) and commercial Pd/C. Theoretical investigations assisted by DFT calculations support the experimental observations with a perfect synergy between the adsorption energies of –OH and –COCH$_3$ to the catalyst surface, which provides unprecedented catalytic activity for REPd as compared to the case of other catalysts. The promotional effects of RE were well exploited in enhancing the activity and stability of Pd towards EOR, as observed by electrochemical studies, X-ray absorption near edge spectroscopy and DFT calculations

Metal Flux Growth, Structural Relations, and Physical Properties of EuCu2Ge2\mathrm{EuCu_{2}Ge_{2}} and Eu3T2In9\mathrm{Eu_{3}T_{2}In_{9}} ( T = Cu and Ag)

Single crystals (SCs) of the compounds Eu$_3$Ag$_2$In$_9$ and EuCu$_2$Ge$_2$ were synthesized through the reactions run in liquid indium. Eu$_3$Ag$_2$In$_9$ crystallizes in the La$_3$Al$_{11}$ structure type [orthorhombic space group (SG) Immm] with the lattice parameters: a = 4.8370(1) Å, b = 10.6078(3) Å, and c = 13.9195(4) Å. EuCu$_2$Ge$_2$ crystallizes in the tetragonal ThCr$_2$Si$_2$ structure type (SG I4/mmm) with the lattice parameters: a = b = 4.2218(1) Å, and c = 10.3394(5) Å. The crystal structure of Eu$_3$Ag$_2$In$_9$ is comprised of edge-shared hexagonal rings consisting of indium. The one-dimensional chains of In6 rings are shared through the edges, which are further interconnected with other six-membered rings forming a three-dimensional (3D) stable crystal structure along the bc plane. The crystal structure of EuCu$_2$Ge$_2$ can be explained as the complex [CuGe]$^{(2+δ)}$– polyanionic network embedded with Eu ions. These polyanionic networks present in the crystal structure of EuCu$_2$Ge$_2$ are shared through the edges of the 011 plane containing Cu and Ge atoms, resulting in a 3D network. The structural relationship between Eu$_3$T$_2$In$_9$ and EuCu$_2$Ge$_2$ has been discussed in detail, and we conclude that Eu$_3$T$_2$In$_9$ is the metal deficient variant of EuCu$_2$Ge$_2$. The magnetic susceptibilities of Eu$_3$T$_2$In$_9$ (T = Cu and Ag) and EuCu$_2$Ge$_2$ were measured between 2 and 300 K. In all cases, magnetic susceptibility data followed Curie–Weiss law above 150 K. Magnetic moment values obtained from the measurements indicate the probable mixed/intermediate valent behavior of the europium atoms, which was further confirmed by X-ray absorption studies and bond distances around the Eu atoms. Electrical resistivity measurements suggest that Eu$_3$T$_2$In$_9$ and EuCu$_2$Ge$_2$ are metallic in nature

Compressive strain induced by multiple phase distribution and atomic ordering in PdCu nanoparticles to enhanced ethanol oxidation reaction performance

The catalytic properties of the materials can be altered with different arrangements of atoms, either in ordered or disordered manner. To study this behavior in detail, we have selected compounds based on Pd and Cu with different atomic arrangements and phase distribution. Nanoparticles of Pd$_{1-x}$Cu$_x$ with different atomic ratios and phase states are obtained by a facile one pot solvothermal method. The multiple combinations of structurally ordered and disordered phases are tuned by optimizing several synthetic strategies, which are qualitatively and quantitatively characterized by X-ray diffraction, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy and transmission electron microscopy measurements. Electrocatalytic ethanol oxidation reaction (EOR) is carried out in alkaline medium for all these synthesized Pd$_{1-x}$Cu$_x$ nanoparticles. It is observed that the EOR activity and stability are enhanced in comparison to the commercial Pd/C catalyst, which can be attributed to the atomic ordering and compressive strain introduced upon optimized phase distribution

Structural, magnetotransport and Hall coefficient studies in ternary Bi2Te2Se, Sb2Te2Se and Bi2Te2S tetradymite topological insulating compounds

International audienceTemperature and magnetic field dependent resistivity studies of topological insulating materials Bi2Te2Se, Sb2Te2Se and Bi2Te2S are examined using various transport characteristics with suitable XRD and Raman structural analyses. The longitudinal resistivity data with decreasing of temperature reveals the insulating phase in Bi2Te2Se and showed profound metallic nature in Sb2Te2Se and Bi2Te2S. We observed a bulk hole carrier concentration of 0.8 × 1019 cm−3 through Hall coefficient study for the compound Sb2Te2Se, whereas Bi2Te2S exhibit an electron carrier density of 0.3 × 1019 cm−3 at 2 K and 9 T. Magnetoresistance results suggest a residual carrier effect from these polycrystalline topological compounds would degrade the actual experimental observation of surface state, instead, bulk insulating behaviour has been realized. With controllable bulk carrier effect, a single crystal approach may unleash the definite Dirac surface state with absence of impurity scattering over topological characteristics

Optimized Metal Deficiency-Induced Operando Phase Transformation Enhances Charge Polarization Promoting Hydrogen Evolution Reaction

Electrochemical water reduction is one of the cleanest ways to produce hydrogen efficiently. Non-noble metal-based intermetallic compounds, particularly Ni-based ones, can be projected as potential water electrocatalysts because of Ni’s low cost and high abundance. In this work, Ni’s crystal structure and electronic properties have been tuned by introducing Sn metal followed by operando structural transformation. The synthetically tuned deficiency of Ni in Ni$_{2–x}$Sn (x = 0.35, 0.50, and 0.63) and operando-induced phase transformation of the room temperature orthorhombic Cmcm space group (Ni$_{2–x}$Sn_RT) to the high-temperature hexagonal P6$_3$/mmc space group (Ni$_{2–x}$Sn_HT) enhance the hydrogen evolution. Several controlled synthesis and electrochemical measurements indicate that Ni$_{1.5}$Sn_HT is the most active and stable HER catalyst because of its unique crystallographic structure and high charge transfer kinetics. The phase Ni$_{1.5}$Sn_RT was electrochemically transformed to the HT phase during the HER process and found to be extremely stable (150 h) in the chronoamperometric study

Synthetically Tuned Atomic Ordering in PdCu Nanoparticles with Enhanced Catalytic Activity toward Solvent-Free Benzylamine Oxidation

Synthesis of ordered compounds with nano size is of particular interest for tuning the surface properties with enhanced activity and selectivity toward various important industrial catalytic processes. In this work, we synthesized ordered PdCu nanoparticles as highly efficient catalyst for the solvent-free aerobic oxidation of benzylamine. The Pd_<i>x</i>Cu_1–<i>x</i> catalysts with different chemical compositions (<i>x</i> = 0, 0.25, 0.4, 0.5, 0.6, 0.75, 1) were prepared by polyol method using NaBH₄ as a reducing agent and were well-characterized by X-ray diffraction (XRD), inductively coupled plasma optical emission spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy (TEM) energy-dispersive analysis of X-rays, and X-ray absorption fine structure. The effect of different metal concentrations of Pd and Cu on the formation of Pd_<i>x</i>Cu_1–<i>x</i> nanoparticles was investigated. The XRD and TEM confirmed the formation of ordered PdCu intermetallic phase with body-centered cubic (BCC) structure for the synthetic composition of Pd/Cu = 1:1. For compositions <i>x</i> = 0, 0.25, 0.75, and 1, Pd_<i>x</i>Cu_1–<i>x</i> alloy with face-centered cubic (FCC) structure was observed, whereas mixed phase of BCC and FCC was observed for <i>x</i> = 0.4 and 0.6. The use of strong reducing agent (NaBH₄) was essential to synthesize PdCu ordered phase compared to weak reducing agents such as oleylamine and ascorbic acid. The PdCu nanocatalyst with ordered structure (BCC) showed excellent catalytic activity compared to Pd_<i>x</i>Cu_1–<i>x</i> alloy nanoparticles with FCC structure. The atomic ordering in the PdCu intermetallic was the driving force for the enhancement in the catalytic activity with high benzylamine conversion of 94.0% and dibenzylimine selectivity of 92.2% compared to its monometallic and alloy counterparts. Moreover, ordered PdCu alloy showed good recyclability and activity toward the oxidation of different amines