Chemical Stoichiometries Research Articles

On-device phase engineering.

In situ tailoring of two-dimensional materials' phases under external stimulus facilitates the manipulation of their properties for electronic, quantum and energy applications. However, current methods are mainly limited to the transitions among phases with unchanged chemical stoichiometry. Here we propose on-device phase engineering that allows us to realize various lattice phases with distinct chemical stoichiometries. Using palladium and selenide as a model system, we show that a PdSe2 channel with prepatterned Pd electrodes can be transformed into Pd17Se15 and Pd4Se by thermally tailoring the chemical composition ratio of the channel. Different phase configurations can be obtained by precisely controlling the thickness and spacing of the electrodes. The device can be thus engineered to implement versatile functions in situ, such as exhibiting superconducting behaviour and achieving ultralow-contact resistance, as well as customizing the synthesis of electrocatalysts. The proposed on-device phase engineering approach exhibits a universal mechanism and can be expanded to 29 element combinations between a metal and chalcogen. Our work highlights on-device phase engineering as a promising research approach through which to exploit fundamental properties as well as their applications.

Predictions of thorium super nitrides and superconductivity under pressure: Ab initio calculations

Thorium nitrides have been the topic of intense studies due to their prospective applications as advanced nuclear fuels. The phase diagram of the Th–N scheme, however, continues unknown at low temperatures and extremely high pressures. In this article, we examine the Th–N system's phase diagram up to 300 GPa from the first-principle approach using universal structure predictor: evolutionary Xtallography (USPEX) method. Apart from the experimentally observed phase (ThN, Th2N3, and Th3N4), there are several unique chemical stoichiometries, i.e., ThN3, ThN4, ThN6, ThN8, ThN10, and ThN12 are found to have stability fields on the Th–N phase diagram at pressure of 3.0, 32, 100, 42, 28, and 236 GPa along with previously predicted composition ThN2 at 3.5 GPa. The structural stability of the predicted compositions is further assessed by evaluating the elastic and dynamic stability. Out of all above mentioned compositions, ThN3 is possibly a metastable one at 0 GPa. Electronic structure calculations predict that all newly discovered compositions are metallic except ThN10, which is semi-metallic at high pressures. Further, we predict that ThN4 and ThN6 have high electron–phonon coupling constant of 1.874 and 0.894 with Tc around 21.22 and 25.02 K, respectively, at 100 GPa.

Activated chemical bonds in nanoporous and amorphous iridium oxides favor low overpotential for oxygen evolution reaction.

To date, the search for active, selective, and stable electrocatalysts for the oxygen evolution reaction (OER) has not ceased and a detailed atomic-level design of the OER catalyst remains an outstanding (if not, compelling) problem. Considerable studies on different surfaces and polymorphs of iridium oxides (with varying stoichiometries and dopants) have emerged over the years, showing much higher OER activity than the conventionally reported rutile-type IrO2. Here, we have considered different metastable nanoporous and amorphous iridium oxides of different chemical stoichiometries. Using first-principles electronic structure calculations, we investigate the (electro)chemical stability, intercalation properties, and electronic structure of these iridium oxides. Using an empirical regression model between the Ir-O bond characteristics and the measured OER overpotentials, we demonstrate how activated Ir-O bonds (and the presence of more electrophilic oxygens) in these less understood polymorphs of iridium oxides can explain their superior OER performance observed in experiments.

Enhancing the Cycling Stability of Transition-Metal-Oxide-Based Electrochemical Electrode via Pourbaix Diagram Engineering

Transition-metal oxides have shown renewed interest as promising electrode materials for high-performance electrochemical energy storage devices. However, applications of these devices are hindered by the instability of the electrode materials, which is originated from the electrochemical redox reactions induced phase transition between materials with different chemical stoichiometries. Herein, the Pourbaix diagram was constructed to predict ways of inhibiting this undesirable phase transition by using iron oxides as a prototypical model. Appropriate metal dopant (e.g., Ni) doping in the iron oxide was theoretically and experimentally found to present single phase under an extra-wide range of applied potential in alkaline aqueous solutions. Typically, the designed material (Ni x Fe 3- x O 4 ) presents high cycling stability with capacitance retention of 94.3% over 5,000 cycles at a current density of 20 A/g when working as a supercapacitor anode. A configured asymmetric supercapacitor with the designed anode exhibits an almost 100% performance retention after 10,000 charging-discharging cycles. This success proposed a new perspective to enhance the stability of electrochemical electrodes for energy storage devices.

Database, Features, and Machine Learning Model to Identify Thermally Driven Metal–Insulator Transition Compounds

Metal-insulator transition (MIT) compounds are materials that may exhibit insulating or metallic behavior, depending on the physical conditions, and are of immense fundamental interest owing to their potential applications in emerging microelectronics. There is a dearth of thermally-driven MIT materials, however, which makes delineating these compounds from those that are exclusively insulating or metallic challenging. Here we report a material database comprising temperature-controlled MITs (and metals and insulators with similar chemical composition and stoichiometries to the MIT compounds) from high quality experimental literature, built through a combination of materials-domain knowledge and natural language processing. We featurize the dataset using compositional, structural, and energetic descriptors, including two MIT relevant energy scales, an estimated Hubbard interaction and the charge transfer energy, as well as the structure-bond-stress metric referred to as the global-instability index (GII). We then perform supervised classification, constructing three electronic-state classifiers: metal vs non-metal (M), insulator vs non-insulator (I), and MIT vs non-MIT (T). We identify two important descriptors that separate metals, insulators, and MIT materials in a 2D feature space: the average deviation of the covalent radius and the range of the Mendeleev number. We further elaborate on other important features (GII and Ewald energy), and examine how they affect classification of binary vanadium and titanium oxides. We discuss the relationship of these atomic features to the physical interactions underlying MITs in the rare-earth nickelate family. Last, we implement an online version of the classifiers, enabling quick probabilistic class predictions by uploading a crystallographic structure file.

High-Performance Second-Harmonic-Generation (SHG) Materials: New Developments and New Strategies.

ConspectusSecond-harmonic-generation (SHG) causes the frequency doubling of light, which is very useful for generating high-energy lasers with specific wavelengths. Noncentrosymmetry (NCS) is the first requirement for an SHG process because the SHG coefficient is zero (χ2 = 0) in all centrosymmetric structures. At this stage, developing novel NCS crystals is a crucial scientific topic. Assembling polar units in an addictive fashion can facilely form NCS crystals with outstanding SHG performance. In this way, our group has obtained many different NCS crystals with extremely large SHG intensities (>5 × KDP or 1 × KTP). In this Account, we first provide a brief review of the development of SHG materials and concisely highlight the features of the excellent SHG materials. Then, we present four facile and rational molecular design strategies: (1) Traditional BO33--based crystals feature short absorption edges but usually suffer from relatively weak SHG performance (<5 × KDP). The combination of two types of pure π-conjugated anions (BO33- and NO3-) in a parallel fashion in the same compound has afforded a metal borate nitrate with a strong SHG effect. (2) To overcome the problems of the weak SHG effect and small birefringence in the less anisotropic QO4-based compounds, highly polarizable cations such as Hg2+ and Bi3+ are introduced into these systems, which greatly enhances both SHG effects and birefringence. (3) Iodate anions can be condensed into polynuclear iodate anions with a higher density of I5+ per unit cell, hence polyiodate anions can serve as excellent SHG-active groups. We developed a novel synthesis method for hydrothermal reactions under a phosphoric acid medium and obtained a series of metal polyiodates with strong SHG effects. In addition, as the number of iodate groups increases, the structural configuration of the polyiodate anion changes from linear to bent. (4) We introduce the concept of aliovalent substitution which features site-to-site atomic displacement at the structural level. Such aliovalent substitution led to new materials that have the same chemical stoichiometries or structural features as their parent compounds. Thus, aliovalent substitution can provide more experimental opportunities and afford new high-performance SHG materials. The introduction of a fluoride anion and the replacement of metal cations in the MO6 octahedron can result in new metal iodates with balanced properties including a large SHG effect, a wide band gap, and a high laser-induced damage threshold (LIDT) value. Finally, we briefly discuss several problems associated with the studies of SHG materials and give some prospects for SHG materials in the future.

Defect-mediated ab initio thermodynamics of metastable γ-MoN(001).

Refractory transition metal nitrides exhibit a plethora of polymorphic expressions and chemical stoichiometries. To afford a better understanding of how defects may play a role in the structural and thermodynamics of these nitrides, using density-functional theory calculations, we investigate the influence of point and pair defects in bulk metastable γ-MoN and its (001) surface. We report favorable formation of Schottky defect pairs of neighboring Mo and N vacancies in bulk γ-MoN and apply this as a defect-mediated energy correction term to the surface energy of γ-MoN(001) within the ab initio atomistic thermodynamics approach. We also inspect the structural distortions in both bulk and surfaces of γ-MoN by using the partial radial distribution function, g(r), of Mo-N bond lengths. Large atomic displacements are found in both cases, leading to a broad spread of Mo-N bond length values when compared to their idealized bulk values. We propose that these structural and thermodynamic analyses may provide some insight into a better understanding of metastable materials and their surfaces.

S/N-doped carbon nanofibers affording Fe7S8 particles with superior sodium storage

Iron sulfides draw much attention as electrode candidates for sodium-ion batteries (SIBs) due to the rich chemical stoichiometries and high capacity. However, they usually exhibit poor cycling performance due to the large volume change during sodiation/desodiation process. In this work, we embed Fe7S8 nanoparticles into sulfur, nitrogen-doped carbon (S/N–C) nanofibers through electrospinning/sulfurization processes. The heteroatom doped carbon matrixes could effectively protect the Fe7S8 from structural collapse, obtaining a stable cycling performance. Moreover, the conductive matrixes with 1D structure can facilitate the diffusion of electrons, leading to good rate capability. As results, the as-designed Fe7S8@S/N–C nanofibers present a discharge capacity of 347 m Ah g−1 after 150 cycles at 1 A g−1 and a high rate capacity of 220 m Ah g−1 at 5 A g−1 in virtue of unique structural characteristics.

Tuning the optical, electrical resistivity and structural properties of DC magnetron sputtered aluminum zinc oxide films by changing the oxygen flow rate

Thin films of aluminum zinc oxide (AZO) were fabricated by DC reactive magnetron sputtering from 50 wt% Al/Zn metallic target. The structure and optical constants of AZO films were controlled by changing the oxygen (O2) flow rate. X-Ray diffraction revealed that the addition of oxygen transforms the crystalline nature of the metallic Al/Zn to completely amorphous film at 4.5 sccm. The film density, surface roughness and deposition rate were evaluated from X-ray reflectometry measurements and they were found to decrease strongly upon transformation from metallic to oxidic sputtering mode. The optical constants were extracted from the ellipsometry measurements. As O2 flow increased from 4.5 to 30 sccm, the optical band gap increased from 4.74 to 5.33 eV whereas the refractive index decreased simultaneously. Low resistivity films were obtained as O2 flow increased from 0 to 4 sccm whereas insulating films were obtained for O2 flows above 4 sccm. The results indicated that by regulating the oxygen flow diverse ZAO films with diverse chemical stoichiometries and properties can be tuned.

Monolayer Transition-Metal Dichalcogenide Mo1–xWxS2Alloys as Efficient Anode Materials for Lithium-Ion Batteries

Intercalation of metal ions in the van der Waals gap of layered materials forms the basis for large-scale electrochemical energy storage. In this work, by means of periodic density functional theory calculations, transition-metal dichalcogenide Mo1–xWxS2 alloys have been explored as efficient materials for lithium storage. Our study reveals that lithium prefers to bind efficiently to the monolayer alloy and diffuses easily with short diffusion distances. All bare phases of alloys are semiconducting, and a semiconducting to metallic phase transition occurs after lithiation, which ensures good electrical conductivity and is crucial for electrode material. We find negligible average open-circuit voltage at different chemical stoichiometries of 0.67, 1.33, and 2. Effects of both strain and concentration on adsorption energy and diffusion barriers are calculated. The effect of strain has manifested significant rise in adsorption energy, whereas in the case of diffusion barrier, this effect is almost negligible...

Stability and Superconductivity of K-P Compounds under Pressure.

We explore the phase diagram, structures, electron properties, and potential superconductivity of the K-P system at pressures of up to 200 GPa through unbiased structure searching techniques and first-principles calculations. Five stable chemical stoichiometries (K4P, K3P, K2P, KP, and KP2) have been theoretically predicted. In particular, P2 units or P-chains are uncovered in K2P, KP, and KP2 compounds with the existence of covalent bonds by analyzing the electron localization functions. And the Bader analysis demonstrates that charges transfer from K atoms to P atoms. Electron-phonon calculations show that the Tc of metallic I4/mmm-KP2 is 22.01 K at 5 GPa. The investigating of the K-P system is in favor of understanding the crystal structures and corresponding properties of potassium phosphides, which also give a strong motivation to search and design new superconductor materials in other phosphides.

Non‐Brownian Particle‐Based Materials with Microscale and Nanoscale Hierarchy

AbstractColloidal crystals are interesting materials owing to their customizable photonic properties, high surface area, and analogy to chemical structures. The flexibility of these materials has been greatly enhanced through mixing particles with varying sizes, compositions, and surface charges. In this way, distinctive patterns or analogies to chemical stoichiometries are produced; however, to date, this body of research is limited to particles with nanoscale dimensions. A simple method is now presented for bottom‐up assembly of non‐Brownian particle mixtures to create a new class of hierarchically‐ordered materials that mimic those found in nature (both in pore distribution as well as stoichiometry). Additionally, these crystals serve as a template to create particle‐based inverted crystalline structures with customizable properties.

Non-Brownian particle-based materials with microscale and nanoscale hierarchy.

Colloidal crystals are interesting materials owing to their customizable photonic properties, high surface area, and analogy to chemical structures. The flexibility of these materials has been greatly enhanced through mixing particles with varying sizes, compositions, and surface charges. In this way, distinctive patterns or analogies to chemical stoichiometries are produced; however, to date, this body of research is limited to particles with nanoscale dimensions. A simple method is now presented for bottom-up assembly of non-Brownian particle mixtures to create a new class of hierarchically-ordered materials that mimic those found in nature (both in pore distribution as well as stoichiometry). Additionally, these crystals serve as a template to create particle-based inverted crystalline structures with customizable properties.

Magnetic and Related Properties of Ternary TmTX Intermetallics

Since many years, rare earth intermetallics R T X, where R = rare earth, T = d-electron and X = p-electron elements, have been a subject of extensive studies due to their intriguing magnetic and electrical properties [1, 2]. Amidst plentitude of such phases with various chemical compositions and stoichiometries, a series of equiatomic compounds RTX received special attention due to simplicity of their crystal structure and large span in their physical behaviors [3]. While much information is available in the literature for the RTX intermetallics with R = Ce Er, relatively little is known on their counterparts with R = Tm. Most recently, we reported on the magnetic properties of TmRhGe and TmRhGa, which were found to order antiferromagnetically below 6.0 K [4] and 3.9 K [5], respectively. Similarly, we established before antiferromagnetic ordering in TmNiIn, TmPtIn and TmAgGe to set at TN = 2.5 [6], 3.4 [7] and 4.2 K [8], respectively. Here, we give the rst account on our on-going studies on a few other Tm-based equiatomic materials, namely TmPdIn, TmAgSn, TmAuGe and TmRuGe. The experimental results are compared with the data previously obtained for the compounds TmNiIn, TmPtIn and TmAgGe.

The chemical physics of unconventional superconductivity

Attempts to explain correlated‐electron superconductivity (SC) have largely focused on the proximity of the superconducting state to antiferromagnetism. Yet, there exist many correlated‐electron systems that exhibit insulator‐superconducting transitions where the insulating state exhibits spatial broken symmetry different from antiferromagnetism. Here, we focus on a subset of such compounds which are seemingly very different in which specific chemical stoichiometries play a distinct role, and small deviations from stoichiometry can destroy SC. These superconducting materials share a unique carrier concentration, at which we show there is a stronger than usual tendency to form local spin‐singlets. We posit that SC is a consequence of these pseudomolecules becoming mobile as was suggested by Schafroth a few years prior to the advent of the Bardeen‐Cooper‐Schrieffer (BCS) theory. © 2014 Wiley Periodicals, Inc.

Nonequilibrium Solid Phase Formation Studied by Lattice Dynamics Calculation and Ion Beam Mixing in an Immiscible Co−Ag System

For the equilibrium immiscible Co-Ag system, a proven realistic ab initio derived n-body potential is applied to study the nonequilibrium solid phase formation at three chemical stoichiometries of Co/Ag = 1:3, 1:1, and 3:1. To predict the structural stability, the elastic constants and the phonon spectra are calculated at the chosen stoichiometries with a total of eight hypothetical crystalline structures. The calculated results suggest that four compounds, that is, D0(3) CoAg3, B1 CoAg, B2 CoAg, and D0(3) Co3Ag, are unstable, as they all feature negative elastic constants as well as imaginary phonons, and that another four compounds of both fcc-type L1(2) and hcp-type D0(19) structures at chemical stoichiometries of Co/Ag = 1:3 and 3:1, respectively, may elastically be favored and therefore obtainable under some specific conditions. It is also found that all the calculated elastic constants and phonon spectra are coincident within the framework of the elastic theory. Moreover, the calculated elastic constants are in good agreement with those acquired directly from ab initio calculations, lending support to the validity of the ab initio derived n-body Co-Ag potential as well as its resultant elastic constants and the phonon spectra. Interestingly, some of the predicted nonequilibrium solid phases, that is, two hcp-type compounds at chemical stoichiometries of Co/Ag = 1:3 and 3:1, respectively, are indeed obtained in ion beam mixing experiments and their lattice constants determined by diffraction analysis are in good agreement with those from calculations.

Counterpoint: What's in a Name?

The author offers a counter argument for the use of the term pseudopolymorphism. He argues that the term polymorph should ideally be restricted to crystals of a single compound. However, he notes that in multicomponent crystals there are too many structural variations and the term polymorphism is unnecessarily restrictive. The term pseudopolymorphism in this context brings to attention the fact that certain related multicomponent crystals, with slightly different chemical compositions or stoichiometries, have slightly different crystal structures in a manner reminiscent of polymorphism.

Prediction of Semiconductor Material Properties by the Properties of Their Constituent Chemical Elements

We demonstrate that it is possible to predict the semiconductor material properties, like lattice constant and band gap energy, from their chemical stoichiometries and fundamental element properties of the constituents. We use a correlation technique to show that band gap energy is strongly correlated to electronegativity and pseudopotential radii whereas the lattice constant is strongly correlated to melting temperature and atomic number. Prediction of band gap energy and lattice constant of some III−V and II−VI semiconductors is made on the basis of the developed correlation model. The technique can also be applied to predict bulk properties of ternary or quaternary semiconductors.

In situ X-ray photoelectron spectroscopy study of the interactions of evaporated magnesium with polyaniline films

X-ray photoelectron spectroscopy was employed for an in situ study of the interactions between thermally evaporated magnesium atoms and polyaniline films of various intrinsic oxidation states. Quantitative changes in the N 1s core-level spectra, the Mg 2p core-level spectra and the surface chemical stoichiometries of these films throughout the magnesium evaporation process were carefully monitored. Although the nitrogen sites appeared to be more attractive to the incoming magnesium atoms, and the emeraldine and nigraniline films underwent an apparent decrease in the intrinsic oxidation state ([N]/[NH] ratio) as a result of magnesium evaporation, there is no direct interaction between the two species. On the other hand, the adsorbed oxygen from the bulk of the polymer film played a dominant role in the interfacial interactions between the polymer and the metal.

In situ XPS study of the interactions of evaporated copper atoms with poly( p-phenylene vinylene) and its ring-substituted derivatives

Angle-resolved X-ray photoelectron spectroscopy was employed for the in situ study of the interactions between thermally evaporated copper atoms and conjugated electroluminescent polymer, such as poly( p-phenylene vinylene) and its soluble derivative, poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV). Quantitative changes in the C 1s and Cu 2p core-level spectra, the Cu LMM Auger line shapes, and the chemical stoichiometries of polymer–metal interfaces with progressive deposition of the copper atoms were carefully monitored. Chemical interactions had occurred between each polymer and the copper atoms, resulting in the formation of an interfacial layer of metal complexes. There appeared to be no significant migration of bulk absorbed oxygen to the polymer surface upon copper deposition unlike that observed during the deposition of metals of lower work function. Diffusion of Cu atoms into the near-surface region of the polymer was observed for PPV and MEH-PPV films. The interactions of the Cu atoms with the polymer surfaces were also compared with the results obtained for the Al/PPV and Al/MEH-PPV interfaces.