High-symmetry Positions Research Articles

Atomic Displacements Enabling the Observation of the Anomalous Hall Effect in a Non-Collinear Antiferromagnet.

Antiferromagnets with non-collinear spin structures display various properties that make them attractive for spintronic devices. Some of the most interesting examples are an anomalous Hall effect despite negligible magnetization and a spin Hall effect with unusual spin polarization directions. However, these effects can only be observed when the sample is set predominantly into a single antiferromagnetic domain state. This can only be achieved when the compensated spin structure is perturbed and displays weak moments due to spin canting that allows for external domain control. In thin films of cubic non-collinear antiferromagnets, this imbalance waspreviously assumed to require tetragonal distortions induced by substrate strain. Here weshow that in Mn3 SnN and Mn3 GaN, spin canting is due to structural symmetry lowering induced by large displacements of the magnetic manganese atoms away from high symmetry positions. These displacements remain hidden in X-ray diffraction when only probing the lattice metric and require measurement of a large set of scattering vectors to resolve the local atomic positions. In Mn3 SnN, the induced net moments enable the observation of the anomalous Hall effect with an unusual temperature dependence, which weconjecture results from a bulk-like temperature-dependent coherent spin rotation within the Kagomeplane. This article is protected by copyright. All rights reserved.

Origin of the variation in lattice thermal conductivities in pyrite-type dichalcogenides

Lattice thermal conductivity is a crucial parameter for thermoelectric (TE) applications. In pyrite-type metal dichalcogenides, it is observed that the type of metal atom has a significant influence in the phonon properties and lattice thermal conductivities, but the physical origin is not clear. In this paper, we show that, for the pyrite-type posttransition metal (such as Zn or Cd) dichalcogenides, because the fully occupied ${d}^{10}$ orbitals inside the valence bands are relatively localized, the enhanced symmetry-controlled $s\text{\ensuremath{-}}d$ coupling effects in the posttransition metal atoms when they vibrate away from the high-symmetry equilibrium positions can lead to soft phonon properties and strong anharmonicity. Thus, they behave as rattlinglike atoms, and these systems have extremely low lattice thermal conductivities. However, for some pyrite-type transition metal dichalcogenides, when the outermost $d$ electrons in transition metal atoms are only occupied by the ${T}_{2g}$-derived states, such as Fe with ${d}^{6}$ configuration, the symmetry-controlled $s\text{\ensuremath{-}}d$ coupling effects in the transition metal atoms are strongly cancelled by the varying crystal field splitting when symmetry is reduced during vibration. Therefore, these compounds exhibit hard phonon properties, weak anharmonicities, and high lattice thermal conductivities. In this paper, we reveal the electronic origin of the puzzling behaviors of pyrite-type dichalcogenides which show a wide range of thermal conductivity properties. The discussed mechanism can also be used to guide researchers in seeking promising TE materials with low lattice thermal conductivity.

Geometric frustration of Jahn-Teller order in the infinite-layer lattice.

The Jahn-Teller effect, in which electronic configurations with energetically degenerate orbitals induce lattice distortions to lift this degeneracy, has a key role in many symmetry-lowering crystal deformations1. Lattices of Jahn-Teller ions can induce a cooperative distortion, as exemplified by LaMnO3 (refs. 2,3). Although many examples occur in octahedrally4 or tetrahedrally5 coordinated transition metal oxides due to their high orbital degeneracy, this effect has yet to be manifested for square-planar anion coordination, as found in infinite-layer copper6,7, nickel8,9, iron10,11 and manganese oxides12. Here we synthesize single-crystal CaCoO2 thin films by topotactic reduction of the brownmillerite CaCoO2.5 phase. We observe a markedly distorted infinite-layer structure, with ångström-scale displacements of the cations from their high-symmetry positions. This can be understood to originate from the Jahn-Teller degeneracy of the dxz and dyz orbitals in the d7 electronic configuration along with substantial ligand-transition metal mixing. A complex pattern of distortions arises in a [Formula: see text] tetragonal supercell, reflecting the competition between an ordered Jahn-Teller effect on the CoO2 sublattice and the geometric frustration of the associated displacements of the Ca sublattice, which are strongly coupled in the absence of apical oxygen. As a result of this competition, the CaCoO2 structure forms an extended two-in-two-out type of Co distortion following 'ice rules'13.

The influence of steric and ligand effects on metal- and alloy-supported oxygenated and hydrogenated Pt(111)

<h2>Abstract</h2> Understanding the mechanisms of the oxygen reduction reaction (ORR) and the hydrogen evolution reaction (HER) is key to the development of modern energy storage and transport materials. The technology inherent in these materials contributes significantly to sustainable chemical production, whose energy footprint is an imperative concern when designing catalysts and networks of catalysts. In this presentation a comparative study of the unreacted and reacted uniaxially strained Pt(111) and the layered (111)-Pt/Ni/Pt3Ni and (111)-Pt/Ni/PtNi3 surfaces has been performed using density functional theory (DFT). Through an analysis of the binding energies of oxygen and hydrogen over the high-symmetry binding positions of all surfaces, the study has shown that O and H tend to bind more strongly to the (111)-Pt/Ni/Pt3Ni surface and less strongly to the (111)-Pt/Ni/PtNi3 surface compared to binding on the equivalently strained Pt(111) surfaces. An estimate the error in the computational binding energies as well as a critical comparative analysis of the binding mechanisms which contribute to these energetic differences has been performed by comparing the predictions of different density functionals. Changes in the surface magnetisation of the surfaces overlaying the ferromagnetic alloys during adsorption are highlighted and discussed, as well as the behaviour of the d-band centre across all surfaces to provide causal explanations of the mechanism changes.

Pressure Induced Disorder-Order Phase Transitions in the Al4Cr Phases

An ordered ω-Al4Cr phase synthesized recently by a high-pressure sintering (HPS) approach was calculated to be stable by density function theory (DFT), implying that high pressure can accelerate the disorder-order phase transitions. The structural building units of the ω-Al4Cr phase as well as the non-stoichiometric disordered ε-Al4Cr and μ-Al4Cr phases have been analyzed by the topological “nanocluster” method in order to explore the structural relations among these phases. Both the ε-and μ-Al4Cr phases contain the typical Macky or pseudo-Macky cluster, and their centered positions were all occupied by Cr atoms, which all occupy the high-symmetry Wyckoff positions. The mechanism of the pressure-induced disorder-order phase transitions from the ε-/μ-Al4Cr to the ω-Al4Cr phase has been analyzed. and the related peritectic and eutectoid reactions have been re-evaluated. All results suggest that the stable ω-Al4Cr phase are transformed from the μ-Al4Cr phase by the eutectoid reaction that is accelerated by high-pressure conditions, whereas the ε-Al4Cr phase should form by the peritectic reaction.

Dynamical study of the origin of the charge density wave in AV3Sb5 ( A=K , Rb, Cs) compounds

Systems containing the ideal kagome lattice can exhibit several distinct and novel exotic states of matter. One example of such systems is a recently discovered $A$V$_{3}$Sb$_{5}$ ($A$ = K, Rb, and Cs) family of compounds. Here, the coexistence of the charge density wave (CDW) and superconductivity is observed. In this paper, we study the dynamic properties of the $A$V$_{3}$Sb$_{5}$ systems in context of origin of the CDW phase. We show and discuss the structural phase transition from $P6/mmm$ to $C2/m$ symmetry that are induced by the presence of phonon soft modes. We conclude that the CDW observed in this family of compounds is a consequence of the atom displacement, from the high symmetry position of the kagome net, in low-temperature phase. Additionally, using the numerical {\it ab initio} methods, we discuss the charge distribution on the $A$V$_{3}$Sb$_{5}$ surface. We show that the observed experimental %$4\times 1$ stripe-like modulation of the surface, can be related to surface reconstruction and manifestation of the three dimensional $2 \times 2 \times 2$ bulk CDW. Finally, the consequence of realization of the $C2/m$ structure on the electronic properties are discussed. We show that the electronic band structure reconstruction and the accompanying modification of density of states correspond well to the experimental data.

Formation and Performance of Diamond (111)/Cu Interface from First-Principles Calculation

The interface formation and properties of composite materials are very important for the preparation of composite materials, and the bonding state and charge transfer between atoms in the interface have a particularly significant effect on the interface formation. In this work, the first-principles calculation method was used to study the adsorption behavior and molecular dynamics of copper atoms on the (111) surface of H-terminated diamond, and the adsorption energy and adhesion work of Cu atoms were calculated. The results show that the adsorption of copper atoms is not sensitive to the diamond (111) surface, the adsorption work is very small at the four high symmetry positions, and the adhesion work is the largest at the T4 position and is 0.6106 J/m2. Furthermore, according to the electron localization function (ELF) analysis, there is no compound formation between Cu and H atoms; only a small amount of charge transfer exists, which belongs to physical adsorption. The diamond–copper interface formed by the growth of adsorption sites is a metastable structure without energy stability. This work provides an important theoretical reference for understanding the formation mechanism of copper-based diamond composites.

Role of oxygen vacancies in colossal polarization in SmFeO3-δ thin films.

The orthorhombic rare-earth manganates and ferrites multiferroics are promising candidates for the next generation multistate spintronic devices. However, their ferroelectric polarization is small, and transition temperature is far below room temperature (RT). The improvement of ferroelectricity remains challenging. Here, through the subtle strain and defect engineering, an RT colossal polarization of 4.14 μC/cm2 is achieved in SmFeO3-δ films, which is two orders of magnitude larger than its bulk and is also the largest one among the orthorhombic rare-earth manganite and ferrite family. Meanwhile, its RT magnetism is uniformly distributed in the film. Combining the integrated differential phase-contrast imaging and density functional theory calculations, we reveal the origin of this superior ferroelectricity in which the purposely introduced oxygen vacancies in the Fe-O layer distorts the FeO6 octahedral cage and drives the Fe ion away from its high-symmetry position. The present approach can be applied to improve ferroelectric properties for multiferroics.

A Comparative Study of Oxygen and Hydrogen Adsorption on Strained and Alloy-Supported Pt(111) Monolayers

A comparative study of the unreacted and reacted uniaxially strained Pt(111) and the layered (111)-Pt/Ni/Pt3Ni and (111)-Pt/Ni/PtNi3 surfaces has been performed using density functional theory (DFT). An in-depth study of the unreacted surfaces has been performed to evaluate the importance of geometric, magnetic and ligand effects in determining the reactivity of these different Pt surfaces. An analysis of the binding energies of oxygen and hydrogen over the high-symmetry binding positions of all surfaces has been performed. The study has shown that O and H tend to bind more strongly to the (111)-Pt/Ni/Pt3Ni surface and less strongly to the (111)-Pt/Ni/PtNi3 surface compared to binding on the equivalently strained Pt(111) surfaces. Changes in the surface magnetisation of the surfaces overlaying the ferromagnetic alloys during adsorption are discussed, as well as the behaviour of the d-band centre across all surfaces, to evaluate the potential mechanisms for these differences in binding. An accompanying comparison of the accessible density functionals has been included to estimate the error in the computational binding energies.

Identification and correction of temporal and spatial distortions in scanning transmission electron microscopy

Scanning transmission electron microscopy (STEM) has become the technique of choice for quantitative characterization of atomic structure of materials, where the minute displacements of atomic columns from high-symmetry positions can be used to map strain, polarization, octahedra tilts, and other physical and chemical order parameter fields. The latter can be used as inputs into mesoscopic and atomistic models, providing insight into the correlative relationships and generative physics of materials on the atomic level. However, these quantitative applications of STEM necessitate understanding the microscope induced image distortions and developing the pathways to compensate them both as part of a rapid calibration procedure for in situ imaging, and the post-experimental data analysis stage. Here, we explore the spatiotemporal structure of the microscopic distortions in STEM using multivariate analysis of the atomic trajectories in the image stacks. Based on the behavior of principal component analysis (PCA), we develop the Gaussian process (GP)-based regression method for quantification of the distortion function. The limitations of such an approach and possible strategies for implementation as a part of in-line data acquisition in STEM are discussed. The analysis workflow is summarized in a Jupyter notebook that can be used to retrace the analysis and analyze the reader's data.

Jahn-Teller and Pseudo-Jahn-Teller Effects: From Particular Features to General Tools in Exploring Molecular and Solid State Properties.

In a generalization of the latest achievements in this field, and as a pattern of massive applications, we present here the Jahn-Teller effect (JTE) and pseudo-JTE (PJTE) as general tools in the study of physical and chemical phenomena related to structural properties of polyatomic systems. We show that the JTE and PJTE are no more specific features of particular (rare) systems (as it was assumed earlier), but virtual properties of all molecular and solid state formations. They occur as a result of vibronic coupling that compensates for the error (inadequacy) introduced in semi-classical definitions of polyatomic configurations by their high-symmetry nuclear positions, thus appending the basic understanding of related phenomena with a new dimension. The implications of the JTE and PJTE in observable properties varies significantly, being especially strong in the states of electronic degeneracy or pseudodegeneracy, but they cannot be a priory fully excluded for any system. After the introductory sections we demonstrate some of the more recent results of the influence of these effects on the observables in physical and chemical phenomena, together with a wide range of applications. The latter are conventionally separated in three parts: intermediate states in chemical and photochemical reactions, manipulation of structural properties of polyatomic systems targeting the JTE and PJTE, and applications in materials science. The illustrative examples include the origin of sudden polarization in photochemical reactions, methods of planarization of puckered (buckled) two-dimensional systems, modification of crystal sublattices by targeting the JTE parameters, the defining role of JTE and PJTE in electronics and spintronics, the origin of ferroelectricity and multiferroicity, as well as a novel property of solids, orientational polarization, and its applications.

The structure solution of the F-type i-ZnMgHf from the X-ray diffraction

ABSTRACTWe report a successful growth of the ZnMgHf F-type icosahedral quasicrystal in the form of faceted single grains. By varying the heat treatment parameters we were able to obtain single crystals with a quality suitable for a single crystal X-ray diffraction which was conducted in a synchrotron facility. Ab initio structure solution by a charge flipping algorithm manifests the existence of two types of Bergman clusters. Clusters are differentiated by the preferential occupation of the high-symmetry positions by hafnium in a rhombic triacontahedron, being the outer shell of the atomic cluster.This paper is part of a Thematic Issue on The Crystallographic Aspects of Metallic Alloys.

One‐Pot Synthesis of Homogeneous EuF3 Nanoplates: A Near‐Ultraviolet Light‐Induced Red‐Emitting Bifunctional Platform for in vitro Cell Imaging and Solid‐State Lighting

AbstractWe reported a new approach to prepare the homogenous EuF3 nanoplates and discussed their potential applications. Upon 396 nm excitation, the EuF3 nanoplates exhibited the visible emissions with high color purity of 84.7%. The monitored photoluminescence emission spectrum indicated that the Eu3+ ions took the high symmetry positions in the host lattices which was further confirmed via the theoretical calculation based on the Judd‐Ofelt theory. The thermal quenching performance of the EuF3 nanoplates was analyzed by the temperature‐dependent photoluminescence emission spectra and the activation energy was decided to be 0.28 eV. With the aid of the near‐ultraviolet chip and EuF3 nanoplates, a red‐emitting light‐emitting diode device was prepared so as to identify the feasibility of the EuF3 nanoplates for solid‐state lighting. Additionally, the cell viability of the EuF3 nanoplates as well as their fluorescence imaging behaviors in the A549 lung cancer cells was also investigated. The observed results demonstrated that the EuF3 nanoplates were bifunctional platform for in vitro cell imaging and solid‐state lighting.

Topological Semimetals Studied by Ab Initio Calculations

In topological semimetals such as Weyl, Dirac, and nodal-line semimetals, the band gap closes at points or along lines in k space which are not necessarily located at high-symmetry positions in the Brillouin zone. Therefore, it is not straightforward to find these topological semimetals by ab initio calculations because the band structure is usually calculated only along high-symmetry lines. In this paper, we review recent studies on topological semimetals by ab initio calculations. We explain theoretical frameworks which can be used for the search for topological semimetal materials, and some numerical methods used in the ab initio calculations.

Predicting displacements of octahedral cations in ferroelectric perovskites using machine learning.

In ferroelectric perovskites, displacements of cations from the high-symmetry lattice positions in the paraelectric phase break the spatial inversion symmetry. Furthermore, the relative magnitude of ionic displacements correlate strongly with ferroelectric properties such as the Curie temperature. As a result, there is interest in predicting the relative displacements of cations prior to experiments. Here, machine learning is used to predict the average displacement of octahedral cations from its high-symmetry position in ferroelectric perovskites. Published octahedral cation displacements data from density functional theory (DFT) calculations are used to train machine learning models, where each cation is represented by features such as Pauling electronegativity, Martynov-Batsanov electronegativity and the ratio of valence electron number to nominal charge. Average displacements for ten new octahedral cations for which DFT data do not exist are predicted. Predictions are validated by comparing them with new DFT calculations and existing experimental data. The outcome of this work has implications in the design and discovery of novel ferroelectric perovskites.

The impact of structural changes in ZrO2-Y2O3 solid solution crystals grown by directional crystallization of the melt on their transport characteristics

This work shows the correlation between the crystal structure, phase composition and transport characteristics of ZrO2 based solid electrolytes depending on the concentration of the stabilizing impurity Y2O3. The crystals ZrO2 stabilized with yttrium oxide in a wide range of compositions (from 2 to 15mol% Y2O3) were studied. The phase composition, twin structure, and conductivity of the crystals we determined for all concentrations by X-ray diffraction, transmission electron microscopy, Raman and impedance spectroscopy. The ionic conductivity of the crystals was changed nonmonotonic, with increasing Y2O3 concentration. The existence of two maxima of ionic conductivity at temperatures 800–900°C for compositions ZrO2-3.2mol% Y2O3 and ZrO2-8mol% Y2O3 was established. We show that twin boundaries do not trigger any additional ionic conductivity acceleration mechanism in ZrO2-Y2O3 crystals. The highest conductivity is observed in ZrO2-(8–10)mol% Y2O3 crystals containing the t″ phase in which the oxygen atoms are shifted from the high symmetry positions that are typical for the cubic phase.

Change in the mechanism of conductivity in ZrO2-based crystals depending on the content of stabilizing Y2O3 additive

The interrelationship between the structure, phase composition, and transport characteristics of solid electrolytes based on ZrO2 has been studied as dependent on the content of stabilizing Y2O3 additive. It is established that twin boundaries do not lead to the appearance of additional mechanism of ionic conductivity acceleration in ZrO2–Y2O3 crystals. The maximum conductivity has been observed in ZrO2–(8–10) mol % Y2O3 crystals containing a t” phase, in which oxygen atoms are displaced from high-symmetry positions characteristic of the cubic phase.

An analysis of the high-temperature phase structure of multiferroic solid solutions of the PFW–PT

The temperature evolution of multiferroic solid solutions of the PFW–PT system, namely a (1–x)Pb(Fe2/3W1/3O3)–(x)PbTiO3 crystal structure where x = 0, 0.2, 0.3, has been studied by neutron powder diffraction in the region of the morphotropic phase boundary. The coexistence of cubic and tetragonal phases in the solutions with x = 0.2, 0.3 was found below T = 259 and 285K, respectively. As a result of the data treatment, the atom coordinates, the occupation factors and the temperature dependences of cell parameters were determined in the cubic perovskite phase. The refinement of the crystal structure in terms of ideal perovskite exhibited anomalously large Debye–Waller factors for the lead cations, indicating the appearance of random static displacements of these cations from the ideal perovskite (000) position. Using the split-ion model we estimated the value of Pb static shifts (∼0.1Å) from their high-symmetry positions along the [110] direction. It was shown that these shifts decrease with increasing the PbTiO3 concentration.

Ab initio studies on the adsorption and implantation of Al and Fe to nitride materials

The formation of transfer material products on coated cutting and forming tools is a major failure mechanism leading to various sorts of wear. To describe the atomistic processes behind the formation of transfer materials, we use ab initio to study the adsorption energy as well as the implantation barrier of Al and Fe atoms for (001)-oriented surfaces of TiN, Ti0.50Al0.50N, Ti0.90Si0.10N, CrN, and Cr0.90Si0.10N. The interactions between additional atoms and nitride-surfaces are described for pure adhesion, considering no additional stresses, and for the implantation barrier. The latter, we simplified to the stress required to implant Al and Fe into sub-surface regions of the nitride material. The adsorption energies exhibit pronounced extrema at high-symmetry positions and are generally highest at nitrogen sites. Here, the binary nitrides are comparable to their ternary counterparts and the average adhesive energy is higher (more negative) on CrN than TiN based systems. Contrary, the implantation barrier for Al and Fe atoms is higher for the ternary systems Ti0.50Al0.50N, Ti0.90Si0.10N, and Cr0.90Si0.10N than for their binary counterparts TiN and CrN. Based on our results, we can conclude that TiN based systems outperform CrN based systems with respect to pure adhesion, while the Si-containing ternaries exhibit higher implantation barriers for Al and Fe atoms. The data obtained are important to understand the atomistic interaction of metal atoms with nitride-based materials, which is valid not just for machining operations but also for any combination such as interfaces between coatings and substrates or multilayer and phase arrangements themselves.

Structural, vibrational, and dielectric properties of Ruddlesden-PopperBa2ZrO4from first principles

Using first-principles computational techniques, we have investigated the structural, vibrational, and dielectric properties of a Ruddlesden-Popper-type layered oxide ${\mathrm{Ba}}_{2}{\mathrm{ZrO}}_{4}$ subjected to a wide range of biaxial strains emulating epitaxial thin-film environment. Under compressive strains, this compound experiences an incommensurate distortion characterized by planar displacements of individual perovskite slabs away from their high symmetry positions. On the other hand, under increasing epitaxial tension, the original centrosymmetric structure becomes unstable---first, with respect to antiferrodistortive oxygen cage rotations and then also with respect to in-plane polar distortions. Both the incommensurate-to-commensurate and the nonpolar-to-polar phase transformations are accompanied by anomalies of the static dielectric response, however, only in the latter case a divergence of the in-plane dielectric constant is observed. Remarkably, even after the transition into the ferroelectric state (with polarization of up to 0.12 ${\mathrm{C}/\mathrm{m}}^{2}$ at 3.5% tension) dielectric permittivity of ${\mathrm{Ba}}_{2}{\mathrm{ZrO}}_{4}$ remains unusually high, which is explained by an emergence of a Goldstone-like excitation in the system manifested through an in-plane libration of the polarization vector. Since ${\mathrm{Ba}}_{2}{\mathrm{ZrO}}_{4}$ displays a yet poorly understood tendency to absorb small molecules, such as water and ${\mathrm{CO}}_{2}$, acquiring better insights into the physical underpinnings of its behavior can produce more efficient functional materials for applications in advanced technologies for carbon sequestration.