Large Unit Cell Research Articles

Lanthanide and yttrium substitution in natural fluorite

Fluorite is one of the most common minerals in the crust and is of widespread economic importance. It shows strong UV-excited luminescence, variously attributed to defects within the fluorite structure and lanthanide substitutions. We present here a detailed chemical characterisation of a suite of natural fluorite samples, chosen to represent the range of compositions observed in nature. We perform X-ray excited luminescence spectroscopy on the samples as a function of temperature (20–673 K) in the wavelength range 250–800 nm to provide insights into physical defects in the lattice and their interactions with lanthanide substituents in natural fluorite. Most broad bands in the UV are attributed to electronic defects in the fluorite lattice, whereas sharp emissions are attributed to intra-ion energy cascades in trivalent lanthanides. Lanthanides are accommodated in fluorite by substitution for Ca2+ coupled with interstitial F−, O2− (substituting for F−) and a variety of electronic defect structures which provide local charge balance. The chondrite-normalised lanthanide profiles show that fluorite accommodates a greater proportion of heavy lanthanides (and Y) as the total Rare Earth Element (REE) concentration increases; whereas cell parameters decrease and then increase as substitution continues. Luminescence intensity also goes through a maximum and then decreases as a function of REE concentration. All three datasets are consistent with a model whereby lanthanides initially act as isolated centres, but, beyond a critical threshold (~ 1000 ppm), cluster into lanthanide-rich domains. Clustering results in shorter REE-O bond distances (favouring smaller heavier ions), a larger unit cell but more efficient energy transfer between lanthanides, thereby promoting non-radiative energy loss and a drop in the intensity of lanthanide emission.

Machine learning guided high-throughput search of non-oxide garnets

Garnets have found important applications in modern technologies including magnetorestriction, spintronics, lithium batteries, etc. The overwhelming majority of experimentally known garnets are oxides, while explorations (experimental or theoretical) for the rest of the chemical space have been limited in scope. A key issue is that the garnet structure has a large primitive unit cell, requiring a substantial amount of computational resources. To perform a comprehensive search of the complete chemical space for new garnets, we combine recent progress in graph neural networks with high-throughput calculations. We apply the machine learning model to identify the potentially (meta-)stable garnet systems before performing systematic density-functional calculations to validate the predictions. We discover more than 600 ternary garnets with distances to the convex hull below 100 meV ⋅ atom−1. This includes sulfide, nitride, and halide garnets. We analyze their electronic structure and discuss the connection between the value of the electronic band gap and charge balance.

Revealing the Effect of Anions on the Formation and Transformation of Zeolite LTA in Caustic solutions: AnIn SituSynchrotron PXRD Study

Zeolite LTA is one of the largest industrial zeolites in both volume and market value. Natural resources or manufacturing wastes such as clay minerals, fly ash, and lithium slag are now used as raw materials for the economically and environmentally friendly synthesis of zeolites. However, soluble impurities such as inorganic anions (CO32–, SO42–, and PO43–) in these feeds have significant effects on the quality of zeolite LTA products, because these impurities enhance or inhibit the formation of other unwanted phases (amorphous zeolites or sodalite) via mechanisms that remain poorly investigated. Here we use in situ synchrotron powder X-ray diffraction to study the effects of different inorganic salts (Na2CO3, Na2SO4, and Na3PO4) on the kinetics of the sequential phase transformations in zeolite LTA during 2.5 h of reaction at 90 °C in both weakly and highly alkaline solutions. The results show that different salts have no effects on the amorphous phase formation stage but do affect the zeolite or sodalite crystallization stage. The addition of Na2CO3 increases the dissolution rate of zeolite, but it barely promotes the crystallization rate of sodalite. The addition of Na3PO4 inhibits sodalite crystallization, whereas the addition of Na2SO4 is most effective in facilitating sodalite formation and improving final sodalite particle size. Also, SO42–-bearing sodalite has a larger unit cell than other types of sodalites. These outcomes suggest that both CO32– and SO42– have negative effects on stabilizing zeolite LTA phases but PO43– has positive effects on these products.

Targeting high symmetry in structure predictions by biasing the potential energy surface

Ground-state structures found in nature are, in many cases, of high symmetry. But structure prediction methods typically render only a small fraction of high-symmetry structures. Especially for large crystalline unit cells, there are many low-energy defect structures. For this reason, methods have been developed where either preferentially high-symmetry structures are used as input or where the whole structural search is done within a certain symmetry group. In both cases, it is necessary to specify the correct symmetry group beforehand. However, it can in general not be predicted which symmetry group is the correct one leading to the ground state. For this reason, we introduce a potential energy biasing scheme that favors symmetry and where it is not necessary to specify any symmetry group beforehand. On this biased potential energy surface, high-symmetry structures will be found much faster than on an unbiased surface and independently of the symmetry group to which they belong. For our two test cases, i.e., a ${C}_{60}$ fullerene and bulk silicon carbide, we get speedups of 25 and 63. In our data, we also find a clear correlation between the similarity of the atomic environments and the energy. In low-energy structures, all the atoms of a species tend to have similar environments.

Thermoelectricity in Ag/Cu-based complex crystal structure minerals with inherent low thermal conductivity

Abstract Inherently poor lattice thermal conductivity (κL) is highly desired for applications like thermoelectricity, thermal management in electronics, thermal barrier coatings and refractories. Recently, complex crystalline materials have drawn serious scientific attention because of various interesting underlying physical phenomena which explain the unique thermal properties. In this review, we have discussed various interesting concepts and their consequences leading to ultralow κL in complex bulk chalcogenide minerals having multiple scattering channels for heat-carrying phonons. The primary focus of this review is on the Ag- and Cu-based large unit cell structures with low heat capacity and liquid-like superionic conduction of cations. The Ag/Cu sublattice of these materials that followed the phonon-liquid electron-crystal concept strongly reduces the transportation of phonons and enhances the scattering process. The presence of a large number of atoms in the unit cell results in low acoustic phonons cut-off frequency, robust acoustic–optical phonons scattering, poor sound velocity and strong crystal anharmonicity inside the crystalline lattice.

Periodic plane-wave electronic structure calculations on quantum computers

A procedure for defining virtual spaces, and the periodic one-electron and two-electron integrals, for plane-wave second quantized Hamiltonians has been developed, and it was validated using full configuration interaction (FCI) calculations, as well as executions of variational quantum eigensolver (VQE) circuits on Quantinuum’s ion trap quantum computers accessed through Microsoft’s Azure Quantum service. This work is an extension to periodic systems of a new class of algorithms in which the virtual spaces were generated by optimizing orbitals from small pairwise CI Hamiltonians, which we term as correlation optimized virtual orbitals with the abbreviation COVOs. In this extension, the integration of the first Brillouin zone is automatically incorporated into the two-electron integrals. With these procedures, we have been able to derive virtual spaces, containing only a few orbitals, that were able to capture a significant amount of correlation. The focus in this manuscript is on comparing the simulations of small molecules calculated with plane-wave basis sets with large periodic unit cells at the Gamma-point, including images, to results for plane-wave basis sets with aperiodic unit cells. The results for this approach were promising, as we were able to obtain good agreement between periodic and aperiodic results for an LiH molecule. Calculations performed on the Quantinuum H1-1 quantum computer produced surprisingly good energies, in which the error mitigation played a small role in the quantum hardware calculations and the (noisy) quantum simulator results. Using a modest number of circuit runs (500 shots), we reproduced the FCI values for the 1 COVO Hamiltonian with an error of 11 milliHartree, which is expected to improve with a larger number of circuit runs.

First-principles based computational framework for the thermal conductivity of complex intermetallics: The case study of MgZn2 and Mg4Zn7

Complex intermetallics usually exist as second phases in metal alloys. How these second phases can affect the thermal conductivity of alloys is generally unknown because the intrinsic thermal transport properties of these complex intermetallic compounds are quite less explored. In this work, we propose a computational framework based on first-principles calculations to study the electron and phonon thermal transport in complex intermetallics. Two typical intermetallics, i.e., MgZn2 and Mg4Zn7, are studied as prototypes. The rigorous mode-level first-principles calculations are first carried out to study the thermal transport of MgZn2. The calculations not only provide accurate thermal conductivity results, but also allow to prove that the constant relaxation time approximation and the Slack model work quite well in complex intermetallics. Then these two models are combined with first-principles calculations to predict the thermal transport properties for Mg4Zn7. Our results show that the directional average thermal conductivities for MgZn2 and Mg4Zn7 are 53.9 and 21.9 W/mK, significantly smaller than those of their elemental counterparts. Electrons are found to be the main heat carriers in these compounds, leading to a nearly temperature-independent thermal conductivity. Phonon thermal conductivity is negligible due to large unit cells and weak metallic bondings. Our work provides reliable thermal conductivity values for MgZn2 and Mg4Zn7. The computational framework developed in this work can also be further extended to study the electrical and thermal transport of other complex intermetallics.

Introduction of a weighting scheme for the X-ray restrained wavefunction approach: advantages and drawbacks.

In a quite recent study [Genoni et al. (2017). IUCrJ, 4, 136-146], it was observed that the X-ray restrained wavefunction (XRW) approach allows a more efficient and larger capture of electron correlation effects on the electron density if high-angle reflections are not considered in the calculations. This is due to the occurrence of two concomitant effects when one uses theoretical X-ray diffraction data corresponding to a single-molecule electron density in a large unit cell: (i) the high-angle reflections are generally much more numerous than the low- and medium-angle ones, and (ii) they are already very well described at unrestrained level. Nevertheless, since high-angle data also contain important information that should not be disregarded, it is not advisable to neglect them completely. For this reason, based on the results of the previous investigation, this work introduces a weighting scheme for XRW calculations to up-weight the contribution of low- and medium-angle reflections, and, at the same time, to reasonably down-weight the importance of the high-angle data. The proposed strategy was tested through XRW computations with both theoretical and experimental structure-factor amplitudes. The tests have shown that the new weighting scheme works optimally if it is applied with theoretically generated X-ray diffraction data, while it is not advantageous when traditional experimental X-ray diffraction data (even of very high resolution) are employed. This also led to the conclusion that the use of a specific external parameter λJ for each resolution range might not be a suitable strategy to adopt in XRW calculations exploiting experimental X-ray data as restraints.

The role of cation size in the ordered–disordered phase transition temperature and cation hopping mechanism based on LiCB11H12

The ordered–disordered transition temperature is reduced significantly with increasing cationic sizes. A possible principle for creating better ion-conducting materials is to have smaller cations in a larger unit cell.

Deuterated Clathrate Hydrates as a Novel Moderator Material for Very Cold Neutrons. Project Aperçu and First Results

Clathrate hydrates are water-based solids with large crystallographic unit cells that show promise as potential moderators for use in new, more intense sources of very cold neutrons (VCN), which would enhance neutron scattering techniques and increase sensitivity of particle physics experiments. These so-called inclusion compounds seem particularly suitable for this application due to the low-energy modes of guest molecules encaged in nano-voids formed by a crystalline network of hydrogen bonded water molecules. In this article we present first results of an ongoing, extensive experimental campaign with the aim of characterizing the scattering properties of deuterated clathrate hydrates relevant for moderator applications. Experiments include neutron diffraction (carried out at Institut Laue-Langevin (ILL) using the instrument D20), as well as measurements of the temperature-dependent dynamical structure factorS(q, ω) in absolute units (carried out on ILL’s Panther and IN5). These measurements will serve as a benchmark for development of new scattering kernels.

Simultaneous intercalation of lithium, potassium and strontium into graphite in molten salts medium

The LiCl–KCl molten salts method has been used in order to intercalate strontium into graphite. Combination of X-Ray Diffraction (XRD) experiments and ion beam analyses attests the obtaining of a monophasic sample. Its chemical composition corresponds to the Li0.2K0.6Sr0.4C6 formula, revealing the synthesis of a quaternary Graphite Intercalation Compound (GIC), in agreement with the high repeat distance of 640 pm determined from its 00l XRD and with the 1D electronic density profile. The very high-quality data obtained by hk0 diffraction and rotating crystal method lead to a 2D organization of the intercalated sheet described by a large hexagonal unit cell containing 78 carbon atoms. Such quaternary phase including simultaneously alkali and alkaline-earth metals is unique and can only be prepared from molten salts method, opening new perspectives concerning graphite-based two-dimensional materials.

Accurate prediction on the lattice thermal conductivities of monolayer systems by a high-throughput descriptor

Due to the vital importance of heat management in micro- and nano-electronic devices, it is quite necessary to evaluate the lattice thermal conductivity () of two-dimensional (2D) materials. However, the accurate prediction on the has been demonstrated to be a rough task, especially for systems with large unit cell and low symmetry. Here, by using the sure independence screening and sparsifying operator (SISSO) approach, we propose a physically interpretable descriptor to quickly determine the of many potential monolayer systems, which are one of the fast-growing class among numerous 2D materials. It should be noted that the Pearson correlation coefficient between the real and predicted is as high as 0.98, suggesting good reliability of the derived descriptor. Beyond the initial training data, the strong predictive power of our descriptor is further confirmed by good agreement between the predicted and those calculated theoretically or measured experimentally. As such a data-driven descriptor contains only elementary properties of the monolayers, it is very beneficial for high-throughput screening of systems with desired .

Letter to the Editor: Structural transitions of the vitreous state

The presence of structural nanoheterogeneity means that sharp structural transitions at a well defined temperature or composition are an anathema to the vitreous state. Hence they should not be expected, but rather an extended transition, whose width is defined by the form and length scale of the relevant nanoheterogeneity. It should also be noted that all of the vitreous state transitions considered here, together with crystallisation at the melting point, are spatially non-uniform. This behaviour should be compared to sharp transitions that occur within the crystalline state, for example the displacive transition between α- and β-quartz, and those from a paramagnetic to a magnetically-ordered state. Such sharp transitions involve simultaneous co-operative movements across a large number identical unit cells, and hence cannot occur in the absence of a periodic structure. As a result, they are sensitive to disorder within individual unit cells, as may be seen from the case of cristobalite, where the presence of disorder reduces the αβ transition temperature, and in extremis inhibits the transition from β- to α-cristobalite.

Steering on-surface reactions through molecular steric hindrance and molecule-substrate van der Waals interactions.

The online version contains supplementary material available at 10.1007/s44214-022-00023-9.

Water Structures Reveal Local Hydrophobicity on the In2O3(111) Surface.

Clean oxide surfaces are generally hydrophilic. Water molecules anchor at undercoordinated surface metal atoms that act as Lewis acid sites, and they are stabilized by H bonds to undercoordinated surface oxygens. The large unit cell of In2O3(111) provides surface atoms in various configurations, which leads to chemical heterogeneity and a local deviation from this general rule. Experiments (TPD, XPS, nc-AFM) agree quantitatively with DFT calculations and show a series of distinct phases. The first three water molecules dissociate at one specific area of the unit cell and desorb above room temperature. The next three adsorb as molecules in the adjacent region. Three more water molecules rearrange this structure and an additional nine pile up above the OH groups. Despite offering undercoordinated In and O sites, the rest of the unit cell is unfavorable for adsorption and remains water-free. The first water layer thus shows ordering into nanoscopic 3D water clusters separated by hydrophobic pockets.

Obtaining and Characterizing Stable Bicontinuous Cubic Morphologies and Their Nanochannels in Lyotropic Liquid Crystal Membranes.

Amphiphilic monomers in polar solvents can self-assemble into lyotropic liquid crystal (LLC) bicontinuous cubic structures under the right composition and temperature conditions. After cross-linking, the resulting polymer membranes with three-dimensional (3D) continuous uniform channels are excellent candidates for filtration applications. Designing such membranes with the desired physical and chemical properties requires molecular-level understanding of the structure, which can be obtained through molecular modeling. However, building molecular models of bicontinuous cubic structures is challenging due to their narrow regime of stability and the difficulty of self-assembly of large unit cells in molecular simulations. We developed a protocol for building stable bicontinuous cubic unit cells involving both parameterization and assembly of the components. We validate the theoretical structure against experimental results for one such LLC monomer and provide insight into the structure missing in experimental data, as well as demonstrate the qualitative nature of water and solute transport through these membranes.

Unconventional skin modes in generalized topolectrical circuits with multiple asymmetric couplings

The eigennmodes in a non-Hermitian system containing nonreciprocal couplings may exhibit an extreme localization, also known as non-Hermitian skin effect (NHSE), under open boundary conditions. Here, we ex- plore unconventional scenarios where the interplay between multiple asymmetric couplings can cause the NHSE to vanish, unlike in known models with nonreciprocal couplings, where the NHSE vanishes only when the non-Hermiticity is turned off. We derive general conditions for the NHSE in a non-Hermitian superlattice in which the overall eigenmode localization is determined by the geometric mean of the cumulative contributions of all asymmetric coupling segments. In the limit of large unit cells, our results provide a route towards the NHSE caused by asymmetric hopping textures, rather than single asymmetric hoppings. We provide explicit electrical circuit setups for realizing our observations, which also extend to other established platforms such as photonics, mechanics, and optics systems, and quantum circuits.

Electronic Configurations of 3d Transition-Metal Compounds Using Local Structure and Neural Networks.

Machine learning (ML) methods extract statistical relationships between inputs and results. When the inputs are solid-state crystal structures, structure-property relationships can be obtained. In this work, we investigate whether a simple neural network is able to learn the 3d orbital occupations for the transition-metal (TM) centers in crystalline inorganic solid-state compounds using only the local structure around the transition-metal centers described by rotationally invariant fingerprints based on spherical harmonics and one-hot elemental encoding. A multilayer neural network trained on density functional theory (DFT) results of about 1800 samples was developed and showed good performance in predicting the TM orbital occupations (for both spin channels). We study in detail how the local structure affects the predictions of the local properties and how they provide physical insights for the design of a future machine learning model for materials chemistry. The proposed ML method is illustrated in practical application by predicting local magnetic moments of the transition-metal atoms in a full set of inorganic structures with large unit cells. Although less accurate compared to the experimental data, the ML results compared well with the DFT results, suggesting the feasibility of electronic property prediction based only on structure input.

Phonons and phase symmetries in bulk CrCl3 from scattering measurements and theory

Phonon-derived behaviors are important indicators of novel phenomena in transition metal trihalides, including spin liquid behavior, two-dimensional magnetism, and spin-lattice coupling. However, phonons and their dependence on spin structure and excitations have not been adequately explored. In this work, we probe and critically examine the vibrational properties of the prototype ferromagnetic honeycomb lattice material CrCl3 using inelastic neutron scattering and density functional theory. We demonstrate that magnetic and van der Waals interactions are essential to describing the structure and phonons in CrCl3; however, the specific spin configuration is unimportant. This provides context for understanding thermal transport measurements as governed by dynamical spin-lattice couplings. More importantly, we introduce an efficient dynamic method that exploits translational symmetries in large conventional unit cells that generates insights into phonon dispersions, interactions, and measured spectra in terms of quantum phase interference conditions. This work opens new avenues for understanding phonons in layered magnets and more generally in conventional cell geometries of a variety of materials.

The high-pressure lithium–palladium and lithium–palladium–hydrogen systems

The lithium–palladium and lithium–palladium–hydrogen systems are investigated at high pressures at and above room temperature. Two novel lithium–palladium compounds are found below {18.7},{mathrm{GPa}}. An ambient temperature phase is tentatively assigned as F{bar{4}}3m,hbox {Li}_{17}hbox {Pd}_{4}, with a = 17.661(1) Å at 8.64 GPa, isostructural with hbox {Li}_{17}hbox {Sn}_{4}. The other phase occurs at high-temperature and is I{bar{4}}3m, hbox {Li}_{11}hbox {Pd}_{2}, a = 9.218(1) Å at 3.88 GPa and 200 ^circ {mathrm{C}}, similar to hbox {Li}_{11}hbox {Pt}_{2}, which is also known at high pressure. The presence of hydrogen in the system results in an I{bar{4}}3m structure with a = 8.856(1) Å at 9.74 GPa. This persists up to {13.3},mathrm{GPa}, the highest pressure studied. Below {2},{mathrm{GPa}} an fcc phase with a large unit cell, a = 19.324(1) Å at 0.39 GPa, is also observed in the presence of hydrogen. On heating the hydrogen containing system at 4 GPa the I{bar{4}}3m phases persists to the melting point of lithium. In both systems melting the lithium results in the loss of crystalline diffraction from palladium containing phases. This is attributed to dissolution of the palladium in the molten lithium, and on cooling the palladium remains dispersed.