Large Unit Cell Research Articles

Influence of Subvalent Twin-Rattler for High n-Type Thermoelectric Performance in Bi13S18Br2 Chalcohalide.

Metal chalcohalides, owing to their higher stability over halides and greater tunability of electronic features over chalcogenides, open new avenues for investigating properties of materials. Complex metal chalcohalides can be a good choice for thermoelectric studies for their halide-like low thermal conductivity and chalcogenide-like high electrical conductivity. Here, we have investigated the thermoelectric properties of n-type Bi13S18Br2 and utilized the concept of Fajans' polarization to describe the formation of a dimer and explained how it can help achieve high thermoelectric figure of merit (zT) of ∼1.0 at 748 K. This zT value is so far the highest-reported value for pristine metal chalcohalides. The existence of subunit in Bi13S18Br2 is experimentally verified by synchrotron X-ray pair distribution function (X-PDF) analysis. The complex structure of Bi13S18Br2 having a large unit cell exhibits simultaneous dimer-cation rattler (i.e., "twin-rattler"), which decreases the lattice thermal conductivity drastically. We observed evidence of such low-energy rattling vibrations from DFT-calculated eigen mode visualizations of the phonon dispersion. The subvalent nature of accommodates an extra electron in Bi(6pz) orbital, which helps form a weakly dispersed donor band just below the Fermi energy (EF), leading to a significant reduction in band gap (0.77 eV), which is favorable for high thermoelectric performance. Consequently, we obtained a semiconducting nature of n-type Bi13S18Br2 with moderate electrical conductivity, as well as a high Seebeck coefficient. Our investigation presents the importance of fundamental chemistry in thermoelectrics and demonstrates the influence of subvalent twin-rattler in triggering high thermoelectric performance in metal chalcohalides.

The Influence of Pressure on the Electronic and Elastic Characteristics of GaP Nanocrystals using Density Functional Theory

The electronic structural and mechanical properties of III-V Gallium Phosphide (GaP) nano-crystals zinc-blende stable (ZB) phase under pressure dependence have been studied via the model of the density functional theory (DFT) in combination with the large unit cell (LUC) approach at (8, 16, 54 & 64) atoms. The exchanges and correlations (XC) energy has been defined in all approaches through generalized gradient approximations of the Perdew-Burke-Ernzerhof (GGA-PBE), and determination of GaP characteristics in the case where the pressure has been increased from 0GPa to ±50GPa. Elastic parameters like elastic constants (C11, C12 & C44), Kleinmann parameter (ξ), Zener anisotropic factor (A), shear modulus (G), Young's modulus (E), Poisson’s ratio (v), Bulk modules (B) of GaP-ZB structure have been calculated and showed systematic variation with a pressure increase. Our calculations show that Bulk modules and sound speed increase with an increase in the number of core atoms. In this systematic approach, it has been found that the B/G ratio is 1.58 for this compound and it exhibits a negative Cauchy pressure, classifying GaP as brittle. The results were discovered to be consistent with other theoretical as well as experimental results. The present study directly relates to the latest experimental works on the III-phosphide compounds. The current results provide information and direction for further investigation into the fundamentals and potential applications of gallium phosphate (GaP) nanocrystals. In addition, we attempted to compare the results of the investigation with some similar types of compounds that are already documented in the literature. We believe that our study will have a significant impact on both the field of research and contemporary technology based on semi-conducting materials.

Large-Angle Rocking Beam Electron Diffraction of Large Unit Cell Crystals Using Direct Electron Detector.

We report a large-angle rocking beam electron diffraction (LARBED) technique for electron diffraction analysis. Diffraction patterns are recorded in a scanning transmission electron microscope (STEM) using a direct electron detector with large dynamical range and fast readout. We use a nanobeam for diffraction and perform the beam double rocking by synchronizing the detector with the STEM scan coils for the recording. Using this approach, large-angle convergent beam electron diffraction (LACBED) patterns of different reflections are obtained simultaneously. By using a nanobeam, instead of a focused beam, the LARBED technique can be applied to beam-sensitive crystals as well as crystals with large unit cells. This paper describes the implementation of LARBED and evaluates the performance using silicon and gadolinium gallium garnet crystals as test samples. We demonstrate that our method provides an effective and robust way for recording LARBED patterns and paves the way for quantitative electron diffraction of large unit cell and beam-sensitive crystals.

Mn-deficient ZnMn2O4/Zn0.5Mn0.5Fe2O4 cathode for enhancing structural reversibility and stability of zinc-ion batteries

Aqueous zinc-ion batteries have surfaced as a viable energy storage technology due to their safety, eco-friendliness, and cost-effectiveness. Binary zinc manganite oxide (ZnMn2O4 or ZMO) stands out as the potential cathode material owing to its considerable theoretical capacity and high operating voltage. However, high electrostatic repulsion of Zn2+ ions within ZMO leads to sluggish diffusion and poor reversibility of Zn2+ ions insertion/extraction that can reduce charge storage and overall cycling performance. This study addresses this challenge by introducing FeCl3 during the synthesis to form ZMO/Zn0.5Mn0.5Fe2O4 (ZMFO) cathode material. This cathode features Mn-deficient structures, which can reduce the electrostatic repulsion via low Mn occupancies and lattice parameters enlargement, thus facilitating Zn2+ ions diffusion. Ex-situ X-ray diffraction analyses further show that the cathode is able to maintain good structural reversibility during the charge-discharge cycles. Additionally, the ZMO/ZMFO cathode exhibits smaller particle sizes than pristine ZMO, providing a wider surface area. The presence of ZMFO with a large unit cell volume can also enhance Zn storage capacity and accelerate Zn2+ ions kinetics. Highlighting those advantages, aqueous zinc-ion batteries with ZMO/ZMFO cathode show a capacity of ~230 mAh g−1 at 0.05 A g−1 and retain 99 % of its initial capacity at 0.2 A g−1 after 200 cycles of charge-discharge.

Synthesis of fullerite (100) through epitaxy on a complex intermetallic surface

We report the first epitaxial growth of bulk-like C60(100) plane stabilized by a size-matching effect on the surface of a complex intermetallic compound. As characterized by low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) techniques, the adsorption of C60 on Au–Si–Ho(100) surface is disordered at room temperature but ordered when the substrate is held at elevated temperature (669 K). In the submonolayer regime, different superstructures are observed depending on the fullerenes coverage. Upon completion of the first monolayer of C60, the thin film of c(1 × 1) superstructure is structurally equivalent to a (100) surface plane of the face-centered cubic structure of C60 bulk crystal. This structure is unusual for C60 monolayers on metal surfaces and appears to be favored by an almost perfect match between the most preferred C60 intermolecular distance and the distances separating specific adsorption sites of the large unit cell compound. The adsorption site selection is dictated by a local symmetry matching between the pentagonal fullerene facets and pseudo-five-fold symmetric atomic arrangements on the substrate. After the completion of the first monolayer, further deposition of C60 at 387 K results in the growth of a (100) oriented thicker epitaxial fullerite crystal.

Computational approach to modeling electronic properties of titanium oxynitride systems

The study presents the use of a novel on-lattice sampling approach to generate titanium oxynitride (TiNxOy) structures with potential applications in photovoltaic and water splitting. This approach presents a simple route to overcome challenges with structure-generating tools like Cluster Approach to Statistical Mechanics (CASM), and Ab initio Random Structure Search (AIRSS), CASM faces difficulty in generating ternary structures with large unit cells. With AIRSS, there is an increase in probability of sampling amorphous sample spaces with increased number of atoms in the unit cell. Here an on-lattice sampling approach was used to model the electronic structure of TiNxOy as a function of composition. We present results for Ti2N2O, Ti5N4O4 and Ti7N4O8, with 33 %, 50 % and 67 % N replaced by O via substitution relative to titanium nitride (TiN), respectively. Koopmans theorem was used correct the Kohn-Sham Density Functional Theory (KS-DFT) bandgaps with corresponding values of 2.68 eV, 3.03 eV, and 3.65 eV for 33, 50 and 67 % O doping respectively. The projected density of states (PDOS) plot for TiN shows that the Fermi level is dominated by the 3d atomic orbitals of Ti, confirming pure TiN’s metallicity. The valence bands of TiNxOy structures were dominated by 2p orbitals of O at lower energy levels, but they were dominated by 2p orbitals of N at energies close to the valence band maximum (VBM). The conduction bands were dominated by the 3d atomic orbitals of Ti, with the bandgap increasing with O composition leading to creation of shallow trap states near the VBM, which negatively impacts carrier mobility. In conclusion, the on-lattice sampling approach is an effective tool to generate highly crystalline structures of large unit cells, also keeping O substitution for N below 33 % as seen in Ti2N2O is crucial for avoiding shallow traps in TiNxOy structures.

ABa6Cu31Te22 (A = K, Rb, Cs) Featuring Polyanionic Copper-Telluride Frameworks with Ultralow Thermal Conductivity.

Three polyanionic tellurides, ABa6Cu31Te22 (A = K, Rb, Cs), were synthesized in salt flux. The isostructural tellurides crystallize in a new structure type, in the cubic Pa3 space group with a Wyckoff sequence of d10c2b1 and large unit cell volumes of over 5500 Å3. The structures feature a framework of [CuTe4] tetrahedra and [CuTe3] trigonal pyramids with disorder in the Cu sites. The polyanionic frameworks have large square antiprism and cuboctahedral voids where Ba and alkali metal cations are situated, forming [BaTe8] and [ATe12], respectively. The overall compositions are close to being charge balanced. The large [ATe12] cuboctahedra allowed for significant anisotropic displacement of the A cations, as observed from both single crystal X-ray diffraction and heat capacity studies. Alkali cations rattling together with Cu atom displacement and disorder leads to the dispersion of phonons, thus softening the lattice and subsequently reducing the thermal conductivity. Evaluations of the electronic band structure revealed the occurrence of a narrow bandgap together with the presence of a flat band near the valence band maximum, giving rise to the high thermopower. The Cs and Rb analogues show a slope change in the temperature dependence of electrical resistivity around room temperature, which is typical for semimetals or degenerate semiconductors. For the as-synthesized and unoptimized materials, high values of the thermoelectric figure-of-merit of ∼0.2 were observed at 623 K.

Vibration suppression of power cabin for underwater vehicle based on mechanical metamaterial flanges

The vibration of underwater vehicles affects their instrumentation and acoustic stealth capabilities. Mechanical metamaterials offer an advanced alternative to conventional vibration control methods. This study develops metamaterial flanges consisting of rectangular star-shaped cells with horizontal ligaments and localized masses. The vibration attenuation performance of metamaterial flanges within the power cabin is validated through simulations and experiments. This research examines the correlation between the bandgaps of large radial span unit cells and the frequencies of vibration attenuation, using the bandgap superposition effect. The findings reveal that the metamaterial cell exhibits exceptional load-bearing capacity and directional bandgaps in the low- to mid-frequency domain. Notably, the vibration attenuation achieved via bandgap superposition reaches 5.1 dB within 823 Hz to 1359 Hz. Specifically, within the range of 1017 Hz to 1146 Hz, the attenuation peaks at an impressive 18.6 dB. The metamaterial structure significantly reduces weight, decreasing the power cabin mass by 36.5%. This research offers new insights and serves as a valuable reference for vibration suppression in small- to medium-sized underwater vehicles.

Tuning Interfacial Water Friction through Moiré Twist.

Foundations of nanofluidics can enable advances in diverse applications such as water desalination, energy harvesting, and biological analysis. Dynamically manipulating nanofluidic properties, such as diffusion and friction, is an area of great scientific interest. Twisted bilayer graphene, particularly at the magic angle, has garnered attention for its unconventional superconductivity and correlated insulator behavior due to strong electronic correlations. The impact of the electronic properties of moiré patterns in twisted bilayer graphene on structural and dynamic properties of water remains largely unexplored. Computational challenges, stemming from simulating large unit cells using density functional theory, have hindered progress. This study addresses this gap by investigating water behavior on twisted bilayer graphene, employing a deep neural network potential (DP) model trained with a data set from ab initio molecular dynamics simulations. It is found that as the twisted angle approaches the magic angle, interfacial water friction increases, leading to a reduced water diffusion. Notably, the analysis shows that at smaller twisted angles with larger moiré patterns, water is more likely to reside in AA stacking regions than AB (or BA) stacking regions, a distinction that diminishes with smaller moiré patterns. This study illustrates the potential for leveraging the distinctive properties of moiré systems to effectively control and optimize interfacial fluid behavior.

Conceptual design of a macromolecular diffractometer for the Jülich high brilliance source.

In this work, a concept for a neutron diffractometer for high-resolution macromolecular structures has been developed within the Jülich High Brilliance Neutron Source (HBS) project. The SELENE optics are adapted to the requirements of the instrument to achieve a tunable low background neutron beam at mm2 scale sample area. With the optimized guide geometry, a low background neutron beam can be achieved at the small sample area with tunable divergence and size. For the 1 × 1mm2 sample, a flux of 1.10 × 107 n/s/cm2 for 0.38° divergence is calculated in the 2-4Å wavelength range, which is about 84.6% of the flux at MaNDi of the high-power spallation source SNS at ORNL. Virtual neutron scattering experiments have been performed to demonstrate the instrument's capabilities for studies of mm scale samples with large unit cells. Results of Vitesse simulations indicate that unit cell sizes of up to 200Å are possible to be resolved with the proposed instrument.

Emittance optimization under magnet strength limitation for a multi-bend achromat lattice

Diffraction-limited storage rings (DLSRs) based on multi-bend achromat (MBA) lattices are being developed towards much lower emittances. For instance, for the future development, MAX IV designed a 19BA lattice to replace the present 7BA lattice, achieving an emittance of 16 pm·rad. While this 19BA lattice, with a significantly increased number of unit cells, requires extremely strong quadrupoles and sextupoles, which will result in very small vacuum chambers and can thus significantly increase beam coupling impedance and potentially enhance beam instabilities. This issue raises a question of how we can reduce magnet strengths while achieving the emittance goal. In this paper, based on a simplified model where an MBA lattice is considered to consist of a number of identical unit cells, emittance optimization is numerically studied under magnet strength limitation. The studied results show that for a given arc length and emittance goal, choosing an appropriate number of unit cells with relatively large horizontal cell tunes can significantly reduce magnet strengths. As a practical example, an 11BA lattice with relatively large unit cell tunes is designed and compared with a 17BA lattice with smaller unit cell tunes. The two lattices have roughly the same emittance with the same energy, ring circumference and number of periods, while the magnets of the 11BA lattice are significantly weaker.

Ground state of the staggered Heisenberg-Γ honeycomb model in a magnetic field

We study the ground state properties of the S=1/2 staggered Heisenberg-\GammaΓ honeycomb model under a magnetic field based on analytical and numerical methods. Our calculations show that the conventional zigzag and stripy phases are favored because of the staggered Heisenberg interaction away from the pure \GammaΓ limit. In our classical analysis, we find that the field induces a series of competing magnetic phases with relatively large unit cells in the region sandwiched between the two magnetic phases with long-range ordering. In the quantum treatment, these large magnetic unit cells are destabilized by strong quantum fluctuations that result in the stabilization of a gapless quantum spin liquid behavior. In a honeycomb \GammaΓ magnet, we disclose an intermediate-field gapless quantum spin liquid phase driven by a tilted field away from the out-of-plane direction only for a narrow region between the low-field zigzag and high-field fully polarized phases.

The axially-loaded behaviours of Schwarz Primitive (SP)-based structures: An experimental and DEM study

This study delves into the axial responses of Schwarz Primitive (SP) structures. Emphasising on bearing capacity, force distribution, cracking phenomena, load-displacement curve and energy absorption. Discrete element method (DEM) is adopted to analyse the axial load-bearing behaviour and fracture mechanics of Main SP and Secondary SP structures under various unit cell configurations after validated by experimental tests. The results indicates that despite sharing the same 1/8th cell structure, the axially-loaded behaviours of the Main SP and Secondary SP structures are not identical, particularly in scenarios involving a large unit cell size and a small number of unit cells. The Main SP structures display an axial load-bearing behaviour that is largely unaffected by variations in the number of unit cells along the x and y axes, while Secondary SP structures show a more pronounced improvement in average strength and energy absorption capacity when the number of unit cells increases. Main SP structures offer superior axial load-bearing capabilities and energy absorption efficiency, which is a significant consideration for their application in scenarios demanding high structural integrity. This distinction underlines the importance of tailored design and selection of SP configurations based on specific engineering requirements.

Global machine learning potentials for molecular crystals.

Molecular crystals are difficult to model with accurate first-principles methods due to large unit cells. On the other hand, accurate modeling is required as polymorphs often differ by only 1kJ/mol. Machine learning interatomic potentials promise to provide accuracy of the baseline first-principles methods with a cost lower by orders of magnitude. Using the existing databases of the density functional theory calculations for molecular crystals and molecules, we train global machine learning interatomic potentials, usable for any molecular crystal. We test the performance of the potentials on experimental benchmarks and show that they perform better than classical force fields and, in some cases, are comparable to the density functional theory calculations.

Diffusion dominant thermal transport in mixed valent Ba4Sn4Se9

New material developments are of paramount importance from the perspective of developing a fundamental understanding of material properties as well as transitioning from materials to devices. Among the different classes of materials of interest, mixed valence Sn compositions have been known to display technologically important properties and are of interest for the design and synthesis of novel compositions for targeted applications. Herein, we report on electronic, optical and thermal properties of the previously unexplored mixed valence composition Ba4Sn4Se9, a material with a direct optical band gap of 2.36 eV. As revealed by single crystal synchrotron X-ray diffraction, Ba4Sn4Se9 crystallizes in the orthorhombic space group Pnma with a large unit cell comprised of 58 atoms. Heat capacity data reveals a low Debye temperature with many low frequency optic modes. Our low temperature thermal conductivity data and analyses show that thermal diffusion dominates thermal transport above 80 K. Furthermore, high lattice anharmonicity contributes to the very low thermal conductivity. Our results and analyses will motivate further investigations of this as well as other compositions in the Ba-Sn-Se system, as well as other mixed valence chalcogenides for technological applications.

Synthesis under high pressure: crystal structure and properties of cubic Dy36O11F50[AsO3]12 ∙ H2O.

The multi-anionic compound with the composition Dy36O11F50[AsO3]12 ∙ H2O, which can be described in the non-centrosymmetric cubic space group F c, already shows an unusually large unit cell with an axis of a = 2587.59(14) pm. Its crystal structure exhibits isolated ψ1-tetrahedral [AsO3]3- anions, but both the coordination numbers and the linking schemes of the Dy3+-centered polyhedra differ significantly from the mostly layered structures described so far in literature. (Dy1)3+ is sevenfold coordinated by oxygen atoms and F- anions, forming a capped trigonal prism [(Dy1)O4.333F2.667]8.333-, and the remaining two cations (Dy2)3+ and (Dy3)3+ both reside in an eightfold coordination of anions. In both cases they form slightly distorted square antiprisms, which have the compositions of [(Dy2)O3.667F4.333]8.667- and [(Dy3)O4.667F3.333]9.667-, respectively. Some of the oxygen atoms are not part of ψ1-[AsO3]3- tetrahedra, but occur as O2- anions and one of these even shares a common crystallographic position with fluoride (F-). It is also worth mentioning that the single crystals were obtained as comparatively large cubes with an edge length of several 100µm providing very good data with regard to single-crystal X-ray diffraction. To verify the simultaneous presence of oxygen and fluorine, electron-beam microprobe analysis was carried out, and a single-crystal Raman spectrum ruled out the presence of hydroxide anions or protonated [AsO3]3- groups, but proved the interstitial crystal-water molecules, which could not be determined precisely by the crystal-structure refinement.

Encoding ordered structural complexity to covalent organic frameworks

Installing different chemical entities onto crystalline frameworks with well-defined spatial distributions represents a viable approach to achieve ordered and complex synthetic materials. Herein, a covalent organic framework (COF-305) is constructed from tetrakis(4-aminophenyl)methane and 2,3-dimethoxyterephthalaldehyde, which has the largest unit cell and asymmetric unit among known COFs. The ordered complexity of COF-305 is embodied by nine different stereoisomers of its constituents showing specific sequences on topologically equivalent sites, which can be attributed to its building blocks deviating from their intrinsically preferred simple packing geometries in their molecular crystals to adapt to the framework formation. The insight provided by COF-305 supplements the principle of covalent reticular design from the perspective of non-covalent interactions and opens opportunities for pursuing complex chemical sequences in molecular frameworks.

Comparative Analysis of the Crystal Structures and Physical Properties of the Complex Group 14 Selenides Sr8Ge4Se17 and Ba8Sn4Se17.

During our group's continued exploration of group 14 chalcogenides, we discovered two new compounds, Sr8Ge4Se17 and Ba8Sn4Se17. Both compounds have an 8:4:17 stoichiometric ratio but adopt different centrosymmetric crystal structures. Sr8Ge4Se17 crystallizes in the triclinic P1̅ space group with a = 11.8429(18) Å, b = 12.172(3) Å, c = 13.624(3) Å, α = 114.472(5)0, β = 97.396(5)0, γ = 107.040(5)0, and Z = 2. Ba8Sn4Se17 crystallizes in the monoclinic C2/c space group with a = 47.286(3) Å, b = 12.6294(5) Å, c = 25.7303(15) Å, β = 104.585(5)0, and Z = 16. The unit cell of Ba8Sn4Se17 is approximately eight times larger than the unit cell of Sr8Ge4Se17, which is a consequence of the differently aligned tetrahedra, resulting in a quadrupled a axis, unchanged b axis, and doubled c axis. The lattice parameters and atomic coordinates were finalized via Rietveld refinements on data collected at a high-energy synchrotron beamline. Both compounds are semiconductors with band gaps in the visible region. Sr8Ge4Se17 and Ba8Sn4Se17 have optical band gaps of 1.88 and 1.93 eV, respectively. Both compounds have remarkably low thermal conductivities owing to their low symmetries and large unit cells. The minimum experimental thermal conductivity values of Sr8Ge4Se17 and Ba8Sn4Se17 are 0.45 W m-1 K-1 at 321 K and 0.31 W m-1 K-1 at 324 K, respectively.

Indications of flat bands driving the δ to α volume collapse of plutonium

On cooling from the melt, plutonium (Pu) undergoes a series of structural transformations accompanied by a ≈ 28% reduction in volume from its δ phase to its α phase at low temperatures. While Pu's partially filled 5f-electron shells are known to be involved, their precise role in the transformations has remained unclear. By using calorimetry measurements on α-Pu and gallium-stabilized δ-Pu combined with resonant ultrasound and X-ray scattering data to account for the anomalously large softening of the lattice with temperature, we show here that the difference in electronic entropy between the α and δ phases dominates over the difference in phonon entropy. Rather than finding an electronic specific heat characteristic of broad f-electron bands in α-Pu, as might be expected to occur within a Kondo collapsed phase in analogy with cerium, we find it to be indicative of flatter subbands. An important role played by Pu's 5f electrons in the formation of its larger unit cell α phase comprising inequivalent lattice sites and varying bond lengths is therefore suggested.

Modelling dynamical 3D electron diffraction intensities. I. A scattering cluster algorithm.

Three-dimensional electron diffraction (3D-ED) is a powerful technique for crystallographic characterization of nanometre-sized crystals that are too small for X-ray diffraction. For accurate crystal structure refinement, however, it is important that the Bragg diffracted intensities are treated dynamically. Bloch wave simulations are often used in 3D-ED, but can be computationally expensive for large unit cell crystals due to the large number of diffracted beams. Proposed here is an alternative method, the `scattering cluster algorithm' (SCA), that replaces the eigen-decomposition operation in Bloch waves with a simpler matrix multiplication. The underlying principle of SCA is that the intensity of a given Bragg reflection is largely determined by intensity transfer (i.e. `scattering') from a cluster of neighbouring diffracted beams. However, the penalty for using matrix multiplication is that the sample must be divided into a series of thin slices and the diffracted beams calculated iteratively, similar to the multislice approach. Therefore, SCA is more suitable for thin specimens. The accuracy and speed of SCA are demonstrated on tri-isopropyl silane (TIPS) pentacene and rubrene, two exemplar organic materials with large unit cells.