Giant Modulation of Refractive Index from Picoscale Atomic Displacements.

It is shown that structural disorder-in the form of anisotropic, picoscale atomic displacements-modulates the refractive index tensor and results in the giant optical anisotropy observed in BaTiS3, a quasi-1D hexagonal chalcogenide. Single-crystal X-ray diffraction studies reveal the presence of antipolar displacements of Ti atoms within adjacent TiS6 chains along the c-axis, and threefold degenerate Ti displacements in the a-b plane. 47/49Ti solid-state NMR provides additional evidence for those Ti displacements in the form of a three-horned NMR lineshape resulting from a low symmetry local environment around Ti atoms. Scanning transmission electron microscopy is used to directly observe the globally disordered Ti a-b plane displacements and find them to be ordered locally over a few unit cells. First-principles calculations show that the Ti a-b plane displacements selectively reduce the refractive index along the ab-plane, while having minimal impact on the refractive index along the chain direction, thus resulting in a giant enhancement in the optical anisotropy. By showing a strong connection between structural disorder with picoscale displacements and the optical response in BaTiS3, this study opens a pathway for designing optical materials with high refractive index and functionalities such as large optical anisotropy and nonlinearity.

Vibrations and Phase Stability in Mixed Valence Antimony Oxide.

α-Sb2O4 (cervantite) and ß-Sb2O4 (clinocervantite) are mixed valence compounds with equal proportions of SbIII and SbV as represented in the formula SbIIISbVO4. Their structure and properties can be difficult to calculate owing to the SbIII lone-pair electrons. Here, we present a study of the lattice dynamics and vibrational properties using a combination of inelastic neutron scattering, Mössbauer spectroscopy, nuclear inelastic scattering, and density functional theory (DFT) calculations. DFT calculations that account for lone-pair electrons match the experimental densities of phonon states. Mössbauer spectroscopy reveals the ß phase to be significantly harder than the α phase. Calculations with O vacancies reveal the possibility for nonstoichiometric proportions of SbIII and SbV in both phases. An open question is what drives the stability of the α phase over the ß phase, as the latter shows pronounced kinetic stability and lower symmetry despite being in the high-temperature phase. Since the vibrational entropy difference is small, it is unlikely to stabilize the α phase. Our results suggest that the α phase is more stable only because the material is not fully stoichiometric.

(Mg,Mn,Fe,Co,Ni)O: A rocksalt high-entropy oxide containing divalent Mn and Fe.

High-entropy oxides (HEOs) have aroused growing interest due to fundamental questions relating to their structure formation, phase stability, and the interplay between configurational disorder and physical and chemical properties. Introducing Fe(II) and Mn(II) into a rocksalt HEO is considered challenging, as theoretical analysis suggests that they are unstable in this structure under ambient conditions. Here, we develop a bottom-up method for synthesizing Mn- and Fe-containing rocksalt HEO (FeO-HEO). We present a comprehensive investigation of its crystal structure and the random cation-site occupancy. We show the improved structural robustness of this FeO-HEO and verify the viability of an oxygen sublattice as a buffer layer. Compositional analysis reveals the valence and spin state of the iron species. We further report the antiferromagnetic order of this FeO-HEO below the transition temperature ~218 K and predict the conditions of phase stability of Mn- and Fe-containing HEOs. Our results provide fresh insights into the design and property tailoring of emerging classes of HEOs.

Magnetically Induced Binary Ferrocene with Oxidized Iron.

Ferrocene is perhaps the most popular and well-studied organometallic molecule, but our understanding of its structure and electronic properties has not changed for more than 70 years. In particular, all previous attempts of chemically oxidizing pure ferrocene by binding directly to the iron center have been unsuccessful, and no significant change in structure or magnetism has been reported. Using a metal organic framework host material, we were able to fundamentally change the electronic and magnetic structure of ferrocene to take on a never-before observed physically stretched/bent high-spin Fe(II) state, which readily accepts O2 from air, chemically oxidizing the iron from Fe(II) to Fe(III). We also show that the binding of oxygen is reversible through temperature swing experiments. Our analysis is based on combining Mößbauer spectroscopy, extended X-ray absorption fine structure, in situ infrared, SQUID, thermal gravimetric analysis, and energy dispersive X-ray fluorescence spectroscopy measurements with ab initio modeling.

Direct Mechanochemical Synthesis, Phase Stability, and Electrochemical Performance of α-NaFeO2.

To better understand polymorph control in transition metal oxides, the mechanochemical synthesis of NaFeO2 was explored. Herein, we report the direct synthesis of α-NaFeO2 through a mechanochemical process. By milling Na2O2 and γ-Fe2O3 for 5 h, α-NaFeO2 was prepared without high-temperature annealing needed in other synthesis methods. While investigating the mechanochemical synthesis, it was observed that changing the starting precursors and mass of precursors affects the resulting NaFeO2 structure. Density functional theory calculations on the phase stability of NaFeO2 phases show that the α phase is stabilized over the ß phase in oxidizing environments, which is provided by the oxygen-rich reaction between Na2O2 and Fe2O3. This provides a possible route to understanding polymorph control in NaFeO2. Annealing the as-milled α-NaFeO2 at 700 °C has resulted in increased crystallinity and structural changes that improved electrochemical performance in terms of capacity over the as-milled sample.

Shape-induced superstructure formation in concentrated ferrofluids under applied magnetic fields.

The field-induced ordering of concentrated ferrofluids based on spherical and cuboidal maghemite nanoparticles is studied using small-angle neutron scattering, revealing a qualitative effect of the faceted shape on the interparticle interactions as shown in the structure factor and correlation lengths. Whereas a spatially disordered hard-sphere interaction potential with a short correlation length is found for ∼9 nm spherical nanoparticles, nanocubes of a comparable particle size exhibit a more pronounced interparticle interaction and the formation of linear arrangements. Analysis of the anisotropic two-dimensional pair distance correlation function gives insight into the real-space arrangement of the nanoparticles. On the basis of the short interparticle distances found here, oriented attachment, i.e. a face-to-face arrangement of the nanocubes, is likely. The unusual field dependence of the interparticle correlations suggests a field-induced structural rearrangement.

Charge Density Wave in Kagome Lattice Intermetallic ScV_{6}Sn_{6}.

Materials hosting kagome lattices have drawn interest for the diverse magnetic and electronic states generated by geometric frustration. In the AV_{3}Sb_{5} compounds (A=K, Rb, Cs), stacked vanadium kagome layers give rise to unusual charge density waves (CDW) and superconductivity. Here we report single-crystal growth and characterization of ScV_{6}Sn_{6}, a hexagonal HfFe_{6}Ge_{6}-type compound that shares this structural motif. We identify a first-order phase transition at 92 K. Single crystal x-ray and neutron diffraction reveal a charge density wave modulation of the atomic lattice below this temperature. This is a distinctly different structural mode than that observed in the AV_{3}Sb_{5} compounds, but both modes have been anticipated in kagome metals. The diverse HfFe_{6}Ge_{6} family offers more opportunities to tune ScV_{6}Sn_{6} and explore density wave order in kagome lattice materials.

RUScal: Software for the analysis of resonant ultrasound spectroscopy measurements.

Resonant ultrasound spectroscopy is used to nondestructively measure the elastic resonances of small solids to elucidate the material's elastic properties or other qualities like size, shape, or composition. Here, we introduce the software RUScal for the purpose of determining elastic properties by analyzing the eigenfrequencies of solid specimens with common shapes, such as rectangular parallelepipeds, cylinders (solid and hollow tube), ellipsoids, and octahedrons, as well as irregularly shaped ellipsoids that can be described analytically. All symmetry classes are supported, from isotropic to triclinic, along with the option to add or remove up to three orthogonal mirror planes as well as the ability to reorient the crystal axes with respect the sample edges via Euler angles. Additional features include tools to help find initial sets of elastic constants, including grid exploration and Monte Carlo methods, a tool to analyze frequencies as a function of sample length or crystal orientation, an error analysis tool to assess fit quality, and formatting of the input and output files for batch fitting, e.g., as a function of temperature. This software was validated with published resonant ultrasound spectroscopy data for various materials, shapes, and symmetries with noted improvements in calculation time compared to finite element methods.

A 51,000-year-old engraved bone reveals Neanderthals' capacity for symbolic behaviour.

While there is substantial evidence for art and symbolic behaviour in early Homo sapiens across Africa and Eurasia, similar evidence connected to Neanderthals is sparse and often contested in scientific debates. Each new discovery is thus crucial for our understanding of Neanderthals' cognitive capacity. Here we report on the discovery of an at least 51,000-year-old engraved giant deer phalanx found at the former cave entrance of Einhornhöhle, northern Germany. The find comes from an apparent Middle Palaeolithic context that is linked to Neanderthals. The engraved bone demonstrates that conceptual imagination, as a prerequisite to compose individual lines into a coherent design, was present in Neanderthals. Therefore, Neanderthal's awareness of symbolic meaning is very likely. Our findings show that Neanderthals were capable of creating symbolic expressions before H. sapiens arrived in Central Europe.

Bronze Age weight systems as a measure of market integration in Western Eurasia.

Weighing technology was invented around 3000 BCE between Mesopotamia and Egypt and became widely adopted in Western Eurasia within ∼2,000 y. For the first time in history, merchants could rely on an objective frame of reference to quantify economic value. The subsequent emergence of different weight systems goes hand in hand with the formation of a continental market. However, we still do not know how the technological transmission happened and why different weight systems emerged along the way. Here, we show that the diffusion of weighing technology can be explained as the result of merchants' interaction and the emergence of primary weight systems as the outcome of the random propagation of error constrained by market self-regulation. We found that the statistical errors of early units between Mesopotamia and Europe overlap significantly. Our experiment with replica weights gives error figures that are consistent with the archaeological sample. We used these figures to develop a model simulating the formation of primary weight systems based on the random propagation of error over time from a single original unit. The simulation is consistent with the observed distribution of weight units. We demonstrate that the creation of the earliest weight systems is not consistent with a substantial intervention of political authorities. Our results urge a revaluation of the role of individual commercial initiatives in the formation of the first integrated market in Western Eurasia.

Deconvoluting the Magnetic Structure of the Commensurately Modulated Quinary Zintl Phase Eu11-xSrxZn4Sn2As12.

The structure, magnetic properties, and 151Eu and 119Sn Mössbauer spectra of the solid-solution Eu11-xSrxZn4Sn2As12 are presented. A new commensurately modulated structure is described for Eu11Zn4Sn2As12 (R3m space group, average structure) that closely resembles the original structural description in the monoclinic C2/c space group with layers of Eu, puckered hexagonal Zn2As3 sheets, and Zn2As6 ethane-like isolated pillars. The solid-solution Eu11-xSrxZn4Sn2As12 (0 < x < 10) is found to crystallize in the commensurately modulated R3 space group, related to the parent phase but lacking the mirror symmetry. Eu11Zn4Sn2As12 orders with a saturation plateau at 1 T for 7 of the 11 Eu2+ cations ferromagnetically coupled (5 K) and shows colossal magnetoresistance at 15 K. The magnetic properties of Eu11Zn4Sn2As12 are investigated at higher fields, and the ferromagnetic saturation of all 11 Eu2+ cations occurs at ∼8 T. The temperature-dependent magnetic properties of the solid solution were investigated, and a nontrivial structure-magnetization correlation is revealed. The temperature-dependent 151Eu and 119Sn Mössbauer spectra confirm that the europium atoms in the structure are all Eu2+ and that the tin is consistent with an oxidation state of less than four in the intermetallic region. The spectral areas of both Eu(II) and Sn increase at the magnetic transition, indicating a magnetoelastic effect upon magnetic ordering.

High frequency atomic tunneling yields ultralow and glass-like thermal conductivity in chalcogenide single crystals.

Crystalline solids exhibiting glass-like thermal conductivity have attracted substantial attention both for fundamental interest and applications such as thermoelectrics. In most crystals, the competition of phonon scattering by anharmonic interactions and crystalline imperfections leads to a non-monotonic trend of thermal conductivity with temperature. Defect-free crystals that exhibit the glassy trend of low thermal conductivity with a monotonic increase with temperature are desirable because they are intrinsically thermally insulating while retaining useful properties of perfect crystals. However, this behavior is rare, and its microscopic origin remains unclear. Here, we report the observation of ultralow and glass-like thermal conductivity in a hexagonal perovskite chalcogenide single crystal, BaTiS3, despite its highly symmetric and simple primitive cell. Elastic and inelastic scattering measurements reveal the quantum mechanical origin of this unusual trend. A two-level atomic tunneling system exists in a shallow double-well potential of the Ti atom and is of sufficiently high frequency to scatter heat-carrying phonons up to room temperature. While atomic tunneling has been invoked to explain the low-temperature thermal conductivity of solids for decades, our study establishes the presence of sub-THz frequency tunneling systems even in high-quality, electrically insulating single crystals, leading to anomalous transport properties well above cryogenic temperatures.

Phonon Spectroscopy in Antimony and Tellurium Oxides.

α-Sb2O3 (senarmontite), ß-Sb2O3 (valentinite), and α-TeO2 (paratellurite) are compounds with pronounced stereochemically active Sb and Te lone pairs. The vibrational and lattice properties of each have been previously studied but often lead to incomplete or unreliable results due to modes being inactive in infrared or Raman spectroscopy. Here, we present a study of the relationship between bonding and lattice dynamics of these compounds. Mössbauer spectroscopy is used to study the structure of Sb in α-Sb2O3 and ß-Sb2O3, whereas the vibrational modes of Sb and Te for each oxide are investigated using nuclear inelastic scattering, and further information on O vibrational modes is obtained using inelastic neutron scattering. Additionally, vibrational frequencies obtained by density functional theory (DFT) calculations are compared with experimental results in order to assess the validity of the utilized functional. Good agreement was found between DFT-calculated and experimental density of phonon states with a 7% scaling factor. The Sb-O-Sb wagging mode of α-Sb2O3 whose frequency was not clear in most previous studies is experimentally observed for the first time at ∼340 cm-1. Softer lattice vibrational modes occur in orthorhombic ß-Sb2O3 compared to cubic α-Sb2O3, indicating that the antimony bonds are weakened upon transforming from the molecular α phase to the layer-chained ß structure. The resulting vibrational entropy increase of 0.45 ± 0.1 kB/Sb2O3 at 880 K accounts for about half of the α-ß transition entropy. The comparison of experimental and theoretical approaches presented here provides a detailed picture of the lattice dynamics in these oxides beyond the zone center and shows that the accuracy of DFT is sufficient for future calculations of similar material structures.

Lithium Iron Aluminum Nickelate, LiNix Fey Alz O2 -New Sustainable Cathodes for Next-Generation Cobalt-Free Li-Ion Batteries.

In recent years, cobalt has become a critical constraint on the supply chain of the Li-ion battery industry. With the ever-increasing projections for electric vehicles, the dependency of current Li-ion batteries on the ever-fluctuating cobalt prices poses serious environmental and sustainability issues. To address these challenges, a new class of cobalt-free materials with general formula of LiNix Fey Alz O2 (x + y + z = 1), termed as the lithium iron aluminum nickelate (NFA) class of cathodes, is introduced. These cobalt-free materials are synthesized using the sol-gel process to explore their compositional landscape by varying aluminum and iron. These NFA variants are characterized using electron microscopy, neutron and X-ray diffraction, and Mössbauer and X-ray photoelectron spectroscopy to investigate their morphological, physical, and crystal-structure properties. Operando experiments by X-ray diffraction, Mössbauer spectroscopy, and galvanostatic intermittent titration have been also used to study the crystallographic transitions, electrochemical activity, and Li-ion diffusivity upon lithium removal and uptake in the NFA cathodes. NFA compositions yield specific capacities of ≈200 mAh g-1 , demonstrating reasonable rate capability and cycling stability with ≈80% capacity retention after 100 charge/discharge cycles. While this is an early stage of research, the potential that these cathodes could have as viable candidates in next-generation cobalt-free lithium-ion batteries is highlighted here.

High-pressure nuclear inelastic scattering with backscattering monochromatization.

The capability to perform high-pressure low-temperature nuclear inelastic scattering on 125Te and 121Sb with a sapphire backscattering monochromator is presented. This technique was applied to measure nuclear inelastic scattering in TeO2 at pressures up to 10 GPa and temperatures down to 25 K. The evaluated partial Te densities of phonon states were compared with theoretical calculations and with Raman scattering measured under the same conditions. The high-pressure cell developed in this work can also be used for other techniques at pressures up to at least 100 GPa.

Thermoelectric Figure-of-Merit of Fully Dense Single-Crystalline SnSe.

Single-crystalline SnSe has attracted much attention because of its record high figure-of-merit ZT ≈ 2.6; however, this high ZT has been associated with the low mass density of samples which leaves the intrinsic ZT of fully dense pristine SnSe in question. To this end, we prepared high-quality fully dense SnSe single crystals and performed detailed structural, electrical, and thermal transport measurements over a wide temperature range along the major crystallographic directions. Our single crystals were fully dense and of high purity as confirmed via high statistics 119Sn Mössbauer spectroscopy that revealed <0.35 at. % Sn(IV) in pristine SnSe. The temperature-dependent heat capacity (C p) provided evidence for the displacive second-order phase transition from Pnma to Cmcm phase at T c ≈ 800 K and a small but finite Sommerfeld coefficient γ0 which implied the presence of a finite Fermi surface. Interestingly, despite its strongly temperature-dependent band gap inferred from density functional theory calculations, SnSe behaves like a low-carrier-concentration multiband metal below 600 K, above which it exhibits a semiconducting behavior. Notably, our high-quality single-crystalline SnSe exhibits a thermoelectric figure-of-merit ZT ∼1.0, ∼0.8, and ∼0.25 at 850 K along the b, c, and a directions, respectively.

Ferromagnetism Near Room Temperature in the Cleavable van der Waals Crystal Fe5GeTe2.

Two-dimensional materials with intrinsic functionality are becoming increasingly important in exploring fundamental condensed matter science and for developing advanced technologies. Bulk crystals that can be exfoliated are particularly relevant to these pursuits as they provide the opportunity to study the role of physical dimensionality and explore device physics in highly crystalline samples and designer heterostructures in a routine manner. Magnetism is a key element in these endeavors; however, relatively few cleavable materials are magnetic and none possess magnetic order at ambient conditions. Here, we introduce Fe5- xGeTe2 as a cleavable material with ferromagnetic behavior at room temperature. We established intrinsic magnetic order at room temperature in bulk crystals ([Formula: see text] = 310 K) through magnetization measurements and in exfoliated, thin flakes ([Formula: see text] ≈ 280 K) using the anomalous Hall effect. Our work reveals Fe5GeTe2 as a prime candidate for incorporating intrinsic magnetism into functional van der Waals heterostructures and devices near room temperature.

Structural, Chemical, Electrical, and Thermal Properties of n-Type NbFeSb.

We report on the structural, chemical, electrical, and thermal properties of n-type polycrystalline NbFeSb synthesized by induction melting of the elements. Although several studies on p-type conduction of this half-Heusler composition have recently been reported, including reports of relatively high thermoelectric properties, very little has been reported on the transport properties of  n-type compositions. We combine transport property investigations together with short- and long-range structural data obtained by Mössbauer spectroscopy of iron-57 and antimony-121 and by neutron total scattering, as well as first-principles calculations. In our investigation, we show that n-type conduction can occur from antiphase boundaries in this material. This work is intended to provide a greater understanding of the fundamental properties of NbFeSb as this material continues to be of interest for potential thermoelectric applications.

Tuning the structure and habit of iron oxide mesocrystals.

A precise control over the meso- and microstructure of ordered and aligned nanoparticle assemblies, i.e., mesocrystals, is essential in the quest for exploiting the collective material properties for potential applications. In this work, we produced evaporation-induced self-assembled mesocrystals with different mesostructures and crystal habits based on iron oxide nanocubes by varying the nanocube size and shape and by applying magnetic fields. A full 3D characterization of the mesocrystals was performed using image analysis, high-resolution scanning electron microscopy and Grazing Incidence Small Angle X-ray Scattering (GISAXS). This enabled the structural determination of e.g. multi-domain mesocrystals with complex crystal habits and the quantification of interparticle distances with sub-nm precision. Mesocrystals of small nanocubes (l = 8.6-12.6 nm) are isostructural with a body centred tetragonal (bct) lattice whereas assemblies of the largest nanocubes in this study (l = 13.6 nm) additionally form a simple cubic (sc) lattice. The mesocrystal habit can be tuned from a square, hexagonal to star-like and pillar shapes depending on the particle size and shape and the strength of the applied magnetic field. Finally, we outline a qualitative phase diagram of the evaporation-induced self-assembled superparamagnetic iron oxide nanocube mesocrystals based on nanocube edge length and magnetic field strength.