High-Pressure X-ray Diffraction and Vibrational Spectroscopy of Polyethylene: Evidence for a Structural Phase Transition
Abstract not provided
Dattelbaum, D. M.↗
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Determining the Effects of Zero-Field Splitting and Magnetic Exchange in Dimeric Europium(II) Complexes
Related BAP [BAP = bis(acyl)phosphide] and Acac (Acac = β-diketonate) molecules perform as robust supports for both lanthanide and actinide metals. Here, a molecular bimetallic Eu 2+ complex was successfully targeted and isolated by employing sodium bis(mesitoyl)phosphide [Na( mes BAP)] in a salt metathesis with EuI 2 , producing [Eu( mes BAP) 2 (et 2 o)] 2 (et 2 o = metal-coordinated diethyl ether). The corresponding Acac-Eu 2+ complex was targeted using mes Acac– (1,3-dimesityl-1,3-propanedione), generating [Eu( mes Acac) 2 (et 2 o)] 2 . Both complexes were characterized by single-crystal X-ray diffraction, UV–vis, IR, and NMR spectroscopies, and variable-temperature magnetic susceptibility. [Eu( mes BAP) 2 (et 2 o)] 2 was persistent under anaerobic, anhydrous conditions, whereas the analogous [Eu( mes Acac) 2 (et 2 o)] 2 showed evidence of decomposition under identical conditions. Finally, variable-temperature magnetic susceptibility and magnetization studies of [Eu( mes BAP) 2 (et 2 o)] 2 and [Eu( mes Acac) 2 (et 2 o)] 2 were performed, resulting in similar magnetic exchange coupling values of J ex = –0.018 and –0.023 cm –1 and axial zero-field-splitting D values of –0.38 and –0.51 cm –1 , respectively.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
Magnetoelastic-coupling-assisted first-order antiferromagnetic transitions in the corrugated honeycomb-lattice compounds (Ca, Sr) Mn 2 P 2
Understanding the origin of unconventional magnetic order is a central theme in the research field of functional quantum materials for spin-dependent applications. In general, a paramagnetic (PM) to antiferromagnetic (AFM) transition is of thermodynamically second order in nature in the absence of a magnetic field. In the magnetically frustrated corrugated-honeycomb lattice compounds (Ca, Sr) Mn 2 P 2 , electrical resistivity 𝜌 measurements in the 𝑎𝑏 plane are reported to exhibit sharp discontinuities at the respective PM to AFM ordering temperatures 𝑇 N = 69.5 and 53 K, respectively, signifying the unusual first-order nature of the AFM transition. To reveal the origin of such anomalous transitions, high-resolution linear thermal-expansion measurements are reported for both CaMn 2 P 2 and SrMn 2 P 2 in the 𝑎𝑏 plane and along the 𝑐 axis using capacitance dilatometry. The thermal expansion measurements show a sharp discontinuity in Δ𝐿/𝐿 at 𝑇 N , yielding a diverging behavior in the thermal expansion coefficient 𝛼(𝑇) associated with strong magnetoelastic coupling, driving the first-order AFM transition in CaMn 2 P 2 . However, this effect is weaker in SrMn 2 P 2 , consistent with the magnetic entropy changes 𝑆(𝑇) obtained from the heat-capacity measurements. Temperature-dependent 𝑎𝑏-plane x-ray diffraction measurements for CaMn 2 P 2 near its 𝑇 N yield a volume thermal expansion coefficient consistent with literature data for other materials. The uniaxial pressure derivatives of 𝑇 N in the two compounds are found to be respectively opposite in sign, presumably due to the different magnetic structures.
Pakhira, Santanu [Maulana Azad National Institute ↗
The High-Pressure Melt Curve and Phase Diagram of Lithium
This study investigates the phase diagram of lithium at temperatures of 200K to 400 K, to pressures over 100GPa using X-ray diffraction in diamond anvil cells, covering the region in which the melting curve is disputed. To overcome degradation of the diamond anvils by dense lithium we utilize a rapid compression scheme taking advantage of the high flux available at modern synchrotrons. Our results show the hR1 and cI16 phases to be stable to higher temperature than previously reported. The melting minima of lithium is found to be close to room temperature between 40GPa and 60GPa, below which the solid is crystalline. Finally, analysis of the stability fields of the cI16 and oC88 phases suggest the existence of a triple point between these and an undetermined solid phase at 60GPa between 220K and 255 K.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Structure and density of silicon carbide to 1.5 TPa and implications for extrasolar planets
There has been considerable recent interest in the high-pressure behavior of silicon carbide, a potential major constituent of carbon-rich exoplanets. In this work, the atomic-level structure of SiC was determined through in situ X-ray diffraction under laser-driven ramp compression up to 1.5 TPa; stresses more than seven times greater than previous static and shock data. Here we show that the B1-type structure persists over this stress range and we have constrained its equation of state (EOS). Using this data we have determined the first experimentally based mass-radius curves for a hypothetical pure SiC planet. Interior structure models are constructed for planets consisting of a SiC-rich mantle and iron-rich core. Carbide planets are found to be ~10% less dense than corresponding terrestrial planets.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Robust superconductivity and the suppression of charge-density wave in the quasi-skutterudites Ca 3 ( Ir 1 – x Rh x ) 4 Sn 13 single crystals at ambient pressure
Single crystals of the quasi-skutterudite compounds Ca 3 (Ir 1-x Rh x ) 4 Sn 13 (3–4–13) were synthesized by flux growth and characterized by x-ray diffraction, energy dispersive x-ray spectroscopy, magnetization, resistivity, and radio frequency magnetic susceptibility techniques. The coexistence and competition between the charge density wave (CDW) and superconductivity was studied by varying the Rh/Ir ratio. The superconducting transition temperature, T c , varies from 7 K in pure Ir (x = 0) to 8.3 K in pure Rh (x = 1). Temperature-dependent electrical resistivity reveals monotonic suppression of the CDW transition temperature, T CDW (x). The CDW starts in pure Ir, x = 0, at T CDW ≈ 40 K and extrapolates roughly linearly to zero at x c ≈ 0.53–0.58 under the superconducting dome. Magnetization and transport measurements show a significant influence of CDW on superconducting and normal states. Meissner expulsion is substantially reduced in the CDW region, indicating competition between the CDW and superconductivity. The low-temperature resistivity is higher in the CDW part of the phase diagram, consistent with the reduced density of states due to CDW gapping. Its temperature dependence just above T c shows signs of non-Fermi liquid behavior in a cone-like composition pattern. We conclude that the Ca 3 (Ir 1-x Rh x ) 4 Sn 13 alloy is a good candidate for a composition-driven quantum critical point at ambient pressure.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Flat-band-induced itinerant ferromagnetism in RbCo 2 Se 2
ACo 2 Se 2 (A=K,Rb,Cs) is a homologue of the iron-based superconductor, AFe 2 Se 2 . From a comprehensive study of RbCo 2 Se 2 via measurements of magnetization, transport, neutron diffraction, angle-resolved photoemission spectroscopy, and first-principle calculations, we identify a ferromagnetic order accompanied by an orbital-dependent spin-splitting of the electronic dispersions. However, we identify the ordered moment to be dominated by a d x 2 –y 2 flatband near the Fermi level, which exhibits the largest spin splitting across the ferromagnetic transition, suggesting an itinerant origin of the ferromagnetism. In the broader context of the iron-based superconductors, we find this d x 2 –y 2 flatband to be a common feature in the band structures of both iron chalcogenides and iron pnictides, accessible via heavy electron doping.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
High-Performance First-Principles Molecular Dynamics for Predictive Theory and Modeling
This project focused on developing high-performance software tools for First-Principles Molecular Dynamics (FPMD) simulations, and applying them in investigations of materials relevant to energy conversion processes. FPMD is an atomistic simulation method that combines a quantum-mechanical description of electronic structure with the statistical description provided by molecular dynamics (MD) simulations. This reliance on fundamental principles allows FPMD simulations to provide a consistent description of structural, dynamical and electronic properties of a material. This is particularly useful in systems for which reliable empirical models are lacking. FPMD simulations are increasingly used as a predictive tool for applications such as batteries, solar energy conversion, light-emitting devices, electro-chemical energy conversion devices and other materials. During the course of the project, several new features were developed and added to the open-source Qbox FPMD code. The code was further optimized for scalable operation of large-scale, Leadership-Class DOE computers. When combined with Many-Body Perturbation Theory (MBPT) calculations, this infrastructure was used to investigate structural and electronic properties of liquid water, ice, aqueous solutions, nanoparticles and solid-liquid interfaces. Computing both ionic trajectories and electronic structure in a consistent manner enabled the simulation of several spectroscopic properties, such as Raman spectra, infrared spectra, and sum-frequency generation spectra. The accuracy of the approximations used allowed for direct comparisons of results with experimental data such as optical spectra, X-ray and neutron diffraction spectra. The software infrastructure developed in this project, as applied to various investigations of solids, liquids and interfaces, demonstrates that FPMD simulations can provide a detailed, atomic-scale picture of structural, vibrational and electronic properties of complex systems relevant to energy conversion devices.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Chalcogenide perovskite BaZrS 3 thin-film electronic and optoelectronic devices by low temperature processing
Owing to its superior visible light absorption and high chemical stability, chalcogenide perovskite barium zirconium sulfide (BaZrS 3 ) has attracted significant attention in the past few years as a potential alternative to hybrid halide perovskites for optoelectronics. However, the high processing temperatures of BaZrS 3 thin films at above 1000 °C severely limits their potential for device applications. Herein, we report the synthesis of BaZrS 3 thin films at temperatures as low as 500 °C, by changing the chemical reaction pathway. The single phase BaZrS 3 thin film was confirmed by X-ray diffraction and Raman spectroscopy. Atomic force microscopy and scanning electron microscopy show that crystalline size and surface roughness were consistently reduced with decreasing annealing temperature. The lower temperatures further eliminate sulfur vacancies and carbon contaminations associated with high temperature processing. The ability to synthesize chalcogenide perovskite thin films at lower temperatures removes a major hurdle for their device fabrication. The photodetectors demonstrate fast response and an on/off ratio of 80. Finally, the fabricated field effect transistors show an ambipolar behavior with electron and hole mobilities of 16.8 cm 2 /Vs and 2.6 cm 2 /Vs, respectively.
36 MATERIALS SCIENCE↗
Dynamic lattice distortions driven by surface trapping in semiconductor nanocrystals
Nonradiative processes limit optoelectronic functionality of nanocrystals and curb their device performance. Nevertheless, the dynamic structural origins of nonradiative relaxations in nanocrystals are not understood. Here, femtosecond electron diffraction measurements corroborated by atomistic simulations uncover transient lattice deformations accompanying radiationless electronic processes in semiconductor nanocrystals. Investigation of the excitation energy dependence shows that hot carriers created by a photon energy considerably larger than the bandgap induce structural distortions at nanocrystal surfaces on few picosecond timescales associated with the localization of trapped holes. On the other hand, carriers created by a photon energy close to the bandgap result in transient lattice heating that occurs on a much longer 200 ps timescale, governed by an Auger heating mechanism. Elucidation of the structural deformations associated with the surface trapping of hot holes provides atomic-scale insights into the mechanisms deteriorating optoelectronic performance and a pathway towards minimizing these losses in nanocrystal devices.
36 MATERIALS SCIENCE↗
Magnetic Order and Its Interplay with Structure Phase Transition in van der Waals Ferromagnet VI 3
We show that Van der Waals magnet VI 3 demonstrates intriguing magnetic properties that render it great for use in various applications. However, its microscopic magnetic structure has not been determined yet. Here, we report neutron diffraction and susceptibility measurements in VI 3 that revealed a ferromagnetic order with the moment direction tilted from the c-axis by ~36° at 4 K. A spin reorientation accompanied by a structure distortion within the honeycomb plane is observed, before the magnetic order completely disappears at $T_C$ = 50 K. The refined magnetic moment of ~1.3$μ_B$ at 4 K is much lower than the fully ordered spin moment of 2$μ_B$/V 3+ , suggesting the presence of a considerable orbital moment antiparallel to the spin moment and strong spin–orbit coupling in VI 3 . This results in strong magnetoelastic interactions that make the magnetic properties of VI 3 easily tunable via strain and pressure.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Quenched disorder in the triangular lattice antiferromagnet YbZn 2 GaO 5
We investigate the crystal electric field (CEF) excitations of Yb 3+ ions in powder samples of the triangular-lattice rare-earth-based antiferromagnet YbZn 2 GaO 5 using inelastic neutron scattering (INS). Three CEF excitations from the ground-state Kramers doublet were observed, each exhibiting significant broadening beyond instrumental resolution. Combining temperature-dependent INS and neutron powder diffraction, we identify a significant static contribution to this broadening and attribute it to heterogeneous coordination of Yb 3+ ions due to Ga 3+ /Zn 2+ site mixing. Rietveld refinement of neutron powder diffraction indicates that 35% of Ga occupies the Zn site and 60% of Zn occupies the Ga site. We show with a point charge model for the CEF Hamiltonian that heterogeneous coordination of Yb3+ ions leads to broadened CEF peaks. First-principles calculations demonstrate that the random Ga 3+ /Zn 2+ distribution can produce the distortions of the YbO 6 octahedra observed from neutron diffraction. Because the documented heterogeneity will extend to exchange interactions, our results suggest that disorder is a significant factor in the unusual magnetism previously reported in YbZn 2 GaO 5 , including broad low-energy magnetic excitations and the absence of magnetic ordering down to 0.3 K.
Crystal field excitations↗
Unraveling the structure and bonding evolution of the newly discovered iron oxide FeO 2
Recently reported synthesis of FeO 2 at high pressure has stimulated great interest in exploring this new iron oxide and elucidating its properties. Here, we present a systematic computational study of crystal structure, chemical bonding, and sound velocity of FeO 2 in a wide range of pressure. Our results establish thermodynamic stability of the experimentally observed pyrite phase (P-phase) of FeO 2 at pressures above 74 GPa and unveil two metastable FeO 2 phases in Pbcn and P4 2 /mnm symmetry at lower pressures. Simulated x-ray diffraction (XRD) spectra of Pbcn and P4 2 /mnm FeO 2 match well with measured XRD data of the decompression products of P-phase FeO 2 , providing compelling evidence for the presence of these metastable phases. Energetic calculations reveal unusually soft O-O bonds in P-phase FeO 2 stemming from a low-frequency libration mode of FeO 6 octahedra, rendering the O-O bond length highly sensitive to computational and physical environments. Calculated sound-velocity profiles of P-phase FeO 2 are markedly different from those of the Pbcn and P4 2 /mnm phases, underscoring their distinct seismic signatures. Here our findings offer insights for understanding the rich structural, bonding, and elastic behaviors of this newly discovered iron oxide.
36 MATERIALS SCIENCE↗
Stoichiometry–Induced Ferromagnetism in Altermagnetic Candidate MnTe
The field of spintronics has seen a surge of interest in altermagnetism due to novel predictions and many possible applications. MnTe is a leading altermagnetic candidate that is of significant interest across spintronics due to its layered antiferromagnetic structure, high Neel temperature (T N ≈ 310 K) and semiconducting properties. The results on molecular beam epitaxy (MBE) grown MnTe/InP(111) films are presented. Here, it is found that the electronic and magnetic properties are driven by the natural stoichiometry of MnTe. Electronic transport and in situ angle-resolved photoemission spectroscopy show the films are natively metallic with the Fermi level in the valence band and the band structure is in good agreement with first-principles calculations for altermagnetic spin-splitting. Neutron diffraction confirms that the film is antiferromagnetic with planar anisotropy and polarized neutron reflectometry indicates weak ferromagnetism, which is linked to a slight Mn-richness that is intrinsic to the MBE-grown samples. Further, when combined with the anomalous Hall effect, this work shows that the electronic response is strongly affected by the ferromagnetic moment. Altogether, this highlights potential mechanisms for controlling altermagnetic ordering for diverse spintronic applications.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Uniformity of GaAs solar cells grown in a kinetically-limited regime by dynamic hydride vapor phase epitaxy
We demonstrate solar cell devices grown on 50 mm diameter GaAs substrates by dynamic hydride vapor phase epitaxy (D-HVPE). In contrast to our prior D-HVPE devices, grown at 650 °C in a transport-limited regime, these devices were grown at 700 °C in a kinetically-limited growth regime in which the growth rate uniformity is controlled by thermally-activated surface reactions. These devices exhibit open-circuit voltages up to 1.07 V, nearly identical performance to the devices grown at lower temperature in a different growth regime. We evaluate the uniformity of device performance and find only a 1% variation in absolute, device efficiency across the majority of the wafer in devices without anti-reflection coating. We analyze the GaAs and GaInP thickness uniformity and GaInP compositional uniformity, using high-tresolution x-ray diffraction mapping, and find that any non-uniformity in device efficiency is not related to variations in these structural parameters. We combine three-dimensional computational fluid dynamics modeling of our reactor with a kinetic model for GaAs growth, and use the combined model to predict spatial GaAs growth rate over a 50 mm wafer area. We compare these predictions with experimental data from our D-HVPE reactor and find excellent agreement. We use the model to the gain insight into the specific mechanisms that control GaAs spatial uniformity in the kinetically-limitedgrowth regime. Furthermore, these results suggest a large parameter window for the growth of high-performance optoelectronic devices by D-HVPE, possibly with large area uniformity.
14 SOLAR ENERGY↗
Structural Evolution of Layered Hybrid Lead Iodide Perovskites in Colloidal Dispersions
Controlling the structure of layered hybrid metal halide perovskites, such as the Ruddlesden-Popper (R-P) phases, is challenging because of their tendency to form mixtures of varying composition. Colloidal growth techniques, such as antisolvent precipitation, forms colloidal dispersions with properties that match bulk layered R-P phases, but controlling the composition of these particles remains challenging. Here, we explore the microstructure of particles of R-P phases of methylammonium lead iodide prepared by antisolvent precipitation from ternary mixtures of alkylammonium cations, where one cation can form perovskite phases (CH 3 NH 3 + ) and the other two promote layered structures as spacers (e.g. C 4 H 9 NH 3 + and C 12 H 25 NH 3 + ). We determine that alkylammonium spacers pack with constant methylene density in the R-P interlayer, and exclude interlayer solvent in dispersed colloids, regardless of length or branching. Using this result, we illustrate how the competition between cations that act as spacers between layers, or as grain- terminating ligands, determines the colloidal microstructure of layered R-P crystallites in solution. Optical measurements reveal that quantum well dimensions can be tuned by engineering the ternary cation composition. Transmission synchrotron wide-angle X-ray scattering and small angle neutron scattering reveal changes in the structure of colloids in solvent and after deposition into thin films. In particular, we find that spacers can alloy between R-P layers if they share common steric arrangements, but tend to segregate into polydisperse R-P phases if they do not mix. In conclusion, this study provides a framework to compare the microstructure of colloidal layered perovskites and suggests clear avenues to control phase and colloidal morphology.
14 SOLAR ENERGY↗
Coherent X-rays reveal the influence of cage effects on ultrafast water dynamics
Here, the dynamics of liquid water feature a variety of time scales, ranging from extremely fast ballistic-like thermal motion, to slower molecular diffusion and hydrogen-bond rearrangements. Here, we utilize coherent X-ray pulses to investigate the sub-100 fs equilibrium dynamics of water from ambient conditions down to supercooled temperatures. This novel approach utilizes the inherent capability of X-ray speckle visibility spectroscopy to measure equilibrium intermolecular dynamics with lengthscale selectivity, by measuring oxygen motion in momentum space. The observed decay of the speckle contrast at the first diffraction peak, which reflects tetrahedral coordination, is attributed to motion on a molecular scale within the first 120 fs. Through comparison with molecular dynamics simulations, we conclude that the slowing down upon cooling from 328 K down to 253 K is not due to simple thermal ballistic-like motion, but that cage effects play an important role even on timescales over 25 fs due to hydrogen-bonding.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Resistive Switching in SrFeO2.5/Nb:SrTiO3 Heterostructures with Growth-Controlled Film Orientation
Resistive switching, a behavior found in many oxide materials, has the potential to enable emerging computer hardware technologies and architectures. We present resistive switching devices fabricated from epitaxial brownmillerite SrFeO2.5 films with two distinct film orientations, wherein facile oxygen ion diffusion planes are aligned parallel (in-plane) and perpendicular (out-of-plane) with the electrodes. SrFeO2.5 films were grown on (001) oriented Nb:SrTiO3 to enable high-quality interfaces and future integration with Si CMOS technologies. Post-growth vacuum annealing and growth pressure were used to control film orientations, as confirmed by transmission electron microscopy and x-ray diffraction measurements. Films grown with diffusion planes oriented in-plane had oxygen-rich, perovskite-like nanodomains spread throughout the film, and fabricated devices exhibited worse switching consistency and more stochasticity. In contrast, films grown with diffusion planes oriented out-of-plane had a more uniform oxygen-rich perovskite interfacial layer above the bottom electrode, and devices built from this film orientation showed significant statistical improvements in switching voltages and cycling consistency.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗