Materials Science (cond-mat.mtrl-sci)

Abstract: Despite the rapid rise in the performance of a variety of perovskite optoelectronic devices with vertical charge transport, the effects of ion migration remain a common and longstanding Achilles heel limiting the long-term operational stability of lead halide perovskite devices. However, there is still limited understanding of the impact of tin (Sn) substitution on the ion dynamics of lead (Pb) halide perovskites. Here, we employ scan-rate-dependent current-voltage measurements on Pb and mixed Pb-Sn perovskite solar cells to show that short circuit current losses at lower scan rates, which can be traced to the presence of mobile ions, are present in both kinds of perovskites. To understand the kinetics of ion migration, we carry out scan-rate-dependent hysteresis analyses and temperature-dependent impedance spectroscopy measurements, which demonstrate suppressed ion migration in Pb-Sn devices compared to their Pb-only analogues. By linking these experimental observations to first-principles calculations on mixed Pb-Sn perovskites, we reveal the key role played by Sn vacancies in increasing the iodide ion migration barrier due to local structural distortions. These results highlight the beneficial effect of Sn substitution in mitigating undesirable ion migration in halide perovskites, with potential implications for future device development.

Abstract: We report the effect of co-doping of In and Ga at low concentrations on the structural, electronic, and thermoelectric properties of SnTe based compositions $Sn_{1.03-2x}In_{x}Ga_{x}Te$ (x = 0, 0.01, 0.02, 0.04) prepared by the solid-state route and spark plasma sintering (SPS). All compositions formed in the fcc structure (Fm-3m) with no other impurity phase. The optical band gap increased with the co-doping, indicative of band convergence effects. First principle electronic structure calculations showed band convergence and the formation of resonant levels, due to Ga and In doping respectively. The carrier concentration increased on hole-doping by In and Ga ions while carrier mobility decreased due to impurity scattering. The resistivity increased with temperature, indicative of the degenerate semiconducting character of the compounds. The Seebeck coefficient of the doped samples increased linearly with temperature, reaching 85 - 95 ${\mu}$V/K at 783 K. Thermal conductivity decreased sharply with co-doping, and the lattice thermal conductivity dropped to 0.42 W$m^{-1}$ $K^{-1}$ above 750 K. The enhanced power factor and low lattice thermal conductivity on doping resulted in a maximum figure of merit ZT = 0.34 at 773 K, twice that of the pristine SnTe.

Abstract: We use ultrafast x-ray diffraction and the time-resolved polar magneto-optical Kerr effect to study the laser-induced metamagnetic phase transition in two FeRh films with thicknesses below and above the optical penetration depth. In the thin film, we identify an intrinsic 8 ps timescale for the lightinduced nucleation of ferromagnetic domains in the antiferromagnetic material that is substantially slower than the speed of sound. For the inhomogeneously excited thicker film, we additionally identify kinetics of out-of-plane domain growth mediated by near-equilibrium heat transport, which we experimentally verify by comparing Kerr effect experiments in front- and backside excitation geometry.

Abstract: The technique known as 4D-STEM has recently emerged as a powerful tool for the local characterization of crystalline structures in materials, such as cathode materials for Li-ion batteries or perovskite materials for photovoltaics. However, the use of new detectors optimized for electron diffraction patterns and other advanced techniques requires constant adaptation of methodologies to address the challenges associated with crystalline materials. In this study, we present a novel image processing method to improve pattern matching in the determination of crystalline orientations and phases. Our approach uses sub-pixelar adaptative image processing to register and reconstruct electron diffraction signals in large 4D-STEM datasets. By using adaptive prominence and linear filters such as mean and gaussian blur, we are able to improve the quality of the diffraction pattern registration. The resulting data compression rate of 103 is well-suited for the era of big data and provides a significant enhancement in the performance of the entire ACOM data processing method. Our approach is evaluated using dedicated metrics, which demonstrate a high improvement in phase recognition. Our results demonstrate that this data preparation method not only enhances the quality of the resulting image but also boosts the confidence level in the analysis of the outcomes related to determining crystal orientation and phase. Additionally, it mitigates the impact of user bias that may occur during the application of the method through the manipulation of parameters.

Abstract: Since the early 1960's, the discovery of Dzyaloshinskii-Moriya interaction (DMI) helped to explain the physical mechanisms behind certain magnetic phenomena, such as net moment in antiferromagnets, or enhanced anisotropy field from heavy metals impurity in dilute Cu:Mn alloy. Since the researchers unveil the key role that DMI plays in stabilizing chiral Neel type magnetic domain wall and magnetic skyrmions, the studies on DMI have received growing interest. Governed by spin-orbit coupling (SOC) and various types of inversion symmetry breaking (ISB) in magnetic systems, DMI drives the forming of distinct morphologies of magnetic skyrmions. Our aim is to briefly introduce the research history of DMI and its significance in the field of modern spintronics.

Abstract: Structure-property relationships in ordered materials have long been a core principle in materials design. However, the intentional introduction of disorder into materials provides structural flexibility and thus access to material properties that are not attainable in conventional, ordered materials. To understand disorder-property relationships, the disorder - i.e., the local ordering principles - must be quantified. Correlated disorder can be probed experimentally by diffuse scattering. The analysis is notoriously difficult, especially if only powder samples are available. Here, we combine the advantages of three-dimensional electron diffraction - a method that allows single crystal diffraction measurements on sub-micron sized crystals - and three-dimensional difference pair distribution function analysis (3D-{\Delta}PDF) to address this problem. 3D-{\Delta}PDFs visualise and quantify local deviations from the average structure and enable a straightforward interpretation of the single crystal diffuse scattering data in terms of a three-dimensional local order model. Comparison of the 3D-{\Delta}PDF from electron diffraction data with those obtained from neutron and x-ray experiments of yttria-stabilized zirconia demonstrates the reliability of the newly proposed approach.

Abstract: The strongly correlated material FeSi exhibits several unusual thermal, magnetic, and structural properties under varying pressure-temperature (P-T) conditions. It is a potential thermoelectric alloy and a materials of several geological implications as a possible constituent at the Earth's core mantle boundary (CMB). The phase transition behavior and lattice dynamics of FeSi under different P-T conditions remain elusive. A previous theoretical work predicted a pressure induced B20-B2 transition at ambient temperature, yet the transition is only observed at high P-T conditions in the experiments. Furthermore, the closing of the electronic gap due to a dramatic renormalization of the electronic structure and phonon anomalies has been reported based on density function calculations. In this study we have performed high pressure powder diffraction and Nuclear Resonant Inelastic X-ray Scattering (NRIXS) measurements up to 120 GPa to understand the phase stability and the lattice dynamics. Our study shows evidence for a nonhydrostatic stress induced B20-B2 transition in FeSi around 36 GPa for the first time. The Fe partial phonon density of states (PDOS) and thermal parameters were derived from NRIXS up to 120 GPa with the density function theoretical (DFT) calculations. These calculations further predict and are consistent with pressure-induced metallization and band gap closing around 12 GPa.

Abstract: The fabrication of bulk delta-phase Zirconium Hydride ($\delta$-ZrH) using Zircaloy-4 as a precursor is herein reported. Characterization using electron-microscopy methods indicate that the fabricated material is of a single-phase. Sn-rich segregation zones have been observed to form as a direct result of the hydriding process. These findings experimentally validate previous \textit{ab initio} calculations on the influence H incorporation in Zircaloy-4 constitutional elements such as Sn, Fe and Cr. The effect of hydriding and Sn segregation on pre-existing Zr(Fe,Cr)$_{2}$ Laves phases is also evaluated. Major implications on the development of moderators for use in microreactors within the nuclear industry are discussed.