Lead halide perovskites hold great promise for photovoltaics and optoelectronics, yet ion migration continues to challenge their long-term stability. Here, combining first-principles calculations and machine learning molecular dynamics, we unravel the interplay between perovskite octahedral lattice dynamics and energy barrier associated with ion migration. Our results show that B-site substitution, particularly with alkaline-earth and lanthanide elements, notably strengthens lattice interactions, restrains octahedral oscillation, and increases iodine-migration barriers, outperforming the commonly used A-site and X-site substitutions and interstitial doping. Moreover, the enhanced barrier aligns with the geometric factor of μτ (tolerance-octahedral product), underlining the superior effectiveness of co- and multiple-element B-site doping in lattice stabilization and ion migration suppression. Experimental validation with exemplary hysteresis-free Eu-Ca–doped perovskite single crystals demonstrates remarkable improvements in ambient stability and transport properties. These findings highlight B-site engineering as an effective microstructural strategy for controlling ion migration, with important implications for stable and lead-reduced perovskite devices.

Segregation of alloying elements and impurities at grain boundaries (GBs) critically influences material behavior by affecting cohesion. In this study, we present an ab initio high-throughput evaluation of segregation energies and cohesive effects for all elements in the periodic table (Z: 1 to 92, H to U) across six model ferritic iron GBs using density functional theory (DFT). From these data, we construct comprehensive elemental maps for solute segregation tendencies and cohesion at GBs, providing guidance for segregation engineering. We systematically assess the cohesive effects of different elements in all segregating positions along multiple fracture paths with a quantum-chemistry bond-order method as well as a modified Rice-Wang theory of interfacial cohesion. The effects of segregants on the cohesion of GBs are shown to vary drastically as a function of site character, and hence their induced cohesive effects must be considered as a thermodynamic average over the spectral energy distribution. Thus, models that overlook these aspects may fail to accurately predict the impacts of varying alloying concentrations, thermal processing conditions, or GB types. The insights presented here, along with our accompanying dataset, are expected to advance our understanding of GB segregation in steels and other materials.

Grain boundaries (GBs) generally exhibit complex structural and compositional features that significantly affect material hardness. Here, we establish a methodology to correlate the local hardness contributions of the GBs with their frequency distribution and their structural and compositional characteristics, using a submicron WC-Co cemented carbide as a model. An exceptional local hardness of (14.68 +/- 0.12) GPa is observed from a 90 degrees WC{0001}/WC{1010} GB, unlike the low contributions from other WC/WC GBs. This is linked to pronounced Cr and Co segregation at this GB, due to Cr affinity at the WC{0001}/Co and WC{1010}/Co phase boundaries and Co infiltration during liquid-phase sintering. Density functional theory results indicate that a large lattice mismatch, strong W-C covalent bonding, and Cr and Co accumulation increase the elastic strain field, resulting in strong atomic distortion near the interface and contributing to exceptional strengthening. Our findings highlight the critical influence of GB complexities on material hardness.

Phase instability poses a serious challenge to the commercialization of formamidinium lead iodide (FAPbI3)-based solar cells and optoelectronic devices. Here, we combine density functional theory and machine learning molecular dynamics simulations, to investigate the mechanism driving the undesired α-δ phase transition of FAPbI3. Prevalent iodine vacancies and interstitials can significantly expedite the structural transition kinetics by inducing robust covalency during transition states. Extrinsically, the detrimental roles of atmospheric moisture and oxygen in degrading the FAPbI3 perovskite phase are also rationalized. Significantly, we discover the compositional design principles by categorizing that A-site engineering primarily governs thermodynamics, whereas B-site doping can effectively manipulate the kinetics of the phase transition in FAPbI3, highlighting lanthanide ions as promising B-site substitutes. A-B mixed doping emerges as an efficient strategy to synergistically stabilize α-FAPbI3, as experimentally demonstrated by substantially higher initial optoelectronic characteristics and significantly enhanced phase stability in Cs-Eu doped FAPbI3 as compared to its Cs-doped counterpart. This study provides scientific guidance for the design and optimization of long-term stable FAPbI3-based solar cells and other optoelectronic devices through defect control and synergetic composition engineering.

AbstractIn this work, the optical transition of self‐trapped excitons (STEs) and the emergent green emission in β‐Ga2O3 samples with/without Sn impurities at various doping levels have been investigated via temperature‐ and power‐dependent photoluminescence. The ultraviolet (UV) emissions ≈ 3.40 eV unanimously exhibit an excitonic nature related to STEs and typical negative thermal quenching (NTQ) characters. The NTQ activation energy decreases from 103.56 to 42.37 meV with the increased electron concentration from 2.1 × 1016 to 6.7 × 1018 cm−3, indicative of the reduced energy barrier that electrons should overcome to form stable STEs due to the lift‐up of Fermi level. In comparison, the green emissions ≈ 2.35 eV with two quenching channels are observed only in samples with Sn impurities at cryogenic temperatures. One channel is the nsnp‐ns2 transition of Sn2+, the other is donor‐acceptor pair recombination via (2VGa‐Sni)2− complex, which is energetically favorable as evidenced by density functional theory calculations. The semi‐classical quantum theory models fitting proves the transition from green to UV emissions with elevated temperature. The enhanced STEs emission with distinguished NTQ effect strengthens evidence that the stable polarons inherently limit the transport of holes in Ga2O3, and also support the potential of Ga2O3 materials for the development of UV optoelectronics.

We report an unexpected yet significant precipitation-strengthening effect with remarkable thermal stability in an otherwise conventionally non-heat treatable AA3104 aluminium alloy. We reveal that a minor addition of Cu plays a critical role in the alloy's strength and thermal stability, and explain the underpinning mechanisms. The primary strengthening β′ phase can be engineered to exhibit distinctive structural characteristics, including regularly-spaced Cu atom columns at the coherent β′/Al interfaces, stacking faults and resultant domains. Our atomistic simulations rationalise that these microscopic characteristics are directly promoted by the Cu addition. The Cu-decorated interfaces promote β′ phase nucleation and result in strong, directional covalent-like Cu-Si bonding at the β′/Al interfaces. Specifically, the interfacial Cu columns increase the work of separation of the habit-plane interfaces and reduce the interface energy, resulting in significant strengthening and stabilising effects. The Cu columns also facilitate the stacking fault formation in the β′ phase. Collectively, these features enhance the strengthening effect and thermal stability of the precipitates. This study demonstrates the potential for exploring "new" features in "old" alloys to advance alloy science and technology.

Titanium alloys are advanced lightweight materials, indispensable for many critical applications1,2. The mainstay of the titanium industry is the α-β titanium alloys, which are formulated through alloying additions that stabilize the α and β phases3-5. Our work focuses on harnessing two of the most powerful stabilizing elements and strengtheners for α-β titanium alloys, oxygen and iron1-5, which are readily abundant. However, the embrittling effect of oxygen6,7, described colloquially as 'the kryptonite to titanium'8, and the microsegregation of iron9 have hindered their combination for the development of strong and ductile α-β titanium-oxygen-iron alloys. Here we integrate alloy design with additive manufacturing (AM) process design to demonstrate a series of titanium-oxygen-iron compositions that exhibit outstanding tensile properties. We explain the atomic-scale origins of these properties using various characterization techniques. The abundance of oxygen and iron and the process simplicity for net-shape or near-net-shape manufacturing by AM make these α-β titanium-oxygen-iron alloys attractive for a diverse range of applications. Furthermore, they offer promise for industrial-scale use of off-grade sponge titanium or sponge titanium-oxygen-iron10,11, an industrial waste product at present. The economic and environmental potential to reduce the carbon footprint of the energy-intensive sponge titanium production12 is substantial.

The segregation of transition metal elements to grain boundaries in steels plays a critical role in determining their cohesion. Here, we investigate the segregation, co-segregation, and cohesion effects of various transition metals (Co, Cr, Cu, Mn, Mo, Ni, Nb, Ti, V and W) to different grain boundary characters in ferritic-iron (α-Fe) through a systematic, brute-force style configurational analysis utilising density functional theory calculations. We demonstrate that differing grain boundary characters change not only transition metal segregation and co-segregation behaviours, but also their effects on cohesion. The effects of co-segregated solutes on cohesion can be substantially different from their summed individual parts. We show that solute-solute interactions at grain boundaries vary significantly as a function of grain boundary character. These interactions are shown to be substantially different from those that occur in the bulk. We introduce a novel quantitative method for assessing effects of segregated elements on interfacial cohesion through calculating the strength of bonds at a grain boundary in the DDEC6 bond-order framework. It is shown that work of separation quantities calculated through rigid separation of surfaces better captures the strength of bonding in most cases, and thus more accurately depicts intergranular fracture. Collectively, these results offer valuable insight towards rational grain boundary engineering in steels.

Oxygen ingression has been shown to substantially decrease the carrier lifetime of Sn-based perovskites, behind which the mechanism remains yet unknown. Our first-principles calculations reveal that in prototypical MASnI3 (MA = CH3NH3), oxygen by itself is not a recombination center. Instead, it tends to form substitutional OI through combining with native I vacancies (VI) and remarkably increases the original recombination rate of VI by 2-3 orders of magnitude. This rationalizes the experimentally observed sharp decline of carrier lifetime in perovskites exposed to air. The significantly enhanced carrier recombination is due to a smaller electron capture barrier of OI, resulting from lattice strengthening and the suppressed structural relaxation upon electron capture. These insights offer a route to further improve device performance via anion engineering in broad Sn-based perovskite optoelec-tronics operating in ambient air. Moreover, our results highlight the important role of lattice relaxation for nonradiative carrier capture in materials in general.

High-magnetization materials play crucial roles in various applications. However, the past few decades have witnessed a stagnation in the discovery of new materials with high magnetization. In this work, Ni/NiO nanocomposites are fabricated by depositing Ni and NiO thin layers alternately, followed by annealing at specific temperatures. Both the as-deposited samples and those annealed at 373 K exhibit low magnetization. However, the samples annealed at 473 K exhibit a significantly enhanced saturation magnetization exceeding 607 emu cm-3 at room temperature, surpassing that of pure Ni (480 emu cm-3 ). Material characterizations indicate that the composite comprises NiO nanoclusters of size 1-2 nm embedded in the Ni matrix. This nanoclustered NiO is primarily responsible for the high magnetization, as confirmed by density functional theory calculations. The calculations also indicate that the NiO clusters are ferromagnetically coupled with Ni, resulting in enhanced magnetization. This work demonstrates a new route toward developing artificial high-magnetization materials using the high magnetic moments of nanoclustered antiferromagnetic materials.

Superplastic deformation of polycrystalline materials is usually accommodated by diffusion-assisted grain boundary (GB) sliding at high temperatures. Lowering the temperature requirement for commercial superplastic forming enables green and cost-effective manufacturing. Recently, room-temperature (RT) superplasticity was realized in ultrafine-grained Al–Zn based alloys, but the underlying mechanism remains unclear. Here, we conducted in-situ tensile straining and post-mortem electron microscopy characterization, and atomistic density functional theory simulation to understand the RT superplasticity of an Al–15 Zn (at%) alloy. Results showed that the superplasticity is achieved by GB sliding and grain rotation, assisted by the continuous diffusion of Zn. In-situ observations showed Zn atoms diffusing from within grains to GBs, resulting in a Zn nanolayer at the GBs that acts as a solid lubricant to decrease the energy barrier of GB sliding. This research advances our understanding of diffusion-assisted deformation mechanism that is a prerequisite for the rational design of new materials with RT superplasticity.

Chemical short-range ordering is an important structural feature that would significantly affect materials properties, including mechanical properties. Chemical short-range ordering has been discovered in many alloy systems and usually forms at high temperatures during the materials synthesis or thermal annealing processes. Here we report chemical short-range ordering in an ultrafine-grained supersaturated Al-Zn alloy introduced by severe plastic deformation (strain > 1) at room temperature. The chemical short-range ordering results in an ordered L1(0)-AlZn structure with the dimensions of 1-3 nm. It is believed that vacancies introduced by severe plastic deformation play an important role in the formation of the chemical short-range ordering at room temperature. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

All‐inorganic halide perovskites have thus far exhibited better thermal stability but lower power conversion efficiency (PCE), compared with their organic–inorganic hybrid counterparts. The experimentally observed nonradiative recombination loss is commonly attributed to the prevalence of native deep defects, yet the exact microscopic origin remains elusive. Based on density functional theory calculations, it is demonstrated that hydrogen impurities may incorporate in the prototypical all‐inorganic perovskite CsPbI3 with a high density and serve as a new source of efficient nonradiative recombination centers. The resultant nonradiative efficiency loss can be significantly higher than those induced by native deep defects, namely interstitials and antisites , contributing to the subdued performance of the CsPbI3‐based devices. Furthermore, it is proposed that the iodine‐moderate growth conditions can effectively reduce the detrimental hydrogen ions. These results highlight the impact of unintentionally incorporated impurities and offer insights into the optimal synthetic route and practical operating protocols in the field of all‐inorganic perovskite solar cells.

Magnetism in layered two dimensional materials has attracted extensive interest. In this work, a variety of concentrations of Co dopants (1, 2 and 4 at.%) were doped into MoTe2 single crystal by ion implantation. Magnetic results indicate that pure MoTe2 displays a diamagnetic behavior. A small amount of Co doping induces a very high saturation magnetization. 4 % Co doping exhibits a saturation magnetization as high as 2231 emu/cm3, higher than pure metallic Fe (1958 emu/cm(3)). In addition, an outsized coercivity of 11 kOe was also detected in the 2 at.% Co doped MoTe2 sample, which may be a consequence of doping-induced defects in the lattice structure, stress, anisotropic geometry of Co-Te ions and pinning effects by the defects in-between the ions. First principles density functional theory calculations reveal that doping-induced structural defects, including substitutional and interstitial Co, nanoholes as well as interstitial Mo, are responsible for the high magnetization. (C) 2022 Elsevier B.V. All rights reserved.

Hydrogen interstitials are expected to be important in organic-inorganic hybrid perovskites; however, the characteristics and behaviors of hydrogen in perovskites remain poorly understood. Here, on the basis of density functional theory calculations, we quantitatively reported that both atomic and molecular hydrogen interstitials can form in hybrid MAPbI(3) and MASnI(3) perovskites. Whereas molecular hydrogen interstitial, H-2, is chemically inert, atomic hydrogen interstitial, Hi, serves as an electrically active negative-U defect. We identify high-density Hi(+) as a significant origin of ionic conductivity in p-type MAPbI(3) under the hydrogen-rich conditions, with the calculated activation energy being comparable to that measured in experiments. The highly diffusive Hi(+) ions are expected to impact hysteresis, charge separation, device polarization, and photogenerated field-screening effect and consequently degrade the solar cell performance. We evaluated approaches for mitigating such detrimental effects and suggested that synthesizing the perovskites with slightly extra iodine addition or tin alloying can effectively suppress the concentration of Hi(+). Our results are important to understand the fundamental aspects of hydrogen in perovskites in general and offer valuable insight for further improving the performance of perovskite solar cells and other optoelectronic devices via defect engineering.

The all-inorganic tin halide perovskite CsSnI3, currently under intensive investigation for photovoltaics and other optoelectronics, characteristically exhibits strong p-type conductivity and consequently poor power conversion efficiency regardless of growth and processing conditions. This has been traditionally attributed to the prevalence of native acceptor defects; however, such a mechanism falls short of explaining the observed high hole concentration under Sn-rich growth conditions in experiments. Here, by using first-principles calculations, we reveal that hydrogen impurities, existing as hydrogen anions, are an important cause for the high p-type character in CsSnI3. Hydrogen anions can be present with high densities and act as shallow acceptors, significantly enhancing the background hole concentrations, even under excess Sn treatment. Careful control and utilization of hydrogen anions are important for improving the performance of CsSnI3-based optoelectronic devices.

Plastic deformation in ceramic materials is normally only observed in nanometre-sized samples. However, we have observed high levels of plasticity (>50% plastic strain) and excellent elasticity (6% elastic strain) in perovskite oxide Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3, under compression along <100>pc pillars up to 2.1 μm in diameter. The extent of this deformation is much higher than has previously been reported for ceramic materials, and the sample size at which plasticity is observed is almost an order of magnitude larger. Bending tests also revealed over 8% flexural strain. Plastic deformation occurred by slip along {110} <1[Formula: see text]0 > . Calculations indicate that the resulting strain gradients will give rise to giant flexoelectric polarization. First principles models predict that a high concentration of oxygen vacancies weaken the covalent/ionic bonds, giving rise to the unexpected plasticity. Mechanical testing on oxygen vacancies-rich Mn-doped Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 confirmed this prediction. These findings will facilitate the design of plastic ceramic materials and the development of flexoelectric-based nano-electromechanical systems.

The hitherto subdued power conversion efficiencies of Sn-based hybrid perovskite solar cells are gener-ally attributed to severe nonradiative recombination; however, the responsible deep-level defects are still unclear. Here, we report an important nonradiative energy loss mechanism in the prototypical FASnI3 [FA = HC(NH2)2, formamidinium]. High-density tin vacancies (VSn) can effectively capture hydrogen to form VSn - H2 complexes that act as highly detrimental nonradiative recombination centers. We quan-titatively show that they can give rise to strong carrier recombination and thus energy loss due to a high nonradiative recombination rate constant. These key findings identify a hidden yet critical origin for the low performance of FASnI3-based devices and highlight the significance of controlling the hydrogen environment in the development of broad high-efficiency nontoxic halide perovskite device applications.

An oxygen-rich face-centred cubic (FCC) Ti phase was engineered in the microstructure of a Ti-6Al-4V alloy via additive manufacturing using laser powder bed fusion. Designated 'C', this oxygen-rich FCC phase has a lattice parameter of 0.406 nm and exhibits an orientation relationship with the parent a0 phase as follows: (000 1)a0//{1 1 1}C, and h1 2 10ia0 //h1 1 0iC. We propose that the formation of the C phase is facilitated by the combined effect of thermal gradients, deformation induced by the martensitic transformation, and local O enrichment. This enables an in-situ phase transformation from the hexagonal close-packed a0 phase to the C phase at elevated temperatures. Our density functional theory calculations indicate that oxygen occupancy in the octahedral interstices of the FCC structure is energetically preferred to corresponding sites in the a0 phase. The in-situ mechanical testing results indicate that the presence of the FCC phase significantly increases the local yield strength from 1.2 GPa for samples with only the a0 phase to 1.9 GPa for samples comprising approximately equal volume fractions of the a0 and FCC phases. No loss of ductility was reported, demonstrating great potential for strengthening and work hardening. We discuss the formation mechanism of the FCC phase and a pathway for future microstructural design of titanium alloys by additive manufacturing.