4H Phase Research Articles

Crystal-Phase-Selective Etching of Heterophase Au Nanostructures.

Selective oxidative etching is one of the most effective ways to prepare hollow nanostructures and nanocrystals with specific exposed facets. The mechanism of selective etching in noble metal nanostructures mainly relies on the different reactivity of metal components and the distinct surface energy of multimetallic nanostructures. Recently, phase engineering of nanomaterials (PEN) offers new opportunities for the preparation of unique heterostructures, including heterophase nanostructures. However, the synthesis of hollow multimetallic nanostructures based on crystal-phase-selective etching has been rarely studied. Here, a crystal-phase-selective etching method is reported to selectively etch the unconventional 4H and 2H phases in the heterophase Au nanostructures. Due to the coating of Pt-based alloy and the crystal-phase-selective etching of 4H-Au in 4H/face-centered cubic (fcc) Au nanowires, the well-defined ladder-like Au@PtAg nanoframes are prepared. In addition, the 2H-Au in the fcc-2H-fcc Au nanorods and 2H/fcc Au nanosheets can also be selectively etched using the same method. As a proof-of-concept application, the ladder-like Au@PtAg nanoframes are used for the electrocatalytic hydrogen evolution reaction (HER) in acidic media, showing excellent performance that is comparable to the commercial Pt/C catalyst.

Antiferromagnetic Semiconductor BaMnO3 Hexagonal Perovskite with a Direct Bandgap.

The unique properties of direct bandgap semiconductors make it important to search for semiconductors exhibiting this phenomenon in perovskite materials. In this study, we employed first-principles calculations to investigate the crystal structures, magnetic configurations, and electronic properties of hexagonal perovskite BaMnO3 in its 4H and 6H phases. The results indicate that both structures exhibit antiferromagnetic characteristics, in which the Mn-O-Mn superexchange plays the dominant role in the 4H phase, although there is a competition between the Mn-Mn direct exchange interaction and the Mn-O-Mn superexchange interaction. In contrast, these two interactions exhibit harmonious coexistence in the 6H phase, and the two antiferromagnetic transitions occurring in the experimental phase should be related to the synergistic effect between them. Despite their different internal arrangements, they exhibit the same charge combination of Ba2+Mn4+O2-3. More importantly, both phases exhibit semiconductor properties with a direct bandgap, making it suitable to serve as an alternative material for photovoltaic and optoelectronic devices. In particular, the band gap of the 4H phase is just the right size to absorb visible light, and the 6H phase should be a potential candidate to absorb light in the ultraviolet region.

In Situ Atomic‐Scale Evidence of Unconventional Plastic Behavior at The Crack Tip in AuCu Nanocrystals

AbstractUnderstanding the plastic behavior of crack tips is crucial for improving the fracture toughness of nanometals. Although many studies are carried out, most previous studies focus on pure metals, and how the crack tip accommodates the plastic deformation of highly concentrated solid‐solution alloys is unclear owing to a lack of direct atomic‐scale evidence. In this study, the atomic‐scale plastic behavior of the crack tip in face‐centered cubic (FCC) AuCu alloy nanocrystals is observed in situ, which provides direct evidence that plastic deformation is governed by the generation of deformation twins and hexagonal close‐packed (HCP) 2H and 4H phases, recurrence of reversible FCC‐HCP phase transitions, and detwinning, which are rarely observed in pure metals. This unusual behavior originates from the inherent chemical inhomogeneity of the AuCu alloy, which inhibits twin thickening via partial dislocations on the adjacent plane, instead of random generation of deformation twins, phase transitions, and reversible processes. This naturally implies a similar behavior at the crack tip in other highly concentrated solid‐solution alloys, including high‐medium‐entropy alloys, providing important insights that greatly improve the understanding of the fracture toughness of metallic materials.

Chemical inhomogeneity–induced profuse nanotwinning and phase transformation in AuCu nanowires

Nanosized metals usually exhibit ultrahigh strength but suffer from low homogeneous plasticity. The origin of a strength–ductility trade-off has been well studied for pure metals, but not for random solid solution (RSS) alloys. How RSS alloys accommodate plasticity and whether they can achieve synergy between high strength and superplasticity has remained unresolved. Here, we show that face-centered cubic (FCC) RSS AuCu alloy nanowires (NWs) exhibit superplasticity of ~260% and ultrahigh strength of ~6 GPa, overcoming the trade-off between strength and ductility. These excellent properties originate from profuse hexagonal close-packed (HCP) phase generation (2H and 4H phases), recurrence of reversible FCC-HCP phase transition, and zigzag-like nanotwin generation, which has rarely been reported before. Such a mechanism stems from the inherent chemical inhomogeneity, which leads to widely distributed and overlapping energy barriers for the concurrent activation of multiple plasticity mechanisms. This naturally implies a similar deformation behavior for other highly concentrated solid-solution alloys with multiple principal elements, such as high/medium-entropy alloys. Our findings shed light on the effect of chemical inhomogeneity on the plastic deformation mechanism of solid-solution alloys.

High-Throughput Predictions of the Stabilities of Multi-Type Long-Period Stacking Ordered Structures in High-Performance Mg Alloys.

The effects of 44 types of elements on the stabilities of I1-constitute multi-type long-period stacking-ordered (LPSO) structures in Mg alloys, such as 4H, 6H, 8H, 9R, 12H, 15R, and 16H phases, are systematically investigated by first-principle high-performance calculations. The intrinsic stacking-fault energies (ISFEs) and their increments are calculated along with the formation enthalpies of solute atoms, and interaction energies between solute atoms and LPSO structures. The results suggest that the 15R phase is the easiest to form and stabilize among these LPSO structures, and 44 types of solute atoms have different segregation characteristics in these LPSO structures. A high temperature inhibits structural stabilizations of the LPSO phases, and these alloying elements, such as elements (Sb, Te, and Cs) for 4H; elements (S, Fe, Sb, and Te) for 6H, 8H, 9R, 15R, and 16H; and elements (S, Sb, and Te) for 12H, can effectively promote the stability of LPSO structures at high temperatures. S and Fe atoms are the most likely to promote the stabilities of the 16H structure with regard to other LPSO phases, but the Fe atom tends to inhibit the stabilities of 4H and 12H structures. This work can offer valuable references to further study and develop high-performance Mg alloys with multi-type LPSO structures.

(Digital Presentation) Development of an Eam-Type Interatomic Potential Model Reproducing Theoretical Energetics in Polytype Structures

The allotropes composed of close-packed (CP) structures that differ only in the stacking order in the system are called the CP polytypes [1]. The CP polytype is also considered one of the physical systems realizing the Kepler conjecture [2]. Since the significant changes in physical properties due to the intentional polytype formation have been reported in various fields of metallic and semiconductive systems [3-5], the material engineering demand for polytype phase control is diverse. The acquisition and deepening of scientific knowledge for the polytype phase control is therefore promising to lead to the creation of new functional materials. However, the scientific control of the polytype stacking order is still incomplete in the field of practical materials engineering. This is because the polytype control technology has many uncertainties since the formation mechanism of polytype is physically unsolved yet. Predicting polytype phase stability for a material has also been a long-standing issue in condensed matter physics and/or materials science. This situation stems from the fact that the atomistic interactions on polytype energetics might be surprisingly quite complex and delicate despite the simplicity of their geometrical structure. In the previous paper, we have presented an analytical theory on the total energetics for the CP polytypes based on the geometrical analysis for the correlation between interlayer interactions and interatomic ones in CP polytypes [6]. Our theoretical studies have shown that the long-range nature of interatomic interactions is an important factor for reproducing the static polytype energetics [6,7]. Based on our theory submitted [6], we have tried to construct an EAM (Embedded Atom Method) [8] type interatomic model for metallic La reproducing the theoretical polytype energetics. Our primary motivation for the present study is to grasp why long periodic polytype structures other than 2H and 3C ones appear as the stable phase under the crystal growth for the specific elemental systems. Therefore, we aim to develop the interatomic potential to reproduce the theoretical polytype energetics and to adopt it to the dynamical simulations for the long-periodic polytypes systems. The ground state for La is the 4H structure which is the simplest long-periodic polytype and La can exist in both 4H phase and the 3C phase under 583K [9]. Thus, it is possible to investigate the essential factors determining the polytype selection rule through the atomistic dynamics for metallic La. From our research so far, we have succeeded in constructing an EAM-type potential that mimics the energetics of metallic La with the 4H as the ground structure [10]. The following three procedures are found to be essentially important for deriving an interatomic interaction model with the 4H structure as the ground state; (i) The cutoff radius should be set to have a relatively longer distance (three or more interlayer distances in the 4H stacking structure). (ii) The equations of state (EOS) for the energetics of 2H, 3C, and 4H structures need to be reproduced as accurately as possible. (iii) The adiabatic potential along important transition paths between crystal structures such as the Bain path for the bcc-fcc transition must be exactly considered [11]. These procedures have made the interaction model describable for not only the energetics but also the lattice dynamics such as dynamical instability, which is known to be generally significant for the phase transition phenomena [11, 12]. The transferability of the associated potential and some results adapted in the molecular dynamics (MD) simulations will be also discussed in the presentation of the day.[1] A. L.Ortiz et al., J. Appl. Cryst. 46, 242 (2013).[2] T. Hales et al., Forum of Mathematics, Pi, 5, E2 (2017).[3] R. Yakimova et al., J. Cryst. Grow. 217, 255 (2000).[4] Z. Fan et al., Nat. Commun. 6, 7684 (2015).[5] E. M. T. Fadaly et al., Nature. 580, 205 (2020).[6] S. Ogane and K. Moriguchi, MRS Advances 6, 170 (2021).[7] K. Moriguchi et al., MRS Advances 6, 163 (2021).[8] Murray S. Daw and M. I. Baskes, Phys. Rev. B 29, 6443 (1984).[9] B. Love, “Metal Handbook”, Vol. 1, American Society for Metals, Novelty, Ohio, 1961, pp 1230-1231.[10] S. Ogane, et al., 2021 MRS Fall Meeting & Exhibit, CH02.13.05 (2021).[11] K Moriguchi and M Igarashi, Phys. Rev. B 74, 024111 (2006).[12] G. Grimvall et al., Rev. Mod. Phys. 84, 945 (2012).[13] S. Baci et al., Phys. Rev. B 81 (2010) 144507. Figure 1

Deformation-Induced Phase Transformations in Gold Nanoribbons with the 4H Phase.

The mechanical stability of metallic nanomaterials has been intensively studied due to their unique structures and promising applications. Although extensive investigations have been carried out on the deformation behaviors of metallic nanomaterials, the atomic-scale deformation mechanism of metallic nanomaterials with unconventional hexagonal structures remains unclear because of the lack of direct experimental observation. Here, we conduct an atomic-resolution in situ tensile-straining transmission electron microscopy investigation on the deformation mechanism of gold nanoribbons with the 4H (hexagonal) phase. Our results reveal that plastic deformation in the 4H gold nanoribbons comprises three stages, in which both full and partial dislocations are involved. At the early deformation stage, plastic deformation is governed by full dislocation activities. Partial dislocations are subsequently activated in regions that have undergone full dislocation gliding, leading to phase transformation from the 4H phase to the face-centered cubic (FCC) phase. At the last stage of the deformation process, the volume fraction of the FCC phase increases, and full dislocation activities in the FCC regions also play an important role.

Restructuring of 4H phase Au nanowires and its catalytic behavior toward CO electro-oxidation

Au nanowires in 4H crystalline phase (4H Au NWs) are synthesized by colloid solution methods. The crystalline phase and surface structure as well as its performance toward electrochemical oxidation of CO before and after removing adsorbed oleylamine molecules (OAs) introduced from its synthesis are evaluated by high-resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), underpotential deposition of Pb (Pb-upd) and cyclic voltammetry. Different methods, i.e. acetic acid cleaning, electrochemical oxidation cleaning, and diethylamine replacement, have been tried to remove the adsorbed OAs. For all methods, upon the removal of the adsorbed OAs, the morphology of 4H gold nanoparticles is found to gradually change from nanowires to large dumbbell-shaped nanoparticles, accompanying with a transition from the 4H phase to the face-centered cubic phase. On the other hand, the Pb-upd results show that the sample surfaces have almost the same facet composition before and after removal of the adsorbed OAs. After electrochemical cleaning with continuous potential scans up to 1.3 V, CO electro-oxidation activity of the 4H Au sample is significantly improved. The CO electro-oxidation activity is compared with results on the three basel Au single crystalline surfaces reported in the literature, possible origins for its enhancement are discussed.

Engineering crystal structure and spin-phonon coupling in Ba1-xSrxMnO3

The interplay between different degrees of freedom such as charge, spin, orbital, and lattice has received a great deal of interest due to its potential to engineer materials properties and their functionalities for device applications. In this work, we have explored the crystallographic phase diagram of Ba1-xSrxMnO3 and studied the correlation between two degrees of freedom, namely phonons and spins using magnetization and inelastic light scattering measurements. The system undergoes a series of crystallographic phase transitions 2H → 9R → 4H as a function of doping (Sr) as observed by X-ray diffraction measurements. Investigation of their temperature-dependent magnetization reveals a para- to antiferro-magnetic transition for all the compositions. An Eg phonon in the 9R phase and an E1g phonon in the 4H phase involving Mn/O-vibrations, show anomalous temperature-dependence in the antiferromagnetic phase arising due to spin-phonon coupling.

Revealing phase evolution mechanism for stabilizing formamidinium-based lead halide perovskites by a key intermediate phase

Revealing phase evolution mechanism for stabilizing formamidinium-based lead halide perovskites by a key intermediate phase

Mixed Mott–Hubbard and charge transfer nature of 4H–SrMnO3 thin film on Si (100)

Room temperature electronic structure of polycrystalline 4H–SrMnO3 thin film grown on Si (100) substrate has been studied using resonance photo emission spectroscopy and soft x-ray absorption spectroscopy measurements. Presence of charge transfer screen Mn 3d n L final state along with the 3d n-1 final state at the valence band edge of 4H–SrMnO3 thin film confirms that the ground state is strongly mixed between Mn 3d and O 2p states. The estimated equivalent values of on-site Coulomb interaction energy (U) and O 2p to Mn 3d- charge transfer energy (Δ) (U ≈ Δ ≈ 4.8 eV) from the combination of occupied and unoccupied spectra further confirm the intermediate Mott–Hubbard and charge transfer insulator nature of 4H–SrMnO3 film. Despite having similar Mn 4+ valence state in 4H–SrMnO3 and cubic SrMnO3, 4H phase is observed to reveal much higher band gap ∼1.5 eV than the cubic phase (0.3 eV), which arises due to different MnO6 octahedra environment.

The role of ligands in pressure-induced phase transition of gold nanoribbons

ABSTRACT The pressure-induced phase transition mechanisms of gold nanoribbons (Au-NRBs) from 4H phase to FCC phase with different surface ligands were studied theoretically. In bulk form, the 4H (hexagonal) phase was found to have nearly the same stability as the face-centered cubic (FCC) phase up to 100 GPa, and does not prefer phase transition at high pressure. However, gold in its nanoribbon form exposing the bare (10) surface shows a clear tendency to phase transition at 15 GPa through the flattening of the exposed (10) surface. On non-polar pentane covered Au-NRBs, the phase transition follows the same mechanism as the clean ones, but with a slightly lowered transition pressure of 11 GPa. Methylamine (CH3NH2), as a strong polar ligand, when adsorbed on the exposed surface of the Au-NRBs, completely changed the phase transition mechanism of the Au-NRBs. This work provided important insights into the controlled synthesis of nanomaterials with desired phases.

Phase transitions and elastic anisotropies of SiC polymorphs under high pressure

The structures, equations of state, phase transitions, mechanical stability, and elastic anisotropies of five SiC polymorphs (wurtzite, 4H, 6H, zinc blende, and rocksalt phases) under pressure were investigated based on first-principles calculations. The optimal structural parameters, densities, and volumes of the five SiC polymorphs under zero and high pressure were obtained and analyzed. The pressures of the phase and stability–instability transitions were predicted using the minimum energy principle and improved Born mechanical stability criteria, respectively, and a new phase transition criterion was developed. The elastic anisotropies of SiC under zero and high pressure were systematically discussed, and the negative Poisson's ratio of zinc blende SiC was verified.

Undercoordinated Active Sites on 4H Gold Nanostructures for CO2 Reduction.

Electroreduction of CO2 is a promising approach toward artificial carbon recycling. The rate and product selectivity of this reaction are highly sensitive to the surface structures of electrocatalysts. We report here 4H Au nanostructures as advanced electrocatalysts for highly active and selective reduction of CO2 to CO. Au nanoribbons in the pure 4H phase, Au nanorods in the hybrid 4H/fcc phase, and those in the fcc phase are comparatively studied for the electroreduction of CO2. Both the activity and selectivity for CO production were found to exhibit the trend 4H-nanoribbons > 4H/fcc-nanorods > fcc-nanorods, with the 4H-nanoribbons achieving >90% Faradaic efficiency toward CO. Electrochemical probing and cluster expansion simulations are combined to elucidate the surface structures of these nanocrystals. The combination of crystal phase and shape control gives rise to the preferential exposure of undercoordinated sites. Further density functional theory calculations confirm the high reactivity of such undercoordinated sites.

Carbon Monoxide Gas Induced 4H-to-fcc Phase Transformation of Gold As Revealed by In-Situ Transmission Electron Microscopy.

The hexagonal 4H phase gold nanostructures shows great potential for catalysis, optical, and biomedical fields. However, its phase stability remains largely unclear. Here, we report the 4H-to-face-centered cubic (fcc) phase transformation of gold induced by CO gas interactions and an electron beam observed through in-situ transmission electron microscopy (in-situ TEM). The atomic scale transformation mechanism is revealed experimentally and supported by first-principle calculations. Density functional theory calculations show that the 4H-to-fcc phase transformation processes via the transition of layer sliding with expanded layer spacing, which can be facilitated by both the adsorbed CO molecules and the extra electron provided by the electron beam. The transformation first takes place at the edges of the nanorods with the collective assistance of both CO and extra electrons, and then the inner portion of the bulk crystal follows with extra electrons as the lubricant. These results promote the understanding of the toxic effect of CO gas and shining light on the structural conversion and atomic migration of noble metal catalysts when they interact with CO molecules.

Efficient Hybrid Mixed‐Ion Perovskite Photovoltaics: In Situ Diagnostics of the Roles of Cesium and Potassium Alkali Cation Addition

Perovskite photovoltaics have made extraordinary progress in power conversion efficiency (PCE) and stability due to process and formulation development. Perovskite cell performance benefits from the addition of alkali metal cations, such as cesium (Cs+) and potassium (K+) in mixed‐ion systems, but the underlying reasons are not fully understood. Herein, the solidification of perovskite layers is studied, incorporating 5%, 10%, to 20% of Cs+ and K+ using in situ grazing incidence wide‐angle X‐ray scattering. It is found that K+‐doped solutions yield nonperovskite 4H phase rather than the 3C perovskite phase. For Cs+‐doped formulations, both 4H and 3C phases are present at 5% Cs+, whereas the 3C perovskite phase is formed in 10% Cs+‐doped formulations, with undesirable halide segregation occurring at 20% Cs+. Postdeposition thermal annealing converts the intermediate 4H phase to the desirable 3C perovskite phase. Importantly, perovskite layers containing 5% of Cs+ or K+ exhibit a reduced concentration of trap states and enhanced carrier mobility and lifetime. By carefully adjusting Cs+ or K+ concentration to 5%, perovskite cells are demonstrated with a ≈5% higher‐average PCE than cells utilizing higher cation concentrations. Herein, unique insights into the crystallization pathways toward perovskite phase engineering and improved cell performance are provided.

Ethylene Selectivity in Electrocatalytic CO2 Reduction on Cu Nanomaterials: A Crystal Phase-Dependent Study.

The crystal phase of metal nanocatalysts significantly affects their catalytic performance. Cu-based nanomaterials are unique electrocatalysts for CO2 reduction reaction (CO2RR) to produce high-value hydrocarbons. However, studies to date are limited to the conventional face-centered cubic (fcc) Cu. Here, we report a crystal phase-dependent catalytic behavior of Cu, after the successful synthesis of high-purity 4H Cu and heterophase 4H/fcc Cu using the 4H and 4H/fcc Au as templates, respectively. Remarkably, the obtained unconventional crystal structures of Cu exhibit enhanced overall activity and higher ethylene (C2H4) selectivity in CO2RR compared to the fcc Cu. Density functional theory calculations suggest that the 4H phase and 4H/fcc interface of Cu favor the C2H4 formation pathway compared to the fcc Cu, leading to the crystal phase-dependent C2H4 selectivity. This study demonstrates the importance of crystal phase engineering of metal nanocatalysts for electrocatalytic reactions, offering a new strategy to prepare novel catalysts with unconventional phases for various applications.

Crystal phase-controlled growth of PtCu and PtCo alloys on 4H Au nanoribbons for electrocatalytic ethanol oxidation reaction

Crystal phase can greatly affect the physicochemical properties and applications of nanomaterials. However, it still remains a great challenge to synthesize nanostructures with the same composition and morphology but different phases in order to explore the phase-dependent properties and applications. Herein, we report the crystal phase-controlled synthesis of PtCu alloy shells on 4H Au nanoribbons (NRBs), referred to as 4H-Au NRBs, to form the 4H-Au@PtCu core-shell NRBs. By tuning the thickness of PtCu, 4H-PtCu and face-centered cubic (fcc) phase PtCu (fcc-PtCu) alloy shells are successfully grown on the 4H-Au NRB cores. This thickness-dependent phase-controlled growth strategy can also be used to grow PtCo alloys with 4H or fcc phase on 4H-Au NRBs. Significantly, when used as electrocatalysts for the ethanol oxidation reaction (EOR) in alkaline media, the 4H-Au@4H-PtCu NRBs show much better EOR performance than the 4H-Au@fcc-PtCu NRBs, and both of them possess superior performance compared to the commercial Pt black. Our study provides a strategy on phase-controlled synthesis of nanomaterials used for crystal phase-dependent applications.

Capturing the Equilibration Pathway of Nanomaterials Metastable in Both Crystal Structure and Morphology

In this issue of Matter, Li et al. used in situ electron microscopy to investigate the equilibration pathway of Au nanoribbons that are metastable in terms of both crystal structure and morphology when they are subjected to thermal stress.

Gas-assisted transformation of gold from fcc to the metastable 4H phase

The metastable hexagonal 4H-phase gold has recently attracted extensive interest due to its exceptional performance in catalysis. However, gold usually crystallizes to its lowest free energy structure called face-centered cubic (fcc). The phase transformation from the stable fcc phase to the metastable 4H phase is thus of great significance in crystal phase engineering. Herein, we report this unusual phenomenon on a 4H gold nanorod template with the aid of CO gas and an electron beam. In situ transmission electron microscopy was used to directly visualize the interface propagation kinetics between the 4H-Au-nanorod and fcc-Au nanoparticle. Epitaxial growth was initiated at the contact interface, and then propagated to convert all parts of these fcc nanoparticles to 4H phase. Density functional theory calculations and ab initio molecular dynamics simulations show that the CO molecules can assist the Au diffusion process and promote the flexibility of Au particles during the epitaxial growth. The phase transformation was driven by the reduction of Gibbs free energy by eliminating the interface between fcc and 4H phases.