Metastable Materials Research Articles (Page 1)

Metallic 1T-phase MoS2 is a promising alternative to platinum-group catalysts for the acidic hydrogen evolution reaction (HER), yet its metastable nature severely limits practical application. Here we present a framework-stabilization strategy for constructing 1T'-phase Mo-Re sulfides by employing the intrinsically distorted 1T' lattice of ReS2 as a structural scaffold. Preferential Mo substitution at Re sites, facilitated by its low formation energy, enables the formation of Re0.5Mo0.5S2 with a high-purity 1T' architecture free of secondary phases. Combined theoretical and experimental investigations reveal that Mo incorporation triggers directional electron transfer and strong Mo 4d/Re 5d/S 3p orbital hybridization, which redistribute charge density, elevate the S 3p band center, and fine-tune the hydrogen adsorption-desorption equilibrium. Consequently, Re0.5Mo0.5S2 achieves exceptional acidic HER performance with a low overpotential of 113 mV at 10 mA cm-2 and remarkable stability exceeding 50 h. This work establishes a versatile paradigm for stabilizing metastable transition metal dichalcogenides, providing new insights into the rational design of metastable functional materials.

Bulk metallic glasses (BMGs), classified as metastable materials, necessitate melt quenching at critical cooling rates higher than 102 K/s to kinetically bypass crystalline phase formation during solidification. Owing to this rapid quenching, the microstructure of BMGs can be regarded as melt quenched. This study examines how their melt state governs the thermal stability, structural characteristics, and plasticity behavior of Zr50Cu40Al10 BMG. Rod samples were prepared via injection casting at controlled melt temperatures and suction casting. Experimental observations demonstrated a positive correlation between elevated melt temperatures and enhanced glass forming ability (GFA) along with improved thermal stability (T-A) in BMGs during processing. Structural analyses confirmed the glassy nature of the prepared BMGs with different melt states and revealed their temperature-dependent atomic-scale heterogeneity: the samples quenched at low melt temperatures exhibited significant Cu-rich clustering as determined via energy-dispersive X-ray spectroscopy (EDS) mapping, and those at high melt temperatures formed homogeneous structures. This structure heterogeneity was directly correlated with good plastic deformation behavior, i.e., the rod sample prepared at the lowest melt temperature achieved 9.7% plastic strain. The transition is attributed to liquid-liquid phase transition (LLPT): below the LLPT threshold, metastable Cu-rich clusters persist in the melt and are retained upon quenching, creating structural defects that facilitate shear band multiplication. These findings highlight melt temperature as a crucial factor in tailoring the structure characteristic and mechanical behavior of Zr50Cu40Al10 BMGs.

O2-type LixCoO2 (O2-LCO) has recently attracted significant attention as a promising cathode material with superior electrochemical performance compared to its conventional O3-type counterpart. Due to its metastable nature, O2-LCO is typically synthesized via ion exchange under relatively mild thermal conditions; however, the structural evolution and resultant phase composition during this process, which critically affect the material's performance, remain insufficiently understood. Here, we systematically elucidate the interplay between composition, structure, and electrochemical performance in metastable LiCoO2 synthesized via molten-salt ion exchange. We show that the Na content in the precursor not only dictates the final Li content in the product but also thermodynamically governs the phase transition pathway. Comprehensive long-range and local structural characterizations reveal the composite nature of ion-exchanged LCO, comprising T#2, O2, and O3 phases, with their relative fractions determined by the initial Na content. Electrochemical measurements, supported by theoretical calculations, indicate that the optimal phase composite maximizes both Li content and T#2 fraction while suppressing O3 formation, thereby enhancing Li+ diffusion kinetics and structural compatibility. These insights provide a fundamental basis for phase engineering in metastable cathode materials and practical guidelines for designing high-performance layered oxide cathodes.

Abstract O2‐type Li x CoO 2 (O2‐LCO) has recently attracted significant attention as a promising cathode material with superior electrochemical performance compared to its conventional O3‐type counterpart. Due to its metastable nature, O2‐LCO is typically synthesized via ion exchange under relatively mild thermal conditions; however, the structural evolution and resultant phase composition during this process, which critically affect the material's performance, remain insufficiently understood. Here, we systematically elucidate the interplay between composition, structure, and electrochemical performance in metastable LiCoO 2 synthesized via molten‐salt ion exchange. We show that the Na content in the precursor not only dictates the final Li content in the product but also thermodynamically governs the phase transition pathway. Comprehensive long‐range and local structural characterizations reveal the composite nature of ion‐exchanged LCO, comprising T # 2, O2, and O3 phases, with their relative fractions determined by the initial Na content. Electrochemical measurements, supported by theoretical calculations, indicate that the optimal phase composite maximizes both Li content and T # 2 fraction while suppressing O3 formation, thereby enhancing Li + diffusion kinetics and structural compatibility. These insights provide a fundamental basis for phase engineering in metastable cathode materials and practical guidelines for designing high‐performance layered oxide cathodes.

The discovery of two-dimensional (2D) ferromagnetic semiconductors holds significant promise for advancing Moore's law and spintronics in-memory computing, sparking tremendous interest. However, the Curie temperature of explored 2D ferromagnetic semiconductors is much lower than room temperature. Although plenty of 2D room-temperature ferromagnetic semiconductors have been theoretically predicted, there have been formidable challenges in preparing such metastable materials with ordered structures and high stability. Here, utilizing a novel template-assisted chemical vapor deposition strategy, we synthesized layered MnS2 microstructures within a ReS2 template. The high-resolution atomic structure representation revealed that monolayer MnS2 microstructures well crystallize into a distorted T-phase. Room-temperature ferromagnetism was confirmed through vibrating sample magnetometer measurements, microzone magnetism imaging techniques, and transport characterization. Theoretical calculations indicated that the room-temperature ferromagnetism originates from the Mn-Mn short-range interaction. Our observation not only offered the experimental confirmation of the intrinsic room-temperature ferromagnetism in layered MnS2, but also provided an innovative strategy for the growth of 2D metastable functional materials.

Thermal quenching has been employed to discover various metastable materials such as hard steels and metallic glasses. More recently, quenching-based phase control has been applied to correlated electron systems that exhibit metal-insulator, magnetic, or superconducting transitions. Despite the discovery of metastable electronic phases, however, the microscopic origin of metastability remains elusive, as the system's degrees of freedom—such as electrons—can vary even at low temperatures. Here, using Monte Carlo simulations, we demonstrate a thermally quenched metastable phase in the Ising model that does not conserve the total magnetization. When multiple types of interactions that stabilize different long-range orders are introduced, the ordering kinetics exhibits significant slowing down, and the characteristic timescale for ordering diverges as the temperature decreases. As a result, the system can reach low temperatures without undergoing ordering if the cooling rate is sufficiently high. Quantitative analysis of the divergent behavior suggests that the energy barrier for eliminating the local structure of competing orders is the origin of this metastability. Thus, the present simulation demonstrates that competing interactions play an essential role in achieving metastability.

The discovery of intrinsic magnetism in layered van der Waals (vdW) magnets has received intensive attention due to their fundamental importance in low-dimensional magnetism and potential device applications. To date, most vdW magnets contain 3d transition metals. Extending vdW magnetism to 4d and 5d transition metal systems is therefore of great interest as it offers opportunities to explore exotic magnetic behaviors arising from the interplay between electronic correlations and strong spin-orbit coupling (SOC). Here, we report the successful synthesis of a metastable layered vdW triangular lattice crystal, 1T-RhO2, through the topochemical reaction from Cs0.5RhO2 single crystals. Electrical transport measurements reveal that 1T-RhO2 is an insulator, while magnetic susceptibility and alternating current susceptibility confirm the absence of long-range magnetic order or spin glass behavior down to 2 K. Raman spectroscopy indicates fermionic excitations consistent with fractionalized Majorana fermions. Angle-resolved photoemission spectroscopy, supported by hybrid functional calculations, reveals a band gap of about 1.0 eV, further confirming the insulating nature. These results collectively suggest that 1T-RhO2 is a possible quantum spin liquid (QSL) candidate. Our work not only sheds light on the research fields of 4d and 5d transition metal vdW magnets but also significantly expands the pathways for discovering QSL candidates in metastable materials.

Abstract High‐entropy metallic glasses (HEMGs) have emerged as promising candidates for developing high‐performance and cost‐effective electrocatalysts for oxygen evolution reaction (OER), owing to their synergistic combination of superior catalytic activity and exceptional compositional tunability. However, a persistent challenge in HEMG synthesis lies in achieving atomic‐level control over microstructural configurations for targeted enhancement of electrocatalytic efficiency. Here, it is presented that a structurally engineered Cantor HEMG self‐supporting catalytic electrode is fabricated through ion beam sputtering (IBS) technology. The electrode features a micromesh integrated with nano‐cone array architecture comprising FeCoNiCrMn alloy, achieving homogeneous elemental distribution and enhanced high‐entropy synergistic effects. The precisely designed hierarchical structure combines the high electrical conductivity of the 3D nickel framework with the catalytically active HEMG surface, delivering exceptional OER performance in alkaline media. The optimized electrode demonstrates a remarkably low overpotential of 296 mV at a current density of 10 mA cm−2, while maintaining stable operation for 30 h under industrial‐grade conditions. The IBS‐derived nano‐cone configuration effectively increases electrochemical active surface area and promotes bubble detachment. This work highlights the dual benefits of IBS‐enabled structure engineering and entropy‐driven electronic modulation, proposing a design strategy for high‐efficiency water splitting systems through integration of metastable materials and 3D architectures.

Abstract Point defects are ubiquitous in solid-state compounds, dictating many functional properties such as conductivity, catalytic activity and carrier recombination. Over the past decade, the prevalence of metastable defect geometries and their importance to relevant properties has been increasingly recognised. A striking example is split vacancies, where an isolated atomic vacancy transforms to a stoichiometry-conserving complex of two vacancies and an interstitial (V X → [V X + X i + V X]), which can be accompanied by a dramatic energy lowering and change in behaviour. These species are particularly challenging to identify from computation, due to the ‘non-local’ nature of this reconstruction. Here, I present an approach for the efficient identification of these defects, through tiered screening which combines geometric analysis, electrostatic energies and foundation machine-learned (ML) models. This approach allows the screening of all solid-state compounds in the Materials Project database (including all entries in the ICSD, along with several thousand predicted metastable materials), identifying thousands of low energy split vacancy configurations, hitherto unknown. This study highlights both the potential utility of (foundation) machine-learning potentials, with important caveats, the significant prevalence of split vacancy defects in inorganic solids, and the importance of global optimisation approaches for defect modelling.

Optimisation of metastable supercooled liquid phase change material for long-term heat energy accumulation

Abstract The four-decade quest for synthesizing ambient-stable polymeric nitrogen - a promising high-energy-density material - remains an unsolved challenge in materials science. We develop a multi-stage computational strategy employing DFTB-based rapid screening combined with DFT refinement and global structure searching, effectively bridging computational efficiency with quantum accuracy. This integrated approach identifies four novel polymeric nitrogen phases (Fddd, P3221, I-4m2, and P6522) thermodynamically stable at ambient pressure. Remarkably, the helical P6522 configuration demonstrates exceptional thermal resilience up to 1500 K, representing a predicted polymeric nitrogen structure maintaining stability under both atmospheric pressure and high-temperature extremes. Our methodology establishes a paradigm-shifting framework for accelerated discovery of metastable energetic materials, resolving critical bottlenecks in theoretical predictions while providing experimentally actionable targets for polymeric nitrogen synthesis.

Metastable materials possess unique properties critical for advanced technologies; however, their synthesis is significantly challenging. Among the TiO2 polymorphs, rutile TiO2 stands out for its exceptional dielectric properties; however, its film growth typically requires high-temperatures or lattice-matched substrates, limiting its practical applications. This article presents a novel sacrificial layer strategy for the atomic layer deposition (ALD) of pure-phase rutile TiO2 films on diverse substrates, including amorphous Al2O3, HfO2, and ZrO2. This approach employs ultrathin Ru sacrificial layers to facilitate the formation of rutile TiO2 seed layers via the in situ generation of a rutile-matched RuO2 lattice. At the same time, it is completely removed as volatile RuO4 under exposure to O3 during the ALD process. This approach eliminates the need for high-temperature annealing and substrate restrictions, enabling low-temperature formation of rutile TiO2 on diverse substrates, including amorphous oxides. Comprehensive characterization reveals the structural stability of the films and their enhanced dielectric performance. Stabilizing rutile TiO2 independently of the underlying layer opens new possibilities for its integration into memory capacitors. Furthermore, this strategy provides a versatile framework for stabilizing other metastable material phases, thereby offering opportunities for diverse applications.

Abstract The metastable P63cm phase of ScFeO3 has potential for multiferroic applications not possible in the ground state if stabilization routes can be identified. This work demonstrates that the P63cm phase of ScFeO3 is stabilized on (0001) Al2O3 by the spontaneous formation of two atomic layers of (111) Fmm FeO during pulsed laser deposition, as observed via scanning transmission electron microscopy and a shift in the Fe L‐edge on approaching the interface. The matching oxygen sublattice and reduced strain of the FeO interlayer enable a ScFeO3 [110] || [2] FeO || Al2O3 [110] orientation relationship despite a −17.1% lattice mismatch. Temperature‐dependent X‐ray diffraction further support interlayer‐mediated stabilization, as FeO forms above 850 °C, preceding the formation of the P63cm ScFeO3 phase at 1000 °C. The identification of the FeO interlayer provides insights into the phase stabilization mechanism of P63cm ScFeO3 and presents a strategy for stabilizing other metastable materials that lack epitaxial substrates.

Metastable carbides and chalcogenides are attractive candidates for wide and promising applications. However, their inherent instability leads to synthetic difficulty and poor durability. Thus, the development of facile strategies for the controllable synthesis and stabilization of metastable carbides is still a great challenge. Here, taking metastable ɛ-Fe2C as a case study, potassium ions (K+) are theoretically predicted and experimentally reported to control the synthesis of metastable ɛ-Fe2C from an Fe2N precursor by increasing the surface carbon chemical potential (μC). The controllable synthesis and improved stability are attributed to the better-matched denitriding and carburizing rates and the impeded spillover of carbon atoms in metastable ɛ-Fe2C with high carbon contents due to the enhanced surface μC. In addition, this strategy is suitable for synthesizing metastable γ’-MoC, MoN, 1T-MoS2, 1T-MoSe2, 1T-MoSe2xTe2(1−x), and 1T-Mo1−xWxSe2, highlighting the universality of the methodology. Impressively, gram-level scalable metastable ɛ-Fe2C remains stable for more than 398 days in air. Furthermore, ɛ-Fe2C exhibits remarkable olefin selectivity and durability for more than 36 h of continuous testing. This work not only demonstrates a facile, easily scalable, and general strategy for accessing various metastable carbides and chalcogenides but also addresses the synthetic difficulty and poor durability challenge of metastable materials.

High-pressure β-Sn germanium may transform into diverse metastable allotropes with distinctive nanostructures and unique physical properties via multiple pathways under decompression. However, the mechanism and transition kinetics remain poorly understood. Here, we investigate the formation of metastable phases and nanostructures in germanium via controllable transition pathways of β-Sn Ge under rapid decompression at different rates. High-resolution transmission electron microscopy reveals three distinct metastable phases with the distinctive nanostructures: an almost perfect st12 Ge crystal, nanosized bc8/r8 structures with amorphous boundaries, and amorphous Ge with nanosized clusters (0.8–2.5 nm). Fast in situ x-ray diffraction and x-ray absorption measurements indicate that these nanostructured products form in certain pressure regions via distinct kinetic pathways and are strongly correlated with nucleation rates and electronic transitions mediated by compression rate, temperature, and stress. This work provides deep insight into the controllable synthesis of metastable materials with unique crystal symmetries and nanostructures for potential applications.

Fracture of metastable materials near absolute zero

Abstract Titanium‐based layered oxides exhibit promising characteristics for negative electrode applications for sodium‐ion batteries (SIBs) owing to their low operating potential and stable redox behavior. However, challenges persist in synthesizing single‐phase NaxCrxTi1–xO2 materials with reduced sodium content (x < 0.58) due to structural instability. In this study, an unconventional approach utilizing potassium analogues to design high Na vacancy concentration compounds is proposed. By exploiting the larger ionic size of K+ ions, a P3‐type K0.5Cr0.5Ti0.5O2 layered material with lower alkali ion concentrations (x = 0.50) is stabilized. Subsequently, through a facile room‐temperature K+/Na+ ion‐exchange process, sodium‐deficient metastable P3‐type Na0.5Cr0.5Ti0.5O2 is successfully synthesized. A similar K+/Li+ ion‐exchange process is also applied to synthesize O3‐type Li0.5Cr0.5Ti0.5O2. X‐ray diffraction combined with electron microscopy reveals the formation of metastable single‐phase P3‐type Na0.5Cr0.5Ti0.5O2 and metastable O3‐type Li0.5Cr0.5Ti0.5O2 with a unique layer arrangement. The high Na vacancy concentration in P3‐type Na0.5Cr0.5Ti0.5O2 results in an increased initial capacity of 125 mA h g−1 at 10 mA g−1. Additionally, Na0.5Cr0.5Ti0.5O2 exhibits Na+/vacancy disordering and high electronic conductivity, enabling a high‐rate charge capability without sodium plating. This work provides new insights into the design of metastable layered materials for durable and safe sodium battery applications with fast‐charge capability.

Gray tin (α-Sn) is an elemental topological material with various topological phases. It is predicted to be a 3D Dirac semimetal when appropriate strain is applied. However, the Dirac semi-metallic properties, such as chiral anomaly, are hard to be observed, probably due to the imperfections in the α-Sn samples. It is even more challenging to manipulate the topological properties in this metastable material without a sacrifice in the crystalline quality. Here, we report a strategy of Fermi level tuning by doping the α-Sn films grown on CdTe (001) substrates by molecular beam epitaxy. The negative magnetoresistance and planar Hall effect, which are attributed to chiral anomaly, are observed in α-Sn when the Fermi level is tuned close to the bulk Dirac point. Detailed analyses of Shubnikov–de Haas oscillations show nontrivial Berry phases for all the samples. Combined with first-principle calculations, these results provide a strong evidence of the three-dimensional Dirac semimetal phase in α-Sn. Furthermore, a clear transition from two-dimensional topological surface states to three-dimensional bulk Dirac cone is demonstrated in α-Sn by precise Fermi level tuning through doping. This study proves the 3D Dirac semimetal phase in α-Sn and provides an effective strategy to manipulate the topological properties for both fundamental studies and device applications.

LiAlH4, characterized by high hydrogen capacity and metastable properties, is regarded as a promising hydrogen source under mild conditions. However, its reversible regeneration from dehydrogenated production is hindered thermodynamically and kinetically. Herein, we demonstrate an active Li–Al–Ti nanocrystalline alloy prepared by melt spinning and cryomilling to enable directly synthesizing nano-LiAlH4. Due to the non-equilibrium preparation methods, the grain/particle size of the alloy was reduced, stress defects were introduced, and the dispersion of the Ti catalyst was promoted. The refined Li–Al–Ti nanocrystalline alloy with abundant defects and uniform catalytic sites demonstrated a high reactivity of the particle surface, thereby enhancing hydrogen absorption and desorption kinetics. Nano-LiAlH4 was directly obtained by ball milling a 5% Ti containing Li–Al–Ti nanocrystalline alloy with a grain size of 17.4 nm and Al3Ti catalytic phase distributed under 20 bar hydrogen pressure for 16 h. The obtained LiAlH4 exhibited room temperature dehydrogenation performance and good reversibility. This finding provides a potential strategy for the non-solvent synthesis and direct hydrogenation of metastable LiAlH4 hydrogen storage materials.

Physical aging intrinsically exists in amorphous materials and refers to the evolution of the nonequilibrium structure toward an equilibrium state. The aging process can significantly affect the thermomechanical properties of the amorphous materials, thereby influencing their macroscopic responses. Aging models not only help in understanding the underlying physical mechanisms of the relaxation behavior but also may provide an effective tool for predicting the physical and mechanical properties of metastable nonequilibrium materials in practical applications. In the current work, based on the measurement of calorimetric data and shear modulus during the heating process of amorphous metallic alloys, we obtained the mechanical and thermal property changes caused by physical aging. By incorporating the characteristic time of their α relaxation into a first-order kinetic equation and considering the coupled evolution between the characteristic time and the structural order parameter, we derived an aging kinetics model based on the hierarchically constrained atomic dynamics theory. This model effectively reproduces the thermal effects in the aging region and the supercooled liquid region observed in the calorimetric data.