Synthesis of paracrystalline diamond.

Solids in nature can be generally classified into crystalline and non-crystalline states1-7, depending on whether long-range lattice periodicity is present in the material. The differentiation of the two states, however, could face fundamental challenges if the degree of long-range order in crystals is significantly reduced. Here we report a paracrystalline state of diamond that is distinct from either crystalline or amorphous diamond8-10. The paracrystalline diamond reported in this work, consisting of sub-nanometre-sized paracrystallites that possess a well-defined crystalline medium-range order up to a few atomic shells4,5,11-13, was synthesized in high-pressure high-temperature conditions (for example, 30 GPa and 1,600 K) employing face-centred cubic C60 as a precursor. The structural characteristics of the paracrystalline diamond were identified through a combination of X-ray diffraction, high-resolution transmission microscopy and advanced molecular dynamics simulation. The formation of paracrystalline diamond is a result of densely distributed nucleation sites developed in compressed C60 as well as pronounced second-nearest-neighbour short-range order in amorphous diamond due to strong sp3 bonding. The discovery of paracrystalline diamond adds an unusual diamond form to the enriched carbon family14-16, which exhibits distinguishing physical properties and can be furthered exploited to develop new materials. Furthermore, this work reveals the missing link in the length scale between amorphous and crystalline states across the structural landscape, having profound implications for recognizing complex structures arising from amorphous materials.

Toughening oxide glasses through paracrystallization.

Glasses, unlike crystals, are intrinsically brittle due to the absence of microstructure-controlled toughening, creating fundamental constraints for their technological applications. Consequently, strategies for toughening glasses without compromising their other advantageous properties have been long sought after but elusive. Here we report exceptional toughening in oxide glasses via paracrystallization, using aluminosilicate glass as an example. By combining experiments and computational modelling, we demonstrate the uniform formation of crystal-like medium-range order clusters pervading the glass structure as a result of paracrystallization under high-pressure and high-temperature conditions. The paracrystalline oxide glasses display superior toughness, reaching up to 1.99 ± 0.06 MPa m1/2, surpassing any other reported bulk oxide glasses, to the best of our knowledge. We attribute this exceptional toughening to the excitation of multiple shear bands caused by a stress-induced inverse transformation from the paracrystalline to amorphous states, revealing plastic deformation characteristics. This discovery presents a potent strategy for designing highly damage-tolerant glass materials and emphasizes the substantial influence of atomic-level structural variation on the properties of oxide glasses.

Pressure-induced amorphization in single-crystal Ta2O5 nanowires: a kinetic mechanism and improved electrical conductivity.

Pressure-induced amorphization (PIA) in single-crystal Ta2O5 nanowires is observed at 19 GPa, and the obtained amorphous Ta2O5 nanowires show significant improvement in electrical conductivity. The phase transition process is unveiled by monitoring structural evolution with in situ synchrotron X-ray diffraction, pair distribution function, Raman spectroscopy, and transmission electron microscopy. The first principles calculations reveal the phonon modes softening during compression at particular bonds, and the analysis on the electron localization function also shows bond strength weakening at the same positions. On the basis of the experimental and theoretical results, a kinetic PIA mechanism is proposed and demonstrated systematically that amorphization is initiated by the disruption of connectivity between polyhedra (TaO6 octahedra or TaO7 bipyramids) at the particular weak-bonding positions along the a axis in the unit cell. The one-dimensional morphology is well-preserved for the pressure-induced amorphous Ta2O5, and the electrical conductivity is improved by an order of magnitude compared to traditional amorphous forms. Such pressure-induced amorphous nanomaterials with unique properties surpassing those in either crystalline or conventional amorphous phases hold great promise for numerous applications in the future.

Folded network and structural transition in molten tin.

The fundamental relationships between the structure and properties of liquids are far from being well understood. For instance, the structural origins of many liquid anomalies still remain unclear, but liquid-liquid transitions (LLT) are believed to hold a key. However, experimental demonstrations of LLTs have been rather challenging. Here, we report experimental and theoretical evidence of a second-order-like LLT in molten tin, one which favors a percolating covalent bond network at high temperatures. The observed structural transition originates from the fluctuating metallic/covalent behavior of atomic bonding, and consequently a new paradigm of liquid structure emerges. The liquid structure, described in the form of a folded network, bridges two well-established structural models for disordered systems, i.e., the random packing of hard-spheres and a continuous random network, offering a large structural midground for liquids and glasses. Our findings provide an unparalleled physical picture of the atomic arrangement for a plethora of liquids, shedding light on the thermodynamic and dynamic anomalies of liquids but also entailing far-reaching implications for studying liquid polyamorphism and dynamical transitions in liquids.

Strengthening in multi-principal element alloys with local-chemical-order roughened dislocation pathways.

High-entropy and medium-entropy alloys are presumed to have a configurational entropy as high as that of an ideally mixed solid solution (SS) of multiple elements in near-equal proportions. However, enthalpic interactions inevitably render such chemically disordered SSs rare and metastable, except at very high temperatures. Here we highlight the wide variety of local chemical ordering (LCO) that sets these concentrated SSs apart from traditional solvent-solute ones. Using atomistic simulations, we reveal that the LCO of the multi-principal-element NiCoCr SS changes with alloy processing conditions, producing a wide range of generalized planar fault energies. We show that the LCO heightens the ruggedness of the energy landscape and raises activation barriers governing dislocation activities. This influences the selection of dislocation pathways in slip, faulting, and twinning, and increases the lattice friction to dislocation motion via a nanoscale segment detrapping mechanism. In contrast, severe plastic deformation reduces the LCO towards random SS.

Robust Formation of Ultrasmall Room-Temperature Neél Skyrmions in Amorphous Ferrimagnets from Atomistic Simulations.

Neél skyrmions originate from interfacial Dzyaloshinskii Moriya interaction (DMI). Recent studies have explored using thin-film ferromagnets and ferrimagnets to host Neél skyrmions for spintronic applications. However, it is unclear if ultrasmall (10 nm or less) skyrmions can ever be stabilized at room temperature for practical use in high density parallel racetrack memories. While thicker films can improve stability, DMI decays rapidly away from the interface. As such, spins far away from the interface would experience near-zero DMI, raising question on whether or not unrealistically large DMI is needed to stabilize skyrmions, and whether skyrmions will also collapse away from the interface. To address these questions, we have employed atomistic stochastic Landau-Lifshitz-Gilbert simulations to investigate skyrmions in amorphous ferrimagnetic GdCo. It is revealed that a significant reduction in DMI below that of Pt is sufficient to stabilize ultrasmall skyrmions even in films as thick as 15 nm. Moreover, skyrmions are found to retain a uniform columnar shape across the film thickness due to the long ferrimagnetic exchange length despite the decaying DMI. Our results show that increasing thickness and reducing DMI in GdCo can further reduce the size of skyrmions at room temperature, which is crucial to improve the density and energy efficiency in skyrmion based devices.

Li-ion battery material under high pressure: amorphization and enhanced conductivity of Li4Ti5O12.

Lithium titanium oxide (Li4Ti5O12, LTO), a 'zero-strain' anode material for lithium-ion batteries, exhibits excellent cycling performance. However, its poor conductivity highly limits its applications. Here, the structural stability and conductivity of LTO were studied using in situ high-pressure measurements and first-principles calculations. LTO underwent a pressure-induced amorphization (PIA) at 26.9 GPa. The impedance spectroscopy revealed that the conductivity of LTO improved significantly after amorphization and that the conductivity of decompressed amorphous LTO increased by an order of magnitude compared with its starting phase. Furthermore, our calculations demonstrated that the different compressibility of the LiO6 and TiO6 octahedra in the structure was crucial for the PIA. The amorphous phase promotes Li+ diffusion and enhances its ionic conductivity by providing defects for ion migration. Our results not only provide an insight into the pressure depended structural properties of a spinel-like material, but also facilitate exploration of the interplay between PIA and conductivity.

Universal structural parameter to quantitatively predict metallic glass properties.

Quantitatively correlating the amorphous structure in metallic glasses (MGs) with their physical properties has been a long-sought goal. Here we introduce 'flexibility volume' as a universal indicator, to bridge the structural state the MG is in with its properties, on both atomic and macroscopic levels. The flexibility volume combines static atomic volume with dynamics information via atomic vibrations that probe local configurational space and interaction between neighbouring atoms. We demonstrate that flexibility volume is a physically appropriate parameter that can quantitatively predict the shear modulus, which is at the heart of many key properties of MGs. Moreover, the new parameter correlates strongly with atomic packing topology, and also with the activation energy for thermally activated relaxation and the propensity for stress-driven shear transformations. These correlations are expected to be robust across a very wide range of MG compositions, processing conditions and length scales.