Twin Crystals Research Articles

Pyrite contact twins.

Two examples of contact twins in pyrite from Peru are described. The first one, from Pasto Bueno ore deposit, shows the pyritohedron {120} as principal form, accompanied by the {111} octahedron and {100} cube as secondary forms, giving a lenticular aspect. (111) is the composition plane, and the twin operation is any one of the three binary axes ⟨110⟩ within this plane. The second one (unknown ore deposit) presents two forms, the octahedron {111} and the pyritohedron {120}; the two crystals in the twin are elongated along [101] and [011], respectively, producing a V profile. It is a reflection twin where the twin plane (110) coincides with the composition plane. These twins are by merohedry. Another contact twin is known in the literature, reported by Gaubert [Bull. Soc. Fr. Minéral. Cristallogr. (1928), 51, 211-212] who described it as a spinel twin, i.e. a reflection twin with twin and composition plane (111); here it is shown that it is actually a rotation twin in which the twin operation is a 180° rotation about any of the three equivalent directions ⟨211⟩, contained in the (111) composition plane. The occurrence of these twins as well as the doubtfulness of the spinel twin in pyrite shows a direct relationship with the structural interpretation based on the pseudo-symmetry of the crystallographic orbits.

Structural tuning of copper sulfide material for sodium-ion batteries.

In this work, single-crystal and twin-crystal copper sulfide materials are constructed in a regulatable and controlled manner. Twin boundaries are engineered into the copper sulphide material to significantly improve its electrochemical performance. The results demonstrate that structure tuning with twin crystals is an effective strategy for enhancing electrochemical reactions, and also sheds light on the design of electrode materials for sodium ion storage.

Crystal Structure Analysis of SH2 Domains in Complex with Phosphotyrosine Peptides.

While the number of tertiary structures solved by cryoelectron microscopy has rapidly increased, X-ray crystallography is still a popular method to determine the tertiary structure of proteins at atomic resolution. However, there are still problems associated with X-ray crystallography, including crystallization and crystal twinning. Indeed, we encountered crystallization and twinning problems in the crystal structure analysis of the SH2 domains complexed with a phosphorylated peptide derived from the oncoprotein CagA. In this chapter, we describe the methods used to overcome these problems. In addition, we provide details of the optimization of the crystallization conditions and cryo-conditions, which are usually not given in published crystal structure analyses.

The origin of twins in the growth of the (100) plane of a β-Ga2O3 crystal using EFG

The twins in the (010) plane of a (100) oriented β-Ga2O3 crystal by EFG. The formation of twin crystals is accompanied by the slip of the (102̄) plane.

Twin suppression effect of dihydroxy-benzene isomers during the secondary growth of b-oriented zeolite MFI nanosheet films

MFI twin crystals on b-axis oriented zeolite MFI nanosheet films were suppressed in the presence of dihydroxybenzene isomers. The resulting films presented an ultra-thin grain thickness of 30–45 nm within hours.

Improved hydrogen storage performance of Sm-Mg composites by adding nano-graphite

Sm5Mg41 + x wt% nano-graphite (NG) (x = 0, 2, 4, 8, 12) composites are synthesized via ball-milling. The influences of NG content on microstructure and hydrogen storage properties of the composites are researched systematically. The composites consist of Mg, Sm5Mg41, SmMg3, and NG. Nano scale Sm3H7 and MgH2 phases are observed after the hydrogen absorbing, and the newly formed Mg phase and the remaining Sm3H7 phase appear after hydrogen desorbing. NG is embedded in the surface of the composites in the form of nanosheets. Nanocrystalline Mg and some microscopic defects, including twin crystal, grain boundaries, and dislocations can be observed in the dehydrogenated sample. Such microscopic defects and nanocrystalline boundaries can reduce the total potential barrier needed to overcome during the hydrogen absorption/desorption reactions and thus the hydrogen storage kinetics of the alloy are improved. For the composites with x = 2, 4, 8 and 12, the activation energies of hydrogen desorption are 132.08, 128.06, 112.90, and 112.35 kJ/mol H2, respectively. The enthalpy changes for the hydrogen absorption are evaluated to be − 79.18, − 78.09, − 76.30, − 76.10 kJ/mol H2, indicating that the addition of NG had no obvious effect on the thermodynamic properties of the composites.

Patterns of crystal organization and calcite twin formation in planktonic, rotaliid, foraminifera shells and spines

The foraminiferal order Rotaliida represents one third of the extant genera of foraminifers. The shells of these organisms are extensively used to decipher characteristics of marine ecosystems and global climate events.It was shown that shell calcite of benthic Rotaliida is twinned. We extend our previous work on microstructure and texture characterization of benthic Rotaliida and investigate shell calcite organization for planktonic rotaliid species. Based on results gained from electron backscattered diffraction (EBSD) and field emission electron microscopy (FESEM) imaging of chemically etched/fixed shell surfaces we show for the planktonic species Globigerinoides sacculifer, Pulleniatina obliquiloculata, Orbulina universa (belonging to the two main planktonic, the globigerinid and globorotaliid, clades): very extensive 60°-{001}-twinning of the calcite and describe a new and specific microstructure for the twinned crystals. We address twin and crystal morphology development from nucleation within a biopolymer template (POS) to outermost shell surfaces. We demonstrate that the calcite of the investigated planktonic Rotaliida forms through competitive growth. We complement the structural knowledge gained on the clade 1 and clade 2 species with EBSD results of Globigerinita glutinata and Candeina nitida shells (clade 3 planktonic species). The latter are significantly less twinned and have a different shell calcite microstructure.We demonstrate that the calcite of all rotaliid species is twinned, however, to different degrees. We discuss for the species of the three planktonic clades characteristics of the twinned calcite and of other systematic misorientations. We address the strong functionalization of foraminiferal calcite and indicate how the twinning affects biocalcite material properties.

Crystal twins: self-supervised learning for crystalline material property prediction

Machine learning (ML) models have been widely successful in the prediction of material properties. However, large labeled datasets required for training accurate ML models are elusive and computationally expensive to generate. Recent advances in Self-Supervised Learning (SSL) frameworks capable of training ML models on unlabeled data mitigate this problem and demonstrate superior performance in computer vision and natural language processing. Drawing inspiration from the developments in SSL, we introduce Crystal Twins (CT): a generic SSL method for crystalline materials property prediction that can leverage large unlabeled datasets. CT adapts a twin Graph Neural Network (GNN) and learns representations by forcing graph latent embeddings of augmented instances obtained from the same crystalline system to be similar. We implement Barlow Twins and SimSiam frameworks in CT. By sharing the pre-trained weights when fine-tuning the GNN for downstream tasks, we significantly improve the performance of GNN on 14 challenging material property prediction benchmarks.

The electrothermal fracture mechanism between aluminum and copper wires

This study investigates the energization and high-temperature damage characteristics of pure aluminum and copper wires of 20 mil (500 μm) wire diameter. Due to the wire drawing annealing process, the base of aluminum wire has an equiaxed grain structure, while the base of copper wire has an equiaxed grain structure containing a twin crystal structure.Normally, in terms of mechanical characteristics, copper wire is inherently stronger than aluminum wire, and both are ductile materials. Electrical experiments found that the voltage of the copper wire showed a stable upward trend with the increase in current; it is worth noting that the resistance of aluminum wire increases abnormally when the current increases continuously, resulting in an overload resistance zone, and the resistance overload zone will cause the wire to appear incandescent.This study discusses the characteristics of the overload resistance zone with various wire lengths. Changing the length of copper wire has no obvious effect on the electrical properties, but decreasing the length of aluminum wire decreases the width of the overload resistance zone. When comparing the energized tensile test and the simulated environmental heat treatment experiment, it can be inferred that the surface temperature of the copper wire is about 900 °C when energized, whereas the surface temperature of aluminum wire is about 600 °C when energized.The tensile mechanical properties of the aluminum wire at different energization stages show that the incandescent phenomenon in the overload resistance zone will deteriorate the wire's mechanical properties. It is confirmed that the incandescent phenomenon is dominated by the micro-AlNi eutectic cladding structure in the aluminum wire, and the eutectic phase is formed in overload resistance zone. These eutectic liquid spherical areas will reduce the ductility (embrittlement) of the wire when energized and stretched, and the eutectic liquid spherical phase will then produce holes after solidification, which will affect the aluminum wire's reliability. Incorporating a trace amount of nickel can improve the strength and corrosion resistance in aluminum wire, but the voltage and current need to be controlled during application to avoid the occurrence of incandescence or affecting the electrical and thermal stability.

TEM observation of nano-twinned precipitation phase Al3Mg2

Friction stir welding (FSW) is a solid-phase bonding technique, which is suitable for bonding aluminium/magnesium dissimilar metals. In the present work, a nano-precipitated phase in 1060 Al/AZ31B Mg FSW joint was observed and analysed by transmission electron microscopy (TEM) and high-resolution electron microscopy (HRTEM). The type and morphology of the precipitated phase were determined. The results showed a nano-twinned precipitation phase, Al3Mg2, which was elliptical in shape and approximately 300 nm in size. In addition, the effective stacking fault energy (SFE) of nano-twinned Al3Mg2, approximately 3.51 mJ m−2, was calculated using the first principles and the effective SFE equation. A low effective SFE was beneficial to the crystal twinning of Al3Mg2.

Mechanical behaviors regulation of triply periodic minimal surface structures with crystal twinning

Triply periodic minimal surface (TPMS) structures have been realized as excellent mechanical materials with high specific strength, energy absorption, and unique layered deformation mechanism due to their saddle shape-surface with non-positive Gaussian curvature. However, the performance of TPMS structures is limited by anisotropic mechanical behavior owing to the oblique shear band with stress concentration when the load beyond the yield stress, resulting in structural catastrophic failure. Herein, a strategy is proposed to manipulating the path of stress transfer by introducing crystal twinning to achieve a designable deformation behavior of the TPMS structures. Various contact reflection twin boundaries for the gyroid (G) and diamond (D) surface structures have been introduced by connecting the ideal structures by mirror-symmetry while maintaining the structural integrity. Compression tests were carried out from various directions of perfect and twinned- G and D surface scaffolds after 3D printing. It was found that the twin boundary can effectively protect the structure from catastrophic failure by deflecting the cracks under compressive loads, and regulate the deformation behavior by various structural design. This study provides new insights in applying the microscopic crystal defects to macroscopic architectural materials, which also contributes to the understanding of these unique microscopic structures. Graphical abstract • Inspired by crystal twinning, deformation programmable TPMS structures were constructed. • Effects of the number, location and orientation of twin boundaries on TPMS structures were investigated. • FE simulations reveal that twin boundaries effectively manipulate the direction and path of stress transfer. • This method can be extended to other type of crystal defects and may yield different phenomena.

A high-withdrawing-rate method to control the orientation of (γ+α2) lamellar structure in a β-solidifying γ-TiAl-based alloy

β-solidifying γ-TiAl-based alloys with well-controlled lamellar orientation possess excellent mechanical properties, and low withdrawing rates are usually used to control the lamellar orientation by directional solidification. Herein, a high-withdrawing-rate method to control the lamellar orientation in a β-solidifying γ-TiAl-based alloy, TNM alloy (Ti-43.5Al–4Nb–1Mo-0.1B at.%), is proposed by Bridgeman directional solidification. Using this method, polysynthetic twinned (PST) single crystal of TNM alloy with a good lamellar controlling result is obtained. The mechanism of lamellar orientation controlling lies in the process of solidification and β/α transformation, which are governed by thermal stabilization treatment and withdrawing rate. Thermal stabilization treatment for 60 min can make the sample maintain stable interfaces of liquid/solid and β/α phase with a high temperature gradient before the withdrawing process starts, and it also leads to the incline of β dendrites; a high withdrawing rate of 100 μm/s can accomplish the grain selection of α phase during β/α transformation and make complete peritectic reaction occur. Compared with as-cast one, PST single crystal specimen has its room-temperature tensile property enhanced greatly, and presents an ultimate tensile strength/strain of 476 MPa/1.75% with a trans-lamellar fracture morphology.

Slip and twinning during deformation of CdWO4 single crystals

Cadmium tungstate, CdWO4, is widely used as a material for scintillating detectors in medicine, prevailingly, in computer tomography, as well as in high energy physics. Many studies consider luminescent properties of these crystals. However, there are few publications concerning structural defects, which can influence practically important properties of cadmium tungstate. In this work strain induced twinning and dislocation slip of CdWO4 single crystals are investigated.

Synergistic optimization of electrical-thermal properties of dual vacancy Bi1−x-yPbyCu1−xSeO by improving mobility and reducing lattice thermal conductivity

We fabricate Bi1−x-yPbyCu1−xSeO (x = 0, 0.03, 0.06, y = 0, 0.10) samples via 4 min-microwave synthesis combined with 5 min-spark plasma sintering. The phase composition, microstructure, valence, and electrical and thermal transport properties of the samples are investigated at 298–873 K. Pb doping provides impurity carriers and increases the concentration to 0.9–3.0 × 1020 cm-³ . Bi and Cu vacancy could provide a carrier transport channel to reduce carrier scattering probability, leading to improved mobility. Twin crystals, stacking faults, and grain boundary segregation are observed in Bi0.87Pb0.10Cu0.97SeO on scanning transmission electron microscopy. Bi and Cu vacancy increase the sample point defects in Pb-doped or undoped samples which results in a decrease in lattice thermal conductivity. The lattice thermal conductivity of Bi0.87Pb0.10Cu0.97SeO is decreased to an extremely low value of 0.13 Wm−1 K−1 and a maximum ZT value of 1.09 is achieved at 873 K.

Morphological and Surface Microtopographic Features of HPHT-Grown Diamond Crystals with Contact Twinning

Gem-grade twinned high-pressure high-temperature (HPHT) synthetic diamond crystals are rare. Hence, few investigations on their morphological features and formation have been reported. In this article, the morphological and surface microtopographic features of HPHT synthetic-diamond crystals contact twinning is detailed and investigated. It indicates that twins of diamond forming and nucleating during the early stages of the growth and the development of {100} and {111} growth sectors on either side of such boundaries proceeds independently, which affects the final morphology of the diamond crystals. According to the different features of crystal macroscopic morphological properties, two kinds of twin model have been established. The formation of twin crystals changed the lattice of diamonds with face-centered cubic dimensions. The type of diamond lattice at the twin boundary is hexagonal and closely packed, which has potential for further developing the application of synthetic diamond twin crystals.

Effects of crystal orientation and twin boundary distance on mechanical properties of FeNiCrCoCu high-entropy alloy under nanoindentation

The effect of crystal orientation and twinning on the mechanical characteristics and microstructure development of the equiatomic FeNiCrCoCu high-entropy alloy is explored and quantified by nanoindentation using molecular dynamics. The substrate hardness and discontinuity in the loading curves are strongly influenced by various crystal orientations due to differences in microstructural evolution and dissimilar dislocation events. The Hall-Petch aspect representing the correlation between the twinning distance and the material strength is found with decreasing the twin boundary distance. The plastic deformation exposes that the propagation of local stress and shear strain into the substrate bulk is dependent on crystal orientation, characterized through the various directions of the slipping bands. Meanwhile, the growth of shear bands into the substrate is easier with the great twin boundary distance, which can enhance the ductility of the alloy. The microstructural evolution suggests that the sliding prismatic dislocation loops in the substrate play a significant role in deformation. The displacement of the dislocation loops into the substrate depends on the crystal orientation, and their propagation is inhibited by the twin boundary. The various surface morphologies of the substrate are dominated by the sliding mechanism of the atoms following the symmetry of <11‾0 >{111} primary sliding system. Besides, the pile-up height tends to increase with the decrease of the twinning distance because the interaction between the sliding system and the twinning occurs more for the small twin boundary distance.

Growth twins and deformation twins of sodalite-type microporous compounds

The tendency of sodalite-group minerals and synthetic materials with the sodalite-type structure to form growth twins has long been known. Judging by faceting, the twin components are connected by a 180-degree turn around the [111] axis of the cubic lattice. In this work, the structure of growth twins, including simple prismatic twins elongated in the [111] direction and V-shaped twins, has been studied in detail at the atomic level. It is shown for elongated twins that the boundary between the twin components passes in the (111) plane through the common vertices of TO4 tetrahedra (T = Al, Si). The coordinates of these vertices are connected by pseudosymmetry operators, each of which can be a twinning operator. This can be either a 180-degree turn around the [111] axis or a reflection in any of the planes (2‾ 11), (12‾ 1), or (112‾). Twinning by a plane is accompanied by smaller distortions of the motif at the twin boundary and, therefore, is energetically more favorable. V-shaped twins are regular intergrowths of two simple twins, the prism axes of which are directed along different ⟨111⟩ directions of the cubic lattice. In addition to growth twins, deformation twins of sodalite and related materials are discussed. Their symmetry is usually lower than cubic, and twinning is often combined with modulation of the crystal structure or the formation of a superstructure. Crystal twinning can be a consequence of a phase transition from a high-temperature cubic phase with a symmetry lowering. So far, no examples are known when growth twinning is combined with deformation twinning, but such a possibility is not ruled out.

Theoretical Diffraction Pattern Characteristics of Cubic Twin Crystal

Twin crystal is a common lattice arrangement of crystal structure. The commonly used characterization methods for crystal structure include transmission electron diffraction (TED) pattern and electron backscatter diffraction (EBSD) pattern, but there is currently no specific reference standard for twin crystal. In this research, the mathematical relationship between crystal structure and TED and EBSD patterns of twin was calculated. The characteristics of twin electron diffraction spectrum, as well as the calibration of TED and EBSD patterns, were discussed and analyzed towards the examples of face-centered cubic crystal and body-centered cubic crystal. Overall, our results establish a theoretical calculation standard of diffraction spectrum, which provides a reference for further explorations to characterization of twin crystal structures.

Formation of contact and multiple cyclic cassiterite twins in SnO2-based ceramics co-doped with cobalt and niobium oxides.

Contact and multiple cyclic twins of cassiterite commonly form in SnO2-based ceramics when SnO2 is sintered with small additions of cobalt and niobium oxides (dual doping). In this work, it is shown that the formation of twins is a two-stage process that starts with epitaxial growth of SnO2 on CoNb2O6 and Co4Nb2O9 seeds (twin nucleation stage) and continues with the fast growth of (101) twin contacts (twin growth stage). Both secondary phases form below the temperature of enhanced densification and SnO2 grain growth; CoNb2O6 forms at ∼700°C and Co4Nb2O9 at ∼900°C. They are structurally related to the rutile-type cassiterite and can thus trigger oriented (epitaxial) growth (local recrystallization) of SnO2 domains in different orientations on a single seed particle. While oriented growth of cassiterite on columbite-type CoNb2O6 grains can only result in the formation of contact twins, the Co4Nb2O9 grains with a structure comparable with that of corundum represent suitable sites for the nucleation of contact and multiple cyclic twins with coplanar or alternating morphology. The twin nucleation stage is followed by fast densification accompanied by significant SnO2 grain growth above 1300°C. The twin nuclei coarsen to large twinned grains as a result of the preferential and fast growth of the low-energy (101) twin contacts. The solid-state diffusion processes during densification and SnO2 grain growth are controlled by the formation of point defects and result in the dissolution of the twin nuclei and the incorporation of Nb5+ and Co2+ ions into the SnO2 matrix in the form of a solid solution. In this process, the twin nuclei are erased and their role in the formation of twins is shown only by irregular segregation of Co and Nb to the twin boundaries and inside the cassiterite grains, and Co,Nb-enrichment in the cyclic twin cores.

Evolution of Cu-rich phase in Al-modified ferrite stainless steel during short-term ageing

In this article, scanning electron microscopy, energy dispersive spectroscopy, and transmission electron microscopy were used to characterize the precipitation behaviour of the Cu-rich phase in Al-free and 1Al Cu-bearing ferrite stainless steels; and further, the effects of Al addition on the precipitation and coarsening of the Cu-rich phase and its mechanism were analysed. The results show that Al increases the nucleation rate of the Cu-rich phase, and refines Cu-rich nanoprecipitates after ageing for 1 min. The Cu-rich phase with four crystal structures-BCC, 9R, twin FCC, and non-twin FCC were found in the Al-free sample during ageing. However, only Cu-rich phase with BCC and BCT structures were observed after Al addition. There are high-density edge dislocations in Al-containing Cu-rich nanoprecipitates. These dislocations can be used as a diffusion channel for solute atoms, increasing the coarsening rate of the Cu-rich phase. However, with the extension of the ageing time, dislocations in the Cu-rich phase will be annihilated owing to the thermal activation and the decrease of the Al concentration in the Cu-rich phase, which will lead to a decrease in the dislocation density in Al-containing Cu-rich phase. Finally, the promotion effect of Al on the coarsening of the Cu-rich phase is weakened.