Twin Lattice Research Articles

Conformal growth of GaP on high aspect ratio Si structured surface via plasma-enhanced atomic layer deposition

This article is concerned with the study of conformal growth of thin gallium phosphide (GaP) layers by plasma-enhanced atomic layer deposition (PE-ALD) on a Si structured surface. GaP layers were deposited on (100) silicon wafers with a structured surface in the form of silicon microcolumns and black silicon. Deposition process were carried out at temperature of 380 °C using trimethylgallium and phosphine precursors. According to transmission electron microscopy (TEM), GaP layers deposited by this method conformally grown on the high aspect ratio structured silicon surface. The energy-dispersive X-ray spectroscopy (EDX) study show that the distribution of the main components of the GaP layer is uniform alongside surface of the silicon microcolumns and wires. High resolution TEM study confirms that GaP layers on the surface of silicon microcolumns and wires are epitaxial with the inclusion of twin lattice defects. Therefore, this deposition method can be used for the conformal deposition of thin crystalline layers of GaP on structured silicon surfaces with a high aspect ratio.

Numerical Investigation of Hypersonic Flat-Plate Boundary Layer Transition Subjected to Bi-Frequency Synthetic Jet

Transition delaying is of great importance for the drag and heat flux reduction of hypersonic flight vehicles. The first mode, with low frequency, and the second mode, with high frequency, exist simultaneously during the transition through the hypersonic boundary layer. This paper proposes a novel bi-frequency synthetic jet to suppress low- and high-frequency disturbances at the same time. Orthogonal table and variance analyses were used to compare the control effects of jets with different positions (USJ or DSJ), low frequencies (f1), high frequencies (f2), and amplitudes (a). Linear stability analysis results show that, in terms of the growth rate varying with the frequency of disturbance, an upstream synthetic jet (USJ) with a specific frequency and amplitude can hinder the growth of both the first and second modes, thereby delaying the transition. On the other hand, a downstream synthetic jet (DSJ), regardless of other parameters, increases flow instability and accelerates the transition, with higher frequencies and amplitudes resulting in greater growth rates for both modes. Low frequencies had a significant effect on the first mode, but a weak effect on the second mode, whereas high frequencies demonstrated a favorable impact on both the first and second modes. In terms of the growth rate varying with the spanwise wave number, the control rule of the same parameter under different spanwise wave numbers was different, resulting in a complex pattern. In order to obtain the optimal delay effect upon transition and improve the stability of the flow, the parameters of the bi-synthetic jet should be selected as follows: position it upstream, with f1 = 3.56 kHz, f2 = 89.9 kHz, a = 0.009, so that the maximum growth rate of the first mode is reduced by 9.06% and that of the second mode is reduced by 1.28% compared with the uncontrolled state, where flow field analysis revealed a weakening of the twin lattice structure of pressure pulsation.

Unusual F3 stacking fault in magnesium

An unusual F3 basal stacking fault resulting from twin-dislocation interaction in magnesium is observed in molecular dynamics simulation. The F3 fault is produced in the twin lattice from the interaction between a migrating (101¯2) twin boundary and a partial dislocation of either a prismatic 〈c〉 edge, or a prismatic 〈c+a〉 mixed dislocation in the matrix. The condition is that the partial dislocation needs to have a negative sign and lie on a plane intersecting a compression site of the twin boundary. The F3 fault can also be generated when a positive basal 〈a〉 mixed dislocation in the twin lattice, with slip plane intersecting a compression site of the twin boundary, interacts with a basal-prismatic twinning disconnection. The F3 fault comprises two I1 faults that have the same character but are separated by two basal layers. It has one end connected to the twin boundary, and the other end bounded by a lattice defect with a Burgers vector identical to that of a 30° Shockley partial dislocation. The formation frequency of the F3 fault is higher at a lower shear stress (below ∼400 MPa) and/or a lower temperature (100 K and 200 K). The F3 fault can decompose into a glissile 30° Shockley and a T2 fault at a temperature above ∼400 K. The relationships between the F3 fault and other types of basal stacking faults such as I2, T2 or paired I1 faults that are separated by multiple basal layers are discussed.

Unveiling the planar deformation mechanisms for improved formability in pre-twinned AZ31 Mg alloy sheet at warm temperature

To investigate the role of pre-twins in Mg alloy sheets during warm planar deformation, the stretch forming is conducted at 200 °C. Results suggest the formability of the pre-twinned AZ31 Mg alloy sheet is enhanced to 11.30 mm. The mechanisms for the improved formability and the deformation behaviors during the planar stretch forming are systematically investigated based on the planar stress states. The Schmid factor for deformation mechanisms are calculated, the results reveal that planar stress states extremely affect the Schmid factor for {10-12} twinning. The detwinning is activated and the prismatic slip is enhanced in the pre-twinned sheet, especially under the planar extension stress state in the outer region. Consequently, the thickness-direction strain is accommodated better. The dynamic recrystallization (DRX) type is continuous DRX (CDRX) regardless of the planar stress state. However, the CDRX degree is greater under the planar extension stress state. Some twin lattices deviate from the perfect {10-12} twinning relation due to the planar compression stress state and the CDRX. The basal texture is weakened when the planar stress state tends to change the texture components.

Accurate intensity integration in the twinned γ-form of o-nitroaniline

o-Nitroaniline, C6H6N2O3, is known to be polymorphic. The α-form is probably amorphous, while the β- and γ-forms are crystalline. Difficulties with the unit-cell determination of the γ-form were reported as a consequence of twinning. In this paper, newly recorded diffraction data of the γ-form of o-nitroaniline are described that were processed taking into account the two twin lattices. Data were partly deconvoluted and much better agreement was obtained in terms of R 1 values and C—C bond precision. The availability of raw data and proper reprocessing using twin lattices is by far superior to efforts to de-twin processed structure factors.

Interaction between migrating [formula omitted] twin boundary and extended 〈 a 〉 dislocations inside twin in pure Mg

Interaction between a matrix prismatic 〈 c 〉 edge dislocation and a migrating (101¯2) twin boundary results in extended basal 〈 a 〉 mixed dislocations in the twin lattice. The interaction between the resultant 〈 a 〉 dislocation and the migrating twin boundary is studied using molecular dynamics simulations. The 〈 a 〉 dislocation tends to detach from or connect to the twin boundary, when its slip plane intersects a compression or extension site of the twin boundary. When the 〈 a 〉 dislocation connects to the twin boundary, connection always occurs with a 90° Shockley partial and a basal-prismatic (BP) or prismatic-basal (PB) two-atomic-layer-height step, depending on whether the Burgers vector direction of the Shockley partial points away from or towards the twin boundary. The BP or PB two-atomic-layer-height step is sessile, in contrast to those that are not connected to such 〈 a 〉 dislocation.

Asymmetric [formula omitted] twin boundary and migration mechanism in hexagonal close-packed titanium

Classical twinning theory predicted that a simple shear was able to create a perfect, twinned structure without the need of atomic shuffles for (112¯1)[112¯6¯] twinning mode in hexagonal close-packed (HCP) metals, and the elementary twinning dislocation should be a two-layer zonal. However, it was revealed in the literature that this particular twinning mode could not have mirror symmetry between the parent and twin, and shuffles should be involved during twin boundary (TB) migration. These conflicting reports indicate that the (112¯1)[112¯6¯] twinning mechanism has not been completely resolved, and what configuration of twinning dislocation mediates twin growth is not well understood either. In this work, atomistic simulation is conducted to further understand the twinning mechanism in titanium. Novel structural analyses are performed to reveal how the parent lattice is transformed to the twin lattice. The results show that the mirror symmetry of a perfect, unrelaxed TB breaks down after relaxation, because of the very high repulsive force between the atom pairs on mirrored positions with a spacing less than half of the lattice constant. As a result, the stacking sequence of the twin differs from that of the parent after relaxation. During twin growth, each {11¯00} prismatic plane of the parent splits into two layers and reorganize into the prismatic planes of the twin, such that the twin prismatic are no longer aligned with the parent prismatic and the mirror symmetry breaks down. This process is accomplished by atoms shuffling in the opposite direction that is perpendicular to the twinning shear. The atomic shuffles spread over multiple consecutive twinning planes to minimize the shuffling magnitude on each twinning plane. Thus, the actual TB is no longer a single twinning plane, but a multi-layer interface zone. Careful examination shows that no well-defined twinning dislocation core can be identified, although single-layer steps can still be observed.

Twinning, Superstructure and Chemical Ordering in Spryite, Ag8(As3+0.50As5+0.50)S6, at Ultra-Low Temperature: An X-Ray Single-Crystal Study

Spryite (Ag7.98Cu0.05)Σ=8.03(As5+0.31Ge0.36As3+0.31Fe3+0.02)Σ=1.00S5.97, and ideally Ag8(As3+0.5As5+0.5)S6, is a new mineral recently described from the Uchucchacua polymetallic deposit, Oyon district, Catajambo, Lima Department, Peru. Its room temperature structure exhibits an orthorhombic symmetry, space group Pna21, with lattice parameters a = 14.984(4), b = 7.474(1), c = 10.571(2) Å, V = 1083.9(4) Å3, Z = 4, and shows the coexistence of As3+ and As5+ distributed in a disordered fashion in a unique mixed position. To analyze the crystal-chemical behaviour of the arsenic distribution at ultra-low temperatures, a structural study was carried out at 30 K by means of in situ single-crystal X-ray diffraction data (helium-cryostat) on the same sample previously characterized from a chemical and structural point of view. At 30 K, spryite still crystallizes with orthorhombic symmetry, space group Pna21, but gives rise to a a × 3b × c superstructure, with a = 14.866(2), b = 22.240(4), c = 10.394(1) Å, V = 3436.5(8) Å3 and Z = 4 (Ag24As3+As5+Ge4+S18 stoichiometry). The twin laws making the twin lattice simulating a perfect hexagonal symmetry have been taken into account and the crystal structure has been solved and refined. The refinement of the structure leads to a residual factor R = 0.0329 for 4070 independent observed reflections [with Fo > 4σ(Fo)] and 408 variables. The threefold superstructure arises from the ordering of As3+ and (As5+, Ge4+) in different crystal-chemical environments.

{101¯2} twinning nucleation in magnesium assisted by alternative sweeping of partial dislocations via an intermediate precursor

Abstract Molecular dynamic simulation and transmission electron microscopy (TEM) characterization was employed to investigate the {10 1 ¯ 2 } twinning mechanism in magnesium. A partial dislocation assisted twinning nucleation mechanism was proposed based on simulation results, in which the twin lattice was reconstructed from parental matrix by two-step sweeping of partial dislocations on different close packed planes from matrix and the subsequently formed twin precursor respectively. A { 10 1 ¯ 2 } twin precursor was observed adjacent to matrix/twin interface by a spherical aberration corrected TEM, which indicated the hexagonal-close-packed (hcp) matrix→quasi face-centered cubic (fcc) twinning precursor→h.c.p twin transformation sequence during {10 1 ¯ 2 } twinning process.

Synthesis, revised crystal structures, and refractive indices of ABW-type CsMTiO4 (M = Al, Fe, Ga) and ANA-type CsTi1.10Si1.90O6.50, and the determination of the electronic polarizability of 4-coordinated Ti4+

Abstract Single crystals of ABW-type CsAlTiO4 (CAT), CsFeTiO4 (CFT), CsGaTiO4 (CGT), and ANA-type CsTi1.1Si1.9O6.5 (CST) were grown and characterized by electron microprobe analyses, single-crystal X-ray diffraction, thermal analyses, and spindle-stage optical investigations to determine the electronic polarizability of 4-coordinated Ti4+, α([4]Ti4+). The crystal structure of CAT was confirmed to crystallize in the highest possible topological symmetry Imma (a = 8.9677(2) Å, b = 5.7322(1) Å, c = 9.9612(3) Å) with tetrahedrally coordinated Al and Ti equally distributed on Wyckoff position 8i. Twinning by reticular merohedry with a twin index of 2 was observed for most of the crystals resulting in a hexagonal twin lattice (a = 11.487(3) Å, c = 8.968(2) Å) with Laue symmetry 6/mmm. Refractive indices measured by immersion methods on an untwinned specimen are n x = 1.716(5), n y = 1.725(2), and n z = 1.727(1) with 2V z = 127.1(6)°. The diffraction patterns of CFT and CGT clearly showed superstructure reflections causing a symmetry lowering of index 4 with a transformation according to 2a, b, c from Imma to Pmab with a = 18.3054(7) Å, b = 5.8083(2) Å, c = 9.9938(4) Å for CFT, and a = 18.2921(6) Å, b = 5.7636(2) Å, c = 9.9210(3) Å for CGT. Refractive indices for CGT are n x = 1.750(3), n y = 1.772(3), and n z = 1.776(2) with 2V z = 132(1)°. The crystal structure of the ANA-type CsTi1.1Si1.9O6.5 was confirmed to crystallize in space group Ia 3 ¯ $\overline{3}$ d (a = 13.8333(4) Å). The extra 0.5 O atoms are needed for charge compensation and to allow the sum of electronic polarizabilities to give a total electronic polarizability calculated from the refractive index n = 1.718(4). The electronic polarizability of [4]Ti4+ was calculated from the difference between the observed total polarizabilities (derived from the mean refractive indices of CAT and CGT) and the sum of electronic polarizabilities of cations and anions omitting the polarizability of Ti resulting in α([4]Ti4+) = 5.15(5) Å3.

Zero-obliquity twin lattice quasi-symmetry threefold twinning in 1-{(R)-1-[(3-oxo-2-isoindolinoyl)methyl]-2-propenyl}-5-methyl-2,3-indolinedione.

Threefold twinning in 1-{(R)-1-[(3-oxo-2-isoindolinoyl)methyl]-2-propenyl}-5-methyl-2,3-indolinedione, C21H16N2O4, has been reported recently [Trost et al. (2020). Org. Lett. 22, 2584-2589] but the twin characterization was not published. This twinning presents several interesting features. The crystal structure is monoclinic, but its lattice is metrically strongly pseudo-orthorhombic and underpins a strongly pseudo-hexagonal sublattice. Several possible twin laws are compatible with these metric specializations, among which the one found experimentally corresponds to a trichromatic point group. Twinning is by reticular pseudo-merohedry with twin index 2 and zero obliquity but a non-zero twin misfit. The twin lattice coincides with the pseudo-hexagonal sublattice of the individual domain, which justifies the adoption of the unconventional setting B21 of the space group.

Nanostructural aspects of the wear process of multilayer tribological coatings

Ex situ and in situ scanning and transmission electron microscopy studies (SEM and TEM respectively) of the wear mechanisms in Ti/TiN with silver nanoparticles were performed herein. Two types of wear mechanisms occur the in multilayer coatings that consist of the layer-by-layer removal of the coating and deformation localization through the formation of shear bands. The shifting of the layers in the shear band area is equal to the layer repetition period, where a period consists of one metallic and one ceramic layer. It is confirmed that the angle in between the c-axis of the parent lattice and twin lattice is approximately 35°, which indicates the operation of the tensile twining mode T2 in the individual Ti crystallites.

Another step toward the solution of the real structure of zinkenite

Abstract The crystal structure of Cu-bearing zinkenite from Saint-Pons (Alpes Maritimes department, France), having idealized chemical composition Cu0.7Pb9.7Sb21.3S42, has been studied. It has a pronounced hexagonal sub-cell, with unit-cell parameters a=22.1219(11), c=4.3207(3) Å, V=1831.2(2) Å3, space group P63, Z=1. The sub-cell crystal structure was refined to R 1=0.072 on the basis of 3905 reflections with F o>4σ(F o) and 133 refined parameters. It can be described as formed by one kind of rod, with walls of columns of (Pb/Sb)-centered polyhedra flanking both the rods and the sites located along the 63 screw axis. Minor Cu is hosted in the tetrahedral voids between the rods and the walls of polyhedra. Alternatively, the crystal structure of zinkenite can be described as formed by trigonal rods, delimited by lone electron-pair micelles, and tunnels hosting (Pb/Sb) atoms. The occurrence of weak superstructure reflections points to a triclinic unit cell with parameters a=38.271(2), b=22.1219(13), c=8.6475(5) Å, α=89.931(3), β=90.030(3), γ=89.957(3)°, V=7323.6(7) Å3, space group P1, Z=4. The twin laws making the twin lattice hexagonal have been taken into account and the crystal structure has been solved and refined. Notwithstanding the very low R 1 value (R 1=0.038 on the basis of 22563 reflections with F o>4σ(F o) and 1194 refined parameters), several shortcomings, mainly due to the low diffraction quality of the available crystals, allow only the description of the main structural features of the superstructure of zinkenite, indicating the correctness of the triclinic model hypothesized by previous authors.

Improved twin detection via tracking of individual Kikuchi band intensity of EBSD patterns

Twin detection via EBSD can be particularly challenging due to the fine scale of certain twin types – for example, compression and double twins in Mg. Even when a grid of sufficient resolution is chosen to ensure scan points within the twins, the interaction volume of the electron beam often encapsulates a region that contains both the parent grain and the twin, confusing the twin identification process. The degradation of the EBSD pattern results in a lower image quality metric, which has long been used to imply potential twins. However, not all bands within the pattern are degraded equally. This paper exploits the fact that parent and twin lattices share common planes that lead to the quality of the associated bands not degrading; i.e. common planes that exist in both grains lead to bands of consistent intensity for scan points adjacent to twin boundaries. Hence, twin boundaries in a microstructure can be recognized, even when they are associated with thin twins. Proof of concept was performed on known twins in Inconel 600, Tantalum, and Magnesium AZ31. This method was then used to search for undetected twins in a Mg AZ31 structure, revealing nearly double the number of twins compared with those initially detected by standard procedures.

Twinning in olivine group revisited

The reported occurrence frequency of twinning in olivine is at odd with the degree of atomic restoration across the composition surface obtained by the analysis of the pseudo-eigensymmetry of the crystallographic orbits: the {0 3 1} twin should be the most frequent one, whereas it is reported as the rarer. The possibility of incorrect indexing, already raised in previous reports, is considered highly probable due to the fact the {0 1 1} and {0 3 1} twins have exactly the same twin lattice and differ for the plane acting as twin element. The rarity of {0 1 2} twinning is explained by the small degree of atomic restoration and the large separation between the planes that produce that restoration. A fourth twin law, reported as (1 0 0), is impossible unless the structure is merohedral and is probably due to errors in the indexing of crystal faces in the old morphological studies.

Unravelling the diffraction pattern of a twin. I. Fundamentals

Abstract Unravelling the diffraction pattern of a twin consists in assigning each observed intensity to the individuals they come from. The unravel process consists in (1) recognizing the presence of twinning; (2) identifying the twin operations; (3) re-indexing the diffraction pattern of the twin. In case of twin lattice quasi symmetry (TLQS) twinning, the presence of twinning appears in the splitting of the diffractions. For twin lattice symmetry (TLS) twinning, instead, the presence of twinning can be overlooked, but the large fraction of unobserved diffractions and the unusual and complex non-space group absences should draw the investigator’s attention to the possible presence of twinning. Some general criteria to identify the presence of twinning are presented and discussed with examples taken from the literature.

Geometrically necessary twins in bending of a magnesium alloy

Evidence for the formation of geometrically necessary twins (GNTs), or twins that accommodate a strain gradient in a multi-axial stress state, in quasi-static, room temperature three-point bending of a rolled magnesium alloy is presented. Electron backscatter diffraction analysis showed that {101¯2}<101¯1¯> extension twins (rather than {101¯1}<101¯2¯> contraction twins) form in arcs in the tension zone, and that twinned grains have very low Schmid factors. The main tensile stress component in the tension zone was nearly perpendicular to the c-axis of the parent grains. The mechanism for such unusual twinning behavior was analyzed from the perspective of strain components that are generated by {101¯2}<101¯1¯> twinning. After twinning, an extension strain component along the c-axis and a contraction strain component perpendicular to the c-axis of the parent lattice are generated simultaneously due to the misfit between the parent and the twin lattice. The contraction strain component by twinning provided an extra strain accommodation for the compressive strain in the tension zone produced by the bending, despite the fact that the local stress state strongly disfavored the {101¯2}<101¯1¯> twinning. Thus, the {101¯2}<101¯1¯> twins in the arcs in the tension zone of the bent specimen present the characteristic of being geometrically necessary, similar to geometrically necessary dislocations and boundaries.

Structural rationale for the occurrence of the elbow twins in cassiterite and rutile

Elbow twins are a distinct feature of cassiterite and rutile. Their occurrence is justified by the reticular theory of twinning because of the existence of a common sublattice- the twin lattice - formed by the overlap of a large fraction of the lattice nodes of the individuals. Yet, the atomic basis for the existence of twins falls outside the possibilities of the reticular theory. We present here the analysis of the pseudo-eigensymmetry of the crystallographic orbits building the structure of the elbow twins and show that their occurrence is justified by the complete restoration of the structure near the composition surface, with a deviation of less than 0.7 angstrom.

Global strain generated by shuffling-dominated [formula omitted] twinning

Recent experimental observations have revealed that {101¯2}<101¯1¯> twinning involves zero shear, confirming this twinning mode is dominated by shuffling rather than shear. This raises the question: how is global strain generated by shuffling in hexagonal close-packed metals without involving shear? This paper addresses this important question. We demonstrate that although shuffling does not contribute to the twinning shear, it generates a normal strain along the c-axis due to the misfit between the parent and the twin lattice.

The candidacy of shuffle and shear during compound twinning in hexagonal close-packed structures

This paper proposes a systematic generalized formulation for calculating both atomic shuffling and shear candidates for a given compound twinning mode in hexagonal closed-packed metals. Although shuffles play an important role in the mobility of twinning dislocations in non-symmorphic crystals, their analytical expressions have not been previously derived. The method is illustrated for both flat planes and corrugated planes which are exemplified by {112¯2} and {101¯2} twinning modes, respectively. The method distinguishes between shuffle displacements and net shuffles. While shuffle displacements correspond to movements between ideal atom positions in the parent and twin lattices, net shuffles comprise contributions from shear on overlying planes which can operate along opposite directions to those of shuffle displacements. Thus, net shuffles in the twinning direction can vanish in a limiting case, as is interestingly the case for those needed in the second plane by the b4 dislocation candidate in {112¯2} twinning. It is found that while shuffle displacement vectors can be irrational when K1 is corrugated, net shuffle vectors are always rational.