Chemical Twinning Research Articles

Composition-dependent photoluminescence in nanocrystalline La2Hf2-xZrxO7:Eu phosphor: role of chemical twin Zr/Hf environments around a luminescent center.

Based on chemical intuition, linear trends are anticipated in Eu3+ photoluminescence performance inside a pyrochlore matrix of the chemical twins, Hf and Zr, owing to probable geometrical and chemical similarity around the luminescent center. The present work reports the drastically fluctuating result of doping Eu3+ in nanocrystalline pyrochlore, La2Hf2-xZrxO7 (LHZO), matrix on composition variation; the variation is counter to the anticipation-based chemical brotherhood of Hf and Zr. Zirconium-enriched samples of LHZO improve asymmetry around Eu3+ ion leading to enhanced photoluminescence quantum yield (PLQY). The samples with compositions 0.7Hf and 1.3Zr depict the lowest non-radiative channels with the highest theoretically calculated PLQY of ∼71% and excellent thermal stability (∼91%). Synergistic experimental and theoretical analysis reveals that Eu does not unbiasedly occupy La-sites in the pyrochlore LHZO matrix towards chemical twins of Hf and Zr; rather, it energetically prefers to occupy Zr-rich vicinal sites. When the composition with Zr is in the low-medium range, Eu has a higher probability of occupying Zr-rich vicinal sites depicting higher lifetime and PLQY. When Zr-content goes beyond 70-80%, the other site occupancies start contributing leading to a reduction in both lifetime and quantum yield. This work paves a great strategy and provides a futuristic potential to utilize europium luminescence in separating chemically close Hf-Zr for various technological applications.

Development of hybrid surrogate model structures for design and optimization of CO2 capture processes: Part I. Vacuum pressure swing adsorption in a confined space

Design and optimization of CO2 capture processes have become a tremendously active area of research particularly in the past decade. In this context, development of intelligent techniques on the basis of first-principles models coupled with data-driven algorithms for such purpose looks very promising. In a series of works, we intend to present mechanism approaches in order to develop applicable structures for design and optimization of the CO2 capture processes of interest via hybrid models. A systematic method is presented for optimizing a process for capturing CO2 from a confined space through a Vacuum Pressure Swing Adsorption (VPSA) operation using a Hybrid Surrogate Model (HSM) and Non-dominated Sorting Genetic Algorithm (NSGA-III). The surrogate model is structured based on the VPSA process model offered in the Aspen Adsorption® environment and an artificial intelligence (AI) data-driven algorithm. The developed HSM is then used to predict the key process outputs including CO2 purity, air recovery ratio and energy consumption rate. Accordingly, the optimized parameters are re-substituted into the VPSA process simulator for further data processing. It is demonstrated that the proposed model architecture provides considerable computational efficiency for the process optimization with only 48 h to complete the corresponding evolutionary search, while the optimization time by the conventional NSGA-direct method is close to 1129 h. The optimization results also show that the CO2 purity changes from 1000 ppm to 399 ppm, the air recovery ratio remains at 93 %, and the energy consumption per unit product (ECP) decreases by 38.5 % to 99.7 kJ·Nm−3 air after an optimized air purification operation. The idea of chemical mechanism and industrial data twin modeling in this study holds substantial importance for the development of digital chemical twin systems and the process optimization of intelligent factory.

Fe- and B-Chains in the Ti5-xFe1-yOs6+x+yB6 Structure Type Derived from Chemical Twinning of the Nb1-xOs1+xB Type: Experimental and Computational Investigations.

The complex metal-rich boride Ti5-xFe1-yOs6+x+yB6 (0 < x,y < 1), crystallizing in a new structure type (space group Cmcm, no. 63), was prepared by arc-melting. The new structure contains both isolated boron atoms and zigzag boron chains (B-B distance of 1.74 Å), a rare combination among metal-rich borides. In addition, the structure also contains Fe-chains running parallel to the B-chains. Unlike in previously reported structures, these Fe-chains are offset from each other and arranged in a triangular manner with intrachain and interchain distances of 2.98 and 6.69 Å, respectively. Density functional theory (DFT) calculations predict preferred ferromagnetic interactions within each chain but only small energy differences for different magnetic interactions between them, suggesting a potentially weak long-range order. This new structure offers the opportunity to study new configurations and interactions of magnetic elements for the design of magnetic materials.

Continental-scale distribution of niobium and tantalum in catchment sediments throughout China: Prospecting implications from the China geochemical Baselines project

Niobium (Nb) and tantalum (Ta), which are chemical twins with very similar geochemical properties, are critical mineral resources for emerging global industries. Owing to increasing demands, understanding their concentrations and distributions in the pedosphere is essential for assessing potential Nb and Ta resources in China. To address this, we used geochemical data obtained from the China Geochemical Baselines (CGB) project, which began in 2008 and has since then yielded continental-scale datasets. With a density of approximately-one sampling site per 3,000 km2, the CGB project involved catchment sediments from two horizons at each site: a top sample collected at a depth of 0–25 cm and a deep sample collected at a depth greater than 100 cm. In total, 3,394 samples each of top and deep catchment sediments were collected, and Nb and Ta concentrations were measured using inductively coupled plasma mass spectrometry under strict quality control conditions. The Nb concentrations in the top and deep sediment samples varied in the ranges 0.04–89.86 ppm with a median value of 12.71 ppm and 1.20–104.12 ppm with a median value of 12.41 ppm, respectively. Further, the Ta concentrations in the top samples ranged from 0.01 to 26.41 ppm with a median of 0.97 ppm, while those in the deep samples ranged from 0.14 to 26.41 ppm with a median of 0.96 ppm. Furthermore, both the top and deep samples had similar 2.5th, 25th, 50th, 75th, and 97.5th percentile Nb and Ta contents, indicating that Nb and Ta in the catchment sediments underwent similar and consistent formation processes. Our results also indicated that the concentrations of both elements varied widely across regions, and their distributions in the catchment sediments throughout China were predominantly governed by the parent rock, particularly granites and alkaline rocks. Mineralization and sedimentation properties also played roles at a regional scale. Overall, the spatial distributions of Nb and Ta in the top and deep catchment sediments were relatively consistent, with concentrations being higher in southeastern China and lower in northwestern China. Twelve large-scale geochemical Nb-Ta belts were delineated based on continuous anomalous point distributions wherein many well known Nb-Ta deposits are already located. These nationwide Nb and Ta concentrations and distributions provide foundational large-scale data for further exploration and geological studies.

Fibonacci Stoichiometry and Superb Performance of Nb&amp;lt;sub&amp;gt;16&amp;lt;/sub&amp;gt;W&amp;lt;sub&amp;gt;5&amp;lt;/sub&amp;gt;O&amp;lt;sub&amp;gt;55&amp;lt;/sub&amp;gt; and Related Super-Battery Materials

In this contribution, two important crystallographic concepts for the formation of a series of block structures associated with channeling have been compared: chemical twinning and crystallographic shear. Twin planes respectively shear planes besides formed channels serve as a sink for charge carriers or, when the oxidation state of metal ions can be reduced, as a reservoir for intercalated lithium. In this way, Wadsley-Roth shear phases such as niobium tungsten oxide exhibit channels for ultra-fast lithium-ion diffusion. They are in focus as anode material for super-batteries, superb in terms of energy respectively power density, charging time, cycle life and safety. It should be noted that the transition metal to oxygen ratio TM/O = 21/55 of the title compound is a Fibonacci number quotient. Also, the crystal lattice can be traced back to Fibonacci geometry. When replacing only 0.0213 tungsten atoms in the formula with less expensive titanium, a TM/O ratio of 0.381966 =ϕ2 can be adapted besides an average valence electron concentration of 2⋅ϕ-2, where represents the most irrational number of the golden mean. The additional disorder caused by even such small titanium replacement and accompanied oxygen vacancies could fasten up the already high lithium diffusion further. Ultrasonic treatment may be applied besides thermal cycling to prepare phase-pure of the highest electrochemical performance. A replacement of oxygen by some fluorine atoms is an obvious synthesis possibility, but the higher binding energy expected between lithium and fluorine in contrast to oxygen may rather hinder than promote lithium diffusion.

Swapping isotopes improves fluorescent dyes

Fluorescence microscopy is a powerful tool for biological research. Researchers would like brighter, more stable fluorophores that enable longer imaging experiments. Rhodamine dyes are particularly bright and stable, and they can be tuned by incorporating various substituents into their structure. But even rhodamines have room for improvement. A team led by Luke D. Lavis of the Howard Hughes Medical Institute’s Janelia Research Campus has upped rhodamines’ game by making a simple switch—replacing hydrogen atoms with their more massive chemical twin, deuterium ( JACS Au 2021, DOI: 10.1021/jacsau.1c00006 ). Instead of deuterating the xanthene core, the researchers deuterated the alkylamino substituents. The deuteration improved the brightness and stability of the dyes without causing unwanted shifts in their fluorescence spectra. The improvements result from isotope effects that slow processes that destabilize and dim the dyes. For cellular imaging, the dyes enabled experiments with longer duration and higher signal. In addition, the researchers

Modularity: The Japanese Pioneering Contribution to the Investigation of Modular Structures (Crystal Research and Technology 5/2020)

Crystal Research and TechnologyVolume 55, Issue 5 2070019 Cover PictureFree Access Modularity: The Japanese Pioneering Contribution to the Investigation of Modular Structures (Crystal Research and Technology 5/2020) First published: 07 May 2020 https://doi.org/10.1002/crat.202070019AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinked InRedditWechat Abstract In article number 1900045, Akihiro Umayahara and Massimo Nespolo present a revised classification of monoarchetypal modular structures. Chemical twins are characterized by altered coordination at the interface between modules and, depending on whether this results or not in a modification of the chemical composition, they are classified in heterochemical and isochemical. Polytypes in which the partial operations mapping the modules are ordinary space-group operations are called Ito twins. OD structures are polytypes in which layer pairs are geometrically equivalent. Volume55, Issue5Special Issue: Mathematical Crystallography 2019May 20202070019 RelatedInformation

Chemical Twinning of Salt and Metal in the Subnitridometalates Ba23 Na11 (MN4 )4 with M=V, Nb, Ta.

The subnitridometalates Ba23 Na11 (MN4 )4 (M=V, Nb, Ta) crystallize in a new structure type, which shows ionic ortho-nitridometalate anions and motifs from simple (inter)metallic packings: Na-centered [Na8 ] cubes as cutouts of the bcc structure of elemental Na and Na-centered [Ba10 Na2 ] icosahedra as found in Laves phases, for example. Single-crystal and powder X-ray diffraction studies in combination with quantum-chemical calculations of the electronic structure and Raman spectroscopy support the characterization of the subnitridometalates as "chemical twins". They consist of independent building units with locally prevalent ionic or metallic bonding in an overall metallic compound.

Coherent intergrowth of simple cubic and quintuple tetragonal perovskites in the system Nd2−εBa3+ε(Fe,Co)5O15−δ

Investigation of the Nd2−εBa3+ε(Fe,Co)5O15−δ system, combining X-ray diffraction and electron microscopy, has allowed a tetragonal quintuple ordered perovskite “ap×ap×5ap” phasoid inter-grown within a single cubic perovskite matrix to be evidenced for ε=0. This nanoscale chemically twinned perovskite is compared with other members, Ln=Sm, Eu, Pr. The unusual long range ordering of the layers develops strains due to size mismatch between Ba2+ and Ln3+ cations. Importantly, two factors allow the strains to be decreased: (i) special intergrowths of double (LnBaFe2O6−δ) and triple (LnBa2Fe3O9−δ) perovskite ribbons/layers oriented at 90°, (ii) nanoscale chemical twinning. The spin locking effect of the nano-domain boundaries upon the magnetic properties of these perovskites is discussed.

Crystal structure, stoichiometry, and dielectric relaxation in Bi 3.32Nb 7.09O 22.7 and structurally related ternary phases

The crystal structure of the phase previously reported to occur at 4:9 Bi 2O 3:Nb 2O 5 has been determined using single-crystal X-ray and powder neutron diffraction ( P6 3/ mmc; a=7.4363(1) Å, c=19.7587(5) Å; Z=2). The structural study combined with phase equilibrium analyses indicate that the actual composition is Bi 3.32Nb 7.09O 22.7. This binary compound is the end-member of a family of four phases which form along a line between it and the pyrochlore phase field in the Bi 2O 3:Fe 2O 3:Nb 2O 5 system. The structures are derived from the parent pyrochlore end-member by chemical twinning, and can also be described as unit-cell intergrowths of the pyrochlore and hexagonal tungsten bronze (HTB) structures. The dielectric properties of the three chemically twinned pyrochlore phases, Bi 3.32Nb 7.09O 22.7, Bi 9.3Fe 1.1Nb 16.9O 57.8 and Bi 5.67FeNb 10O 35, were characterized. All exhibit low-temperature, broad dielectric relaxation similar to that of the Bi–Fe–Nb–O pyrochlore. At 1 MHz and ≈175 K the observed relative permittivites were 345, 240, and 205, respectively, compared to 125 for the Bi–Fe–Nb–O pyrochlore. The higher relative permittivities observed for the chemically twinned pyrochlore derivatives are ascribed to the presence of HTB blocks in their structures: The Bi atoms located in the HTB blocks feature highly asymmetric coordination environments compared to pyrochlore, and the magnitude of the relative permittivity increases with the proportion of Bi located within the HTB portions of the structures.

Pd0.213Cd0.787 and Pd0.235Cd0.765 Structures: Their Long c Axis and Composite Crystals, Chemical Twinning, and Atomic Site Preferences

We present single-crystal studies of Pd(0.213)Cd(0.787) and Pd(0.235)Cd(0.765), synchrotron powder studies of Pd(1-x)Cd(x), 0.755> or =x> or =0.800, and LDA-DFT and extended Hückel (eH) calculations on these or related phases. The two single-crystal structures have a, b, and c axis lengths of 9.9013(7), 14.0033(10), 37.063(24) and 9.9251(3), 14.0212(7), 60.181(3) A, respectively and they crystallize in the space groups Ccme and F2mm, respectively (solved as (3+1)-dimensional crystals their most convenient superspace group is Xmmm(00gamma)s00). The structures have two different structural components each with their own separate axis parameters. Powder data shows that the ratio of these separate axes (S/L) varies from 1.615 to 1.64, values near the golden mean (1.618). For Pd(0.213)Cd(0.787), different Pd and Cd site occupancies lead to variation in the R factor from 2.6-3.6 %. The site occupancy pattern with the lowest R factor (among the 26 820 variants studied) is the exact site occupancy pattern predicted by LDA-DFT parameterized eH Mulliken charge populations. The phases can be understood through a chemical twinning principle found in gamma-brass, the parent structure for the above phases (a relation with the MgCu(2) Laves phase is also noted). This twinning principle can be used to account for Cd and Pd site preferences. At the same time there is a clean separation among the Cd and Pd atoms for the two separate chain types at height b=0 and 1/2. These results indicate that Cd:Pd stoichiometry plays a role in phase stability.

Chemical twinning of the pyrochlore structure in the system Bi 2O 3–Fe 2O 3–Nb 2O 5

New ternary bismuth iron niobates having structures based on chemical twinning of pyrochlore are described. Bi 5.67Nb 10FeO 35 has hexagonal symmetry, P6 3 /mmc, a = 7.432 ( 1 ) Å , c = 31.881 ( 2 ) Å , Z = 2 and Bi 9.3Nb 16.9Fe 1.1O 57.8 has rhombohedral symmetry, R-3 m, a = 7.433 ( 1 ) Å , c = 77.488 ( 2 ) Å , Z = 3 . The structures of both phases were determined and refined to R 1 = 0.04 using single-crystal X-ray data. They can be described as being derived from the pyrochlore structure by chemical twinning on (111) py oxygen planes. The chemical twin operation produces pairs of corner-connected hexagonal tungsten bronze (HTB) layers as in the HTB structure, so the structures may alternatively be described as pyrochlore:HTB unit-cell intergrowth structures. In the hexagonal phase the pyrochlore blocks have a width of 12 Å, whereas the rhombohedral phase has pyrochlore blocks of two widths, 6 and 12 Å, alternating with HTB blocks. It is proposed that the previously reported binary 4Bi 2O 3:9Nb 2O 5 phase has a related structure containing pyrochlore blocks all of width 6 Å. A feature of the structures is partial occupancy (∼65%) of the Bi sites and displacement of the Bi atoms from the ideal pyrochlore A sites towards the surrounding oxygen atoms, as observed in Bi-containing pyrochlores.

Unit-cell intergrowth of pyrochlore and hexagonal tungsten bronze structures in secondary tungsten minerals

Structural relations between secondary tungsten minerals with general composition A x [(W,Fe)(O,OH) 3] · y H 2O are described. Phyllotungstite ( A=predominantly Ca) is hexagonal, a = 7.31 ( 3 ) Å , c = 19.55 ( 1 ) Å , space group P6 3/ mmc. Pittongite, a new secondary tungsten mineral from a wolframite deposit near Pittong in Victoria, southeastern Australia ( A=predominantly Na) is hexagonal, a = 7.286 ( 1 ) Å , c = 50.49 ( 1 ) Å , space group P-6 m2. The structures of both minerals can be described as unit-cell scale intergrowths of (111) py pyrochlore slabs with pairs of hexagonal tungsten bronze (HTB) layers. In phyllotungstite, the (111) py blocks have the same thickness, 6 Å, whereas pittongite contains pyrochlore blocks of two different thicknesses, 6 and 12 Å. The structures can alternatively be described in terms of chemical twinning of the pyrochlore structure on (111) py oxygen planes. At the chemical twin planes, pairs of HTB layers are corner connected as in hexagonal WO 3.

Ba5(Al/Ga)5(Sn/Pb): Neue Verbindungen an der Zintl-Grenze / Ba5(Al/Ga)5(Sn/Pb): New Compounds at the Zintl Border

The new isotypic intermetallic phases Ba5MIII 5MIV (MIII = Al, Ga; MIV = Sn, Pb) have been synthesized from stoichiometric amounts of the elements at maximum temperatures of 900 to 1000 ◦C. They crystallize in the hexagonal space group P6̄m2 (Ba5Al5Sn: a = 605.05(8), c = 1109.0(2) pm, R1 = 0.0137; Ba5Ga5Sn: a = 599.45(5), c = 1086.00(7) pm, R1 = 0.0485; Ba5Al5Pb: a = 606.9(2), c = 1112.0(4) pm, R1 = 0.0409 and Ba5Ga5Pb: a = 601.76(7), c = 1091.51(13) pm, R1 = 0.0295), forming a new structure type. Similar to the Zintl phases Ba2MIV (Co2Si structure type, orthorhombic, space group Pnma; Ba2Sn: a = 861.52(14), b = 569.85(9), c = 1056.9(2) pm, R1 = 0.0217 and Ba2Pb: a = 865.12(13), b = 569.1(2), c = 1061.8(2) pm, R1 = 0.0470), these new ternary phases contain isolated MIV atoms (coordinated by 11 Ba atoms). In addition, sheets of 3- and 4-bonded Al/Ga atoms similar to those in Ba3Al5 are present. In accordance with this, a formal subdivision of Ba5MIII 5MIV into Ba3MIII 5 ・ Ba2MIV can be performed to describe the observed intergrowth or chemical twinning of two different binary intermetallics to give the new ternary compounds. Beyond structural aspects, also the nature of the chemical bonding (as studied by FP-LAPW calculations) in these new, non-electron precise compounds in the vicinity of the Zintl border can be interpreted in this vein.

Synthesis, Structure, and Magnetic Properties of CeNiIn2

Single crystals of the ternary indide CeNiIn2 were synthesized from an indium flux. An arc-melted button of the starting composition CeNiIn2 was annealed in a zirconia crucible with a slight excess of indium at 1200 K followed by slow cooling (5 K/h) to 870 K. The new indide has been investigated by X-ray diffraction on powders and single crystals: PrNiIn2 type, Cmcm, a = 440.78(4), b = 1834.3(1), c = 2178.8(2) pm, wR2= 0.0753 for 1471 F2 values and 66 variable parameters. The structure contains three crystallographically independent nickel positions which have a distorted trigonal prismatic coordination. The CeNiIn2 structure is related to the MgCuAl2 structure by chemical twinning. The shortest interatomic distances occur for Ni-In and In-In contacts. The nickel and indium atoms form a complex three-dimensional [NiIn2] network in which the cerium atoms fill distorted pentagonal channels. Magnetic susceptibility measurements indicate Curie-Weiss behavior with an experimental magnetic moment of 2.44(3) μB/Ce atom. At 3.4(3) K CeNiIn2 orders ferro- or ferrimagnetically. The experimental saturation magnetization at 2K and 5.5 T is 0.95(2) μB/Ce.

New periodic and aperiodic triangular prismatic sheet carbides obtained by crystallization of Mn 1− xC x amorphous films and described by the chemical twinning model

Mn 1− x C x amorphous alloys with x varying from 0.25 to 0.34 were prepared by RF magnetron sputtering. Their crystallization studied by in situ transmission electron microscopy leads at nearly T=650 K to carbides with feather-shaped or lath-shaped morphologies. The SAED patterns show that it is matter of a set of new triangular prismatic sheet (TPS) carbides deriving from Mn 5C 2 with either periodic or one-dimensional aperiodic structures. For the periodic structures, we identify three new carbides: Mn 7C 3, Mn 9C 4 and Mn 2C. Aperiodic TPS structures transform with an increase of the temperature into the concentration-nearest periodic TPS structures. All these structures are well-interpreted by the chemical twinning model.

A comparative analysis of quantitative ratios in formulas of chemical compounds and twins

A comparative analysis is performed for the distribution and the frequency of occurrence of the quantitative ratios between the chemical coefficients of binary chemical compounds, symbols of twinning faces, etc. Comparison is carried out using the modified Weiss-Goldschmidt prognostic ranked scheme. It is demonstrated that the Pauling parsimony law is combined with the Goldschmidt complication principle.

Indides LnNiIn2 (Ln=Pr, Nd, Sm) and Ferromagnetic PrRhIn

The title compounds were synthesized by reacting the elements in an arc-melting apparatus under purified argon and subsequent annealing at 970 K. The nickel containing compounds crystallize with a new structure type which was determined for PrNiIn2: Cmcm, a=440.0(2) pm, b=1833.9(6) pm, c=2164.6(5) pm, wR2=0.0550, 1451 F2 values, and 66 parameters. From a geometrical point of view, the structure of PrNiIn2 may be described as an intergrowth of small distorted CaCu5-, CsCl-, and Cu3Au-related slabs. The structure is also related to the MgCuAl2 type by chemical twinning. The shortest interatomic distances within the PrNiIn2 structure occur for the Ni–In and In–In contacts. The nickel and indium atoms form a three-dimensional [NiIn2] polyanion in which the praseodymium atoms fill distorted pentagonal channels. To a first approximation the formula may be written as Pr3+[NiIn2]3−. PrRhIn adopts the ZrNiAl type structure: P62m, a=755.6(2) pm, c=404.8(1) pm, wR2=0.0285, 361 F2 values, and 14 parameters. Structural motifs of PrRhIn are tricapped trigonal prisms [Rh1In6Pr3] and [Rh2Pr6In3]. The rhodium and indium atoms form a three-dimensional [RhIn] polyanion in which the praseodymium atoms are located in distorted hexagonal channels. Susceptibility measurements reveal Curie–Weiss behavior with a magnetic moment of 3.69(5) μB/Pr. PrRhIn orders ferromagnetically at TC=5.8(6) K and shows a saturation magnetization of 1.60(5) μB/Pr at 2 K and 5.5 T. Resistivity measurements indicate metallic behavior with a specific resistivity of 105±20 μΩ cm at room temperature.

Topological relationships and building blocks in Zintl phases of the composition Ba(n+1)(Mg,Li)2nSi2(n+1).

The structures of two novel Zintl phases, Ba6Mg5.2Li2.8Si12 and BaMg0.1Li0.9Si2, are presented. Both compounds contain chains in cis-trans conformation. The silicon partial structure of Ba6Mg5.2Li2.8Si12 (C2/m; a = 1212.0(1), b = 459.78(4), c = 1129.10(9) pm; beta = 91.77(2) degrees; Z = 1) is built of unbranched, planar Si6 chains while BaMg0.1Li0.9Si2 (Pnma; a = 725.92(5), b = 461.36(3), c = 1169.08(8) pm; Z = 4) consists of infinite Si(n) chains. The compounds show all electronic and structural characteristics that are typical for the special subset of Zintl phases with highly charged planar anions. The structures of the new compounds, as well as that of Ba2Mg3Si4, can be derived from the common parent type BaMg2Si2. It is shown that a comprehensive picture of a chemical twinning based on BaMg2Si2 can be derived.

Al–Cu approximants and associated B2 chemical-twinning modes

The Al3Cu4 alloy, with an e/a ratio of 1.86 being close to ternary Al–Cu–TM (transition metal) quasicrystals, has been chosen for the search of Al–Cu approximants. Phase structures and compositions were studied using TEM, X-ray diffraction and EPMA techniques. Two new phases were found: face-centered orthorhombic oF-Al43.2Cu56.8 (a=0.8166, b=1.4149, c=0.9995nm) and body-centered orthorhombic oI-Al41.3Cu58.7 (oI, a=0.4083, b=0.7074, c=0.9995nm). Their e/a ratios are the same as that of the Al–Cu–Fe icosahedral quasicrystal. Both are B2 superstructures and their unit cell components can be expressed approximately as oF-Al36Cu48□12 and oI-Al8Cu12□4 (□: vacancies). They both exist in twinning variants of the types 120°/[001] and 180°/[310]. Such twinning modes indicate that these orthorhombic phases are the decomposition products of a high-temperature parent phase ϵ2-Al2Cu3, the atomic structure of which shows pentagonal atomic arrangements. Further analysis on the twinning modes of oF and oI leads to the recognition of the chemical-twinning mode of the basic B2 structure as 180°/〈111〉B2. This kind of chemical twinning mode is responsible for the pentagonal atomic configuration in the Al–Cu approximants as well as for the pseudo-5-fold B2 twinning.