Polysynthetic Twinning Research Articles

Alexandrite and colour-change chrysoberyl from the Lake Manyara alexandrite-emerald deposit in northern Tanzania

Mineralogical and gemmological properties of alexandrite from the Mayoka deposit, Lake Manyara mining area, Tanzania, are described. The alexandrite crystals are located in a phlogopite- bearing schist which is part of a metamorphic-metasomatic association of mafi c or ultramafi c rocks in contact with pegmatitic intrusions. Emeralds are associated with the pegmatites. Single crystals, contact twins, penetration twins and cyclic twins (trillings) are all present in this area and are developed in two major habits, one tabular parallel to the a pinacoid and the other columnar along the a-axis. Microscopic features of faceted and rough alexandrites are characterized by internal growth structures refl ecting the external morphology of the samples. Common mineral inclusions are phlogopite, apatite and zircon. Associated with series of channels parallel to the a-axis three groups of elongated or tabular negative crystals oriented in a plane parallel to the a pinacoid form milky white zones in some samples. The complexity of alexandrite colours is clarifi ed to some extent by correlation of colorimetric parameters with trace element contents and with the orientation of the alexandrites. The designation of samples as alexandrites and a possible distinction from colour-change chrysoberyl is discussed. Alexandrite and green chrysoberyl from Lake Manyara is compared with samples from other occurrences in phlogopite-bearing schist such as the 'classical' alexandrite deposits in the Urals, Russia, and from Novello, Zimbabwe.

Mineralogy and paragenesis of Ca-dachiardite in Cretaceous zeolitized volcanic rocks, Esquel, Chubut, Argentinian Patagonia.

Ca-dachiardite is present in Cretaceous volcanic breccias affected by very low grade metamorphism. The cement of the breccia shows quartz, Ca-dachiardite, mordenite, cristobalite, calcite and siderite, while in the clasts there are pectolite, prehnite, yugawaralite, Ca-dachiardite, albite, adularia, interestratified smectite/chlorite (S/C), quartz and cristobalite. Most Ca-dachiardites are found in the cement of the breccia. It occurs principally as fibrous tubes of parallel or lightly divergent bladed and tabular crystals, arranged in concentrical layers with length along 'b' and the fibers; the crystals are flattened on {001} and rarely on {100}. It shows cyclic twins in cross section normal to the length and multiple twins parallel to {001}, but single crystals are also present. The optical character shows: a=b; g:c= 43°; 2Va=55°/60°; strong dispersion r>v. The structural formulae is: Ca1.54-1.72 K0.82-0.98 Na0 (Al3.65-3.98 Si19.95-20.25 O48) 13H2O ; TSi= 0.83-0.85; R2+/(R+ + R2+)=0.61/0.68; DEC=Ca and K. The strong diffraction lines are: 3.460 Å; 3.190 Å and 1.870 Å; a0 =18.679Å; b0=7.488 Å; c0 10.267 Å; b=107°86'. The secondary mineralogical succession deposited according with the decrease in temperature and Xco2, could be: calcite (step I); pectolite, prehnite, quartz, yugawaralite, Ca-dachiardite, S/C, adularia, albite (step II, zeolite facies); mordenite and cristobalite (step III) and then, siderite and calcite (step IV). The metamorphic assemblages suggests that the crystallization conditions of Ca-dachiardite would be 200-220°C at pressures smaller than 0.5 kb, from high aSi and aCa fluids.

Slip Transfer Across Hetero-Interfaces in Two-Phase Titanium Aluminum Intermetallics

The role of slip transfer processes across the heterophase interfaces in two-phase TiAl intermetallics has been studied. Polysynthetically twinned (PST) crystals of TiAl (PST-TiAl) have been used as model systems for individual grains in technologically relevant polycrystalline lamellar TiAl alloys. Compressive plastic loads have been applied for orientations of the lamellar interfaces parallel and perpendicular to the loading directions to produce hard mode slip activity in both the γ and the α2 phases. Transmission electron microscopy has been used to determine the active deformation modes in the constituent phases and to study details of the hard mode of the slip transfer across heterophase interfaces. The results are discussed with respect to the mechanical behavior of PST-TiAl.

Directional solidification of Ti-45Al-8Nb-(W,B,Y) alloy

Using a Bridgman vertical vacuum furnace, Ti-45Al-8Nb-(W,B,Y) (at.%) bars, which were prepared from a plasma arc melting (PAM) ingot, were directionally solidified at growth rates of 10, 15, and 20 μm/s. Polysynthetic twinned (PST) crystal with an aligned lamellar microstructure was obtained at the growth rate of 15 μm/s because of high Nb addition. The principle of PST crystal growth and the effect of Nb element were discussed. The results of investigations on microstructure and micromechanical properties of the directionally solidified (DS) bars of Ti-45Al-8Nb-(W,B,Y) alloy are briefly summarized.

Cyclic twin nucleation in tin-based solder alloys

The microstructure and Sn crystal orientations of lead-free solder alloys such as near-eutectic SnAgCu have a significant influence on the mechanical response of a solder joint to service conditions. Thus solidification processes were examined in SnAgCu solder joints. Distinct evidence of sixfold cyclic growth twinning of Sn during solidification from the melt was observed in Sn–Ag, SAC and Sn–Cu solders. Three orientations of Sn grains, each having a common 〈1 0 0〉 direction, were found in each of these systems, though the morphologies of these cyclic twinned microstructures differed. Analysis of dendrite arm spacing in cyclically twined structures with a beach ball morphology implies that the common 〈1 0 0〉 axis intersects with the region of the nucleation event. Models are presented for two pseudo/metastable hexagonal unit cells based upon {1 0 1} or {3 0 1} twins that introduce the cyclic twinning structure at the nucleation stage. Formation of these hexagonal unit cells may be facilitated by the presence of alloy elements. Subsequent epitaxial growth of the tetragonal unit cell on this nucleus can account for all three types of morphologies observed in microstructures of Sn-rich solder alloys.

Atomistic studies of interactions between the dominant lattice dislocations and γ/γ-lamellar boundaries in lamellar γ-TiAl

This paper reports on atomistic simulations of the interactions between the dominant lattice dislocations in γ-TiAl (ordinary screw 1 2 〈 1 1 0 ] and 〈1 0 1] superdislocations) with all three kinds of γ/γ-lamellar boundaries in polysynthetically twinned (PST) TiAl. The purpose of this study is to clarify the early stage of lamellar boundary controlled plastic deformation in PST TiAl. The interatomic interactions in our simulations are described by a bond order potential for L1 0-TiAl which provides a proper quantum mechanical description of the bonding. We are interested in the dislocation core geometries that the lattice produces in proximity to lamellar boundaries and the way in which these cores are affected by the elastic and atomistic effects of dislocation-lamellar boundary interaction. We study the way in which the interfaces affect the activation of ordinary dislocation and superdislocation slip inside the γ-lamellae and transfer of plastic deformation across lamellar boundaries. We find three new phenomena in the atomic-scale plasticity of PST TiAl, particularly due to elastic and atomic mismatch associated with the 60° and 120° γ/γ-interfaces: (i) two new roles of the γ/γ-interfaces, i.e. decomposition of superdislocations within 120° and 60° interfaces and subsequent detachment of a single ordinary dislocation and (ii) blocking of ordinary dislocations by 60° and 120° interfaces resulting in the emission of a twinning dislocation.

The Complex Hydrothermal History of Granitic Rocks: Multiple Feldspar Replacement Reactions under Subsolidus Conditions

Recurring subsolidus re-equilibration of granitic feldspars induced by fluid infiltration events provides a record of fluid^rock interactions that affect large volumes of the Earth’s continental crust.This has a direct bearing on the interpretation of the present-day granitic rock mineralogy and geochemistry. We examine Palaeoproterozoic grey and red-stained granitoids from the Simpevarp and Laxemar areas in SE Sweden, particularly focusing on consecutive feldspar replacement reactions, to provide an in-depth understanding of subsolidus re-equilibration of granitic rocks with hydrothermal fluids. The apparently most unaltered grey granitoids contain highly porous oligoclase grains that enclose crystallographically continuous microcline relicts. This texture suggests that the oligoclase is already secondary and may be a replacement product of original microcline. Oligoclase is progressively replaced by albite (� An9) along polysynthetic twinning and intragranular fractures. The features of this replacement are characteristic of a dissolution^reprecipitation mechanism. Finegrained mica (sericite) is closely associated with the albite porosity within micron-sized pores observable with scanning electron microscopy as well as in nanopores imaged with transmission electron microscopy. The reddening phenomenon in the vicinity of fractures is contemporaneously related to the K-feldspathization of sericite, which is restricted to the altered oligoclase. Submicron size hematite precipitation within orthoclase pores at the replacement front results in the red coloration. The complex associations between the fluid^ feldspar reactions indicate that the replacement reactions may be due to sequential fluid infiltration events and that the granitoids have undergone extensive subsolidus re-equilibration, changing the original magmatic mineralogy. Therefore, the effects of large-scale re-equilibrations of granitic rocks through hydrothermal convection systems should be more closely considered.

Constrained Deformation of TiAl PST Crystals

AbstractTiAl polysynthetically twinned (PST) crystals were deformed under plane strain condition, in which the anisotropic macroscopic deformation of PST crystals is restricted with a channel die, in order to clarify the deformation behavior of TiAl/Ti3Al lamellar structure under constraint conditions. TEM analysis of deformation modes together with the Taylor analysis reveals that all TiAl orientation variants deform to yield the relaxed-constraint-type plastic strain, where three shear strain components are not zero for each TiAl variant but are macroscopically compensated to zero by the existence of twin-related TiAl lamellae at the early stage of deformation. The Taylor analysis assuming the relaxed constraint conditions is found to be adaptable for predicting the operative deformation modes in TiAl at the early stage of deformation and also for correlating quantitatively the stress-strain behavior of PST crystals under external constraint with those under the unconstraint condition.

Northwest Africa 2526: A partial melt residue of enstatite chondrite parentage

Abstract— NWA 2526 is a coarse‐grained, achondritic rock dominated by equigranular grains of polysynthetically twinned enstatite (˜85 vol%) with frequent 120° triple junctions and ˜10–15 vol% of kamacite + terrestrial weathering products. All other phases including troilite, daubreelite, schreibersite, and silica‐normative melt areas make up <˜1 vol% of the rock Oxygen isotopic analyses are well within the range of those for enstatite chondrites and aubrites. We show that the “enstatite achondrite” (Russell et al. 2005) Northwest Africa (NWA) 2526 is actually a partial melt residue of an enstatite chondrite‐like lithology that experienced ˜20 vol% partial melting. We suggest that the heat source was internal to the parent body. The FeS‐Fe,Ni and plagioclase‐enstatite partial melts were removed from the parent lithology, leaving NWA 2526 as a residue highly depleted in troilite and lacking plagioclase. Sub‐solidus slow cooling and annealing is responsible for the coarsegrained, recrystallized texture of the rock. We also suggest that the parent lithology of NWA 2526, prior to partial melting, experienced a shock event which formed the curvilinear trails of blebs of minor troilite and rare metal that are enclosed in enstatite crystals; thus, these represent relicts. After partial melting and annealing, NWA 2526 experienced a second, relatively mild impact event (<˜20 GPa) that caused formation of the polysynthetic twinning in the enstatite.We suggest that the meteorite Zaklodzie, which has been referred to as a “primitive enstatite achondrite” (Przylibski et al. 2005), did not form from a magma of internal origin, but that it is an impact‐melt breccia of enstatite chondrite‐like parentage, as previously discussed by Burbine et al. (2000) and Keil (2007).Finally, the “metal‐rich enstatite meteorite with achondritic texture” Itqiy (Patzer et al. 2001) formed by processes very similar to those responsible for formation of NWA 2526 and is also the residue of ˜20 vol% partial melting of an enstatite chondrite‐like parent lithology, with the FeS‐Fe,Ni and plagioclase‐enstatite partial melts having been removed from the residue. It also experienced an impact event after partial melting that was responsible for the formation of the mixed Mg‐Mn‐Fe‐sulfides and the shock stage S3 features of the enstatite. These similarities indicate that NWA 2526 and Itqiy may have formed on the same parent body. This body was different from the EH, EL, Shallowater and aubrite parent bodies, and NWA 2526 and Itqiy may represent samples from yet another, fifth enstatite meteorite parent body.

Features of evolution of wedge-shaped twins in bismuth single crystals subjected to polysynthetic twinning

Features of evolution of wedge-shaped twins in bismuth single crystals with polysynthetic twins are examined. Polysynthetic twins are found to promote an increase in number, and a decrease in length, of indentation-induced wedge-shaped twins. The latter quantities depend on the density of twins in a polysynthetic twin. Based on the dislocation model, stress fields in the vicinity of wedge-shaped and polysynthetic twins are calculated at a mesoscopic level.

Unexpected High Stiffness of Ag and Au Nanoparticles

We studied the compressibility of silver (10 nm) and gold (30 nm) nanoparticles, n-Ag and n-Au, suspended in a methanol-ethanol mixture by x-ray diffraction (XRD) with synchrotron radiation at pressures up to 30 GPa. Unexpectedly for that size, the nanoparticles show a significantly higher stiffness than the corresponding bulk materials. The bulk modulus of n-Au, K(0)=290(8) GPa, shows an increase of ca. 60% and is in the order of W or Ir. The structural characterization of both kinds of nanoparticles by XRD and high-resolution electron microscopy identified polysynthetic domain twinning and lamellar defects as the main origin for the strong decrease in compressibility.

Tammann paradox and amorphization of substances under pressure

It is shown that amorphization of substances under pressure can be related to the fundamental property of the isobaric-isothermal potential surface as a function of temperature and pressure: its convexity. Amorphization is cold melting. The peaks in the pressure dependences of the melting temperature are not anomalies but a manifestation of the general case, whose implementation is limited by polymorphism. A model of the amorphous state near the absolute zero of temperature is proposed: specifically, nanoceramics formed according to the mechanism of polysynthetic twinning.

PIZGRISCHITE, (Cu,Fe)Cu14PbBi17S35, A NEW SULFOSALT FROM THE SWISS ALPS: DESCRIPTION, CRYSTAL STRUCTURE AND OCCURRENCE

Pizgrischite, (Cu,Fe)Cu14PbBi17S35, is a new mineral species named after the type locality, Piz Grisch Mountain, Val Ferrera, Graubunden, Switzerland. This sulfosalt occurs as thin, striated, metallic lead-grey blades measuring up to I cm in length, embedded in quartz and associated with tetrahedrite, chalcopyrite, pyrite, sphalerite, emplectite and derivatives of the aikinite-bismuthinite series. In plane-polarized light, the new species is brownish grey with no perceptible pleochroism; under crossed nicols in oil immersion, it presents a weak anisotropy with dark brown tints. Minimum and maximum reflectance values (in %) in air are: 40.7-42.15 (470 nm), 41.2-43.1 (546 nm), 41.2-43.35 (589 nm) and 40.7-43.3 (650 nm). Cleavage is perfect along 001 I and well developed on {010}. Abundant polysynthetic twinning is observed on (010). The mean micro-indentation hardness is 190 kg/mm(2) (Mohs hardness 3.3), and the calculated density is 6.58 g/cm(3). Electron-microprobe analyses yield (wt%; mean result of seven analyses): Cu 16.48, Pb 2.10, Fe 0.77, Bi 60.70, Sb 0.35, S 19.16, Se 0.04, total 99.60. The resulting empirical chemical formula is (Cu15.24Fe0.80Pb0.60)(Sigma 16.64)(Bi17.07Sb0.17)(Sigma 17.24)(S35.09Se0.03)(Sigma 35.12), in accordance with the formula derived from the single-crystal refinement of the structure, (Cu,Fe)Cu14PbBi17S35. Pizgrischite is monoclinic, space group C2/m, with the following unit-cell parameters: a 35.054(2), b3.91123(I), c43.192(2) angstrom, beta 96.713(4)degrees, V5881.24 angstrom(3), Z=4. The strongest seven X-ray powder-diffraction lines [d in angstrom (I)(hkl)] are: 5.364(40)((6) over bar 04), 4.080(50)((8) over bar 05), 3.120(40)(118), 3.104(68)((3) over bar 18), 2.759(53) ((9) over bar 11),2.752(44)(910) and 1.956(100)(020). The crystal structure is an expanded monoclinic derivative of kupcikite. Pizgrischite belongs to the cuprobismutite series of bismuth sulfosalts but, sensu stricto, it is not a homologue of cuprobismutite. At the type locality. pizarischite is the result of the Alpine metamorphism under greenschist-facies conditions of pre-Tertiary hydrothermal Cu-Bi mineralization.

Lamellar structures in neodymium borides

Samples with the nominal composition Nd 2B 5 contain strongly disordered crystals composed of nanosized (100) lamellae. Most commonly, a polysynthetic twinning based on a fourfold rotation separates the lamellae at coherent boundaries. The structure of the twin interface contains a pseudo fourfold arrangement of B atoms as derived from electron microscopy techniques. Such pseudo-symmetry serves for a rationalization of the twinning. Additionally, a chemical intergrowth of Nd 2B 5 and NdB 4 lamellae based on related structural building units can be observed. Further HRTEM examinations allow the identification of a new structural variant of neodymium borides.

The ICDP Lake Bosumtwi impact crater scientific drilling project (Ghana): Core LB‐08A litho‐log, related ejecta, and shock recovery experiments

Abstract— The 1.07 Myr old Lake Bosumtwi impact crater in Ghana was drilled within the framework of the International Continental Scientific Drilling Project (ICDP). Hole LB‐08A, drilled into the outer flank of the central uplift and with a total depth of 451 m, yielded 215.71 m of impact‐related rocks. This paper summarizes observations of the lithological logging on core LB‐08A. Between a depth of 235.6 and ∼260 m, the section consists of a melt‐bearing allochthonous, polymict, and mostly clast‐supported impact breccia. Down to ∼418 m, the section comprises a rather uniform unit of meta‐graywacke alternating with phyllite to slate (lower greenschist facies); few (par‐) autochthonous impact breccia bodies and rare impact dike breccias are present. The lowermost part of the section contains several centimeter‐ to decimeter‐thick melt‐bearing breccia dikes in country rocks identical to those occurring above. Omnipresent fracturing was mapped in a qualitative manner. Most prominent shock effects in the uplifted target rocks comprise planar fractures and deformation elements in quartz and polysynthetic twinning in carbonate minerals; the maximum shock pressure as evidenced by quartz is below 26 GPa. The allochthonous breccias occasionally contain a few vol% of melt particles. Suevites occur outside the crater rim, carrying diaplectic crystals, coesite, and ballen quartz as well as true melt glasses and a variety of lithic clasts, among those spectacular staurolite‐rich mica‐schists. The recorded shock level in the uplifted target rocks is lower than expected and modeled. Shock recovery experiments with analogue carbonaceous graywackes at 34 and 39.5 GPa yielded nearly complete transformation of quartz into diaplectic glass. We therefore exclude a specific shock behavior of the soft, fluid‐rich target material (carbonaceous graywackes, shales, slates) in core LB‐08A as the prime or only reason for the melt deficit and the generally low shock levels recorded inside the Lake Bosumtwi impact crater.

Crack propagation behavior in TiAl–Nb single and Bi-PST crystals

The fracture toughness of directional solidified Ti–(45,47)Al–3Nb, Ti–(45,47)Al–3Nb–0.2Si–0.1C, Ti–(45,47)Al–3Nb–0.3Si–0.2C type I alloys and their contribution to crack growth resistance of TiAl–Nb alloys were studied using PST (polysynthetically twinned) crystals produced by directional solidification in FZ (floating zone) furnace. Lamellar orientations in the individual colonies are described using two angles defined with respect to the notch orientation: an in-plane kink angle and a through-thickness twist angle. Therefore, lamellar misorientation across an individual colony boundary is quantified as differences in these angles across the boundary. Crack growth resistance in colony boundary was identified by three-point bend test and crack advance was monitored by interrupted in situ test. From three-point bend test, it was found that the colony boundary could offer significant resistance to crack growth under large twist angle difference. Fracture toughness of type I specimens (in which crack propagates against lamellae boundaries) of the alloys decreased slightly with increasing Si and C contents and increased rapidly with decreasing Al content. The toughness for type I specimens was controlled by α 2–α 2 spacing in which the delamination-type separation occurred. Compared to 47Al alloys, α 2–α 2 spacing in 45Al alloys increased by decreasing Al content, therefore, fracture toughness increased rapidly. These results are discussed and the ability to improve toughness by changing Al content, Si and C addition in TiAl–Nb alloys produced by directional solidification is suggested.

Extended defects in organic molecular semiconductors: the role of crystal growth mechanisms

AbstractAtomic force and optical microscopy analysis of topographic features on the {001} faces of the organic semiconductor quaterthiophene is employed to establish connections between bulk and surface structure. Observation of interlaced step patterns on growth spirals is indicative of the presence of layers rotated by 180° about the normal of the (001) crystal face. This result, together with the height of the exposed step ledges of the order of 10‐100 nm, suggests the complex polytypic nature of these crystals due to periodic polysynthetic twinning. Therefore, the basic monoclinic structure of quaterthiophene is modified giving rise to many different periodicities along [001] even in one crystallite. The consequences on physical properties of these phenomena arising from growth mechanisms are briefly discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The crystal chemistry of welshite, a non-centrosymmetric (P1) aenigmatite-sapphirine-surinamite group mineral

Previous attempts to refine the crystal structure of welshite, known only from Långban, Sweden, have been foiled by its extensive polysynthetic twinning, and without a structure determination, derivation of a reasonable formula has been difficult due its eclectic chemical composition. We report a successful refinement [wR(F2) = 0.0566 for 8048 unique reflections] in the non-centrosymmetric space group P1 of a relatively little twinned crystal (4% chiral) from sample NRM040068: a = 10.394(3) Å, b = 10.777(3) Å, c = 8.896(2) Å, α = 105.953(4)°, β = 96.294(4)°, γ = 124.948(3)°, V = 738.8(3) Å3, Z = 1. The refined formula, Ca3.78Mg7.87Sb3.00Mn1.35Si5.73Be3.33Al1.71Fe3+0.96As0.27O40, is in reasonable agreement with a formula determined by electron microprobe and Mössbauer spectroscopy (Be from the refinement), Ca3.81Mg7.84Sb3.03Mn1.17Zn0.04Fe2+ 0.05Si5.57Be3.24Al1.54Fe3+1.25As0.37O40. The P1 symmetry of welshite 040068 is mainly a result of its cation distribution, which is driven by charge ordering on both the octahedral and tetrahedral sites as supported by calculation of electrostatic site potentials. Welshite can accommodate up to 3.46 Be per 40 O atoms by including several T sites with 100% Be occupancies without the formation of unfavorable tetrahedral Be-O-Be linkages that would result in the related centrosymmetric structures of sapphirine, khmaralite, makarochkinite, and høgtuvaite, none of which contain >2.1 Be per 40 O. A generalized formula for welshite is (Ca,Mn)4(Mg,Mn,Fe2+,Fe3+)9(Sb,Fe3+)3O4[(Si,As)6(Be,Al)4(Al,Fe3+)2O36]. Compositions of most of the samples, including NRM040068, can be expressed in terms of two idealized Al and Fe end-members Ca4Mg9Sb3O4[Si6Be3Al3O36] and Ca4Mg9Sb3O4[Si6Be3AlFe2O36], respectively; the latter is the most representative for welshite and could be used in databases. One sample is far more heterogeneous and is irregularly zoned on scales from <1 to 10 μm. Compositional variation in this sample and in a crystal from another lie between the end-members Ca4Mg8.55Fe0.45Sb3.0O4[Si5.15As0.45Be3.45Al0.5Fe2.45O36] and Ca4Mg7.05Fe3.45Sb1.5O4[Si6.65As0.45Be3.45Al0.5Fe0.95O36], which are related by the coupled substitution MSb5+ + VIMg2+ + IVFe3+ = MFe3+ + VIFe3+ + IVSi4+, where M refers specifically to the M7, M3A, and M4A octahedral sites. The lowest measured Sb is 1.454/40 O, which may be close to the limit imposed by the need to maintain local charge balance.

A Thermodynamic Model for the Shape and Stability of Twinned Nanostructures

Although thermodynamically metastable, planar defects are often observed in many faceted nanomaterials including nanocrystals, nanorods, and nanowires, even after annealing. These planar defects include contact twins and (intrinsic or extrinsic) stacking faults, and are usually neglected by most analytical models. For example, many bulk metals have the face-centered cubic structure, but small nanocrystals and nanorods of the same material often exhibit various structural and morphological modifications such as single or multiple symmetric twinning, as well as 5-fold cyclic twinning resulting in decahedral and truncated decahedral nanostructures. Presented here is a general analytical model for the investigation of nanomaterials of arbitrary shape, and with any configuration of planar defects. The model is tested for the case of twinning in unsupported gold nanocrystals and nanorods, and is shown to give results in excellent agreement with experimental and computational studies reported in the literature.

Reflections on symmetry and formation of axial quasicrystals

Abstract Some ideas are presented on the geometrical factors governing structure and formation of axial quasicrystals with 5- and 7-fold symmetry, respectively. First, the importance of thin atomic layers is discussed for the growth of decagonal quasicrystals. They are the only long-range ordered structural units carrying information on the local non-crystallographic symmetry of the constituent clusters. Second, the consequences of local polysynthetic twinning for the formation of pentagonal and heptagonal quasicrystals are demonstrated. Third, the special relationship of the β-phase, (Co,Ni)1–xAl1+x, to the decagonal phase and its periodic average structure is discussed as well as its role as template for the formation and growth of decagonal Al—Co—Ni.