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Studying intraspecific geographic variation in communication signals represents a good approach to understand the evolution of species diversification in animals. The poison frog Andinobates bombetes (Dendrobatidae) exhibits intraspecific geographic variation in the spectral traits of its advertisement call. However, it is still unknown whether there is concomitant variation in temporal call traits, and whether call variation is somehow linked to differences in warning coloration (red and yellow morphs). We analyzed spectral and temporal traits of the advertisement calls recorded from 100 males belonging to twelve populations of A. bombetes: nine bearing the red coloration, and three bearing the yellow one. After controlling for eventual effects of temperature and body size of males in call traits, we found geographic variation in both spectral and temporal call traits, yet such variation was unrelated to differences in warning coloration between populations. Previous research indicated that geographic variation in the spectral properties of this species advertisement call can be explained by concomitant differences in the acoustic abiotic environment. Nonetheless, a positive relationship between geographic distance and call divergence also supports the role of genetic drift in the evolution of acoustic signaling of A. bombetes. [ABSTRACT FROM AUTHOR]

As part of a systematic investigation into the ternary system CaO-Fe2O3-SiO2, we discovered a phase with a general chemical composition of A14O20 (where A represents Ca, Fe, and Si) and previously unknown symmetry. Synthesis experiments were conducted at 1200 °C in air with an oxide ratio of CaO:Fe2O3:SiO2 = 3:5:1 in the starting mixture. In addition to β-Ca2SiO4, and hematite (Fe2O3), powder X-ray diffraction analysis indicated the presence of compounds related to the aluminum-free counterpart of a silico-ferrite of calcium and aluminum (SFCA), one of the major bonding phases in iron ore sinter. Single crystals of this so-called SFC phase, large enough for diffraction experiments, were found in the sintered pellet fragments under a digital microscope. The compound with composition Ca2.68Fe10.32Si1.00O20 crystallizes in space group I2/c and has the following basic crystallographic data: a = 10.4643(10) Å, b = 15.2740(11) Å, c = 5.3050(5) Å, β = 110.017(9)°, V = 796.69(13) Å3 and Z = 2. The final composition, as determined by crystal structure refinement, differs slightly from the weight-specified starting mixture of Ca3Fe10SiO20 and suggests the presence of both Fe(III) and small amounts of Fe(II) in the sample. The crystal structure is related to that of the triclinic polytype-1A of SFC, but exhibits a higher degree of disorder due to the partial occupation of additional octahedrally and tetrahedrally coordinated sites. This results in a smaller unit cell and an increased space-group symmetry. The described phase can be regarded as a basic or family structure, from which the two simplest polytypes (1A and hypothetical 2M) can be derived through ordering processes of cations among different possible vacancies. The chemical similarity to one of the phases of the SFCA-family suggests that such disordered compounds could also occur in industrial sinters. [ABSTRACT FROM AUTHOR]

As part of a systematic investigation into the ternary system CaO-Fe2O3-SiO2, we discovered a phase with a general chemical composition of A14O20 (where A represents Ca, Fe, and Si) and previously unknown symmetry. Synthesis experiments were conducted at 1200 °C in air with an oxide ratio of CaO:Fe2O3:SiO2 = 3:5:1 in the starting mixture. In addition to β-Ca2SiO4, and hematite (Fe2O3), powder X-ray diffraction analysis indicated the presence of compounds related to the aluminum-free counterpart of a silico-ferrite of calcium and aluminum (SFCA), one of the major bonding phases in iron ore sinter. Single crystals of this so-called SFC phase, large enough for diffraction experiments, were found in the sintered pellet fragments under a digital microscope. The compound with composition Ca2.68Fe10.32Si1.00O20 crystallizes in space group I2/c and has the following basic crystallographic data: a = 10.4643(10) Å, b = 15.2740(11) Å, c = 5.3050(5) Å, β = 110.017(9)°, V = 796.69(13) Å3 and Z = 2. The final composition, as determined by crystal structure refinement, differs slightly from the weight-specified starting mixture of Ca3Fe10SiO20 and suggests the presence of both Fe(III) and small amounts of Fe(II) in the sample. The crystal structure is related to that of the triclinic polytype-1A of SFC, but exhibits a higher degree of disorder due to the partial occupation of additional octahedrally and tetrahedrally coordinated sites. This results in a smaller unit cell and an increased space-group symmetry. The described phase can be regarded as a basic or family structure, from which the two simplest polytypes (1A and hypothetical 2M) can be derived through ordering processes of cations among different possible vacancies. The chemical similarity to one of the phases of the SFCA-family suggests that such disordered compounds could also occur in industrial sinters. [ABSTRACT FROM AUTHOR]

Crystals of 1-nitronaphthalene were grown by evaporation of a methanol solution. The structure [P21/c, a = 13.2780(13) Å, b = 3.8131(3)° Å, c = 31.851(3) Å, β = 91.173(8), V = 1,612.3 Å3] was solved from a crystal twinned by twofold rotation about [100]. Twinning is explained by the polytype character: layers with (idealized) p2111 symmetry can connect via either 1‾ or 21 operations, leading to geometrically distinct pairs of layers. In the twin individuals, the two kinds of contacts alternate, at the composition plane two subsequent contacts of the same type are realized. [ABSTRACT FROM AUTHOR]

General characteristics of the crystal-chemical features of the sulfide class of minerals, including the genesis, chemical bonding, different approaches to the classification, etc., are given. The structures of the main representatives of tetrahedral sulfides, disulfides, cluster sulfides, sulfides of elements with incomplete valence shells, and sulfosalts are considered. Mineralogically possible iron sulfides in the Earth's core are presented. It is noted that the geophysical interest in sulfide Fe5S2 is related to its possible presence in the core of not only the Earth but also in the other planets in the solar system. [ABSTRACT FROM AUTHOR]

K3FeTe2O8(OH)2(H2O)2 was synthesized under hydrothermal conditions from Te(OH)6, FeSO4·7H2O and 85 wt% KOH in a 1:2:6 molar ratio. The crystal structure is built of a triperiodic network. One disordered water molecule per formula unit is located in a channel and can be partially removed by heating. Systematic one‐dimensional diffuse scattering indicates a polytypic character, which is best described by application of the order–disorder theory. The major polytype is monoclinic with pseudo‐orthorhombic metrics. It is interrupted by fragments of an orthorhombic polytype. The diffraction intensities are analyzed using structure factor calculations. [ABSTRACT FROM AUTHOR]

K3FeTe2O8(OH)2(H2O)2 was synthesized under hydrothermal conditions from Te(OH)6, FeSO4·7H2O and 85 wt% KOH in a 1:2:6 molar ratio. The crystal structure is built of a triperiodic network. One disordered water molecule per formula unit is located in a channel and can be partially removed by heating. Systematic one‐dimensional diffuse scattering indicates a polytypic character, which is best described by application of the order–disorder theory. The major polytype is monoclinic with pseudo‐orthorhombic metrics. It is interrupted by fragments of an orthorhombic polytype. The diffraction intensities are analyzed using structure factor calculations. [ABSTRACT FROM AUTHOR]

In the context of investigating isostructural relationships between sulfates and monofluorophosphates, crystals of the double salts (NH4)2PO3F·NH4NO3 (AFP·AN) and (NH4)2XO4·3NH4NO3 (AX·3AN; X = Se, Cr) were grown from aqueous solutions and structurally characterized using X-ray diffraction and thermal analysis. Whereas the high-temperature forms of the two AX·3AN double salts are in fact isostructural with the sulfate analogue, AFP·AN crystallizes with a reduced amount of NH4NO3 and thus has a unique crystal structure. Both AFP·AN and the two AX·3AN compounds exhibit reversible structural phase transitions. Upon cooling, the monofluorophosphate double salt transforms from the monoclinic room-temperature polymorph (I; P21/n, Z = 4) to the intermediate triclinic polymorph (II; P1, Z = 4) that in turn transforms to the monoclinic low-temperature polymorph (III; P21/n, Z = 4). The two phase transitions (I) → (II) and (II) → (III) are characterized by a significant increase of the unit cell volumes upon cooling. The two AX·3AN double salts transform upon cooling from a disordered monoclinic crystal structure (P21, Z = 2) to a monoclinic polymorph with a doubled unit cell (P21/c, Z = 4). Such a phase transition is not observed for the sulfate analogue. All molecular moieties are fully ordered at −93 °C for the selenate double salt, whereas one of the nitrate anions remains disordered for the chromate double salt even at −173 °C. In all AFP·AN and AX·3AN crystal structures, the nitrate anions play a crucial role during the phase transitions, and an extensive network of N–H···O hydrogen-bonding interactions is responsible for the cohesion of the crystal. [ABSTRACT FROM AUTHOR]

The modular approach is a powerful tool in current inorganic crystal chemistry. It enables not only a more detailed analysis of the known structures and the determination of structural relationships between them, but also the prediction of potentially novel structures that can be applied in modern materials science. A large number of examples of compounds with modular structures allows us to state that structural modularity is a widely spread phenomenon among natural and synthetic compounds. The use of the formalism of OD theory makes it possible to analyze the symmetry of polytypes with different crystal structures. In this review, we collected new data published in the last 15 years about OD structures and phenomena of polytypism and modularity in inorganic compounds, as well as the topological approach to the analysis of crystal structures. [ABSTRACT FROM AUTHOR]

GaAs-based nanowires are among the most promising candidates for realizing a monolithic integration of III–V optoelectronics on the Si platform. To realize their full potential for applications as light absorbers and emitters, it is crucial to understand their interaction with light governing the absorption and extraction efficiency as well as the carrier recombination dynamics determining the radiative efficiency. Here, we study the spontaneous emission of zincblende GaAs/(Al,Ga)As core/shell nanowire arrays by μ-photoluminescence spectroscopy. These ordered arrays are synthesized on patterned Si(111) substrates using molecular beam epitaxy and exhibit an exceptionally low degree of polytypism for interwire separations exceeding a critical value. We record emission spectra over more than five orders of excitation density for both steady-state and pulsed excitation to identify the nature of the recombination channels. An abrupt Mott transition from excitonic to electron–hole plasma recombination is observed, and the corresponding Mott density is derived. Combining these experiments with simulations and additional direct measurements of the external quantum efficiency using a perfect diffuse reflector as a reference, we are able to extract the internal quantum efficiency as a function of carrier density and temperature, as well as the extraction efficiency of the nanowire array. The results vividly document the high potential of GaAs/(Al,Ga)As core/shell nanowires for efficient light emitters integrated on the Si platform. Furthermore, the methodology established in this work can be applied to nanowires of other materials systems of interest for optoelectronic applications. [ABSTRACT FROM AUTHOR]

We report a study of nanophases in the La2O3–MO3 (M = Mo, W) systems, which are known to contain a variety of good oxygen-ion and proton conductors. Mechanically activated La2O3 + MO3 (M = Mo, W) mixtures and the final ceramics have been characterized by differential scanning calorimetry (DSC) and X-ray diffraction (XRD) with Rietveld refinement. The microstructure of the materials has been examined by scanning electron microscopy (SEM), and their conductivity in dry and wet air has been determined using impedance spectroscopy. In both systems, the formation of hexagonal La15M8.5O48 (phase II, 5H polytype) (M = Mo, W) nanophases is observed for the composition 1:1, with exothermic peaks in the DSC curve in the range ~480–520 °C for La15Mo8.5O48 and ~685–760 °C for La15W8.5O48, respectively. The crystallite size of the nanocrystalline tungstates is ~40 nm, and that of the nanocrystalline molybdates is ~50 nm. At higher temperatures (~630–690 and ~1000 °C), we observe irreversible reconstructive phase transitions of hexagonal La15Mo8.5O48 to tetragonal γ-La2MoO6 and of hexagonal La15W8.5O48 to orthorhombic β-La2WO6. We compare the temperature dependences of conductivity for nanoparticulate and microcrystalline hexagonal phases and high-temperature phases differing in density. Above 600 °C, oxygen ion conduction prevails in the coarse-grained La18W10O57 (phase I, 6H polytype) ceramic. Low-density La15W8.5O48 and La15Mo8.5O48 (phase II, 5H polytype) nanoceramics exhibit predominantly electron conduction with an activation energy of 1.36 and 1.35 eV, respectively, in dry air. [ABSTRACT FROM AUTHOR]

The thermodynamic and mechanic anomalies within the region 0.1–0.5 K experimentally observed in hexagonal close-packed solid 4He crystal are discussed and interpreted on the basis of a polytypic HCP structure (close-packed stack of 2D crystalline planes on the triangular lattice). It elucidated the principal role of the stacking faults and Shockley's partial dislocations in the quite unexpected low-temperature behavior of the solid 4He. [ABSTRACT FROM AUTHOR]

Owyheeite [Cu0.09 (1)Ag2.77 (4)Pb10.23 (4)Sb10.89 (5)S28.00 (5)] crystallizes as a twofold superstructure with P21/n symmetry and pseudo‐orthorhombic metrics [a = 8.1882 (3) Å, b = 27.2641 (7) Å, c = 22.8679 (7) Å, β = 90.293 (3)°, V = 5105.0 (3) Å3, Z = 4]. Owyheeite is systematically twinned by reflection at (021) or equivalently (021). Twinning is explained by describing a simplified Pmcn archetype structure as polytype built of two kinds of rods, which contact via electron‐pair micelles. A procedure of generating hypothetical polytypes by tiling space with partially overlapping equivalent regions is described. [ABSTRACT FROM AUTHOR]

Glass containing chromium is a promising material for use in various modern fields of application (laser technology, optoelectronic devices, and luminescent resources). Chromium oxides are well-known nucleating agents that can cause crystallization. One of the most commonly observed crystalline phases in silicate glasses is cristobalite, which lowers their mechanical strength, leading to the destruction of the material. The objective of this investigation was to study in detail the crystallization of cristobalite in sodium borosilicate glass in the presence of 2 mol% Cr2O3, depending on the thermal history of the glass. The glass was studied using XRD, SEM, EPR, FTIR-spectroscopy, XPS, and solid-state NMR. Eskolaite, α-Cr2O3, which had crystallized in this glass, stimulated the bulk crystallization of cristobalite at 550 °C after isothermally treating it for 72 h, due to the phase-separated structure of the glass with its interpenetrating phase morphology. Polytypism, resulting in the incorporation of alkalis into the cristobalite structure, was observed. Cr2O3 causes the catalytic crystallization of cristobalite at an extremely low temperature, which is at lower concentrations and temperatures than in glass containing Fe2O3 with a similar composition. The crystal growth rate and the incubation time for the crystallization of cristobalite were roughly estimated. [ABSTRACT FROM AUTHOR]

Compounds with the general formula Ln3+(SeO3)(HSeO3)·2H2O, where Ln = Sm3+, Tb3+, Nd3+ and Lu3+, are characterized by orthorhombic symmetry with space group P212121 and unit‐cell parameters in the ranges a ∼ 6.473–6.999, b ∼ 6.845–7.101, c ∼ 16.242–16.426 Å. Light‐purple irregularly shaped crystals of a new monoclinic polytype of neodymium selenite Nd(SeO3)(HSeO3)·2H2O have been obtained during a mild‐condition hydrothermal synthesis. The monoclinic unit‐cell parameters are: a = 7.0815 (2), b = 6.6996 (2), c = 16.7734 (5) Å, β = 101.256 (1)°, V = 780.48 (6) Å3; space group P21/c. The crystal structures of Nd(SeO3)(HSeO3)·2H2O polymorphs show order–disorder (OD) character and can be described using the same OD groupoid family, more precisely a family of OD structures built up from two kinds of non‐polar layers (category IV). The first monoclinic maximum degree order (MDO) structure (MDO1‐polytype) with space group P21/c can be obtained when the inversion centre is active in the L2n‐type layers, while the second MDO structure (MDO2‐polytype) is orthorhombic with space group P212121 and can be obtained when the [21‐‐] operation is active in the L2n‐type layers. The structural complexity parameters and DFT calculations of both polytypes show that the polytype structures are extremely close energy‐wise and almost equally viable from the point of total energy of the structure. [ABSTRACT FROM AUTHOR]

In the context of investigating isostructural relationships between sulfates and monofluorophosphates, crystals of the double salts (NH4)2PO3F·NH4NO3 (AFP·AN) and (NH4)2XO4·3NH4NO3 (AX·3AN; X = Se, Cr) were grown from aqueous solutions and structurally characterized using X-ray diffraction and thermal analysis. Whereas the high-temperature forms of the two AX·3AN double salts are in fact isostructural with the sulfate analogue, AFP·AN crystallizes with a reduced amount of NH4NO3 and thus has a unique crystal structure. Both AFP·AN and the two AX·3AN compounds exhibit reversible structural phase transitions. Upon cooling, the monofluorophosphate double salt transforms from the monoclinic room-temperature polymorph (I; P21/n, Z = 4) to the intermediate triclinic polymorph (II; P1, Z = 4) that in turn transforms to the monoclinic low-temperature polymorph (III; P21/n, Z = 4). The two phase transitions (I) → (II) and (II) → (III) are characterized by a significant increase of the unit cell volumes upon cooling. The two AX·3AN double salts transform upon cooling from a disordered monoclinic crystal structure (P21, Z = 2) to a monoclinic polymorph with a doubled unit cell (P21/c, Z = 4). Such a phase transition is not observed for the sulfate analogue. All molecular moieties are fully ordered at −93 °C for the selenate double salt, whereas one of the nitrate anions remains disordered for the chromate double salt even at −173 °C. In all AFP·AN and AX·3AN crystal structures, the nitrate anions play a crucial role during the phase transitions, and an extensive network of N–H···O hydrogen-bonding interactions is responsible for the cohesion of the crystal.

We consider the structural features and symmetry of alkali metal (Li, Na, K, Rb, Cs) and thallium hydroxides crystallizing in LiOH, α-NaOH, β-NaOH, TlOH structure types. The complexities of crystal structures are calculated. The structure of these compounds is based on the alternation of dense layers with strong metal–oxygen bonds and the interlayer space. The common features of hydroxide structures is the location of atoms at square network nodes. Atoms are bound the networks by both true symmetry elements of space groups and additional pseudo-symmetry elements, which indicates the order-disorder (OD) character of hydroxide structures. The arrangement of atoms by the square network pattern allows the consideration of these compounds as a uniform group whose structures retain the tetragonal character of the initial LiOH structure, which is expressed in the similarity of horizontal parameters of their unit cells. [ABSTRACT FROM AUTHOR]

Our systematic explorations of the complex rare earth tellurite halide family have added several new [Ln12(TeO3)12][M6X24] (M = Cd, Mn, Co) representatives containing strongly deficient and disordered metal-halide layers based on transition metal cations. The degree of disorder increases sharply with decrease of M2+ radius and the size disagreements between the cationic [Ln12(TeO3)12]+12 and anionic [M6Cl24]−12 layers. From the crystal chemical viewpoint, this indicates that the families of both rare-earth selenites and tellurites can be further extended; one can expect formation of some more complex structure types, particularly among selenites. Analysis of the polytypism of compounds have been performed using the approach of OD ("order–disorder") theory. [ABSTRACT FROM AUTHOR]

1,3‐di(tert‐butyl)cyclopentadienyl pentaphosphaferrocene (Cp′′FeP5) crystallizes as polytypes, where adjacent layers are related by either a c‐ or an n‐glide reflection. In the simplest possible polytypes only one kind of glide reflections is realized, leading to overall Pca21 or Pna21 symmetry respectively. Crystals isolated from the same crystallization experiment feature a varying degree of stacking disorder. One‐dimensional diffuse scattering of a crystal with particularly strong disorder was modelled with closed‐form expressions derived from a growth model. A range of interaction of s = 2 was necessary to satisfactorily describe the observed diffraction intensities. The crystal can be described as a disordered analogue of an allotwin, that is an intergrowth of the Pca21 or Pna21 polytypes with a distinct preference for the former. [ABSTRACT FROM AUTHOR]

We report a study of nanophases in the La2O3–MO3 (M = Mo, W) systems, which are known to contain a variety of good oxygen-ion and proton conductors. Mechanically activated La2O3 + MO3 (M = Mo, W) mixtures and the final ceramics have been characterized by differential scanning calorimetry (DSC) and X-ray diffraction (XRD) with Rietveld refinement. The microstructure of the materials has been examined by scanning electron microscopy (SEM), and their conductivity in dry and wet air has been determined using impedance spectroscopy. In both systems, the formation of hexagonal La15M8.5O48 (phase II, 5H polytype) (M = Mo, W) nanophases is observed for the composition 1:1, with exothermic peaks in the DSC curve in the range ~480–520 °C for La15Mo8.5O48 and ~685–760 °C for La15W8.5O48, respectively. The crystallite size of the nanocrystalline tungstates is ~40 nm, and that of the nanocrystalline molybdates is ~50 nm. At higher temperatures (~630–690 and ~1000 °C), we observe irreversible reconstructive phase transitions of hexagonal La15Mo8.5O48 to tetragonal γ-La2MoO6 and of hexagonal La15W8.5O48 to orthorhombic β-La2WO6. We compare the temperature dependences of conductivity for nanoparticulate and microcrystalline hexagonal phases and high-temperature phases differing in density. Above 600 °C, oxygen ion conduction prevails in the coarse-grained La18W10O57 (phase I, 6H polytype) ceramic. Low-density La15W8.5O48 and La15Mo8.5O48 (phase II, 5H polytype) nanoceramics exhibit predominantly electron conduction with an activation energy of 1.36 and 1.35 eV, respectively, in dry air.

Single crystals of the two alkali metal zinc oxidotellurates(IV), Rb2Zn(TeO3)(CO3)·H2O and Na2Zn2Te4O11, were obtained by reactions of mixtures of ZnO, TeO2, Rb2CO3 (molar ratios 2:3:6) and ZnO, TeO2, Na2CO3 (molar ratios 2:3:10), respectively, with small amounts of water as a mineralizer. Both compounds crystallize as order-disorder (OD) structures of layers and feature a high stacking fault probability. The crystal structure of Rb2Zn(TeO3)(CO3)·H2O is composed of layers extending parallel to (100). The structure is composed of two kinds of non-polar OD layers consisting of trigonal-pyramidal [TeO3]2−, tetrahedral [ZnO4]6−, Rb1+, and CO32−, H2O, Rb2+, respectively. Different centrings of the layer groups lead to an ambiguity in the stacking arrangement. The crystal structure of Na2Zn2Te4O11 is built from layers extending parallel to (001). Trigonal-pyramidal [TeO3]2− and bisphenoidal [TeO4]4− polyhedra form [Te4O11]6− groups, which are connected by longer Te–O-contacts to form 1∞[Te8O22]12− double chains oriented along either [100] or [010]. These chains form non-polar layers, which appear alternatingly in two orientations related by a fourfold rotoinversion. The Zn2+ and Na+ cations are located at the layer interface. The stacking ambiguity is due to different lattices of adjacent layers. [ABSTRACT FROM AUTHOR]

Single crystals of the two alkali metal zinc oxidotellurates(IV), Rb2Zn(TeO3)(CO3)·H2O and Na2Zn2Te4O11, were obtained by reactions of mixtures of ZnO, TeO2, Rb2CO3 (molar ratios 2:3:6) and ZnO, TeO2, Na2CO3 (molar ratios 2:3:10), respectively, with small amounts of water as a mineralizer. Both compounds crystallize as order-disorder (OD) structures of layers and feature a high stacking fault probability. The crystal structure of Rb2Zn(TeO3)(CO3)·H2O is composed of layers extending parallel to (100). The structure is composed of two kinds of non-polar OD layers consisting of trigonal-pyramidal [TeO3]2−, tetrahedral [ZnO4]6−, Rb1+, and CO32−, H2O, Rb2+, respectively. Different centrings of the layer groups lead to an ambiguity in the stacking arrangement. The crystal structure of Na2Zn2Te4O11 is built from layers extending parallel to (001). Trigonal-pyramidal [TeO3]2− and bisphenoidal [TeO4]4− polyhedra form [Te4O11]6− groups, which are connected by longer Te–O-contacts to form 1∞[Te8O22]12− double chains oriented along either [100] or [010]. These chains form non-polar layers, which appear alternatingly in two orientations related by a fourfold rotoinversion. The Zn2+ and Na+ cations are located at the layer interface. The stacking ambiguity is due to different lattices of adjacent layers. [ABSTRACT FROM AUTHOR]

Experimentally observed physical properties of condensed He II phase of the simplest atomic matter 4 He are reviewed and analysed in view of the governing Shubnikov's idea about spatial and spin ordering in 4 He below λ transition. The matter has been considered within the framework of the unitary quantum mechanical concept for interatomic interaction between helium atoms based on principal role of the spin subsystem which is a real nature of the commonly accepted helium quanticity widely discussed over the last century. [ABSTRACT FROM AUTHOR]

In this work, the crystal structure of a natural specimen of the "Messbach-type" variscite from Tooele Co. (Utah, U.S.A.) was determined, and the dehydration process leading to the transition to a new AlPO SB 4 sb phase was monitored at high temperature. Crystal structure description of variscite 2O and comparison with variscite 1O Like variscite I 1O i , the variscite I 2O i crystal structure belongs to the family of framework 3D MT structures in which octahedra (M) and tetrahedra (T) are linked by bridging O atoms ([30]). The Al-coordination change from octahedral to tetrahedral that accompanies dehydration, clearly demonstrated by the SP 27 sp Al NMR data, confirms the removal of the Al-bound H SB 2 sb O molecules as the dehydration mechanism, transforming the variscite octahedral-tetrahedral framework to a purely tetrahedral framework. Keywords: Variscite; X-ray powder diffraction; charge-flipping; nuclear magnetic resonance; polytypism; AlPO4 EN Variscite X-ray powder diffraction charge-flipping nuclear magnetic resonance polytypism AlPO4 1385 1395 11 07/05/22 20220701 NES 220701 Introduction Interest in hydrated aluminophosphates and in compounds that derive from their dehydration spans a wide spectrum of disciplines. [Extracted from the article]

The hexagonal‐2H crystal phase of Ge has recently emerged as a promising direct bandgap semiconductor in the mid‐infrared range providing new prospects of additional opto‐electronic functionalities of group‐IV semiconductors (Ge and SiGe). The controlled synthesis of such hexagonal‐2H Ge phase is a challenge that can be overcome by using wurtzite GaAs nanowires as a template. However, depending on growth conditions, unusual basal stacking faults (BSFs) of I3‐type are formed in the metastable 2H structure. The growth of such core/shell heterostructures is observed in situ and in real time by means of environmental transmission electron microscopy using chemical vapor deposition. The observations provide the first direct evidence of a step‐flow growth of Ge‐2H epilayers and reveal the growth‐related formation of I3‐BSFs during unstable growth. Their formation conditions are dynamically investigated. Through these in situ observations, a scenario can be proposed for the nucleation of I3‐type BSFs that is likely valid for any metastable hexagonal 2H or wurtzite structures grown on m‐plane substrates. Conditions are identified to avoid their formation for perfect crystalline synthesis of SiGe‐2H. [ABSTRACT FROM AUTHOR]

The sp2-bonded layered compound boron nitride (BN) exists in more than a handful of different polytypes (i.e., different layer stacking sequences) with similar formation energies, which makes obtaining a pure monotype of single crystals extremely tricky. The co-existence of polytypes in a similar crystal leads to the formation of many interfaces and structural defects having a deleterious influence on the internal quantum efficiency of the light emission and on charge carrier mobility. However, despite this, lasing operation was reported at 215 nm, which has shifted interest in sp2-bonded BN from basic science laboratories to optoelectronic and electrical device applications. Here, we describe some of the known physical properties of a variety of BN polytypes and their performances for deep ultraviolet emission in the specific case of second harmonic generation of light. [ABSTRACT FROM AUTHOR]

The crystal structure of Na2Mn3(SO4)4 [unit‐cell parameters a = 14.8307 (18), b = 9.9107 (18), c = 8.6845 (12) Å, space group Cmc21] displays order–disorder (OD) character and can be described using the OD groupoid family, more precisely a family of OD structures built up by two types of non‐polar layers, with layer symmetry P(m)c21 (L2n+1 type) and P(b)cm (L2n type) (category IV). A new hypothetical MDO2 polytype has been proposed and the geometry optimization demonstrates its reasonability as another possible stable polytype. Compounds Na2Mn3–xMgx(SO4)4 with the unit‐cell parameters a ∼ 29.2–29.7 Å, b ∼ 9.5–9.9 Å, c ∼ 8.7 Å and space group Pbca can be described in terms of modularity as a sequence of A, S1 and S2 modules:...|AS1AS2AS1AS2|... or (AS1AS2), together with MDO1 (AS1AS1) and MDO2 (AS2AS2). The crystal structures of itelmenite, NaCaFe3+3(PO4)4, and Ca2MgFe3+2(PO4)4 are crystal‐chemical isotypic to Na2Mn3–xMgx(SO4)4 and should be considered as (A*S1A*S2) derivatives of the (AS1AS2)‐type structure. [ABSTRACT FROM AUTHOR]

Peculiar and unique growth mechanisms involved in semiconductor nanowires (NWs) pave the way to the achievement of new crystallographic phases and remarkable material properties, and hence, studying polytypism in semiconductor NWs arouses a strong interest for the next generation of electronic and photonic applications. In this context, the growth of ZnS nanowires has been investigated, as bulk ZnS compound exhibits numerous unstable polytypes at high temperatures, but their stable occurrence is highly anticipated in a nanowire due to its special quasi-dimensional shape and growth modes. In this work, the idea is to provide a change in the growth mechanism via the physical state of catalyst droplet (liquid or solid) and hence, study the induced structural modifications in ZnS nanowires. The HRTEM images of VLS (via liquid alloyed catalyst) grown ZnS NWs show periodic stacking faults, which is precisely identified as a stacking sequence of cubic or hexagonal individual planes leading to an astonishing 15R crystal polymorph. This crystallographic phase is observed for the first time in nanowires. Contrastingly, NWs grown with VSS (via solid catalyst) show crystal polytypes of zinc blende and wurtzite. We calculate and discuss the role of cohesive energies in the formation of such ZnS polytypes. Further, we present the selection rules for the crystallization of such 15R structure in NWs and discuss the involved VLS and VSS growth mechanisms leading to the formation of different crystal phases. [ABSTRACT FROM AUTHOR]

The platinum-group elements (PGMs) owing their high chemical stability, and unique properties, have recently attracted great research interest in various promising applications. Herein using state-of-the-art first-principles evolutionary algorithms, we explore the thermodynamic and mechanical properties of Ir, Os, and their alloys. Hitherto unknown polytype-phases of Ir (6H, 4H, 2H) and Os (6H, 4H) have been predicted, two of these (4H-Ir, and 4H-Os) were recently observed experimentally. Os and, to more extent, Ir polytypes divulge weak thermodynamic instabilities. Phonon dispersion and elastic constants calculations were carried out to demonstrate the dynamical and mechanical stabilities of these phases. For the Ir-Os alloys, four ordered structures (Pm-3 m (IrOs3, Ir3Os), P-6 m2 (IrOs), P-3 m1 (IrOs2)) have been predicted, which are found mechanically and dynamically stables and manifest insignificantly thermodynamic instabilities. Furthermore, our obtained results show that Ir-Os systems evince outstanding mechanical properties, IrOs3 has high bulk modulus (∼421GPa), large shear modulus (∼266GPa), and Young’s modulus (659 GPa); whereas IrOs2 possesses a remarkable large elastic constants C11 (779GPa) and C33 (814GPa). The anisotropic calculations show that IrOs3 exhibits that greatest anisotropy in the universal elastic anisotropy index AU, shear modulus (AG) and Young’s modulus (AE); while IrOs2 has the smallest elastic anisotropy. Moreover, IrOs3 reveals distinguished hardness of 27.6GPa, which is 3% weaker than 2H-Os (28.6GPa), and is so far one of the hardest materials among the PGMs binary alloys.

The present study deals with accurate identification of polytypes, twins, and allotwins – oriented crystal associations of more than one polytype. The trioctahedral 1:1 layered silicate cronstedtite was studied using single-crystal X-ray diffraction data collected with a four-circle diffractometer equipped with a CCD detector. The sample from the skarn occurrence, Ouedi Beht, El Hammam, Morocco, was explored. It contains cronstedtite in fibrous massive aggregates in the central part, and euhedral crystals in surrounding veinlets and druses. The reciprocal space (RS) sections created by the diffractometer software and presented here were used to determine the OD (ordered-disordered) subfamilies (Bailey's group A, B, C, D) and to identify polytypes. The chemical compositions of some crystals were determined thereafter by electron probe microanalysis (EPMA-WDS). Some crystals studied are more or less complicated allotwins. Polytypes were thus separated by cleaving crystals into smaller parts in many cases. All polytypes found belong to subfamilies A or D. The following polytypes of the subfamily A were identified: 2M1 (a = 5.49, b = 9.51, c = 14.40 Å, β = 97.30°, space group Cc), 1M (a = 5.51, b = 9.54, c = 7.33 Å, β = 104.5°, Cm), 3T (a = 5.51, c = 21.32 Å, P31), 6T2 (a = 5.50, c = 42.60 Å, P31). 2M1 and 3T were present as isolated crystals or separated by cleaving, otherwise all these polytypes are parts of allotwins. The 2M1 polytype is sometimes twinned by reticular pseudo merohedry with twin index n = 3 and 120° rotation about the chex axis as the twin operation. Allotwins of 1M + twinned 2M1 polytypes are also present. Another kind of twinning, with rotation by (2n+1)×60° about chex is rare. The subfamily D is represented mostly by 2H1 and 2H2 polytypes, a = 5.50, c = 14.25 Å, space groups P63cm (2H1), P63 (2H2). In addition, several six-layer (a = 5.49, c = 42.80 Å), mostly non-MDO polytypes were separated from allotwins by cleaving. In order to identify them, 24 possible stacking sequences were modeled, diffraction patterns calculated, graphical identification diagrams constructed, and comparisons made with actual RS sections. This simulation revealed that five pairs of sequences provided identical diffraction patterns. Polytypes actually found correspond to the following sequences: 1 (6T1), 5 (proposed Ramsdell's symbol 6T3), 8+10 (6T5), 11+12 (6T4), 24 (6T6, trigonal polytypes, space group type P3), 22 (6R1), and 23 (6R2, rhombohedral polytypes, space group type R3c and R3, respectively). The RS section corresponding to the hexagonal polytype 6H2 (sequence 14) was also found. However, diffraction patterns geometrically indistinguishable can be produced by the twin with rotation by 180° about chex as a twin operation of the rhombohedral polytype 6R2 (sequence 23). Several aggregates with fiber texture of polytype 2H2 were separated from the central part. Use of EPMA-WDS revealed Fe and Si along with significant amounts of Mn and Mg. Crystals from veins were more Mn- and Mg-rich than these from the central part. Traces of Cl, S, and Al are present also. [ABSTRACT FROM AUTHOR]

The layered iron silicate cronstedtite was encountered in ore veins in the exploration shaft mined in the Neoproterozoic black shale-hosted pyrite-manganese deposit near Litošice (Eastern Bohemia, Czech Republic) around 1955. It forms up to 2 mm thick black double or single bands in symmetrically zoned hydrothermal veins cutting shales. The specimens selected from available material were studied by single-crystal X-ray diffraction using the four-circle diffractometer with an area detector. The chemical composition of some of the specimens was determined by the electron probe microanalysis (EPMA) in the WDS mode. Furthermore, a polished section of the ore material with cronstedtite bands was prepared, and the mineral association was analyzed with the aid of back-scattered electron (BSE) images. The interpretation of reciprocal space (RS) sections produced by the diffractometer software allowed the determination of OD subfamilies (Bailey's groups) A, B, C, D, and polytypes. The 1T polytype (subfamily C), a = 5.52, c = 7.12 Å, space group P31m, is the most abundant in the occurrence. In rare cases, it forms oriented crystal associations (allotwins) with the 1M polytype (subfamily A), a = 5.52 Å, b = 9.55, c = 7.136 Å, ß = 104.4°, space group Cm. Fully disordered allotwinned crystals of the A + C subfamilies were found, too. In addition, few allotwins of the polytype 2H1 (subfamily D) with a small amount of 2H2, were identified. Unit cell parameters are a = 5.49, c = 14.21 Å, space groups are P63cm (2H1), and P63 (2H2). EPMA-WDS of selected crystals of prevailing 1T polytype revealed elevated contents of Mn (0.19-0.62 apfu) and low contents of Mg (up to 0.13 apfu) and Cl (up to 0.05 apfu), respectively. More rare 2H1 (+ 2H2) polytypes show elevated contents of Mg in the range of 0.19-0.62 apfu and distinctly lower Mn (up to 0.07 apfu) and Cl contents (up to 0.01 apfu). The BSE images reveal that cronstedtite bands are associated with multiple generations of carbonates (rhodochrosite, siderite, rarely magnesite and calcite), quartz, opal, pyrite and carbonate-fluorapatite. Intense metasomatic replacement of cronstedtite by opal and siderite appeared especially around the center of the studied vein. [ABSTRACT FROM AUTHOR]

Our systematic explorations of the complex rare earth tellurite halide family have added several new [Ln12(TeO3)12][M6X24] (M = Cd, Mn, Co) representatives containing strongly deficient and disordered metal-halide layers based on transition metal cations. The degree of disorder increases sharply with decrease of M2+ radius and the size disagreements between the cationic [Ln12(TeO3)12]+12 and anionic [M6Cl24]−12 layers. From the crystal chemical viewpoint, this indicates that the families of both rare-earth selenites and tellurites can be further extended; one can expect formation of some more complex structure types, particularly among selenites. Analysis of the polytypism of compounds have been performed using the approach of OD (“order–disorder”) theory.

Crystals of the hydrous magnesium orthotellurate(VI) Mg(H2O)2[TeO2(OH)4] were grown by slow diffusion of an aqueous MgCl2 solution into a KOH/Te(OH)6 solution immobilized in gelatin. The crystal structure is built of sheets of nearly regular corner‐sharing [MgO6] and [TeO6] octahedra. Half of the bridging O atoms are connected to disordered H atoms, which are located in rhomboidal voids (long and short diameters of ∼5.0 and ∼2.5 Å, respectively) of these layers. Moreover, the TeVI atom connects to two OH− ions and the MgII atom to two H2O molecules. The OH− ions and H2O molecules connect adjacent layers forming a disordered hydrogen‐bonding network. In a given layer, an adjacent layer may be positioned in four ways, which can be characterized by one of two origin shifts and one of two orientations with respect to [100]. The crystals feature a disordered stacking arrangement, leading to rods of diffuse scattering in the diffraction pattern. The polytypism is explained by application of the order–disorder (OD) theory. Different refinement models are compared and the diffuse scattering is evaluated with structure factor calculations. The correlation coefficient of subsequent origin shifts is ∼ −0.33, whereas the orientation of the layers is essentially random. Determining the latter is particularly difficult owing to a small contribution to the diffraction pattern and virtually indistinguishable diffraction patterns for pairs of correlations with the same absolute value. On longer standing in a glass vial, an ordered polytype forms. [ABSTRACT FROM AUTHOR]

Abstract We report on the growth of Te-doped catalyst-free InAs nanowires by molecular beam epitaxy on silicon (111) substrates. Changes in the wire morphology, i.e. a decrease in length and an increase in diameter have been observed with rising doping level. Crystal structure analysis based on transmission electron microscopy as well as X-ray diffraction reveals an enhancement of the zinc blende/(wurtzite+zinc blende) segment ratio if Te is provided during the growth process. Furthermore, electrical two-point measurements show that increased Te-doping causes a gain in conductivity. Two comparable growth series, differing only in As-partial pressure by about 1 × 10−5 Torr while keeping all other parameters constant, were analyzed for different Te-doping levels. Their comparison suggests that the crystal structure is strongly affected and the conductivity gain is more distinct for wires grown at a comparably higher As-partial pressure.

Phenotypic variation among populations, as seen in the signaling traits of many species, provides an opportunity to test whether similar factors generate repeated phenotypic patterns in different parts of a species' range. We investigated whether genetic divergence, abiotic gradients, and sympatry with closely related species explain variation in the dewlap colors of Amazon Slender Anoles, Anolis fuscoauratus. To this aim, we characterized dewlap diversity in the field with respect to population genetic structure and evolutionary relationships, assessed whether dewlap phenotypes are associated with climate or landscape variables, and tested for nonrandom associations in the distributions of A. fuscoauratus phenotypes and sympatric Anolis species. We found that dewlap colors vary among but not within sites in A. fuscoauratus. Regional genetic clusters included multiple phenotypes, while populations with similar dewlaps were often distantly related. Phenotypes did not segregate in environmental space, providing no support for optimized signal transmission at a local scale. Instead, we found a negative association between certain phenotypes and sympatric Anolis species with similar dewlap color attributes, suggesting that interactions with closely related species promoted dewlap divergence among A. fuscoauratus populations. Amazon Slender Anoles emerge as a promising system to address questions about parallel trait evolution and the contribution of signaling traits to speciation. [ABSTRACT FROM AUTHOR]

The sp2-bonded layered compound boron nitride (BN) exists in more than a handful of different polytypes (i.e., different layer stacking sequences) with similar formation energies, which makes obtaining a pure monotype of single crystals extremely tricky. The co-existence of polytypes in a similar crystal leads to the formation of many interfaces and structural defects having a deleterious influence on the internal quantum efficiency of the light emission and on charge carrier mobility. However, despite this, lasing operation was reported at 215 nm, which has shifted interest in sp2-bonded BN from basic science laboratories to optoelectronic and electrical device applications. Here, we describe some of the known physical properties of a variety of BN polytypes and their performances for deep ultraviolet emission in the specific case of second harmonic generation of light.

The present study provides an example of the accurate identification of polytypes of trioctahedral 1:1 layered silicates from single-crystal X-ray diffraction data collected with the aid of a four-circle diffractometer equipped with an area detector. Single crystals of the mineral cronstedtite from the Nagybörzsöny gold ore deposit, northern Hungary, were studied. The chemical composition of some crystals was determined by electron probe microanalysis (EPMA). The precession-like images of the reciprocal space (RS) sections created by the diffractometer software and presented in the study were used to determine the OD (ordered-disordered) subfamilies (Bailey's groups A, B, C, D) and particular polytypes. With one exception, all crystals studied belong to subfamily A. The rare polytype 1M, a = 5.51, b = 9.54, c = 7.33 Å, β = 104.5°, space group Cm is relatively abundant in this occurrence. Another polytype 3T, a = 5.51, c = 21.32 Å, space group P31 was also found. Both polytypes occur separately or in mixed, mostly 1M dominant crystals. Some 1M polytype crystals are twinned by order 3 reticular merohedry with a 120° rotation along the chex axis as the twin operation. A rare 1M+3T mixed crystal with 1M part twinned also contains a small amount of subfamily C. A possible presence of the most common 1T polytype of this subfamily cannot be confirmed because of overlap of the characteristic reflections with those of 3T. Several completely disordered crystals produced diffuse streaks instead of discrete characteristic reflections on the RS sections. The EPMA revealed Fe, Si, traces of Mg, Al, S, and Cl. One black crystal originally considered to be cronstedtite was identified as (111) twinned sphalerite. Some crystals of cronstedtite are covered partially by a honey-brown crust or small crystals of siderite. [ABSTRACT FROM AUTHOR]

The effect of elastic stresses on the Ga(As, P) crystalline phase of nanowires is studied. It is shown that the elastic stresses can lead to the stable growth of whisker nanocrystals in the metastable phase. The possibility of crystal-phase switching inside a GaP nanowire after the appearance of a Ga(As, P) insert is described. [ABSTRACT FROM AUTHOR]

Modular structures are crystal structures built by subperiodic (zero‐, mono‐ or diperiodic) substructures, called modules. The whole set of partial operations relating substructures in a modular structure build up a groupoid; modular structures composed of identical substructures are described by connected groupoids, or groupoids in the sense of Brandt. A general approach is presented to describe modular structures by Brandt's groupoids and how to obtain the corresponding space groups, in which only the partial operations that have an extension to the whole crystal space appear. [ABSTRACT FROM AUTHOR]

[Cu2ZnTeO4]2+[SO4·H2O]2− crystallizes as a category IV order‒disorder (OD) structure with two kinds of nonpolar layers with the formal composition [Cu2ZnTeO4]2+ and [SO4·H2O]2−, respectively. Reflection symmetry that is only valid for the [Cu2ZnTeO4]2+ layers leads to two ways of placing the [SO4·H2O]2− layers. Two of the polytypes are of a maximum degree of order (MDO1: P21/m and MDO2: Pnma). The crystal under investigation is mostly of the MDO2 polytype. Remaining positive electron density peaks in the difference Fourier maps indicate fragments of the MDO1 polytype. [ABSTRACT FROM AUTHOR]

RbSbO3 crystallizes in different polytypes composed of trigonal [SbO3]− and [Rb]+ layers, which can appear in two orientations with respect to m<100> and m<120>, respectively. The order–disorder (OD) polytype family comprises four polytypes of a maximum degree of order (MDO), of which two are realized in actual crystals: MDO1 (R3¯, c ≈ 19.8 Å, isotypic to ilmenite‐like KSbO3) and MDO2 (R3¯c, c ≈ 39.2 Å, isotypic to AgRuO3). Crystals grown from RbF:Sb2O5 in a 2:1 ratio are MDO1:MDO2 allotwins, whereas crystals grown from a 12:1 mixture are composed of MDO2 only. The stacking ambiguity leads to twinning by rotation about <100> or <120>. KSbO3 crystals grown from KF:Sb2O5 are composed of the MDO1 polytype only. The crystals are fourfold twins owing to stacking faults. A detailed description of the analysis of the diffraction data is given. [ABSTRACT FROM AUTHOR]

[Cu2ZnTeO4]2+[SO4·H2O]2− crystallizes as a category IV order‒disorder (OD) structure with two kinds of nonpolar layers with the formal composition [Cu2ZnTeO4]2+ and [SO4·H2O]2−, respectively. Reflection symmetry that is only valid for the [Cu2ZnTeO4]2+ layers leads to two ways of placing the [SO4·H2O]2− layers. Two of the polytypes are of a maximum degree of order (MDO1: P21/m and MDO2: Pnma). The crystal under investigation is mostly of the MDO2 polytype. Remaining positive electron density peaks in the difference Fourier maps indicate fragments of the MDO1 polytype. [ABSTRACT FROM AUTHOR]

Synchysite-(Ce) at Cinquevalli occurs as fine needles intergrown with quartz in quartz-dikes and in association with altered K-feldspar and oxidized chalcopyrite as major constituents. Synchysite-(Ce) [Ca1.00(Ce0.43La0.26Nd0.17Y0.07Pr0.04Sm0.02Gd0.01)Σ=1.00(CO3)2(F0.58(OH)0.42)], shows an overgrowth rim of bastnäsite-(Ce) [(Ce0.34La0.25Nd0.17Pb0.07C a0.06Y0.06Pr0.04S m0.02Gd0.01)Σ=1.00C O3(F0.75(OH)0.25)]. Unit cell refinement of synchysite (C2/c) and bastnäsite (P62c) led to a = 12.272(4), b = 7.100(2), c = 18.640(5) Å, β = 102.71(5)° , and a = 7.085(1), c = 9.746(2) Å, respectively. Polysomatic faults are sporadic, but polytypic disorder is widespread. High resolution transmission electron microscopy images taken along [100] or h130i show an apparent order and the related diffraction patterns are streak-free. Conversely, along [010] or h110i, a high density of stacking faults is observed and the related diffraction patterns show hhl rows with h , 3n affected by streaks. No ordered domain larger than a few unit cells was detected. The stacking sequence of (Ca,REE)-fluorcarbonates can be compared with subfamily-B mica polytypes (2M2, 2O and 6H), which are characterized by n·60° (n = odd) rotations. Subfamily-A polytypes (1M, 2M1 and 3T), characterized by n·60° (n = even) rotations, should not be possible. Synchysite, characterized by ±60° rotations, can be likened to the 2M2 polytype. [ABSTRACT FROM AUTHOR]

Denisovite is a rare mineral occurring as aggregates of fibres typically 200–500 nm diameter. It was confirmed as a new mineral in 1984, but important facts about its chemical formula, lattice parameters, symmetry and structure have remained incompletely known since then. Recently obtained results from studies using microprobe analysis, X-ray powder diffraction (XRPD), electron crystallography, modelling and Rietveld refinement will be reported. The electron crystallography methods include transmission electron microscopy (TEM), selected-area electron diffraction (SAED), high-angle annular dark-field imaging (HAADF), high-resolution transmission electron microscopy (HRTEM), precession electron diffraction (PED) and electron diffraction tomography (EDT). A structural model of denisovite was developed from HAADF images and later completed on the basis of quasi-kinematic EDT data by ab initio structure solution using direct methods and least-squares refinement. The model was confirmed by Rietveld refinement. The lattice parameters are a = 31.024 (1), b = 19.554 (1) and c = 7.1441 (5) Å, β = 95.99 (3)°, V = 4310.1 (5) Å3 and space group P12/a1. The structure consists of three topologically distinct dreier silicate chains, viz. two xonotlite-like dreier double chains, [Si6O17]10−, and a tubular loop-branched dreier triple chain, [Si12O30]12−. The silicate chains occur between three walls of edge-sharing (Ca,Na) octahedra. The chains of silicate tetrahedra and the octahedra walls extend parallel to the z axis and form a layer parallel to (100). Water molecules and K+ cations are located at the centre of the tubular silicate chain. The latter also occupy positions close to the centres of eight-membered rings in the silicate chains. The silicate chains are geometrically constrained by neighbouring octahedra walls and present an ambiguity with respect to their z position along these walls, with displacements between neighbouring layers being either Δz = c/4 or −c/4. Such behaviour is typical for polytypic sequences and leads to disorder along [100]. In fact, the diffraction pattern does not show any sharp reflections with l odd, but continuous diffuse streaks parallel to a* instead. Only reflections with l even are sharp. The diffuse scattering is caused by (100) nanolamellae separated by stacking faults and twin boundaries. The structure can be described according to the order–disorder (OD) theory as a stacking of layers parallel to (100).

A part of the system CaO‐SiO2–Al2O3–Fe2O3–MgO which is of relevance to iron‐ore sintering has been studied in detail. For a bulk composition corresponding to 10.45 wt% CaO, 5.49 wt% MgO, 69.15 wt% Fe2O3, 13.37 wt% Al2O3 and 1.55 wt% SiO2 synthesis runs have been performed in air in the range between 1100 and 1300°C. Products have been characterized using reflected‐light microscopy, electron microprobe analysis and diffraction techniques. At 1250°C, an almost phase‐pure material with composition Ca2.99Mg2.67Fe3+14.58Fe2+0.77Al4.56Si0.43O36 has been obtained. The compound corresponds to the first Si‐containing representative of the M14+6nO20+8n polysomatic series of so‐called SFCA phases (Silico‐Ferrites of Calcium and Aluminum) with n = 2 and is denoted as SFCA‐III. Single‐crystal diffraction investigations using synchrotron radiation at the X06DA beamline of the Swiss Light Source revealed that the chemically homogenous sample contained both a triclinic and monoclinic polytype. Basic crystallographic data are as follows: triclinic form: a = 10.3279 (2) Å, b = 10.4340 (2) Å, c = 14.3794 (2) Å, α = 93.4888 (12)°, β = 107.3209 (14)° and γ = 109.6626 (14)°, V = 1370.49 (5) Å3, Z = 2, space group ; monoclinic form: a = 10.3277 (2) Å, b = 27.0134 (4) Å, c = 10.4344 (2) Å, β = 109.668 (2)°, V = 2741.22 (9) Å3, Z = 4, space group P21/n. Structure determination of both modifications was successful using diffraction data from the same allotwinned crystal. A description of the observed polytypism within the framework of OD‐theory is presented. Triclinic and monoclinic SFCA‐III actually correspond to the two possible maximum degree of order structures based on OD‐layers containing three spinel (S) and one pyroxene (P) modules (⟨S3P⟩). The existence of SFCA‐III in industrial iron‐ore sinters has yet to be confirmed. Polytypism is likely to occur in other SFCA‐members (SFCA, SFCA‐I) relevant to sintering as well, but has so far been neglected in the characterization of industrial samples. Our results shed light on this phenomenon and may therefore be also helpful for better interpretation of the powder diffraction patterns that are used for phase analysis of iron‐ore sinters. [ABSTRACT FROM AUTHOR]

As is typical of chemically defended animals, poison frogs present high variability in their alkaloid-based defenses. Previous studies have shown that geographically separated color morphs of Oophaga and Dendrobates species differ in both alkaloid composition and arthropod palatability. Here, we tested the generality of that finding by studying the alkaloid composition and palatability of geographically separated blue and orange morphs of the splash-backed poison frog, Adelphobates galactonotus. We identified and quantified the alkaloid composition of each individual frog using gas chromatography–mass spectrometry and evaluated the palatability of individual secretions to arthropods conducting feeding trials with Drosophila melanogaster. Despite their conspicuous differences in color and separation on opposite sides of a large aquatic barrier, the two morphs did not differ in alkaloid composition or palatability. This result shows that both color morphs are equally chemically protected and suggests that the color variation is not driven by predator selection. [ABSTRACT FROM AUTHOR]

Crystals with the La18W10O57‐type structure (6H and 5H polytypes) were obtained by a self‐flux method from high‐temperature solutions. Some of the crystal samples were studied by single‐crystal X‐ray structure analysis. The diffraction patterns indicated that two phases co‐exist in each sample. The hexagonal lattices have a common period of a ≈ 9.0 Å and are non‐equal in length but have equally oriented superstructure periods 6c (phase I) and 5c (phase II), c ≈ 5.4 Å. The structures of phases I and II were solved in the symmetry groups P2c and P321, respectively, based on the X‐ray data for crystals I and II, with predominant content of the first and second phase. The motif of isolated WO6 prisms with W atoms on the cell edges is common to both phases. WO6 octahedra, both isolated and joined by faces, are distributed along the c axis within the unit cells. Phase I contains extra layers of isolated WO6 octahedra compared to phase II. Tungsten sites in joined octahedra are disordered and partially occupied. Disordering is more expressed in phase II, which in return contains rather more W and O per atom of La. The refined chemical compositions are La18W10O57 for I and La15W8.5O48 for II. [ABSTRACT FROM AUTHOR]