Syntactic Intergrowths Research Articles

A multi-methodological study of the bastnäsite-synchysite polysomatic series: Tips and tricks of polysome identification and the origin of syntactic intergrowths

Abstract In this paper, we evaluated the potentialities of Raman spectroscopy and electron backscattered diffraction (EBSD) in the microscopic characterization of Ca-REE fluorcarbonates (CRFC) belonging to the bastnäsite-synchysite series to provide a “road map” for further investigations with transmission electron microscopy (TEM). EBSD was effective in establishing the sample orientation, setting up the oriented cuts, and ascertaining the effective syntactic relationship among all the detected CRFC phases; however, it failed to distinguish between different polysomes. On samples with different orientations that were preventively ascertained by EBSD and characterized by scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS), micro-Raman spectroscopy allows for distinguishing between polysomes based on the differences in intensity and position of the symmetric stretching vibration (ν1) of the carbonate group (CO32–) in the region around 1080–1099 cm–1. However, as evidenced by TEM-EDS, what appears as a homogeneous polysome in backscattered electrons (BSE) images may be a disordered intergrowth of compositional faults with a bulk composition being matched with that of a real polysome only by accident. Therefore, we conclude that the Raman signal is sensitive to different Ca/(Ca+REE) ratios but not to any ordered distribution of Ca-poor and Ca-rich lamellae within the analyzed volume, making the unambiguous identification of a polysome tricky. Finally, several ordered polysomes were detected at the TEM scale, including a B2S and a long-range polytype with a 32 nm repeat distance along c. The possible implications of the detected microstructure for ore mineral formation are discussed.

Improved control of atomic layering in perovskite-related homologous series

Homologous series are layered phases that can have a range of stoichiometries depending on an index n. Examples of perovskite-related homologous series include (ABO3)nAO Ruddlesden–Popper phases and (Bi2O2) (An−1BnO3n+1) Aurivillius phases. It is challenging to precisely control n because other members of the homologous series have similar stoichiometry and a phase with the desired n is degenerate in energy with syntactic intergrowths among similar n values; this challenge is amplified as n increases. To improve the ability to synthesize a targeted phase with precise control of the atomic layering, we apply the x-ray diffraction (XRD) approach developed for superlattices of III–V semiconductors to measure minute deviations from the ideal structure so that they can be quantitatively eradicated in subsequent films. We demonstrate the precision of this approach by improving the growth of known Ruddlesden–Popper phases and ultimately, by synthesizing an unprecedented n = 20 Ruddlesden–Popper phase, (ATiO3)20AO where the A-site occupancy is Ba0.6Sr0.4. We demonstrate the generality of this method by applying it to Aurivillius phases and the Bi2Sr2Can–1CunO2n+4 series of high-temperature superconducting phases.

Regular intergrowths of minerals in pegmatites from the Il’meny mountains

This paper summarizes the data on oriented intergrowths of minerals in pegmatites of the Il’meny Mountains. Two-feldspar, two-mica, biotite-amphibole, biotite-pyroxene, and pyroxene-amphibole intergrowths have been revealed in syenitic and miaskitic pegmatites. The orientation of exsolution products has been studied in sunstone (aventurine) and moonstone feldspars, nepheline, and cancrinite. Corundum-biotite-feldspar and two-mica intergrowths and exsolution lamellae occur in stellate corundum from corundum-feldspar veins. Syntactic intergrowths of various amphiboles and chlorites have been detected in pegmatoid ultramafic rocks. Regular quartz-feldspar (graphic) and two-feldspar intergrowths are typical of granitic pegmatites. Two-mica, columbite-samarskite, columbite-ilmenorutile, zircon-xenotime, cassiterite-ilmenorutile, cassiterite-ixiolite, cassiterite-tantalite, tantalite-stibiotantalite, fergusonite-columbite, pyrochlore-fergusonite, and beryl-cordierite intergrowths have been found in granitic pegmatites as well. New intergrowth laws have been established for many pairs of minerals.

Spinel-anthophyllite rocks of the Kokchetav massif ( northern Kazakhstan)

Spinel-anthophyllite rocks that may be classified as ultrabasic low-Ca spinel amphibolites have been first discovered in the Kokchetav collision zone (northern Kazakhstan). They outcrop 2 km west of Enbek-Berlyk Village among schists and quartzites and are closely associated with spinel harzburgites and garnet pyroxenites. The main hosted minerals are spinel (hercynite) and anthophyllite. The rocks bear magnetite-hornblende-spinel-anthophyllite pseudomorphs with rounded and polygonal sections, which might have been resulted from the replacement of garnet grains. The prismatic anthophyllite crystals and scarce olivine relics contain elongate parallel spinel inclusions resembling spinel-olivine syntactic intergrowths in the Enbek-Berlyk spinel harzburgites. The spinel-anthophyllite rocks are similar to the associated spinel harzburgites in CaO, MnO, SiO 2, and Al 2O 3 contents but are richer in FeO and poorer in MgO ( F = FeO/(FeO + MgO) = 57% against 35% in the harzburgites). Geological, mineralogical, and geochemical data suggest that the spinel-anthophyllite rocks formed during the isochemical contact metamorphism of garnet-bearing spinel harzburgites, which contained more FeO and less MgO than garnet-free harzburgites of the same area. Variations in FeO and MgO contents in both types of harzburgites seem to be due to different chemical compositions of the chlorite protoliths of these rocks.

Mechanism of formation, fine structure, and superconducting properties of high-temperature superconductors and superconducting composites

A review of available literature data is given and our own results (mainly obtained using transmission electron microscopy) are presented that concern some important problems in the structure of high-Tc superconducting materials, including superconducting composites based on the Bi-containing ceramics, namely, mechanisms of the formation of the basic superconducting 2223 phase from precursors (liquid and diffusion mechanisms); syntactic intergrowths of homologous 2223, 2212, and 2201 phases; mechanisms of penetration of silver atoms from the silver sheath into the ceramics; and location of carbon in the Bi-ceramics/Ag composite. Effects of all above factors on the superconducting characteristics of high-Tc superconducting materials is discussed.

Polytypism and microstructures of the mixed-layer member B2S, CaCe3(CO3)4F3 in the bastnaesite-(Ce)–synchysite-(Ce) series

The crystal structures and polytypism of bastnaesite-(Ce) (CeCO3F, B layer) and synchysite-(Ce) (CeCO3F·CaCO3, S layer) mineral series are very complicated in Sichuan Province, Southwest China. Four new polytypes: 6R, 2H2, 4H and 12H of the mixed-layer member CaCe3(CO3)4F3 (B2S) of this series were discovered by X-ray diffraction analyses and high resolution transmission electron microscopy, structural symmetry, cell and subcell parameters, possible space group, and structural stacking mode (dededgfg/…). They reveal that the mixed-layer member B2S is formed by the ordered stacking of two structural unit layers of bastnaesite-(Ce) (B layer) and one unit layer of synchysite-(Ce) (S layer) along the c direction. The forming of the new polytypes may depend on distributions and the periodical variation of orientation of the [CO3] groups between the unit layers of B2S. We have observed the {000l} microtwin of the 6R polytype and the syntactic intergrowths among the different polytypes formed by stacking faults in B2S.

Crystal Chemistry of α-AlB12 and γ-AlB12

The structures of α-AlB12 and γ-AlB12 are both characterized by a rigid boron framework derived from a combination of three-dimensional networks of icoasahedral B12 and B20 units. Crystals of γ-AlB12 (P21212; a=16.57, b=17.51, c=10.14 Å) are almost always obtained as syntactically intergrown crystals with α-AlB12 (P41212 or P43212; a=10.16, c=14.27 Å) from high-temperature Al–B solutions. Examination of structural details of α-AlB12 and γ-AlB12 demonstrated that there are close similarities between them, notwithstanding marked differences in the crystal data. The syntactic intergrowth between α-AlB12 and γ-AlB12 is always effected so as to have the (101)α and the (100)γ planes parallel to each other and the aα or bα axis and the cγ axis in the same direction; these planes are identical in unit area and in the orientation inside the intergrown crystals. This fact, as well as the fact that the icosahedral B12 networks of both crystals which are parallel to the syntactic intergrowth plane are identical, may facilitate the formation of the intergrown crystal. Structural details of one crystal are compared to the corresponding ones of the other.

New occurrence of Ba-REE fluorcarbonate minerals at Bayan Obo, Inner Mongolia, North China, and their mineralogical features

Ba-REE fluorcarbonate minerals from a carbonatite dyke at Bayan Obo, Inner Mongolia, North China, are first reported in this contribution, in which chemical composition, rare earth element (REE) patterns, and intergrowth relationships for these minerals have been investigated. Syntactic intergrowth or syntaxy between cebaite and cordylite, as well as cordylite and huanghoite were observed. This syntactic texture resulted from the variation of chemical composition of crystallizing agents for those minerals that crystallized directly from carbonatite magmas. It is worth noting that REE patterns of the Ba-REE fluorcarbonate minerals in the dyke are similar to those of the corresponding minerals from the ore hosted dolomite marble of the Bayan Obo giant REE-Nb-Fe mineral deposit, which implies their relation in origin.

Powder X-Ray Diffraction Data for the Superconducting Phase TlCaBa2Cu2O7−δ

High temperature superconducting phases in the Tl-Ca-Ba-Cu-O system are ideally represented by the formula TlmCan−1Ba2CunO2(n+1)+m, with m either 1 or 2 and n = 1 to at least 3 (Parkin et at., 1988). Each of these phases contains one or more of the nearly planar CuO2sheets common to the cuprate superconductors. A single Ca atom separates adjacent CuO2sheets (n > 1). Single or double rock salt-like Tl-O layers are separated from the Can−1CunO2nregions by single Ba-O layers. Each of the Ca-containing members of this family crystallizes in a tetgragonal unit cell, with space group 14/mmm for the m = 2 series and P4/mmm for the m = 1 series.Despite the general interest in this family of superconductors, little has been reported about the m = 1, n = 2 member, TlCaBa2Cu2O7−δ, hereafter called 1122. This lack of work is due at least in part to the difficulty in synthesizing the pure compound (Michel et at., 1991). Additionally, technological interest has focused on members of the family with higher superconducting transition temperatures, particularly Tl2Ca2Ba2Cu3Oywith Tcup to 125 K. The critical temperature of 1122 has been reported from as low as 50 K (Hervieu et al., 1988) to as high as 103 K (Morosin et al., 1988), and at several values in between (Ganguli et al., 1988; Liang et al., 1988). Most of the samples had other superconducting phases in addition to 1122. Because of the nearly identical a axis lengths of the unit cells of the Tl-family of superconductors, syntactic intergrowths may be present in such multiphase samples.

Low level mobile liquid droplet mechanism allowing development of large platelets of high- Tc “Bi-2223” phase within a ceramic

A VLS-like mechanism is illustrated of how 3-Cu-layer Bi-HTSC (high temperature superconductor) is slowly formed from precursors by small, Bi/Pb rich, liquid droplets migrating over growing platelets, depositing ledges (steps) of product in their wakes. The difficulty of obtaining a pure 3-layer, i.e. without syntactic intergrowths, is explained.

Monitoring Sintering / Densification and Crystallization / Grain-Growth in Tl-Based High Temperature Superconductors by Electrical Conductivity Measurements

An “in situ” electrical resistance method has been used to monitor sintering to best achieve a high percentage of Tl-2223 structure. By this means, the sequential formation of syntactic intergrowths, 2201, 2212 and 2223 structures, was followed as a function of temperature, time, seeding and gaseous atmosphere. At 895°C, slight melting assists the formation of ∼95% pure 2223; the presence of carbonate is probably deleterious.

Electron microscopy studies of high Tc phase development in melt-quenched Bi-Ca-Sr-Cu oxides

A Bi-Ca-Sr-Cu oxide composition (2:4:2:5) was rapidly solidified from the melt, and the crystallization behavior examined on heat-treatment. Annealing conditions were 865°C for up to 11 days. The high Tc 2223 phase (105 K) evolved from the 2122 phase (80 K), which in turn developed from the 2021 phase (12 K). The high Tc phase developed only in the presence of a liquid phase at 865 °C. Lattice imaging was used to follow the conversion of 2122 phase to 2223. Data are reported for syntactic intergrowths, which became less frequent with time at temperature. EDS results are consistent with the conversion of 2122 to 2223. Crystals of 2223 could not be grown from the melt, nor crystallized from the solid at temperatures below 820 °C. The presence of a Cu- and Ca-rich liquid was essential for the development of 2223 at 865 °C. A tentative model for the formation of 2223 via a liquid mediated reaction is proposed. EDS confirmed the liquid was rich in Ca and Cu near the solid-liquid interface, and precipitates of secondary phases were identified by SEM, TEM, and XRD methods. The presence of CuO and (Ca,Sr)2CuO3 verified the enrichment of Cu and Ca at the solid-liquid interface. The results are consistent with the evolution of structure of a 2223 from a 2425 starting composition.

Comment upon “crystal growth of the new superconducting compound Pb0.6Bi3Sr2CaCu3Ox by the flux method”, by H. Wang, S. Shang, Z. Wang and X. Chen

A suggestion is made that syntactic intergrowths in a “single crystal” had led to the authors [Wang et al., J. Crystal Growth 96 (1989) 221] mistaking the X-ray pattern for that of a new phase.

Growth and oxygenation of single-crystal BiSrCaCuO and TlBaCaCuO superconductors

Self-flux methods for growing millimeter-sized crystals of Tl2Ba2CaCu2O8 and centimeter-sized crystals of Bi2Sr2CaCu2O8 superconductors are described. Preparation protocols and post-synthesis oxygen treatments were devised to optimize sample quality, as manifested in dc magnetic susceptibility measurements. Other analytical methods, including Laue diffraction and secondary ion mass spectrometry (SIMS), have been brought to bear to assess the quality of our samples and to evaluate the extent of contamination from crucibles. It was found that the TlBaCaCuO material, while occasionally comprising true single crystals, often consists of syntactic intergrowth crystals with two or three phases coexisting.

Growth of crystals and effects of oxygen annealing in the Bi-Ca-Sr-Cu-O and Tl-Ca-Ba-Cu-O superconductor system

The growth of mm-sized crystals in the Tl-Ca-Ba-Cu-O and Bi-Ca-Sr-Cu-O superconductor systems is discussed. The presence of polycrystals, which can be described as syntactic intergrowth crystals, is widely found. In the Tl-Ca-Ba-Cu-O s crystals of three distinct phases with superconducting transitions at 103, 106–108 and 112–114 K have been grown. In the Bi-Ca-Sr-Cu-O system crystals have been grown for two phases that exhibit transition temperatures at 80 and 92 K. Oxygen annealing is shown to substantially improve the superconducting properties in both systems.

Structural and compositional characterization of polycrystals and single crystals in the Bi- and Tl-superconductor systems: Crystal structure of TlCaBa2Cu2O7

The growth of mm-sized crystals in the Tl-Ca-Ba-Cu-O and Bi-Ca-Sr-Cu-O superconductor systems shows the common presence of polycrystals, which can be described as syntactic intergrowth crystals, as well as normal single crystals. Three distinct phases with superconducting transitions at 103 K, 106–108 K and 112–114 K have been grown in the Tl-Ca-Ba-Ca-O system. The newly identified 103 K phase is TlCaBa 2 Cu 2 O 7 which is structurally similar to YBa 2 Cu 3 O 7 but without the presence of chains. Transmission electron diffraction and electron microprobe analysis on several polycrystals show that they are composed of interlayers of these distinct phases grown epitaxially along the c -axis. Crystals have been grown for two phases in the Bi-Ca-Sr-Cu-O system that exhibit transition temperatures at 80 K and at 92 K. Oxygen annealing substantially improves the superconducting properties of most of the crystals.

Syntactic Intergrowth Problems with Bcsco and Fabrication Difficulties Therefrom

Bismuth containing oxide superconductors were first encountered in the BaPb1−xBxO3 perovskites [1]. In the the YBa2Cu3Ox family, bismuth was a possible substitute for yttrium and was attempted by us many months ago (and undoubtedly by many other groups) with the result that, under the necessary oxidizing conditions, BaBiO3 formed as a separate phase. Attempts to substitute bismuth into the La2−xSrxCuO4 family were reported [2], with the intriguing result of a 42K superconductor with Tc higher than the previous 38K high for the family. A second paper [3] discussed the new compound “Bi2Sr2Cu7+δ, a = 5.32Å, b = 26.6Å, C = 48.8Å”, with TC of 7K to 22K; this was confirmed, but C = 24.4Å was found [4], and it is now clear that this compound is ideally Bi2Sr2CuO6+δ. A higher Tc than in BaPb1−xBixO3 was next detected in the Ba-K-Bi-O system [5]. Work at the Japanese National Research Institute of Metals [6] then disclosed T_ of as high as ∼ 105K in the Bi-Ca-Sr-Cu-O (BCSCO) system; both Ca and Sr must be present to produce these higher values.

Structure and non-stoichiometry of calcium aluminates

Abstract Lattice imaging of reaction-sintered ceramic compounds in the CaO‒Al2O3 system showed 00.1 syntactic intergrowths in the matrix CaO. 6Al2O3 phase. The structure of these intergrowths could be interpreted in terms of the concepts used to describe various magnetoplumbite phases (Braun 1957). The observed formation of syntactic phases reveals that calcium aluminates can accommodate calcium oxide/alumina non-stoichiometry in a relatively wide range by altering the stacking sequence of three sub-unit cell 00.1 slabs.

Microsyntactic intergrowth in V nO 2n−1

Characteristics of microsyntactic intergrowth of V 8O 15 in V 9O 17 prepared by a chemical transport method are investigated by transmission electron microscopy. There is observed a range of syntactic intergrowth that can be described as two phase coexistence down to that on a unit cell scale which supports the concept that the microsyntactic intergrowth is a syntactic intergrowth of different compounds on a unit cell scale. A note on the significance of microsyntactic intergrowth to the understanding of long periods in (electronically) insulating substances is added.