Intergrowth Layers Research Articles

Unravelling the Nature of the Intrinsic Complex Structure of Binary-Phase Na-Layered Oxides.

The layered sodium transition metal oxide, NaTMO2 (TM = transition metal), with a binary or ternary phases has displayed outstanding electrochemical performance as a new class of strategy cathode materials for sodium-ion batteries (SIBs). Herein, an in-depth phase analysis of developed Na1-x TMO2 cathode materials, Na0.76 Ni0.20 Fe0.40 Mn0.40 O2 with P2- and O3-type phases (NFMO-P2/O3) is offered. Structural visualization on an atomic scale is also provided and the following findings are unveiled: i) the existence of a mixed-phase intergrowth layer distribution and unequal distribution of P2 and O3 phases along two different crystal plane indices and ii) a complete reversible charge/discharge process for the initial two cycles that displays a simple phase transformation, which is unprecedented. Moreover, first-principles calculations support the evidence of the formation of a binary NFMO-P2/O3 compound, over the proposed hypothetical monophasic structures (O3, P3, O'3, and P2 phases). As a result, the synergetic effect of the simultaneous existence of P- and O-type phases with their unique structures allows an extraordinary level of capacity retention in a wide range of voltage (1.5-4.5 V). It is believed that the insightful understanding of the proposed materials can introduce new perspectives for the development of high-voltage cathode materials for SIBs.

Directed assembly of layered perovskite heterostructures as single crystals

The precise stacking of different two-dimensional (2D) structures such as graphene and MoS2 has reinvigorated the field of 2D materials, revealing exotic phenomena at their interfaces1,2. These unique interfaces are typically constructed using mechanical or deposition-based methods to build a heterostructure one monolayer at a time2,3. By contrast, self-assembly is a scalable technique, where complex materials can selectively form in solution4-6. Here we show a synthetic strategy for the self-assembly of layered perovskite-non-perovskite heterostructures into large single crystals in aqueous solution. Using bifunctional organic molecules as directing groups, we have isolated six layered heterostructures that form as an interleaving of perovskite slabs with a different inorganic lattice, previously unknown to crystallize with perovskites. In many cases, these intergrown lattices are 2D congeners of canonical inorganic structure types. To our knowledge, these compounds are the first layered perovskite heterostructures formed using organic templates and characterized by single-crystal X-ray diffraction. Notably, this interleaving of inorganic structures can markedly transform the band structure. Optical data and first principles calculations show that substantive coupling between perovskite and intergrowth layers leads to new electronic transitions distributed across both sublattices. Given the technological promise of halide perovskites4, this intuitive synthetic route sets a foundation for the directed synthesis of richly structured complex semiconductors that self-assemble in water.

Relationship between photoluminescence intensity, Mn ion oxidation, and crystal structure of new phosphors Li-M-Ti-O:Mn4+ (M = Nb or Ta)

In this study, new phosphors Li-M-Ti-O:Mn4+ (LMT, M = Nb or Ta) were synthesized using a solid state reaction. In a specific composition range, the Li1+x-yM1-x-3yTix+4yO3 solid solution forms a unique periodical structure, i.e., a superstructure, by the periodical insertion of an intergrowth layer into a trigonal structure matrix. The Ta system showed a narrower composition range of the superstructure than the Nb system. Using LMT as the host material, phosphor-activated Mn ions showed emission and excitation spectra at approximately 685 and 493 nm, respectively. These photoluminescence (PL) intensities were closely related to the ratio of Mn4+/Mn3+ ions because the ratio easily changed owing to the sintering temperature, Ti content, superstructure, and MgO co-doping. The relationship between PL intensity, Mn ion oxidation, and crystal structure was clarified using X-ray diffraction, transmission electron microscopy, and X-ray absorption spectroscopy to observe the X-ray absorption fine structure.

Applications of Smart Material of Li<sub>2</sub>O-(Nb/Ta)<sub>2</sub>O<sub>5</sub>-TiO<sub>2</sub> Solid Solution Having a Unique Periodical Structure

In the Li2O-M2O5-TiO2 system, Li1+x-yM1-x-3yTix+4yO3 (M = Nb, or Ta, 0.06 ≤ x ≤ 0.33, 0 ≤ y ≤ 0.175 (LMT) forms a superstructure, known as smart material. The superstructure is formed by periodical insertion of a corundum-type intergrowth layer of [Ti2O3]2+ in a matrix having a trigonal structure during the grain growth. To apply this unique structure as a host material of phosphor, new phosphors doped with Mn4+ ion with a red emission colour, which had a broad peak around 685 nm excited by 493 nm. In order to improve the PL intensity, we investigated the compositions, Mn4+ ratio and crystal structure. Results showed that PL intensity was closely related to Mn4+ ratio and its crystal structure.

Unique crystal structures and their applications as materials for Li1+x-yM1-x-3yTix+4yO3 (M = Nb or Ta, 0.07 ≤ x ≤ 0.33, 0 ≤ y ≤ 0.175)

In the Li2O–M2O5–TiO2 (M = Nb or Ta) system, Li1+x-yM1-x-3yTix+4yO3 (LMT) forms with a superstructure known as the M-phase, in which the periodical intergrowth layers are inserted in a matrix having a trigonal structure of LiMO3. We have been investigating this unique material focusing on its crystal structure, photoluminescence, and electrical properties.First, the formation area of the superstructure was compared between Nb- and Ta- systems and the difference was accurately analyzed.Second, to apply this unique material as a host material of phosphor, we synthesized new phosphors with various emission colors. Red phosphor, having a high PL intensity with an internal quantum efficiency of 98%, was successfully synthesized and used as a host material of the solid solution for the Ta system. Its high value was the result of the large centroid-to-cation distance of the Eu3+ position in the [(Li, Eu)O12] polyhedral.Further still, toward application of the unique qualities of an electro-ceramic, we successfully fabricated oriented balk ceramics for the Nb system by slip casting in a strong magnetic field of 12 T. As a result, anisotropic electric properties were found along the c-axis, which were caused by the superstructure. We first clarified the mechanism showing that the anisotropic Qf value was due to anisotropic electron conductivity and anisotropic bonding strength within the superstructure.

Rapid Synthesis of Li<sub>2</sub>O-(Nb/Ta)<sub>2</sub>O<sub>5</sub>-TiO<sub>2</sub> Solid Solution Having Superstructure by Smart Processing Techniques

We successfully synthesized Li1+x-yM1-x-3yTix+4yO3 solid solutions (M = Nb or Ta, LMT, 0.07 ≤ x ≤ 0.33, 0 ≤ y ≤ 0.175) that have a superstructure. The materials formed a superstructure known as the M-phase, which is formed by the periodical insertion of an intergrowth layer in a matrix having a trigonal structure. The homogeneous and periodical structure of M-phase in LNT needed to sinter at 1393 K for 40 h-200 h using a conventional electric furnace. In order to form the homogeneous intergrowth layers rapidly, millimeter-wave or air-pressure control atmos furnaces were also used as smart processing techniques. We concluded that atomic diffusion was promoted in the useful reaction-fields: millimeter-wave radiation or high oxygen pressure.

Rapid Sintering of Li₂O-Nb₂O₅-TiO₂ Solid Solution by Air Pressure Control and Clarification of Its Mechanism.

We first successfully synthesized Li1+x−yNb1−x−3yTix+4yO3 (LNT) solid solutions (0.13 ≤ x ≤ 0.18, 0 ≤ y ≤ 0.06) rapidly at 1373 K for one hour under 0.35 MPa by the controlling of air pressure using an air-pressure control atmosphere furnace. The composition is a formation area of a superstructure for LNT, in which the periodical intergrowth layer was formed in the matrix, and where it can be controlled by Ti content. Therefore, the sintering time depended on Ti content, and annealing was repeated for over 24 h until a homogeneous structure was formed using a conventional electric furnace. We clarified the mechanism of the rapid sintering using various microscale to nanoscale characterization techniques: X-ray diffraction, a scanning electron microscope, a transmission electron microscope (TEM), a Cs-corrected scanning TEM equipped with electron energy-loss spectroscopy, and X-ray absorption fine structure spectroscopy.

Self-doped iron-based superconductors with intergrowth structures

The key structural unit of iron-based superconductors (FeSCs) is the Fe2X2 (X refers to a pnictogen or a chalcogen element) layer which stacks alternately along the crystallographic c axis with other spacer layers. This structural feature makes it possible to find FeSCs via rational material design. In this paper, we first review the crystal structure of FeSCs along with the relevant progress. Then we summarize several rules for designing the intergrowth structures. The rules include the following points. 1) Lattice match between the intergrowth layers should be good enough. Quantitatively, the lattice mismatch, defined as =2(aA-aB)/(aA + aB), where aA and aB are respectively the lattice parameters of the two constituent compounds, should be no larger than~2%. 2) The charge transfer between the intergrowth layers is mostly essential, which acts as the glue that combines the constituent layers together. Such a charge transfer also induces the extra charge carriers in the superconducting key layer to give rise to superconductivity without extrinsic doping (so-called self doping). 3) For the structure with similar yet crystallographically distinct sites, one needs to avoid forming solid solutions. 4) Each intergrowth layer is preferably thermodynamically stable. 5) The designed structure can be preliminary evaluated with the hard and soft acids and bases conception and ab initio calculations. Following these empirical rules, we introduce and analyze five examples, namely, (Li0.8Fe0.2OH)FeSe, Ba2Ti2Fe4As4O, 42214-type Ln4Fe2As2Te1-xO4 (Ln=Pr, Sm, Gd), 1144-type AkAeFe4As4 (Ak=K, Rb, Cs; Ae=Ca, Sr, Eu), and 12442-type AkCa2Fe4As4F2 and AkLn2Fe4As4O2 (Ak=K, Rb, Cs; Ln=Nd-Ho). For the last 12442-type compounds, we also discuss the unusual relation between superconducting transition temperature and crystallographic parameters. We conclude that the structural-design approach may serve as an effective route, not only for discovering new FeSCs but also for exploring other relevant functional materials with similar crystal structures.

Fabrication and anisotropic electronic property for oriented Li1+x−yNb1−x−3yTix+4yO3 solid solution by slip casting in a high magnetic field

The Li1+x−yNb1−x−3yTix+4yO3 (0.06≤x≤0.33, 0≤y≤0.09) (hereafter LNT), forms a unique and periodical structure in the Li2O-Nb2O5-TiO2 ternary system. In this work, toward application of the unique qualities of an electro-ceramic, we fabricated oriented LNT balk ceramics by slip casting in a strong magnetic field of 12 T using various sizes of particles. The c-axis of the LNT powders was aligned parallel to the magnetic field. As a result, we found anisotropic- and unique- electric properties which were caused by a superstructure with intergrowth layers of corundum-type [Ti2O3]2+. The Qf value parallel to the c-axis was about five times greater than that of perpendicular to the c-axis. We first clarified the mechanism showing that the anisotropic Qf value was caused by the anisotropic electron conductivity and the anisotropic bonding strength in the superstructure.

Microstructures and formation history of melilite-rich calcium–aluminum-rich inclusions from the ALHA77307 CO3.0 chondrite

We have studied four melilite-rich calcium–aluminum-rich inclusions (CAIs) from the Allan Hills A77307 CO3.0 chondrite using transmission electron microscopy with the focused ion beam sample preparation technique. This type of CAI represents one of the dominant types of refractory inclusions in CO3 chondrites. Individual melilite-rich CAIs 04–07 record complex formational histories involving high-temperature gas–solid condensation that occurred under both equilibrium and disequilibrium conditions. CAI 04 contains two texturally- and compositionally-distinct occurrences of perovskite: fine-grained perovskite within a melilite-rich core and aggregates of perovskite grains that surround the core. The perovskite in the core was probably involved in a disequilibrium reaction with early equilibrium condensates (e.g., melilite and spinel) and a nebular gas to form Al-Ti-rich diopside, followed by a later condensation of the perovskite aggregates under equilibrium conditions. CAI 05 has a compact melilite-rich core surrounded by a porous mantle, and likely formed by at least two different condensation events under equilibrium and disequilibrium conditions. In CAI 06, complex intergrowth layers of spinel and diopside surrounding a melilite-rich core indicate disequilibrium reaction of spinel and melilite with a nebular gas to form Al–Ti-rich diopside following core formation by equilibrium condensation. CAI 07 is dominated by melilite with a narrow compositional range and equilibrated textures, suggesting its formation by equilibrium condensation over a limited temperature range. Collectively, we infer that the melilite-rich inclusions formed by a generalized sequence of high-temperature gas–solid condensation that involved: (1) formation of CAI cores by aggregation of primary equilibrium condensates (i.e., perovskite, spinel, and melilite), (2) back-reactions of the primary core minerals with a nebular gas under disequilibrium conditions, forming diopside that evolves in composition from Al–Ti-rich at the interface with the inclusion core to Al–Ti-poor on the exterior of the inclusions. The change in formation conditions may have been achieved by transport and injection of the core materials into a region of a partially-condensed gas that still contained refractory elements in the gas phase.

Microstructural constraints on complex thermal histories of refractory CAI-like objects in an amoeboid olivine aggregate from the ALHA77307 CO3.0 chondrite

We have carried out a FIB/TEM study of refractory CAI-like objects in one AOA from the ALHA77307 CO3.0 chondrite. The CAI-like objects in the AOA consist of a zoned sequence with a spinel-rich core through an intergrowth layer of spinel and Al-Ti-rich diopside to a diopside rim. The spinel-rich core consists of polycrystalline aggregates of spinel and ±minor melilite showing equilibrated grain boundary textures. The intergrowth layer contains fine-grained diopside and spinel with minor anorthite with highly curved and embayed grain boundaries. The diopside rim consists of polycrystalline aggregates of diopside. The compositions of pyroxene change significantly outward from Al-Ti-rich diopside in contact with the spinel-rich core to Al-Ti-poor diopside next to the surrounding olivine of the AOA. Overall microstructural and chemical characteristics suggest that the spinel-rich core formed under equilibrium conditions whereas the intergrowth layer is the result of reactions that occurred under conditions that departed significantly from equilibrium. The remarkable changes in formation conditions of the CAI-like objects may have been achieved by transport and injection of refractory objects into a region of a partially-condensed, Ca,Ti-saturated gas which reacted with spinel and melilite to form Al-Ti-rich diopside.Crystallographically-oriented TiO2 nanoparticles decorate the grain boundaries between spinel grains and between spinel and Al-Ti-rich diopside grains. During the disequilibrium back-reaction of spinel with a partially-condensed, Ca,Ti-saturated gas, metastable TiO2 nanoparticles may have condensed by an epitaxial nucleation mechanism and grown on the surface of spinel. These TiO2 nanoparticles are disordered intergrowths of the two TiO2 polymorphs, anatase and rutile. These nanoparticles are inferred to have nucleated as anatase that underwent partial transformation into rutile. The local presence of the TiO2 nanoparticles and intergrowth of anatase and rutile imply that the disequilibrium back-reaction of spinel with the gas occurred on a short timescale, i.e., minutes to hours at maximum.

Microstructural comparison between Nb- and Ta-systems in Li1+x−yM1−x−3yTix+4yO3 (M = Nb5+, Ta5+) solid solution with superstructure

We successfully synthesized Li1+x−yM1−x−3yTix+4yO3 solid solutions (M=Nb or Ta, LMT, 0.07⩽x⩽0.33, 0⩽y⩽0.175) that have a superstructure, using a conventional electric furnace. The synthesizing time depended on the Ti content, and annealing was repeated for 24–264h until a homogeneous structure was formed by the insertion of periodical intergrowth layers. We characterized the LMT solid solutions, from micro-scale to nano-scale, using X-ray diffraction, a scanning electron microscope, and a transmission electron microscope. The period of the intergrowth layer was controlled by the Ti content. LNT’s period of the intergrowth layer was narrower than that of LTT in the real lattice when the Ti content was the same. The narrowest interplanar distance (1.82nm) of the intergrowth layer was formed in Li0.935Ta0.365Ti0.810O3. The widest distance (14.3nm) appeared in Li1.11Nb0.89Ti0.11O3, but no such wide period formed in the Ta-system.

Microstructure and luminescence of rare earth doped Li(Nb,Ti)O3 solid solutions

Li1+x−yNb1−x−3yTix+4yO3 (0.11≤x≤0.33, 0≤y≤0.09) solid solutions in the Li2O-Nb2O5-TiO2 ternary system are known as M Phase with superstructure. Eu doped Li1+x−yNb1−x−3yTix+4yO3 (LNT) solid solutions were prepared by firing mixed powders at temperatures of 1120 °C for up to 24 hours. The interval of intergrowth layers of Eu doped LNT solid solution was as twice as that of no doped LNT solid solution. Another rare earth (Dy, Sm, Tm, Er) doped LNT solid solutions were also prepared each other. These LNT solid solutions with 20 mol%-TiO2 did possess a disorder structure, but did not possess a superstructure. PL intensity of Eu doped LNT solid solution was higher than Eu doped LiNbO3 (LN) solid solution. However, PL intensity of another rare earth doped LNT solid solutions was less than LN solid solutions each other. It was thought that the distribution of Eu and Ti and a relative distance between Eu and Ti were optimized by the formation of the superstructure and PL intensity of Eu doped LNT solid solutions increased.

Rapid synthesis and structural analysis of Li-Nb-Ti-O solid solutions with superstructure by millimeter-wave heating

We have succeeded in synthesizing Li1+x−yNb1−x−3yTix+4yO3 solid solutions (0.11 ≤ x ≤ 0.18, 0 ≤ y ≤ 0.09) having a superstructure by millimeter-wave heating for only 1 h. Synthesizing temperatures were attempted in the range from 873 to 1273 K. A homogeneous LNT solid solution with a superstructure was formed above 1173 K. Such LNT solid solutions were characterized using X-ray diffraction, a scanning electron microscope, and a transmission electron microscope (TEM) from micro-scale to nano-scale. Element analysis revealed that the superstructure was formed by periodical insertion of intergrowth layers with a high concentration of Ti ions. The structural stability of the LNT solid solution was estimated by a first-principles calculation. The results revealed that the structure was stable due to the formation of the superstructure, which is composed through an anisotropic arrangement of the Ti ions.

Studies on the Reorganization of Extended Defects with Increasingn in the Perovskite-Based La4Srn-4TinO3n+2 Series

Perovskite titanates with nominal stoichiometry ABO3+δ often exhibit quite interesting properties, but their structural characterization is not always rigorous. Herein, we demonstrate how excess oxygen can be incorporated in a titanate perovskite-based lattice. A new family of layered perovskites La4Srn–4TinO3n+2 has been investigated by means of X-ray diffraction, neutron diffraction, transmission electron microscopy, thermogravimetric analysis, and density and magnetic measurements. Such layered perovskites are known to be able to accommodate extra oxygen beyond the parental ABO3 perovskite in crystallographic shears. The structure evolves with increasing n. Firstly, the perovskite blocks become more extensive and the oxygen intergrowth layers move further apart; then the spacing between the intergrowth layers increases further and their repetition becomes more sporadic. Finally, the layered structure is lost for the n = 12 member (La2Sr4Ti6O19–δ). In this structure, excess oxygen is accommodated within the perovskite framework in randomly distributed short-range linear defects. These defects become more dilute as the cubic perovskite, that is, n = ∞, composition is approached.

Competitiveness of porosity and phase purity in melt processed Bi2212/Ag conductors

The porosity remaining in Bi2212/Ag monocore tapes after melt processing is shown to be a function of the conductor geometry. The effect of other processing variables on the final microstructure was studied. It was found that increasing the length of the soak time, t m, at the maximum processing temperature increases the size of second-phase particles and the fraction of 2201 intergrowth layers within the 2212 grains.

Microstructural study of the Ni-doped 2212-phase of BiSrCaCuO superconductor single crystals

The microstructure of Bi 2Sr 2Ca 1Cu 2− x Ni x O y ( x=0.0, 0.02, 0.04) single crystals grown by the travelling solvent floating zone method have been investigated using transmission electron microscopy. High-resolution observations and diffraction patterns at the [100] zone axis show a very small change in the direction of the modulation wave with increasing of Ni concentration. Streak-shaped diffuse scattering along the row of modulation reflections was observed in all the specimens. Convergent beam diffraction patterns at the [001] zone axis showed two distinct symmetries: either a very strong asymmetry or a weak asymmetry in the b-plane. The strong asymmetry was attributed to the presence of intergrowth layers on the c-plane. The positions of Ni atoms within the structure of the crystal and their influence on the electric resistivity was discussed by considering the present experimental observation results.

Microscopic segregation in intergrowth layers of (Hg, Re)Ba 2Ca n−1 Cu nO y

Microscopic structures of (Hg, Re)Ba 2Ca n−1 Cu n O y (nominally n = 4) have been investigated by means of high-resolution electron microscopy. Remarkable structural changes were detected during the observations. The lattice images viewed along the a-axis showed an ordinary intergrowth of members with mostly n > 4 in the matrix of the n = 4 host lattice. On heating crystals by a weak electron beam, dense rows of nm size domains accompanied by large strain fields appeared selectively at the intergrowth layers. However, further heating by an intense electron beam wiped off these domains, and strain-free lattices were recovered. These successive changes in the microstructure are interpreted in terms of the two-dimensional disproportionation of Re content on the Hg sites. It is suggested that the observed small domains with a typical size of 3 nm parallel to a 10 nm normal to intergrowth layers may provide promising pinning centers for flux lines in the mixed state.

Perovskite stacking in the structures of the high temperature cuprate superconductors

Most of the known cuprate superconductors belong structurally to one family and are closely related to each other. Ignoring odd structural details and considering the idealized structures only, most of them adopt a Ruddlesden-Popper (R-P) or an oxygen-deficient R-P structure with or without intergrowth layers which are composed of either a rock-salt type atomic arrangement, chains, bare cations, or a mixture of them. The rock-salt layers found in R-P structures are simply the result of the stacking of perovskite unit cells. Sometimes a fluorite type structural unit can be found, but the structures containing fluorite blocks may also be considered to be of the R-P structure type with minor readjustment of anions in the rock-salt layers to stabilize the structure. A general formula can be used to describe the basic structures of all the cuprate superconductors, even those having fluorite units.

Statistical theory of x-ray diffraction by one-dimensional randomly disordered TlBa2Ca2Cu3O9/Tl2Ba2Ca2Cu3O10 superconducting thin films

Textured TlBaCaCuO superconducting thin films identified by electron microscopy as consisting of a mixture of TlBa2Ca2Cu3O9 and Tl2Ba2Ca2Cu3O10 layers with no long range crystalline order normal to the basal planes have been prepared. The Tl1+xBa2Ca2Cu3O9+x thin films yield complex x-ray diffraction patterns with a nonrational series of basal reflections. In order to further understand the structure of this material, a statistical model for x-ray diffraction by partially disordered layered structures was formulated for a material consisting of a random mixture of two types of layers in which both the layer thickness and scattering power of the layers differ. The scattering equation was solved computationally for x-ray scattering from the basal planes of layered materials consisting of random mixtures of TlBa2Ca2Cu3O9 and Tl2Ba2Ca2Cu3O10 layers. The results were applied to the modeling of the x-ray diffraction scattering intensity for 1.0-μm-thick textured thin films of TlBa2Ca2Cu3O9, Tl2Ba2Ca2Cu3O10, and Tl1+xBa2Ca2Cu3O9+x. The computational results support the identification of one such Tl1+xBa2Ca2Cu3O 9+x thin film as consisting predominantly of a nearly random mixture of 50% TlBa2Ca2Cu3O9 and 50% Tl2Ba2Ca2Cu3O10 layers (x≂0.5). The model was also used to examine the effect of intergrowth layers in TlBaCaCuO compounds on the intensity, location, and width of the basal plane reflections.