Octahedral Layers Research Articles

Paramagnetic Iron and Manganese Ions as Paired Spin Probes of Structural Transformations in Octahedral Layers of Phyllosilicates

Paramagnetic Iron and Manganese Ions as Paired Spin Probes of Structural Transformations in Octahedral Layers of Phyllosilicates

Morphology, mineralogical composition and genesis of vertisols in liman Bolshoi (Volgograd region)

The morphological structure of the profiles of two vertisols on microelevations of the gilgai complex of the Bolshoi liman (Svetloyarsk district of the Volgograd region) was studied. Despite the same microrelief position and equal relative heights of microelevations, morphological differences in the profiles at the subtype level were revealed. In one of the profiles, due to the higher humidity, the signs of vertisol genesis were less pronounced. Whereas in the second, more pronounced signs of vertic processes were described – slickensides with 15–30 cm length, smaller slickensides up to 10 cm, wedge-shaped aggregates. The proportion of the clay fraction in the mixed horizons, obtained by the Gorbunov method, was 45– 52%. Analysis of the qualitative mineralogical composition of the clay (<1μm) fractions showed that the smectite phase is represented mainly by disordered and mixed-layered smectite-illite formations. Individual illite belongs to the biotite variety according to the filling of the octahedral layer. Analysis of the ratio of the main mineral phases showed the predominance of illite (53–87%) in all soil samples. In the first pit an increase in the proportion of the smectite component with depth (from 1 to 35%) is noted. In the second pit, there are no regularities in the distribution of mineral phases (fluctuations within 24–37%), which may be associated with the involvement of chocolate clays layers in the profile during soil formation. Mineralogical analysis of chocolate clays (parent rocks) showed their high heterogeneity depending on the depth of the studied layer. No correlation was found between the severity of the signs of vertic processes and the proportion of the swelling component. Illustration of the stages of the formation of the soil cover on the investigated plot of the liman is given. The main reasons that led to the formation of different soils on microelevations are the unevenness of the “squeezing out” of soil blocks upward in the process of microrelief formation in combination with local erosion.

Nonradical induced degradation of bisphenol AF by NaBiO3 coupled peroxymonosulfate process: Performance and mechanism

It has been shown previously that the NaBiO3 (NBO) could efficiently photocatalytically degrade organic contaminants because of its excellent photocatalytic property. In this study, we proposed a heterogeneous catalytic system of NBO for peroxymonosulfate (PMS) without light irradiation. The results showed that NBO/PMS exhibited excellent performance toward bisphenol AF (BPAF) degradation and mineralization. The effectiveness of the NBO/PMS system was also investigated in different multicomponent systems. The effects of initial pH, NBO dosages, and PMS concentration for BPAF degradation were also investigated. Radical quenching experiments combined with electron spin resonance analysis indicated that singlet oxygen (1O2) was the main reactive oxygen species, and ·SO4- and ·OH– radicals participated in the process. X-ray photoelectron spectroscopy revealed the main catalytic mechanism: lattice oxygen (Ovac) was extruded during the transformation of Bi(V) to Bi(III) to form and activate oxygen (O*), and the generated O* between the [BiO6] regular octahedral layers reacted with PMS on the NBO surface to form 1O2. Based on the results of the Bi element, it is suggested that the activation process proceeded through electron transfer from NBO to PMS. The stability of NBO and applicability of the NBO/PMS system in a natural water environment were explored. This work provides a novel approach to PMS activation and the potential use of NBO for the decontamination of organic pollutants.

Power-Dependent Photoluminescence Efficiency in Manganese-Doped 2D Hybrid Perovskite Nanoplatelets.

Substitutional metal doping is a powerful strategy for manipulating the emission spectra and excited state dynamics of semiconductor nanomaterials. Here, we demonstrate the synthesis of colloidal manganese (Mn2+)-doped organic-inorganic hybrid perovskite nanoplatelets (chemical formula: L2[APb1-xMnxBr3]n-1Pb1-xMnxBr4; L, butylammonium; A, methylammonium or formamidinium; n (= 1 or 2), number of Pb1-xMnxBr64- octahedral layers in thickness) via a ligand-assisted reprecipitation method. Substitutional doping of manganese for lead introduces bright (approaching 100% efficiency) and long-lived (>500 μs) midgap Mn2+ atomic states, and the doped nanoplatelets exhibit dual emission from both the band edge and the dopant state. Photoluminescence quantum yields and band-edge-to-Mn intensity ratios exhibit strong excitation power dependence, even at a very low incident intensity (<100 μW/cm2). Surprisingly, we find that the saturation of long-lived Mn2+ dopant sites cannot explain our observation. Instead, we propose an alternative mechanism involving the cross-relaxation of long-lived Mn-site excitations by freely diffusing band-edge excitons. We formulate a kinetic model based on this cross-relaxation mechanism that quantitatively reproduces all of the experimental observations and validate the model using time-resolved absorption and emission spectroscopy. Finally, we extract a concentration-normalized microscopic rate constant for band edge-to-dopant excitation transfer that is ∼10× faster in methylammonium-containing nanoplatelets than in formamidinium-containing nanoplatelets. This work provides fundamental insight into the interaction of mobile band edge excitons with localized dopant sites in 2D semiconductors and expands the toolbox for manipulating light emission in perovskite nanomaterials.

Iridate Li8IrO6: An Antiferromagnetic Insulator.

A polycrystalline iridate Li8IrO6 material was prepared via heating Li2O and IrO2 starting materials in a sealed quartz tube at 650 °C for 48 h. The structure was determined from Rietveld refinement of room-temperature powder neutron diffraction data. Li8IrO6 adopts the nonpolar space group R3̅ with Li atoms occupying the tetrahedral and octahedral sites, which is supported by the electron diffraction and solid-state 7Li NMR. This results in a crystal structure consisting of LiO4 tetrahedral layers alternating with mixed IrO6 and LiO6 octahedral layers along the crystallographic c-axis. The +4 oxidation state of Ir4+ was confirmed by near-edge X-ray absorption spectroscopy. An in situ synchrotron X-ray diffraction study of Li8IrO6 indicates that the sample is stable up to 1000 °C and exhibits no structural transitions. Magnetic measurements suggest long-range antiferromagnetic ordering with a Néel temperature (TN) of 4 K, which is corroborated by heat capacity measurements. The localized effective moment μeff (Ir) = 1.73 μB and insulating character indicate that Li8IrO6 is a correlated insulator. First-principles calculations support the nonpolar crystal structure and reveal the insulating behavior both in paramagnetic and antiferromagnetic states.

Tunable bandgap and luminescence characters in single-phase two-dimensional perovskite AVA2PbClxBr4-x alloys

Two-dimensional (2D) lead halide perovskites have garnered increasing interest due to their high photoluminescence quantum yields and tunable luminescence characters, making them excellent materials in light-emitting, especially for white-light emission. In contrast to their 3D analogues, they are formed by inserting large organic cations between the anionic layers of lead halide octahedra. Herein, we designed and prepared AVA2PbClxBr4-x (x = 0, 1, 2, 3, 4) alloy perovskites, in which AVA (5-aminovaleric acid) was used as an organic cation to separate the layers of chlorine-substituted lead bromide 2D perovskite, in order to identify the role of Cl substitution in tuning the bandgap and luminescence characters of the 2D AVA2PbBr4 perovskite. X-ray diffraction measurements revealed achievement of single-phase thin films of AVA2PbClxBr4-x perovskites with a (001) orientation. With increasing Cl/Br ratio from 0 to 1, the lattice constant c was found to decrease from 1.672 nm to 1.597 nm following c(x) = 1.674 – 0.076x, while the bandgap increased from 3.11 eV to 3.76 eV as approximated with Eg(x) = 3.08 + 0.62x. Correspondingly, the alloy luminescence character evolved from narrow-band blue-violet to broadband white-light emission, which was attributed to transformation from free-exciton to self-trapped-exciton emission with increasing octahedral distortion caused by Cl substitution. Based on an alloy system with continuously varying content designed for the first time, the work highlighted the role of Cl incorporation in changing the crystal lattice structure, and tuning bandgap as well as the light emission characters in 2D lead halide perovskites.

High Efficiency and Narrow Emission Band Pure-Red Perovskite Colloidal Quantum Wells.

Colloidal quantum wells (CQWs) have excellent optical performance, such as ultranarrow emission, due to strong quantum confinement in the vertical direction. However, there are few reports on metal halide perovskite CQWs with high-purity red emission (∼630 nm) for the display application, owing to the broadened photoluminescence spectrum and beyond the red emission range. Herein, we successfully synthesize high-quality CsPbI3 CQWs in a strong electronegative solvent (mesitylene), which cut the chemical reaction rate of the precursor system to slow down the crystal nuclei growth. The number of PbI64- octahedral layers (n) of CQWs can be regulated to achieve the emission of the CQW shift from orange (596 nm, n = 3) to red (626 nm, n = 4). The pure-red-emitting CQWs have a high photoluminescence quantum yield of 99% and a narrow emission bandwidth (28 nm). The Commission Internationale de l'Eclairage coordinates (0.69, 0.31) meet the requirement of the Rec. 2020 standard.

Microstructure and electrical properties of La2Ti2O7 thin films on SrTiO3 substrates

La2Ti2O7 has recently received extensive interest due to its attractive ferroelectric and photocatalytic properties. Here, high-quality La2Ti2O7 thin films on (110) SrTiO3 substrates are prepared via oxidation of LaTiO3 thin films that are fabricated by pulsed-laser deposition. X-ray diffraction analyses reveal that the La2Ti2O7 thin film grows epitaxially on the (110) SrTiO3 substrate and has a good crystallinity. Atomic force microscopy investigations suggest that the surface of La2Ti2O7 thin film turns into a stepped and terraced structure after the annealing. X-ray photoelectron spectroscopy analyses indicate that the La2Ti2O7 thin film has a high purity and contain only Ti4+ ions (i.e., without Ti3+ ions). The band gap of the La2Ti2O7 film is measured to be ∼3.2 eV by UV–visible spectra. Atomic and electronic structures of the La2Ti2O7/SrTiO3 heterointerface have been studied by a combination of transmission electron microscopy and first-principles calculations. The La2Ti2O7/SrTiO3 heterointerface is atomically abrupt and coherent. Electronically, this heterointerface contributes to the decrease of band gap of La2Ti2O7 near the interface. The reduction of spontaneous polarization in La2Ti2O7 is localized in two layers of TiO6 octahedra near the interface due to the discontinuous Ti-O bonds between the perovskite layers of La2Ti2O7.

High-Pressure Synthesis and Magnetism of the 4H-BaMnO3 Single Crystal and Its 6H-Type Polymorph.

A 4H-type BaMnO3 single crystal was prepared by combining the floating zone method with high-pressure treatment at 5 GPa and 1023 K. The crystal crystallizes to a hexagonal structure with space group P63/mmc and lattice parameters a = 5.63723(5) Å and c = 9.22355(8) Å. In this structure, face-sharing MnO6 octahedral dimers connect with each other by corner O atoms along the c-axis direction, forming an -A-B-A-C-type 4H arrangement. A long-range antiferromagnetic (AFM) phase transition is found to occur at TN ≈ 263 K. When the synthesis pressure increases to 20 GPa, a new polymorphic phase is obtained. This higher-pressure phase still possesses the hexagonal P63/mmc symmetry, but the lattice parameters change to be a = 5.61349(2) Å and c = 13.66690(9) Å with a unit cell volume reduction of 2.05%. In this new phase, the c-axis MnO6 dimers are separated by MnO6 octahedral layers in the ab plane, forming an -A-B-C-A-C-B-type 6H structure. The 6H phase exhibits two long-range AFM orderings at TN1 ≈ 220 K and TN2 ≈ 25 K, respectively. The different magnetic properties are discussed on the basis of the detailed structural constitutions of 4H- and 6H-BaMnO3.

Superior Performance and Stability of 2D Dion–Jacobson Halide Perovskite Photodetectors Operated under Harsh Conditions without Encapsulation

AbstractThe practical utilization of electronic and optoelectronic materials such as perovskites not only relies on their good device performance and moisture stability but also requires stability under other operating conditions. Herein, the operational stability of the Dion–Jacobson perovskite (DJP) photodetector under different harsh conditions is carefully studied and compared with that of its widely studied counterpart, i.e., the Ruddlesden–Popper perovskite (RPP) photodetector. The DJP‐based photodetector can maintain its responsivity after storage in humid environments for two months and can ensure stable operation without any photocurrent degradation under continuous light illumination for 20 000 s. Moreover, the DJP film does not exhibit any changes in its absorbance spectrum and surface morphology when it is heated at 100 °C for 18 h, and the film is hardly affected by high‐energy electron beam irradiation. In addition, the mechanical stability of the DJP film is also found to be superior based on the cyclic bending measurements of the fabricated flexible photodetectors. The excellent performance and stability of the DJP‐based photodetector are found to be the result of the elimination of weak van der Waals bonds among the octahedral PbI4 layers of DJP films, which is typically observed in RPP films.

Geochemistry of barium ions associated with biogenic manganese oxide nanoparticles generated by a fungus strain: Implications for radium sequestration in uranium mill tailings

Biogenic Mn oxides are reactive and ubiquitous in many Earth surface environments, yet their role in radionuclide sequestration at U mill-tailings sites still require an improved understanding at the nano- and molecular-scales. This study concerns the uptake of Ba, utilized as a safe and chemically appropriate surrogate for radioactive Ra, by biogenic Mn oxides produced by a fungal species, Coprinopsis urticicola, isolated from the mine water of the Ningyo-toge U mine, Okayama, Japan. The biogenic Mn oxides were identified as birnessite nanocrystals <10 nm in width having many vacancy sites in the MnO6 octahedral layers. Ba2+ adsorption experiments using biogenic birnessite were conducted at pH 6.00 ± 0.02 with the initial Ba concentrations ranging from 10–8 to 10–3 mol/L. The apparent distribution coefficient, Kd, was calculated to be 103.8 L/g for biogenic birnessite, which is comparable to the apparent Kd of Ra in the mill tailings pond at Ningyo-toge. Both in adsorption and coprecipitation experiments, extended X-ray absorption fine edge structure analysis of Ba L3 edge revealed that the Ba atom forms an inner-sphere complex with O atoms of the MnO6 layer. A slightly greater coordination number for coprecipitated Ba may be attributed to the formation of BaO binding to the newly overlying MnO6 layer during coprecipitation. Static desorption experiments for 7 days reveal that the steady-state release rate of adsorbed Ba is ∼1.4 times faster than that of the coprecipitated Ba, when the Ba concentration in the initial loading solution was ∼10–8 mol/L, indicating that the release of intercalated (coprecipitation) Ba to solution is retarded. The present study demonstrates the importance of fungus-generated Mn oxides as an efficient absorber of Ba2+, and likely Ra2+, among soil compounds in U mill-tailings. This may be applicable to other contaminated sites due to the ubiquitous occurrence of fungi in the environment.

Structure and Optical Properties of Hybrid-Layered-Double Perovskites (C8H20N2)2AgMBr8 (M = In, Sb, and Bi).

The Pb-free hybrid-layered-double perovskites (HLDPs) have been proposed as green and stable semiconducting materials for optoelectronic devices, but the synthesized members are still limited. Here, we report the synthesis of three new HLDPs, (C8H20N2)2AgMBr8 (M = In, Sb, and Bi), by a solution method using 1,4-bis(ammoniomethyl)cyclohexane (C8H20N22+) as the organic spacing cation. All three of these HLDPs show ⟨100⟩-oriented layered structures with Ag and In/Sb/Bi order arranged in corner-sharing octahedral layers. The first-principle calculations indicate the indirect-gap nature of (C8H20N2)2AgInBr8 and (C8H20N2)2AgSbBr8, while their Bi counterpart shows a direct band gap after considering spin-orbit coupling. The band gaps obtained by diffuse-reflectance spectroscopy are 3.11, 2.60, and 2.70 eV for M = In, Sb, and Bi, respectively. (C8H20N2)2AgInBr8 shows a broadband red emission centered at 690 nm, and it is mainly attributed to the self-trapped-excitons mechanism. Our results not only provide a series of new "Pb-free" HLDPs with chemical diversity but also help us to further understand the structure-property relationships of HLDP materials.

Relationships between Distortions of Inorganic Framework and Band Gap of Layered Hybrid Halide Perovskites

The unprecedented structural flexibility and diversity of inorganic frameworks of layered hybrid halide perovskites (LHHPs) raise a wide range of useful optoelectronic properties thus predetermining the extraordinary high interest in this family of materials. Nevertheless, the influence of different types of distortions of their inorganic framework on key physical properties such as band gaps has not yet been quantitatively identified. We provided a systematic study of the relationships between LHHPs’ band gaps and six main structural descriptors of inorganic framework, including interlayer distances (dint), in-plane and out-of-plane distortion angles in layers of octahedra (θin, θout), layer shift factor (LSF), and axial and equatorial Pb–I bond distances (dax, deq). Using this set on the selected structural distortions, we realized the inverse materials design based on multistep density functional theory and a machine learning approach to search LHHPs with target values of the band gap. The analysis of calculated descriptors versus band gap dependences for the wide range of generated model structures of (100) single-layered LHHPs results in the following descending order of their importance: dint > θin > dax > LSFmin > θout> deq > LSFmax, and it also implies a strong interaction value for some pairs of structural descriptors. Moreover, we found that the structures with completely different distortions of inorganic frameworks could have similar band gaps, as illustrated by a number of both experimental and model structures.

Catalytic performances of natural Ni-bearing clay minerals for production of syngas from dry reforming of methane

The catalytic activity of Ni-bearing clay minerals obtained from natural nickel lateritic ores from Niquelândia, Brazil was evaluated in methane dry reforming reaction at 800 °C. These phyllosilicates materials are Ni/Mg smectites with a Ni content of about 23 wt %. The mineralogy and the crystallochemistry of the clay minerals were characterized using X-Ray diffraction and FTIR spectroscopy respectively. Prior to any catalytic test, the clay fraction was extracted from the lateritic ore to concentrate the smectites and remove primary minerals (quartz, pyroxenes) or impurities. In these samples, the Ni is localized in the octahedral layers (structural Ni) and in the interlayer space (exchangeable Ni). To evidence the impact of octahedral or exchangeable Ni, the clays were saturated either with Mg or with Ni. The activity in methane dry reforming is higher with the clay minerals saturated with Ni compared to those saturated with Mg. These results support the hypothesis that both octahedral and exchangeable Ni play a role in the conversion of CH4 and CO2 into H2 and CO. Finally, in situ XRD and TEM analyses were crucial to explain the strong influence of the reduction temperature on the H2 yield. In these natural nickel lateritic ores, the presence of Fe leads to strong interaction with Ni during the reduction step, which depending on its temperature can result in the loss of catalytic activity.

Elucidating the Role of Substrates on Domain Distribution of Quasi-2D Perovskites for Blue Light-Emitting Diodes

Perovskite light-emitting diodes (PeLEDs) have been developed at a rapid pace with high external quantum efficiencies (EQEs) surpassing 20% for red, infrared, and green emissions; nevertheless, the performance of their blue-emitting cousins remains relatively quite low. Among various crucial factors that could impact these solution-processed devices, selection of a suitable underlayer (usually a hole-transport layer) is critical to the quality of resulting perovskite films. Particularly, the role of the substrate is highly important in the case of quasi-2D perovskite films where the emission occurs as a result of cascade energy transfer among various perovskite domains with different numbers of lead-halide octahedral layers (n) sandwiched between long-chain organic molecules. Herein, we explored the impact of the substrate on domain distribution of quasi-2D perovskite films, and a series of efficient and color-stable blue PeLEDs is attained through precise stoichiometric engineering. By employing a range of characterization tools, we thoroughly investigated the crystallization mechanism of quasi-2D perovskites on different substrates, including poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and poly[(9,9-dioctylfluorenyl-2,7-diyl)- co -(4,4′-( N -(4-sec-butylphenyl)diphenylamine)] (TFB). Perovskite films spin-coated on PEDOT:PSS exhibited cyan emission from the n ≥ 4 quasi-2D phase with uniformly distributed small grains. In contrast, perovskite layers formed on TFB featured an island morphology and showed blue emission from the quasi-2D phase with n-values between 2 and 4. In addition, we examined how the variation of A-site cations affects the optical properties of quasi-2D perovskites. Consequently, PEDOT:PSS-based champion PeLED exhibited cyan emission at 495 nm with a maximum EQE of 6.73%, whereas TFB-based counterparts manifested blue emission at 486 nm with a peak EQE of 3.95%. We anticipate that our work will provide a fundamental guideline to rationally selecting the substrates for the optimized growth of quasi-2D perovskites toward high-efficiency PeLEDs.

Comparative Acid Leaching Study of Mongolian Muscovite and Montmorillonite Clay Minerals

Acid activation is the most commonly used method to enhance the chemical and physical properties of clay minerals. Porous structure formation behavior depends on the crystal structure of clay minerals. Within the same structure of clay minerals, their resistance to acid also varies. Acid leaching has been used to increase the surface area of clay minerals and obtain solids with high porosity and many acidic sites. This paper is focused on the results of acid leaching of Mongolian clay minerals (montmorillonite and muscovite). Both clay minerals belong to a group of phyllosilicates with the 2:1 crystal structure. The influence of acid concentration and leaching time on the porous properties of silica was studied. Initially, the montmorillonite was pre-treated by a simple physical purification methods. The montmorillonite and muscovite were acid leached by a 10% hydrochloric acid solution in an autoclave at 120°C for 10h. X-ray diffraction (XRD), X-ray fluorescence analysis (XRF), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and the surface area used for characterization of the raw and leached samples. The surface area of leached montmorillonite increased up to 77% and muscovite up to 63%. Clay mineral’s swelling character and isomorphic substitution of the octahedral layer show the main influence on porous structure formation.

Investigation of the Crystal Structure and Ionic Pathways of the Hexagonal Perovskite Derivative Ba3–xVMoO8.5–x

The hexagonal perovskite derivatives Ba3NbMoO8.5, Ba3NbWO8.5, and Ba3VWO8.5 have recently been reported to exhibit significant oxide ion conductivity. Here, we report the synthesis and crystal structure of the hexagonal perovskite derivative Ba3-xVMoO8.5-x. Rietveld refinement from neutron and X-ray diffraction data show that the cation vacancies are ordered on the M2 site, leading to a structure consisting of palmierite-like layers of M1Ox polyhedra separated by vacant octahedral layers. In contrast to other members of the Ba3M'M″O8.5 family, Ba3-xVMoO8.5-x is not stoichiometric and both barium and oxygen vacancies are present. Although synthesized in air at elevated temperatures, Ba3-xVMoO8.5-x is unstable at lower temperatures, as illustrated by the formation of BaCO3 and BaMoO4 by heat treatment in air at 400 °C. This precludes measurement of the electrical properties. However, bond-valence site energy (BVSE) calculations strongly suggest that oxide ion conductivity is present in Ba3-xVMoO8.5-x.

Asymmetric Spacer in Dion–Jacobson Halide Perovskites Induces Staggered Alignment to Direct Out‐of‐Plane Carrier Transport and Enhances Ambient Stability Simultaneously

AbstractDion–Jacobson (DJ)‐type 2D halide perovskites present superior environmental stability and a narrower bandgap, yet a contradiction between charge transport and stability remains to be resolved. Herein, it is shown that both symmetry and substitution of the organic spacer in DJ perovskites synergistically direct the narrow interlayer spacing, staggered spacer alignment, and regular phase arrangement, thereby promoting out‐of‐plane carrier transport and ambient stability. Compared to its symmetric para‐xylylenediamine (PDMA) counterpart, the asymmetric 2‐(4‐aminophenyl)ethylamine (PMEA) spacer largely aids in compressing the inorganic octahedra layer to form a non‐confinement structure with decreased exciton binding energy, while stacked benzene rings enable a staggered alignment of spacers. Such non‐confined structures are less remarkable in meta‐substituted diamine‐based DJ perovskites than those para‐ones, which retard carrier transport from 2D to quasi‐2D phases. The preferential PMEA spacer however requires a long relaxation time to form a dense and ordered staggered alignment, which is realized by a slight addition of strong‐coordinating DMSO into the DMF solvent, thus decelerating crystallization and further optimizing lamellar orientation. As a result, a best efficiency of ≈12% is achieved in (PMEA)MA3Pb4I13 based p‐i‐n type planar solar cells. Importantly, such unencapsulated devices can maintain 81% initial efficiencies after storage in ambient conditions ( ≈60% relative humidity, ≈20 °C) for 700 h.

SrCoO3-δ microstructures in Perovskite-Brownmillerite heterostructure

SrCoO3-δ (0 < δ < 0.5) film has shown great potential in applications for interface engineering of spintronic devices due to its multiformity of crystal structures and magnetic properties. In this work, we have studied the microstructures of SrCoO3-δ in SrCoO2.5-La0.67Sr0.33MnO3 (SCO-LSMO) double-layered films by using scanning transmission electron microscopy. Our fabricated SCO film is determined to have a structure with the Ima2 space group (No. 46). Perpendicular domain and stack-fault structures are observed to coexist in the SCO layer. In the joint of domains, oxygen content remains a constant for octahedra layers but a slight rise for tetrahedra layers. Continuous changes of oxygen content are also detected in the same level layer in the stack-fault region near the domain boundary. Besides, a peculiar nanocrystalline is observed at the interface of SCO-LSMO. The electron energy-loss spectroscopy results show that the Co ions in nanocrystalline have a lower valence, indicating a chemical compositon of SrCoO2.5-δ (0 < δ < 0.5).

Low-temperature phase transition and magnetic properties of K3YbSi2O7

The new ambient-temperature hexagonal (space group P63 /mmc) polymorph of tripotassium ytterbium(III) disilicate (β-K3YbSi2O7) has been synthesized by the high-temperature flux method and subsequently structurally characterized. In the course of the temperature-dependent single-crystal diffraction experiments, a phase transformation of β-K3YbSi2O7 to a novel low-temperature orthorhombic phase (β′-K3YbSi2O7, space group Cmcm) has been observed at about 210 K. β-K3YbSi2O7 is isostructural with K3ErSi2O7, whereas β′-K3YbSi2O7 adopts a new type of structure. Both compounds can be built up from a regular alternation of layers of two types, which are parallel to the (001) plane. In the octahedral layer, YbO6 octahedra are isolated and linked by K1O6+3 polyhedra. The second, slightly thicker sorosilicate layer is formed by a combination of Si2O7 dimers and K2O6+3 polyhedra. The boundary between the layers is a pseudo-kagome oxide sheet based on 3.6.3.6 meshes. The phase transition is due to a tilt of the two SiO4 tetrahedra forming a single dimer which induces a decrease of the Si—O—Si angle between bridging Si—O bonds from 180° (dictated by symmetry in space group P63/mmc) to ≃164°. Magnetic characterization indicates that K3YbSi2O7 remains paramagnetic down to 2 K, showing no apparent influence of the phase transformation on its magnetic properties. Analysis of the magnetization data revealed the positions of the three lowest crystal field levels of the Yb3+ cations, as well as the corresponding projections of their angular momentum on the direction of the magnetic field.