6H Phase Research Articles

Antiferromagnetic Semiconductor BaMnO3 Hexagonal Perovskite with a Direct Bandgap.

The unique properties of direct bandgap semiconductors make it important to search for semiconductors exhibiting this phenomenon in perovskite materials. In this study, we employed first-principles calculations to investigate the crystal structures, magnetic configurations, and electronic properties of hexagonal perovskite BaMnO3 in its 4H and 6H phases. The results indicate that both structures exhibit antiferromagnetic characteristics, in which the Mn-O-Mn superexchange plays the dominant role in the 4H phase, although there is a competition between the Mn-Mn direct exchange interaction and the Mn-O-Mn superexchange interaction. In contrast, these two interactions exhibit harmonious coexistence in the 6H phase, and the two antiferromagnetic transitions occurring in the experimental phase should be related to the synergistic effect between them. Despite their different internal arrangements, they exhibit the same charge combination of Ba2+Mn4+O2-3. More importantly, both phases exhibit semiconductor properties with a direct bandgap, making it suitable to serve as an alternative material for photovoltaic and optoelectronic devices. In particular, the band gap of the 4H phase is just the right size to absorb visible light, and the 6H phase should be a potential candidate to absorb light in the ultraviolet region.

High-Throughput Predictions of the Stabilities of Multi-Type Long-Period Stacking Ordered Structures in High-Performance Mg Alloys.

The effects of 44 types of elements on the stabilities of I1-constitute multi-type long-period stacking-ordered (LPSO) structures in Mg alloys, such as 4H, 6H, 8H, 9R, 12H, 15R, and 16H phases, are systematically investigated by first-principle high-performance calculations. The intrinsic stacking-fault energies (ISFEs) and their increments are calculated along with the formation enthalpies of solute atoms, and interaction energies between solute atoms and LPSO structures. The results suggest that the 15R phase is the easiest to form and stabilize among these LPSO structures, and 44 types of solute atoms have different segregation characteristics in these LPSO structures. A high temperature inhibits structural stabilizations of the LPSO phases, and these alloying elements, such as elements (Sb, Te, and Cs) for 4H; elements (S, Fe, Sb, and Te) for 6H, 8H, 9R, 15R, and 16H; and elements (S, Sb, and Te) for 12H, can effectively promote the stability of LPSO structures at high temperatures. S and Fe atoms are the most likely to promote the stabilities of the 16H structure with regard to other LPSO phases, but the Fe atom tends to inhibit the stabilities of 4H and 12H structures. This work can offer valuable references to further study and develop high-performance Mg alloys with multi-type LPSO structures.

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.

Revealing phase evolution mechanism for stabilizing formamidinium-based lead halide perovskites by a key intermediate phase

Revealing phase evolution mechanism for stabilizing formamidinium-based lead halide perovskites by a key intermediate phase

SrIrO3 modified with laminar Sr2IrO4 as a robust bifunctional electrocatalyst for overall water splitting in acidic media

Strontium iridate perovskites are attractive electrocatalysts for acidic water oxidation, but suffer from serious Sr-leaching during catalysis, which causes surface rearrangements that damage long-term applications. Herein, 6H phase SrIrO3 modified with laminar Sr2IrO4 (SrIrO) have been developed. Unlike the previous extreme chemical state changes of Ir, SrIrO maintained relatively stable surface Ir states during electrocatalysis. The layered nature of Sr2IrO4 enables the interlaminar Sr easily extracted from the crystal while remaining the in-plane IrO6 octahedral framework, thereby stabilizing the Ir state. The SrIrO displays extremely low overpotential of 245 mV for OER and 18.2 mV for HER at 10 mA cm−2, and outstanding catalytic stability. For acidic water splitting, SrIrO requires only 1.50 V to achieve 10 mA cm−2, surpassing most reported electrocatalysts. This work offers a new route for alleviating the surface rearrangements of perovskites, and exploring efficient and stable electrocalysts for hydrogen production via electrochemical water electrolysis.

Phase transitions and elastic anisotropies of SiC polymorphs under high pressure

The structures, equations of state, phase transitions, mechanical stability, and elastic anisotropies of five SiC polymorphs (wurtzite, 4H, 6H, zinc blende, and rocksalt phases) under pressure were investigated based on first-principles calculations. The optimal structural parameters, densities, and volumes of the five SiC polymorphs under zero and high pressure were obtained and analyzed. The pressures of the phase and stability–instability transitions were predicted using the minimum energy principle and improved Born mechanical stability criteria, respectively, and a new phase transition criterion was developed. The elastic anisotropies of SiC under zero and high pressure were systematically discussed, and the negative Poisson's ratio of zinc blende SiC was verified.

Therapeutic effects of hydro-alcoholic leaf extract of Withania somnifera on age-induced changes in daily rhythms of Sirt1, Nrf2 and Rev-erbα in the SCN of male Wistar rats.

The temporal expression pattern of the circadian clock genes are known to be altered/attenuated with advance in age. Withania somnifera (WS) essentially consists of numerous active constituents including withanolides is known to have antioxidant, anti-inflammatory and adaptogenic properties. We have earlier demonstrated therapeutic effects of hydro-alcoholic leaf extract of WS on the age-induced alterations in the levels and daily rhythms of various clock genes such as rBmal1, rPer1, rPer2 and rCry1. We have now studied effects of hydro-alcoholic leaf extract of WS on the age-induced alterations in the levels and daily rhythms of expression of SIRT1 (an NAD+ dependent histone deacetylase and a modulator of clock) and NRF2 (a clock controlled gene and a master transcription factor regulating various endogenous antioxidant enzymes) in addition to rRev-erbα in SCN of adult [3months (m)], middle-aged (12m) and old-aged (24m) male Wistar rats. The daily rhythms of rNrf2 expression showed 6h phase delay in middle age and 12h phase advance in old age. WS restored rSirt1 daily rhythms and phase in old age whereas it restored the phase of rNrf2 in the SCN of both middle and old aged animals. At protein level, SIRT1 expression showed phase advances in 12m and 24m whereas NRF2 daily rhythms were abolished in both the age groups. WS restored the phase and daily rhythms of SIRT1 as well as NRF2 in 12m old rats. However, rRev-erbα expression was found insensitive to WS treatment in all the age groups studied. Pairwise correlation analysis demonstrated significant stoichiometric interactions among rSirt1, rNrf2 and rRev-erbα in 3m which altered with aging significantly. WS treatment resulted in differential restorations of such interactions.

Effects of Rb Intercalation on NbSe2: Phase Formation, Structure, and Physical Properties.

Here, we report the crystal structures and properties of Rb xNbSe2, with 0 ≤ x ≤ 0.5. With Rb intercalation, Rb xNbSe2 evolves from 2H (Phase I) for 0 ≤ x ≤ 0.025, to 6R (Phase II) for x ∼ 0.2 with space group R3 m (no. 166), and finally to 2H (Phase IV) for 0.375 ≤ x ≤ 0.5 with space group P63/ mmc (no. 194). In addition, Phase II is found to transform to a rare 6H structure (Phase III) with space group P63/ mmc (no. 194) by annealing at a relatively low temperature. We show the first 6H phase in the intercalated transition-metal dichalcogenides (TMDs) family obtained through a solid-state reaction. Moreover, both 6R and 6H phases are new polymorphs in the NbSe2 system. For the range 0.2 ≤ x ≤ 0.5 in Rb xNbSe2, we show metallic electronic transport behavior and a paramagnetic feature. The lack of superconductivity (SC) down to 2 K is most probably due to the decrease of hole carrier density with increasing Rb content. Through careful analysis of the structural data, we were able to assemble a phase diagram covering the range of 0 ≤ x ≤ 0.5 in Rb xNbSe2.

Shock induced phase transition in SiC polytypes

The effect of shock on 3C and 6H phases of silicon carbide was investigated within our earlier developed scaled binding energy based Debye–Gru¨neisen model. Required input parameters for the equation of state (EOS) model were obtained from density functional theory calculations using PBE-GGA functional as well as a nonlocal exchange-correlation functional B3LYP. In our earlier calculations using PBE-GGA functional, we predicted the phase transition from 3C to B1 phase at about 68 GPa, which deviated significantly from the experimental value of about 100 GPa. An empirical energy correction was applied in that case to obtain the experimental transition pressure. However, in the present work, we observed that if hybrid B3LYP functional is employed to calculate EOS parameters, shock induced phase transitions from 3C to B1 and 6H to B1 phases occur at about 102 GPa and 105 GPa, respectively, in close agreement with experiments without invoking any empirical correction. We obtained good agreement of our predictions with experimental shock data using a simple model of metastability and the hybrid B3LYP functional, which accounts for exchange-correlation effects better than the PBE-GGA functional in the case of SiC.

Effects of aluminum, boron and carbon additives on microstructure of porous silicon carbide ceramics and their properties

Abstract Porous SiC ceramics with in-situ plate-like grain growth were fabricated by pressureless-sintering using Al, B and C additives, and the effects of Al, B and C additives on the microstructure of porous SiC ceramics and their properties were investigated. Whereas the porous SiC ceramics sintered with B-C additives mostly consisted of fine SiC grains without grain growth, Al additives with B-C system enhanced plate-like grain growth of SiC in the porous SiC ceramics. Notably, the porous SiC ceramics sintered with 0.1 wt% Al and B-C additives consisted of plate-like SiC grains of around 10-20 μm, and the plate-like SiC grains stuck out of the walls of pores and the sintered surface in the porous SiC ceramics. XRD results suggested that the plate-like SiC grains formed in the porous SiC ceramics sintered with Al-B-C additives mainly consisted from 6H (α-SiC) phase. Compressive strength and thermal conductivity of the porous SiC ceramics sintered with 0.1 wt% Al and B-C additives at room temperature were 50 MPa and 10 W/m·K, respectively. From these results, it is concluded that high performance porous SiC ceramics with unique morphology of entangled large plate-like SiC grains were successfully achieved based on in-situ grain growth.

OR-018 The study of acupuncture intervention on the dynamic changes of Ca2+, Na+, H2O2 flux at earlier time points of rat skeletal muscle regeneration following eccentric exercise

OR-018 The study of acupuncture intervention on the dynamic changes of Ca2+, Na+, H2O2 flux at earlier time points of rat skeletal muscle regeneration following eccentric exercise

Polytypism-induced stabilization of hexagonal 2H, 4H and 6H phases of gold

Recent experiments reveal that, contrary to traditional expectations, face-centered cubic Gold known, in many respects, as the “noblest” of all metals evincing remarkable repertoire of chemistry, can adopt in the nano-scale regime unusual hexagonal close-packed polytypes (2H, and 4H) form. Through first-principles study, we show evidence of global phase stability (via thermodynamic, mechanic and dynamic calculations) of the synthesized 2H and 4H with distinguished structural correspondence to experimental measurements. In addition, we found that polytypism can induce stable higher polytype hexagonal 6H phase of Au.

Synthesis of 6H-SiC nanowires on bamboo leaves by carbothermal method

6H silicon carbide (SiC) nanowires were fabricated on bamboo leaves infiltrated with tetraethyl orthosilicate (TEOS) by carbothermal method at 1300–1400°C. The bamboo leaves were the carbon source and template for the growth of SiC. The TEOS was the silicon source. The crystalline structure, morphology, and the distribution of the prepared SiC were characterized by scanning electron microscopy, X-ray diffraction, Raman spectroscopy and high resolution transmission electron microscopy. The SiC had a hexagonal 6H- structure with diameter of 60 to 160nm and length up to tens of microns. The yield of SiC grown on the top surface of the bamboo leaf was higher and had a branched structure. The SiC on the bottom surface showed a bamboo-like structure. The nanowires were mainly 6H phase, however cubic 3C-SiC phase was found on the divisions in the branch structure and the nodes in the bamboo-like structure. The difference in density of SiC nanowires between the two surfaces is proposed to be related to the structural and compositional differences of the two surfaces. TEOS was preferred to attach the hydrophilic top surface, which led to larger amount of SiC. Meanwhile, the Al content inside the bottom surface prohibited the growth of the SiC nanowires. The growth mechanism of the SiC nanowires is also discussed.

Direction-dependent effects of chronic “jet-lag” on hippocampal neurogenesis

Disruptions in circadian rhythms, as seen in human shift workers, are often associated with many health consequences including impairments in cognitive functions. However, the mechanisms underlying these affects are not well understood. The objective of the present study is to explore the effects of circadian disruption on hippocampal neurogenesis, which has been implicated in learning and memory and could serve as a potential pathway mediating the cognitive consequences associated with rhythm disruption. Circadian rhythm disruptions were introduced using a weekly 6h phase shifting paradigm, in which male Wistar rats were subjected to either 6h phase advances (i.e. traveling eastbound from New York to Paris) or 6h phase delays (i.e. traveling westbound from Paris to New York) in their light/dark schedule every week. The effects of chronic phase shifts on hippocampal neurogenesis were assessed using doublecortin (DCX), a microtubule binding protein expressed in immature neurons. The results revealed that chronic disruption in circadian rhythms inhibits hippocampal neurogenesis, and the degree of reduction in neurogenesis depends upon the direction and duration of the shifts. In two cohorts of animals that experienced phase shifts for either 4 or 8 weeks, a greater decrease in neurogenesis was observed when the phase was advanced versus delayed in both groups. The direction-dependent effect mirrors the findings on clock gene expression in the SCN, suggesting a causal link between the reduction in hippocampal neurogenesis and a disrupted SCN circadian clock.

Oxygen-deficiency-induced 6H-polymorph of hexagonal perovskite Ba4YMn3O11.5−δ: synthesis, structure and properties

The 12-layer hexagonal perovskite Ba4YMn3O11.5 (Rm, referred to as 12R) was transformed to a 6-layer mixed valent Mn3+/4+ hexagonal perovskite Ba4YMn3O10.7 (P63/mmc, referred to as 6H) via a partial-reduction in a N2 flow. This phase transformation between the 12R and 6H phases is redox-reversible. In contrast with the 12R-type Ba4YMn3O11.5 structure containing c-BaO2.75 and h-BaO3 layers with a (cchh)3 stacking, the 6H-type Ba4YMn3O10.7 structure consists of the cubic (c) BaO2.87 and hexagonal (h) BaO2.33 layers with a (cch)2 stacking, showing a preference of oxygen vacancy distribution in the hexagonal layers over the cubic layers. The h-BaO2.33 layer in the 6H-type Ba4YMn3O10.7 transforms two-thirds of face-sharing octahedral Mn2O9 dimers into edge-sharing pyramidal Mn2O8 units, sharing corners with Y octahedra/pyramids. Impedance measurements suggested that the 6H-type material is insulating with a bulk electrical resistivity of ∼107 Ω cm at 303 K, significantly higher than that for the 12R-type Ba4YMn3O11.5 by ∼4 orders of magnitude.

Suppressing the Powdering of Zirconia Fibers after High-Temperature Application by Large Additions of Yttria

Zirconia fibers with different Y2O3 additions were prepared by sol-gel method. After the fiber was thermally treated at 1650°C for 6h, the microstructure and crystalline phase were investigated by scanning electron microscope and X-ray diffraction. It was found that the fibers with 5mol% of Y2O3 seriously powdered due to volume expansion caused by the phase transformation form tetragonal phase to monoclinic phase. The powdering of zirconia fibers after high–temperature application could be depressed when the amount of Y2O3 was more than 10mol%.

Synthesis of α-SiC/α-SiAlON composites by spark plasma sintering: Phase formation and microstructures development

α-SiC/α-SiAlON composites with 80 wt% α-SiC (6H phase) were fabricated by spark plasma sintering at 1800–2000 °C in a 0.6 atm nitrogen atmosphere. The effects of the temperatures on the phase development, microstructures and mechanical properties were investigated. The results showed the Si 3N 4, AlN, Al 2O 3, and Y 2O 3 particles were isolated by the 6H-SiC to prevent α-SiAlON formation at 1800 °C. The Si 3N 4 decomposed at 1900 °C and above, thus added Si in the phase compositions. The α-SiC grains grew anisotropic in the sintering liquids at 1800 °C and 1900 °C, forming the self-reinforcing microstructures, and accordingly increased the flexural strength and fracture toughness. In cooling down immediately after the temperature reached 2000 °C, a transitory hold at 1700 °C transformed the 6H-SiC into the 3C polytype in 30 s. The electric current was suspected of activating this polytype transformation.

Structural and Magnetic Properties of Ba0.7Sr0.3Ru1−xMnxO3 Hexagonal Perovskites

The structures and properties of Ba(0.7)Sr(0.3)Ru(1-x)Mn(x)O(3) perovskites have been investigated in samples prepared at 1300 degrees C in air. Two polytypes are found, a 6H phase for 0.2 < or = x < or = 0.4 and a 4H type for 0.6 < or = x < or = 1. The cell parameters and volume vary linearly in both solid solution ranges. Spin-freezing transitions up to 60 K are observed for the 6H samples, but the 4H materials show high temperature Néel transitions at 210 to 280 K.

A New Perovskite Polytype in the High-Pressure Sequence of BaIrO3

The high-pressure sequence of the perovskite polytypes of BaIrO(3) has been investigated in the pressure range up to 10 GPa. At ambient pressure the so-called "9R" polytype has been prepared by solid-state reaction and slow cooling in air to yield an almost fully oxygen-stoichiometric BaIrO(2.96(1)) composition. The crystal structure has been refined from XRD data in the monoclinic C2/m space group with a = 10.0046(3) A, b = 5.75362(14) A, c = 15.1839(4) A, beta = 103.27(1) degrees ; it contains trimers of face-sharing octahedra (or Ir(3)O(12) trioctahedra) that are linked by their vertices to form columns parallel to the c-axis with a stacking of layers of corner sharing (c) and face sharing (h) IrO(6) octahedra along the sequence hhchhc. This structure is stable up to 3 GPa; at 4 GPa a new 5H polytype has been stabilized as a pure phase. The crystal structure has been solved by ab initio procedures from powder XRD data. It is monoclinic with a = 9.9511(2) A, b = 5.7503(1) A, c = 13.71003(3) A, beta = 118.404(2) degrees , and it was refined in the C2/m space group from NPD data collected at room temperature. This polytype can be described as a stacking of IrO(6) octahedra along the sequence hchcc. The structure contains chains of double dimer units of face-sharing octahedra; the twin dimers are connected to single layers of vertex-sharing octahedra, forming infinite chains along c. This is a unique stacking that, with this repetition length, has never been described before among the hexagonal polytypes of ABO(3) perovskites. The 5H polytype is stable in a narrow pressure range; at 5 GPa the 6H structure is formed, stable up to 10 GPa. The 6H-BaIrO(3) polytype is monoclinic, space group C2/c, with a = 5.7483(2) A, b = 9.9390(3) A, c = 14.3582(5) A, beta = 91.319(2) degrees . The structure consists of dimers of face-sharing octahedra separated by single corner-sharing octahedra, showing the sequence hcchcc along the c-axis. At 10 GPa the cubic 3C perovskite structure could be identified as a minority phase, with a = 4.0611(7) A, defined in the Pm3m space group. The precarious stability of the 5H polytype, as well as the novel pressure sequence displayed by BaIrO(3) that is distinct from the classical sequence 9R-4H-6H-3C exhibited by many transition metal oxides, for instance BaRuO(3), is a result of the particular stability of the "9R" ambient-pressure structure, which is reinforced by a strong Ir-Ir bond across the octahedral faces, and the Ir-Ir Coulombic repulsion across shared faces that destabilizes the 4H polytype relative to the 6H phase to allow stabilization of the hybrid 5H polytype in a narrow presure range.

3C–6H phase transition in BaTiO3 induced by Fe ions: an electron paramagnetic resonance study

X-ray diffraction (XRD) patterns and electron paramagnetic resonance (EPR) powder spectra (9 and34 GHz) of BaTiO3+0.04 BaO+0.5xFe2O3 (0.0005≤x≤0.02) ceramics were studied to investigate the development of the hexagonal phase(6H modification) of Fe-doped material in dependence upon doping levelx and sinteringtemperature Ts. Three axially symmetric EPR spectra assigned toFe3+ ions substituted at Ti lattice sites corresponding to the different distorted octahedra intetragonal and hexagonal modification are observed at room temperature. Thepresence of a hexagonal phase is shown by the XRD pattern and the EPR spectra.The 6H modification begins to occur at a nominal Fe concentration of betweenx = 0.005 and 0.01 and increases with increasing sintering temperature.BaTiO3 ceramicswith x = 0.02 sintered at Ts = 1400 °C is hexagonal. As discussed in the case of other 3d ions we propose the Jahn–Teller(JT) distortion as the driving force for the cubic–hexagonal transition at hightemperatures whereas a sufficiently high concentration of oxygen vacancies isthe second condition for this transformation process. In the case of Fe-dopedBaTiO3 a minimumconcentration of Fe4+ in the ceramic grains could trigger the transmutation into the hexagonal phase.