Tetrahedral atom ordering in a zeolite framework: a key factor affecting its physicochemical properties

Three gallosilicate natrolites with closely similar chemical composition but differing in the distribution of Si and Ga over crystallographically different tetrahedral sites (T-sites) show striking differences in their cation exchange performance. The ability to exchange Na(+) by the larger alkali metal cations decreases upon increasing the size of the cation, as expected, but also with the degree of T-atom ordering. To seek an insight into this phenomenon, the crystal structures of 11 different zeolites, which show variations in degree of T-atom ordering, nature of countercation, and hydration state, have been refined using synchrotron diffraction data. While the three as-made sodium materials were characterized to have a low, medium, and high degree of ordering, respectively, their pore sizes are close to the size of the bare Na(+) cation and much smaller than that of the larger alkali cations, which are nonetheless exchanged into the materials, each one at a different level. Interestingly, large differences are also manifested when the Na(+) back-exchange is performed on the dehydrated K(+) forms, with crystallographic pore sizes too small even to allow the passage of Na(+). Although the thermodynamic data point to small differences in the enthalpy of the Na(+)/K(+) exchange in the three materials, comparison of the "static" crystallographic pore sizes and the diameter of the exchanged cations lead us to conclude that during the exchange process these zeolites undergo significant deformations that dynamically open the pores, allowing cation traffic even for Cs(+) in the case of the most disordered material. In addition to the very large topological flexibility typical of the natrolite framework, we propose as a hypothesis that there is an additional flexibility mechanism that decreases the rigidity of the natrolite chain itself and is dependent on preferential siting of Si or Ga on crystallographically different T-sites.     

Dehydration-induced water disordering in a synthetic potassium gallosilicate natrolite

A new potassium gallosilicate zeolite with a natrolite topology (approximate formula K8.2Ga8.2Si11.8O40.11.5H2O) was synthesized under hydrothermal conditions and characterized as a function of temperature using monochromatic synchrotron X-ray powder diffraction and Rietveld analyses. Unlike the previously known tetragonal K8Ga8Si12O40.6H2O phase, the as-synthesized material contains twice the amount of water molecules in an ordered arrangement throughout the channels in an orthorhombic (I212121) symmetry. The ordered configuration of water molecules is stabilized below 300 K, whereas heating above 300 K results in a selective dehydration and subsequent disordering of water molecules in a tetragonal (I2d) symmetry. Above 400 K, the material transforms to a fully dehydrated tetragonal phase with a concomitant volume reduction of ca. 15%. The fully dehydrated material transforms back to its original state when rehydrated over a period of up to 2 weeks. The distribution of potassium cations within the channels remains largely unperturbed during the water rearrangements and their order-disorder transition within the channels.     

Intraframework Migration of Tetrahedral Atoms in a Zeolite

The transformation from a disordered into an ordered version of the zeolite natrolite occurs on prolonged heating of this material in the crystallizing medium, but not if the mother liquor is replaced by water or an alkaline solution. This process occurs for both aluminosilicate and gallosilicate analogues of natrolite. In cross experiments, the disordered Al‐containing (or Ga‐containing) analogue is heated while in contact with the mother liquor of the opposite analogue, that is, the Ga‐containing (or Al‐containing) liquor. Therefore, strong evidence for the mechanism of the ordering process was obtained, which was thus proposed to proceed by intraframework migration of tetrahedral atoms without diffusion along the pores. Migration is first triggered, then fuelled by surface rearrangement through reactions with the mother liquor, and stops when an almost fully ordered state is attained. Classical dissolution–recrystallization and Ostwald ripening processes do not appear to be relevant for this phase transformation.     

分子筛中TO4四面体的电荷效应

以AlCl₃和PCl₅与APO-5分子筛气固相反应结果为出发点,应用Pauling静电规则和量子化学计算考察了硅铝分子筛和磷铝分子筛同晶置换反应中的电性因素和能量因素。比较Loewenstein规则和静电规则在分子筛中的应用,得出结论:Loewensteio规则的本质是静电规则,这一规则的内容具有较大的局限性,应扩展为带同种电荷的TO₄四面体不相连规则。量子化学的计算证明了上述结论的正确性。     

Phase transformation studies on Mn−Al−C alloy,by DSC and electrical resistivity measurements

Phase transformation studies carried out on Mn?Al?C permanent magnet alloy employing DSC and electrical resistivity measurements, are reported and discussed. The transformation of the hexagonal Mn?Al phase (disordered and non-magnetic) to the ferromagnetic fct phase proceeds via the formation, in at least two stages, of the ordered orthorhombic phase. The activation energy for the formation of the fct phase is ≈34.65 kcal/mol. Microstructural changes occurring at various stages of the transformation are also studied.     

Synthesis and characterization of ordered and disordered polycrystalline La(2)NiMnO(6) thin films by sol-gel

Polycrystalline La(2)NiMnO(6) thin films are prepared on Pt/Ti/SiO(2)/Si substrates by the sol-gel method. Through controlling the processing parameters, the cation ordering can be tuned. The disordered and ordered thin films exhibit distinct differences for crystal structures as well as properties. The crystal structure at room temperature characterized by X-ray diffraction and Raman spectra is suggested to be monoclinic (P2(1)/n) and orthorhombic (Pbnm) for the ordered and disordered thin films, respectively. The ferromagnetic-paramagnetic transition is 263 K and 60 K for the ordered and disordered samples respectively, whereas the saturation magnetic moment at 5 K is 4.9 μ(B) fu(-1) (fu = formula unit) and 0.9 μ(B) fu(-1). The dielectric constant as well as magnetodielectric effect is higher for the ordered La(2)NiMnO(6) thin films. The magnetodielectric effect for the ordered thin film is dominantly contributed to the intrinsic coupling of electric dipole ordering and fluctuations and magnetic ordering and fluctuations, while it is mainly contributed to Maxwell-Wagner (M-W) effects for the disordered thin film. The successful achievements of ordered and disordered polycrystalline La(2)NiMnO(6) thin films will provide an effective route to fabricate double-perovskite polycrystalline thin films by the sol-gel method.     

The chromium tungsten nitride system: evidence of a disorder-order phase transformation

A detailed investigation of the synthesis of CrWN₂ from a chemical precursor has revealed that it forms via an intermediate disordered nitride. The kinetics of this disordered to ordered phase transformation have been determined and suggest that the layered compound nucleates and grows epitaxially out of the disordered nitride via a two-dimensional transformation mechanism in which cation ordering takes place at the interface between the two structures. Results from high-resolution transmission electron microscopy analysis were used to identify the interfacial relationship between the disordered and ordered phases. In combination with our kinetic data, this information implies that ordering occurs through a short-range, interfacially controlled diffusional process.     

On the high-temperature phase transition of Gd5Si2Ge2

The first-order monoclinic-to-orthorhombic (beta-->gamma) phase transition of the giant magnetocaloric material Gd(5)Si(2)Ge(2) was studied using in situ high-temperature single-crystal X-ray diffraction. A special crystal mounting procedure was developed to avoid crystal contamination by oxygen or nitrogen at high temperatures. The elastic beta-->gamma transformation occurs at 300-320 degrees C during heating, and it is reversible during fast and slow heating and slow cooling but irreversible during rapid cooling. Contrary to theoretical predictions, the macroscopic distribution of the Si and Ge atoms remains the same in both the orthorhombic gamma-polymorph and the monoclinic beta-phase. It appears that interstitial impurities may affect stability of both the monoclinic and orthorhombic phases. In the presence of small amounts of air, the beta-->gamma transformation is complete only at 600 degrees C. The interslab voids, which can accommodate impurity atoms, have been located in the structure, and an effect of partially filling these voids with oxygen or nitrogen atoms on the beta-gamma transition is discussed.     

Pressure-induced amorphization in Y2(WO4)3: in situ X-ray diffraction and Raman studies

The high-pressure behavior of Y₂(WO₄)₃ has been investigated at room temperature by in situ X-ray diffraction and Raman scattering measurements. Both the studies show that beyond ∼3 GPa, this compound smoothly transforms from the ambient orthorhombic phase to a disordered phase. The structural modifications are found to be reversible up to ∼4 GPa but become irreversible at higher pressures. Low pressures of transformation imply that these changes are intrinsic and not due to non-hydrostatic stresses. In addition, the correlation between the stability range of orthorhombic phase and counter cation size supports that this compound has a large field of negative thermal expansion in this family of compounds.     

Gd5-xYxTt4 (Tt = Si or Ge): effect of metal substitution on structure, bonding, and magnetism

A crystallographic study and theoretical assessment of the Gd/Y site preferences in the Gd 5- x Y x Tt 4 ( Tt = Si, Ge) series prepared by high-temperature methods is presented. All structures for the Gd 5- x Y x Si 4 system belong to the orthorhombic, Gd 5Si 4-type (space group Pnma). For the Gd 5- x Y x Ge 4 system, phases with x < 3.6 and x >or= 4.4 adopt the orthorhombic, Sm 5Ge 4-type structure. For the composition range of 3.6 相似文献   

Synthesis of thermally stable highly ordered nanoporous tin oxide thin films with a 3D face-centered orthorhombic nanostructure

Thin films of nanoporous tin oxide with a 3D face-centered orthorhombic nanostructure have been synthesized by self-assembly that is controlled by post-coating thermal treatment under controlled humidity. In contrast to the conventional evaporation-induced self-assembly (EISA), the films here have no ordered nanostructure after dip-coating. However, the initial coatings are formed under conditions that inhibit significant hydrolysis and condensation for extended periods. This allows the use of postsynthesis thermal vapor treatments to completely control the formation of the nanostructure. With EO106-PO70-EO106 (Pluronic F127) triblock copolymer as the template, highly ordered nanostructures were generated by exposing the disordered films to a stream of water vapor at elevated temperature, which rehydrates the films and allows the formation of the thermodynamically favored phase. Further exposure to water vapor drives the condensation reaction through the elimination of HCl. The X-ray diffraction pattern from the nanostructure was indexed in the space group Fmmm as determined by analysis of 2D small-angle X-ray scattering patterns at various angles of incidence. The nanostructure is then stabilized and made nanoporous by extended controlled thermal treatments. After self-assembly and template removal, the films are thermally stable up to 600 degrees C and retain an ordered, face-centered orthorhombic nanostructure.     

β-AgAl(1-x)Ga(x)O2 solid-solution photocatalysts: continuous modulation of electronic structure toward high-performance visible-light photoactivity

A series of β-AgAl(1-x)Ga(x)O(2) solid-solution materials were explored as novel visible-light-sensitive photocatalysts. These Ag-based solid solutions crystallize in a homogeneous crystal structure with orthorhombic symmetry but possess continuously modulated band gaps from 2.19 to 2.83 eV by decreasing the ratios of Ga/Al. Their photoactivities for iso-propanol degradation were found to be dependent on the variation of chemical compositions. Among them, the β-AgAl(0.6)Ga(0.4)O(2) sample showed the highest photocatalytic performance, which simultaneously exhibited 35 and 63 times higher activities than two terminus materials, β-AgAlO(2) and β-AgGaO(2), respectively. The apparent quantum efficiency of this sample for iso-propanol photodegradation achieved up to 37.3% at the wavelength of 425 ± 12 nm. The theoretical calculation based on density functional theory demonstrated that the levels of valence band maximum of β-AgAl(1-x)Ga(x)O(2) are similar, but the levels of conduction band minimum are gradually negatively shifted with the increase of the ratio of Ga/Al, thereby continuously narrowing the band gap. Nevertheless, the highest activity observed on β-AgAl(0.6)Ga(0.4)O(2) may be attributed to its optimized band structure, which adapts the balance between effective visible-light absorption and adequate redox potentials.     

On stimuli and mechanisms of crystallization

The crystallographic analysis of a series of compounds with complex composition was carried out from mechanical wave nature of the crystalline state and, as a consequence, a preferential energetic stability of the atomic groups with the local ordering of the atomic positions (point and translational symmetry). Such ordered atomic groups were shown to be nuclei of the crystal structure resulting from the coherent assemblage of the groups. A predominance of monoclinic and orthorhombic crystals in nature is explained by equally probable nuclei formation of right and left atomic configurations as well as the presence of glide plane operations for the mutual packing.     

Disorder Engineering in Monolayer Nanosheets Enabling Photothermic Catalysis for Full Solar Spectrum (250–2500 nm) Harvesting

A persistent challenge in classical photocatalyst systems with extended light absorption is the unavoidable trade‐off between maximizing light harvesting and sustaining high photoredox capability. Alternatively, cooperative energy conversion through photothermic activation and photocatalytic redox is a promising yet unmet scientific proposition that critically demands a spectrum‐tailored catalyst system. Here, we construct a solar thermal‐promoted photocatalyst, an ultrathin “biphasic” ordered–disordered D‐HNb₃O₈ junction, which performs two disparate spectral selective functions of photoexcitation by ordered structure and thermal activated conversion via disordered lattice for combinatorial photothermal mediated catalysis. This in situ synthetically immobilized lattice distortion, constrained to a single‐entity monolayer structure not only circumvents interfacial incompatibility but also triggers near‐field temperature rise at the catalyst–reactant complexes’ proximity to promote photoreaction. Ultimately, a generic full solar conversion improvement for H₂ fuel production, organic transformation and water purification is realized.     

"Nanoscale zippers" in Gd5(SixGe(1-x))4: symmetry and chemical influences on the nanoscale zipping action

One critical parameter influencing the structural nature of the phase transitions in magnetocaloric materials Gd5(SixGe(1-x))4 is the Si/Ge ratio (x/1-x), because transition temperatures and structures depend crucially on this value. In this study, single-crystal X-ray diffraction indicates that Si and Ge atoms are neither completely ordered nor randomly mixed among the three crystallographic sites for these elements in these structures. Ge atoms enrich the T sites linking the characteristic slabs in these structures, while Si atoms enrich the T sites within them. Decomposition of the total energy into site and bond energy terms provides a rationale for the observed distribution, which can be explained by symmetry and electronegativity arguments. For any composition in Gd5(SixGe(1-x))4, a structure map is presented that will allow for a rapid assessment of the specific structure type.     

Ga+ primary ion ToF-SIMS fragment pattern of metals and inorganic compounds.

The appearance regularity of the Ga+ primary ion ToF-SIMS fragment pattern of metals and inorganic compounds is discussed. For inorganic compounds formulated like M-A, where the valence of the cation M is +n, that of the anion A is -p; also, the chemical composition of the ToF-SIMS fragment is MxAy, the rule nx > or = (py + 1) is satisfied for positive ion fragments and nx < or = (py + 1) for negative ones. For example, for an oxide fragment of chemical composition MxOy (valence of M is +n), the chemical composition of the fragment appears obeying the rule nx > or = (2y + 1) for positive ions and nx < or = (2y + 1) for negative ones, respectively. The regularity of ToF-SIMS fragment patterns of sulfides, nitrates, sulfates etc. is discussed. Further, when the Ga+ primary ion ToF-SIMS fragment patterns of metals are observed, it can be inferred that the overlapped particle emission occurs from metal surfaces through alloying and/or clustering of Ga with metal on surfaces.     

B、Al、Ga同晶取代丝光沸石的从头计算

用从头计算Hartree Fock方法研究了B、Al、Ga等同晶取代进入丝光沸石骨架后可能存在的位置,确定了与电荷平衡质子结合的氧位置,考察了B、Ga等杂原子进入骨架对丝光沸石Brnsted酸性的影响.能量分析表明B、Al、Ga在丝光沸石骨架中最容易进入T3和T4位;当Al、Ga在T4位时,质子与O10结合为能量最低即最稳定结构,而当B在T4位时,质子与O2或O10结合比较稳定.质子亲合势分析表明与硅铝丝光沸石相比, B和Ga进入骨架导致丝光沸石分子筛的Brnsted酸性有所减弱,其酸性依次为：B ZSM 5Ga ZSM 5< Al ZSM 5.     

二维六方p6mm有序介孔WO_3-TiO_2复合材料的制备及其可见光光催化性能

以非离子型表面活性剂为模板剂,采用蒸发诱导自组装法制备了一系列不同WO3含量的有序介孔WO3-TiO2复合材料,并表征了其孔结构、形貌、孔隙率、光谱性质及组成.结果表明,该材料呈二维六方p6mm对称和锐钛矿晶相结构;与无序WO3-TiO2复合材料相比,其比表面积(152～154m2/g)更大,孔径更均一(5.3nm),且比纯TiO2的帯隙宽度(3.0eV)更窄.将该WO3-TiO2样品用于可见光光催化降解水相中罗丹明B和2,4-二氯苯氧乙酸的反应中,发现WO3含量适当的有序介孔WO3-TiO2样品的光催化活性比无序的样品和纯TiO2的更高.     

Surface characterization of functional iron–gallium alloys annealed under different conditions

Fe–Ga alloys are functional magnetostrictive materials, which are promising for application in actuators and sensors. Because surface properties of these alloys such as corrosion resistance are important in technological applications, it is required to characterize the chemical composition and state of the surface of these alloys, which depend on annealing conditions. In this study, X-ray absorption spectroscopy (XAS) and secondary ion mass spectrometry (SIMS) were used to characterize surface layers formed on the Fe–Ga alloys annealed under different atmospheric conditions. The XAS spectra of the annealed sample showed that the amount of gallium in the surface layers increased due to annealing, whereas the XAS spectra of the as-polished alloys revealed that the amounts of iron and gallium arise from the bulk composition. The XAS spectra of the alloys annealed in argon–hydrogen with residual oxygen showed that gallium is increased for its preferential oxidation. SIMS depth profile also showed the enrichment of gallium on the surface and the inhomogeneous distribution of iron on the surface layers.