Metastable Reaction Products Research Articles

One of Nature's Puzzles Is Assembled: Analog of the Earth's Most Complex Mineral, Ewingite, Synthesized in a Laboratory.

Through the combination of low-temperature hydrothermal synthesis and room-temperature evaporation, a synthetic phase similar in composition and crystal structure to the Earth’s most complex mineral, ewingite, was obtained. The crystal structures of both natural and synthetic compounds are based on supertetrahedral uranyl-carbonate nanoclusters that are arranged according to the cubic body-centered lattice principle. The structure and composition of the uranyl carbonate nanocluster were refined using the data on synthetic material. Although the stability of natural ewingite is higher (according to visual observation and experimental studies), the synthetic phase can be regarded as a primary and/or metastable reaction product which further re-crystallizes into a more stable form under environmental conditions.

Synthesis of ZIF-11 – Influence of the synthesis parameters on the phase purity

The synthesis of ZIF-11 in N,N-Diethylformamide (DEF) at varying reaction temperatures, times and different molar compositions of the reaction mixture was investigated in detail in order to identify ranges of the formation of phase-pure ZIF-11. Since it is known that water in the solvent DEF leads to the formation of the phase impurity ZIF-7-III during the ZIF-11 synthesis, all reactions were carried out in DEF exhibiting water contents below 0.1 wt%. The variations of the molar ratio of substrates Zn:bIm:DEF = 1:x:y as well as the synthesis temperature and time revealed a number of phase transitions within the ZIF-11 synthesis. Highly crystalline ZIF-11 was obtained for a molar ratio Zn:bIm:DEF of 1:15:1400 already after 3 h of reaction at 60 °C and 100 °C. Depending on the reaction vessel, the water content of the solvent as well as the temperature, ZIF-11 transformed to the dense phase ZIF-7-III within different reaction times. It was concluded that ZIF-11 represents a metastable reaction product. SEM images of the samples obtained after different synthesis times showed that ZIF-11 undergoes successive dissolution-recrystallization cycles in combination with heterogeneous nucleation. For a reduced amount of solvent, the formation of ZIF-11 proceeded faster, but ZIF-11 with lower maximum relative crystallinity was formed. For lower ratios of solvent to zinc, the time range of stable ZIF-11 formation was smaller as compared to higher solvent amounts. The variations of the ratio of zinc to the linker benzimidazole showed that ZIF-11 formation was slower for lower molar ratios, e.g. 1:2:1400 and 1:4:1400. ZIF-11 with the highest relative crystallinity and a pore volume of 0.43 cm3 g−1 was formed after 3–12 h of reaction at molar ratios Zn:bIm:DEF of 1:8:1400 and 1:12:1400. Another phase transition was observed for synthesis times of 6–96 h for a molar ratio of 1:2:1400. Under those conditions, an unidentified layered crystalline product was found and similarities to the layered ZIF-L were identified.

The Thermal Oxidation of TiAlN High Power Pulsed Magnetron Sputtering Hard Coatings as Revealed by Combined Ion and Electron Spectroscopy

The thermal oxidation of TiAlN hard coatings deposited by High Power Pulsed Magnetron Sputtering (HPPMS) is investigated at room temperature and 800 K at oxygen pressures ranging from 10−6 to 10−2 Pa by means of in situ X‐ray and Ultraviolet Photoelectron Spectroscopy as well as Low Energy Ion Scattering. The spectra reveal that oxygen binds selectively to titanium during the initial chemisorption step and simultaneously some oxygen is dissolved into subsurface layers, which stay metallic. Enhanced oxidation results into continuous formation of a multilayered oxide film including oxynitride TiAl(O,N) as a metastable reaction product buried below an oxidic top layer. This top layer is either composed of mixed TiAlO after oxidation at 800 K or of segregated TiO2 and Al2O3 when oxidizing at 293 K. Additionally, evaluation of UV‐valence bands reveals nitrogen doping of the surface oxide films. The results are of high relevance for tailoring of the surface characteristics of TiAlN after deposition, for the design of TiAlN multilayers and for an improved understanding of the interactions of gas particles with these coatings.

From Colloidal Aggregates to Layered Nanosized Structures in Polymer−Surfactant Systems. 1. Basic Phenomena

In this work, we examine the rich crystallization behavior that occurs in PbII/S-II/poly(ethylene oxide) (PEO)/sodium dodecyl sulfate (SDS) systems, in which the anionic surfactant interacts strongly with the polymer molecules, forming micellar aggregates attached to the polymer chains above the critical association concentration. Lead sulfide crystallites are formed in the vicinity of polymer-bound micelles by adding lead and sulfide ions to the polymer−surfactant solution. Surfactant-stabilized inorganic particles adsorbed on the polymer chains combine through a polymer-mediated bridging flocculation mechanism to produce characteristic rodlike colloidal aggregates. Under certain conditions, these evolve into a range of metastable structures, composed of lead sulfide, PbS, and lead dodecyl sulfate, Pb(DS)2. XRD analysis of the metastable reaction products allows us to follow the slow kinetics of their formation and reveals a well-defined layered structure, based on lead dodecyl sulfate, the thickness of ...

Phase development in annealed zirconia-titania nanolaminates

Phase development was studied in sputter-deposited and annealed ZrO2–TiO2 nanolaminate films on fused silica substrates. The goal was to understand phase evolution as these structures moved toward thermodynamic equilibrium. The results show that diffusive amorphization to form α-ZTO was the first reaction of the as-deposited constituents at low temperature (700 °C). Growth of α-ZTO was self-limited, and a second metastable reaction product with an incommensurate α-PbO2-type lattice, ss-ZTO, formed with time at temperature. Terminal phases formed only when the annealing temperature was raised to 1000 °C, thereby lifting kinetic constraints to diffusion. The terminal phases were ZrTiO4 or ZrTi2O6, depending upon film stoichiometry. This study demonstrates that in the absence of a physical driving force to promote layer registration upon low temperature annealing, constituents react to lower the system’s free energy via a series of metastable phases that involve limited atomic rearrangement. Equilibrium phases are formed only after the kinetic constraints to diffusion are relaxed.

Reaction of H 2 and CH 3OH with YBa 2Cu 3O 7 and related compounds

The oxygen content of YBa 2Cu 3O 7, (LaSr) 8Cu 8O 20, La 4BaCu 5O 13, La 2Sr 6Cu 8O 16 and La 2CuO 4 is reduced by hydrogen in different steps to metastable reaction products at 160–450 °C and the stable decomposition product of metallic Cu and different oxides above 420–730 °C. Similar reactions were also observed for CH 3OH, which reacts catalytically above ≈ 200 °C. The behaviour of CH 4 is similar to an inert gas below ≈ 450 °C.

Reaction of H 2 gas with perovskite related cuprates

The oxygen content of (La, Sr) 8Cu 8O 20, La 4BaCu 5O 13, La 2Sr 6Cu 8O 16, La 2CuO 4, LaBa 2Cu 3O 7 and YBa 2Cu 3O 7 is reduced by reaction with hydrogen at 160–450°C in different steps of metastable reaction products with perovskite related crystal structures. The minimum oxygen content x = 16.6, 11, 15 and 3.75 of the first to fourth compound is lower than the equilibrium values. Thermogravimetric experiments show a decomposition to hydroxides or oxides and Cu metal at increased temperatures.

Detection of oxygen by PEELS in ribbon-like defects of thermally aged CZ-Si

Oxygen precipitation during post growth thermal annealing of CZ-Si has been a subject of practical and fundamental interest for several decades. Precipitation in the lower temperature part of the range, below about 1000K, where the products are small and metastable. is of highest interest at present. The recent general availability of HREM instruments with interpretable resolution limits of ∽0.25nm facilitated many imaging and diffraction investigations of low temperature precipitation reaction products during the past three years. Many metastable reaction products were observed, including ribbon-like defects (RLD’s) extended along [011] Si with habit plane variable between {001} and {311} Si. loopites. blobs, decorated dipoles and small features visible only in strain contrast thought to be nuclei for the larger amorphous SiOx precipitates observed after aging at higher temperature. The RLD’s contained a crystalline phase along their cores differing in structure from Si. This phase was first identified as coesite, a phase of SiO2 stable only under high pressure, but later the HREM results were reinterpreted as hexagonal Si. not involving oxygen. This controversy remains unresolved, and it was suggested that independent determination of the presence or absence of oxygen in RLD’s by electron energy loss spectroscopy may be the most useful method for its resolution. This note presents our first results.

Alloying reaction in thin nickel films deposited on GaAs

The alloying reaction in a thin nickel film deposited on a GaAs substrate was investigated using microprobe Auger spectroscopy, reflection high energy electron diffraction and transmission electron diffraction. A nickel film reacts with the substrate above 200°C to form a metastable hexagonal reaction product with the composition Ni:Ga:As = 2:1:1, which is monocrystalline with the orientation relation 〈0001〉 hexagonal // 〈111〉 GaAs and 〈11 2 0〉 hexagonal // 〈110〉 GaAs. Above 400°C the metastable reaction product decomposes into NiAs and β-NiGa, both of which are also monocrystalline with the same orientation relation as the metastable reaction product. The role of nickel in GaAs contact systems is explained by the high reactivity of nickel with GaAs in the solid-solid phase.

Laser induced fluorescence detection and characterization of ionic and metastable reaction products

Ionic and metastable products of reactions involving metastable inert gas atoms have been detected and characterized by the laser induced fluorescence (LIF) method. The LIF technique has been used to measure product vibrational distributions which can be compared with predictions based on observations of the chemiluminescence produced in these reactions. Good agreement between the LIF measurements and the chemiluminescence predictions has been obtained for A 3Σ+u N2, a 4Π O2+, and a 3Π CO. However, in the case of X 2Σg+ N2+, a surprisingly high v=0 population is detected by LIF. This observation may be explained by either extremely efficient vibrational relaxation or a ’’dark’’ channel for the production of v=0, X 2Σ+g N2+.

Peraluminous sapphirine as a metastable reaction product in kyanite—gedrite—talc schist from Sar e Sang, Afghanistan

SummaryMinute crystals of pure Mg-sapphirine approximating (Mg3.23Al4.74)vi(Al4.69Si1.31)ivO20, and thus having more aluminous compositions than the hitherto accepted 7:9:3 end member, were formed together with Mg-cordierite and corundum along narrow zones of the schist through reaction of the pre-existing high-pressure assemblages kyanite-gedrite and kyanite-talc. These sapphirines are considered to be metastable intermediate products growing under the influence of the structurally similar host mineral kyanite. Under equilibrium conditions they continued to react with kyanite to form the stable assemblage cordierite-corundum. Peraluminous sapphirines including those most recently synthesized may be metastable phases under all conditions of crystallization.