Photoinduced bending of needle crystals caused by photochemical transformation can be used as an extremely sensitive method for studying the kinetics of the transformation. However, the determination of the absolute value of the quantum yield of the reaction requires an accurate value of the intensity of light penetrating the crystal, in contrast to reactions in solutions where only the value of the total absorbed irradiation dose is sufficient. To address this problem, this study utilizes the effect of photothermal bending of a crystal due to its heating by light, occurring simultaneously with the bending due to transformation and proportional to the same value of light intensity. The ratio of the amplitudes of the two effects is independent of the light intensity, which allows the quantum yield to be determined without knowledge of the intensity value. In addition, the method allows the light intensity and thermal conductivity of the crystal to be estimated. The method is applied to measure wavelength dependence of the quantum yield of nitro-to-nitrito photoisomerization in [Co(NH3)5NO2]Cl(NO3) crystals. A monotonically decreasing value of the quantum yield φ from 0.19 to 0.04 in the range of λ from 403 to 523 nm was obtained. This result indicates the qualitative differences in the transformation mechanism in crystals and in solutions, where φ = 0.03 independent of λ in the same wavelength range.
This paper is devoted to the study of phase composition and kinetic and thermodynamic characteristics of Mo-doped strontium ferrite SrFe0.85Mo0.15O3-δ (SFM15) under oxygen-conducting membrane working conditions. Single-phase SFM15 with a cubic Pm3Ìm structure was synthesized using a ceramic method. It was shown that the molybdenum introduction stabilizes the perovskite cubic structure over a wide range of oxygen pressures and temperatures, preventing the bulk phase transition at high temperatures. Oxygen exchange constants, diffusion coefficients and activation energy of oxygen exchange were obtained using oxygen relaxation and isotopic exchange techniques, and the obtained values are consistent with known literature data. It was shown that the surface reaction rates obtained using chemical and tracer relaxation methods are quantitatively comparable with each other, despite significantly different experimental conditions. This result not only confirms the reliability of the data obtained by independent methods, but also allows one to expand the area of physical conditions for studying the kinetics of oxygen transfer where another method has technical or methodological limitations.
Exposure of a photoreactive single crystal to light with a wavelength offset from its absorption maximum can have two distinct effects. The first is the "direct" effect, wherein the excited state generated in individual chemical species is influenced. The second is the "indirect" effect, which describes the penetration of light into the crystal and hence the spatial propagation and completeness of transformation. We illustrate using the nitro-nitrito isomerization of [Co(NH3)5NO2]Cl(NO3) as an example that the direct and indirect effects can be independently determined. This is achieved by comparing the dynamics of macroscopic crystal deformation (bending curvature and crystal elongation) induced by the photochemical reaction when irradiating a crystal at the absorption maximum and at different band edges (above or below the maximum) of the same band. Quantitative description of the macroscopic strain dynamics in comparison with experiments allowed us to suggest that irradiation at different tails of the same absorption band causes isomerization to proceed via different excited states and an additional photochemical reaction (presumably, reverse nitrito-nitro isomerization) can occur on irradiation at the ligand-field band edges.
Perovskite related oxides ABO3-δ exhibiting mixed ionic-electronic conductivity (MIEC) possess large deviations from the oxygen stoichiometry. When providing excellent application potential, this feature also makes it very difficult to study the reaction mechanism between such oxides and molecular oxygen, also known as the oxygen reduction reaction. The complexity of the theoretical interpretation of kinetic experiments originates from the significant dependence of the kinetic and equilibrium properties of MIEC oxides on δ. It is proposed to consider such grossly nonstoichiometric oxides having different oxygen nonstoichiometry as chemical homologues participating in the oxygen exchange reaction and forming a series continuous in δ. The continuous homologous series approach is considered using the example of SrCo0.9Ta0.1O3-δ, an SOFC cathode material. The equilibrium and kinetic properties of the oxide were studied by new methods of oxygen partial pressure relaxation and oxygen release. Linear free-energy relationships have been discovered in the homologous series: thermodynamic and kinetic enthalpy-entropy compensations, as well as the Brønsted-Evans-Polanyi relation. A relationship has been established between the change in the observed LFERs and the morphotropic phase transition in the oxide.
Mechanical response of single crystals to light, temperature, and/or force-an emerging platform for the development of new organic actuating materials for soft robotics-has recently been quantitatively described by a general and robust mathematical model ( Chem. Rev . 2015 , 115 , 12440 - 12490 ). The model can be used to extract accurate activation energies and kinetics of solid-state chemical reactions simply by tracking the time-dependent bending of the crystal. Here we illustrate that deviations of the macroscopic strain in the crystal from that predicted by the model reveal the existence of additional, "hidden" chemical or physical processes, such as sustained structural relaxation between the chemical transformation and the resulting macroscopic deformation of the crystal. This is illustrated with photobendable single crystals of 4-hydroxy-2-(2-pyridinylmethylene)hydrazide, a photochemical switch that undergoes E-to-Z isomerization. The irreversible isomerization in these crystals results in amorphization and plastic deformation that are observed as poor correlation between the transformation extent and the induced strains. The occurrence of these processes was independently confirmed by X-ray diffraction and differential scanning calorimetry. An extended mathematical model is proposed to account for this complex mechanical response.
A novel methodology for the analysis of oxygen exchange in practically important nonstoichiometric oxides with mixed ionic electronic conductivity (MIEC) is suggested. It is based on the fact that the kinetic and thermodynamic properties of such oxides vary continuously with oxygen stoichiometry. This allows MIEC oxides to be considered as a homologous series, with the difference that traditional series are discrete in their chemical composition whereas MIEC oxides are continuous in oxygen stoichiometry. Analysis of the relations between Gibbs energies of reactions and activation barriers traditionally performed for homologous series can be useful in studies of oxygen exchange in MIEC oxides. To demonstrate the approach, thermodynamic and oxygen-exchange kinetics parameters are measured as functions of oxygen nonstoichiometry δ for two perovskites, SrCo0.8Fe0.2O3-δ and SrFeO3-δ, having metal-like and p-type semiconducting conductivities, respectively. Both oxides are shown to obey linear free energy relationships of the Brønsted-Evans-Polanyi form in spite of their different types of electronic structures. The results open up new possibilities for understanding the mechanism of the rate determining step of oxygen exchange in MIEC oxides.
Photomechanically reconfigurable elastic single crystals are the key elements for contactless, timely controllable and spatially resolved transduction of light into work from the nanoscale to the macroscale. The deformation in such single-crystal actuators is observed and usually attributed to anisotropy in their structure induced by the external stimulus. Yet, the actual intrinsic and external factors that affect the mechanical response remain poorly understood, and the lack of rigorous models stands as the main impediment towards benchmarking of these materials against each other and with much better developed soft actuators based on polymers, liquid crystals and elastomers. Here, experimental approaches for precise measurement of macroscopic strain in a single crystal bent by means of a solid-state transformation induced by light are developed and used to extract the related temperature-dependent kinetic parameters. The experimental results are compared against an overarching mathematical model based on the combined consideration of light transport, chemical transformation and elastic deformation that does not require fitting of any empirical information. It is demonstrated that for a thermally reversible photoreactive bending crystal, the kinetic constants of the forward (photochemical) reaction and the reverse (thermal) reaction, as well as their temperature dependence, can be extracted with high accuracy. The improved kinematic model of crystal bending takes into account the feedback effect, which is often neglected but becomes increasingly important at the late stages of the photochemical reaction in a single crystal. The results provide the most rigorous and exact mathematical description of photoinduced bending of a single crystal to date.
For martensitic transformations the macroscopic crystal strain is directly related to the corresponding structural rearrangement at the microscopic level. In situ optical microscopy observations of the interface migration and the change in crystal shape during a displacive single crystal to single crystal transformation can contribute significantly to understanding the mechanism of the process at the atomic scale. This is illustrated for the dehydration of samarium oxalate decahydrate in a study combining optical microscopy and single-crystal X-ray diffraction.
