Search results for "Stability"
showing 10 items of 3085 documents
A new monomeric interpretation of intrinsic optical bistability observed in Yb3+-doped bromide materials
2003
We present a mechanism able to show intrinsic bistable behaviour involving single Yb3+ ions embedded into bromide lattices, in which intrinsic optical bistability (IOB) has been observed. The mechanism is based on the experimentally found coupling between the Yb3+ ion and the totally symmetric local mode of vibration of the [YbBr6]3- coordination unit. The model reproduces the IOB observed in CsCdBr3:1% Yb3+ and allows to understand the experimentally found presence of the phenomenon in the other bromides, but its absence in Cs3Lu2Cl9:Yb3+.
Levy flights in confining environments: Random paths and their statistics
2013
We analyze a specific class of random systems that are driven by a symmetric L\'{e}vy stable noise. In view of the L\'{e}vy noise sensitivity to the confining "potential landscape" where jumps take place (in other words, to environmental inhomogeneities), the pertinent random motion asymptotically sets down at the Boltzmann-type equilibrium, represented by a probability density function (pdf) $\rho_*(x) \sim \exp [-\Phi (x)]$. Since there is no Langevin representation of the dynamics in question, our main goal here is to establish the appropriate path-wise description of the underlying jump-type process and next infer the $\rho (x,t)$ dynamics directly from the random paths statistics. A pr…
Low temperature plasma treatment of monomolecular Langmuir-Blodgett films
1993
Abstract The structure of the surface layer of materials is usually quite different from the bulk. Detailed information about the surface structure is often not available. This, together with the complex nature of low temperature plasma treatment, hinders the investigations of plasma chemical processes on solid surfaces. Monomolecular Langmuir-Blodgett (LB) structures offer a unique opportunity to prepare model surfaces with known thickness and molecular architecture. From the data obtained, the depth of the Ar plasma influence on solid organic surfaces was estimated to be about 400–1000 A depending on the nature of the surface layer. An oxygen plasma penetrates deeper with a penetration li…
Assessment of the Potential Energy Hypersurfaces in Thymine within Multiconfigurational Theory: CASSCF vs. CASPT2
2016
The present study provides new insights into the topography of the potential energy hypersurfaces (PEHs) of the thymine nucleobase in order to rationalize its main ultrafast photochemical decay paths by employing two methodologies based on the complete active space self-consistent field (CASSCF) and the complete active space second-order perturbation theory (CASPT2) methods: (i) CASSCF optimized structures and energies corrected with the CASPT2 method at the CASSCF geometries and (ii) CASPT2 optimized geometries and energies. A direct comparison between these strategies is drawn, yielding qualitatively similar results within a static framework. A number of analyses are performed to assess t…
Revised Atomistic Models of the Crystal Structure of C–S–H with high C/S Ratio
2016
Abstract The atomic structure of calcium-silicate-hydrate (C1.67–S–H x ) has been studied. Atomistic C–S–H models suggested in our previous study have been revised in order to perform a direct comparison of energetic stability of the different structures. An extensive set of periodic structures of C–S–H with variation of water content was created, and then optimized using molecular dynamics with reactive force field ReaxFF and quantum chemical semiempirical method PM6. All models show organization of water molecules inside the structure of C–S–H. The new geometries of C–S–H, reported in this paper, show lower relative energy with respect to the geometries from the original definition of C–S…
Thermodynamic stability of stoichiometric LaFeO 3 and BiFeO 3 : a hybrid DFT study
2017
BiFeO3 perovskite attracts great attention due to its multiferroic properties and potential use as a parent material for Bi1−xSrxFeO3−δ and Bi1−xSrxFe1−yCoyO3−δ solid solutions in intermediate temperature cathodes of oxide fuel cells. Another iron-based LaFeO3 perovskite is the end member for well-known solid solutions (La1−xSrxFe1−yCoyO3−δ) used for oxide fuel cells and other electrochemical devices. In this study an ab initio hybrid functional approach was used for the study of the thermodynamic stability of both LaFeO3 and BiFeO3 with respect to decompositions to binary oxides and to elements, as a function of temperature and oxygen pressure. The localized (LCAO) basis sets describing th…
Capillary Hysteresis in Nanopores: Theoretical and Experimental Studies of Nitrogen Adsorption on MCM-41
1995
Capillary hysteresis in cylindrical nanopores has been studied using MCM-41 as the prime example of a mesoporous material. These materials, due to their regular pore structure, can be considered to be candidates for reference adsorbents for standardizing adsorption measurements and methods for characterization of porous solids. They provide a unique opportunity for verification of theoretical models employed for predicting phase equilibrium in confined geometry. Three samples with monodisperse pore channels have been synthesized and examined using X-ray diffraction (XRD). Nitrogen adsorption isotherms were modeled using nonlocal density functional theory (NLDFT) in a wide range of pore size…
Mechanistic and kinetic insight into spontaneous cocrystallisation of isoniazid and benzoic acid
2015
Solid-state cocrystallisation is of contemporary interest, because it offers an easy and efficient way to produce cocrystals, which are recognized as prospective pharmaceutical materials. Research explaining solid-state cocrystallisation mechanisms is important, but still too scarce to give a broad understanding of factors governing and limiting these reactions. Here we report an investigation of the mechanism and kinetics of isoniazid cocrystallisation with benzoic acid. This reaction is spontaneous; however its rate is greatly influenced by environmental conditions (humidity and temperature) and pre-treatment (milling) of the sample. The acceleration of cocrystallisation in the presence o…
Interplay between spin crossover and proton migration along short strong hydrogen bonds
2021
The iron(ii) salt [Fe(bpp)2](isonicNO)2·HisonicNO·5H2O (1) (bpp = 2,6-bis(pyrazol-3-yl)pyridine; isonicNO = isonicotinate N-oxide anion) undergoes a partial spin crossover (SCO) with symmetry breaking at T1 = 167 K to a mixed-spin phase (50% high-spin (HS), 50% low-spin (LS)) that is metastable below T2 = 116 K. Annealing the compound at lower temperatures results in a 100% LS phase that differs from the initial HS phase in the formation of a hydrogen bond (HB) between two water molecules (O4W and O5W) of crystallisation. Neutron crystallography experiments have also evidenced a proton displacement inside a short strong hydrogen bond (SSHB) between two isonicNO anions. Both phenomena can al…
Hyperthermal stability of neuroglobin and cytoglobin
2005
Neuroglobin (Ngb) and cytoglobin (Cygb), recent additions to the globin family, display a hexa-coordinated (bis-histidyl) heme in the absence of external ligands. Although these proteins have the classical globin fold they reveal a very high thermal stability with a melting temperature (Tm) of 100 °C for Ngb and 95 °C for Cygb. Moreover, flash photolysis experiments at high temperatures reveal that Ngb remains functional at 90 °C. Human Ngb may have a disulfide bond in the CD loop region; reduction of the disulfide bond increases the affinity of the iron atom for the distal (E7) histidine, and leads to a 3 °C increase in the Tm for ferrous Ngb. A similar Tm is found for a mutant of human Ng…