Search results for "Kite"
showing 10 items of 1123 documents
Quantum chemical modelling of "green" luminescence in ABO $ \mathsf {_3}$ perovskites
2002
The origin of the intrinsic excitonic (“green”) luminescence in ABO3 perovskites remains a hot topic over the last quarter of a century. We suggest as a theoretical interpretation for the “green” luminescence in these crystals, the recombination of electron and hole polarons forming a charge transfer vibronic exciton. In order to check quantitatively the proposed model, we performed quantum chemical calculations using the Intermediate Neglect of Differential Overlap (INDO) method combined with the periodic defect model. The luminescence energies calculated for four perovskite crystals are found to be in good agreement with experimental data.
Analytic continuation and numerical evaluation of the kite integral and the equal mass sunrise integral
2017
We study the analytic continuation of Feynman integrals from the kite family, expressed in terms of elliptic generalisations of (multiple) polylogarithms. Expressed in this way, the Feynman integrals are functions of two periods of an elliptic curve. We show that all what is required is just the analytic continuation of these two periods. We present an explicit formula for the two periods for all values of $t \in {\mathbb R}$. Furthermore, the nome $q$ of the elliptic curve satisfies over the complete range in $t$ the inequality $|q|\le 1$, where $|q|=1$ is attained only at the singular points $t\in\{m^2,9m^2,\infty\}$. This ensures the convergence of the $q$-series expansion of the $\mathr…
Perovskite CH3NH3PbI3–XClx Solar Cells and their Degradation (Part 1: A Short Review)
2021
Development of hybrid organic-inorganic perovskite solar cells (PSC) has been one of the hottest research topics since 2013. Within brief literature review, we would like to achieve two objectives. Firstly, we would like to indicate that a whole set of physical properties, such as high change carrier mobility, very low recombination rates, large carrier life time and diffu-sion length, large absorption coefficients and very weak exciton binding energies, are defining high power conversion efficiency (PCE) of methyl ammonium lead trihalide SC. The second objective is to draw attention to some, in our opinion, important aspects that previously have not been satisfactory addressed in literatur…
Magnetic transitions in double perovskiteSr2FeRe1−xSbxO6(0⩽x⩽0.9)
2006
The double perovskites ${\mathrm{Sr}}_{2}\mathrm{Fe}M{\mathrm{O}}_{6}$ $(M=\mathrm{Re},\mathrm{Mo})$ belong to the important class of half-metallic magnetic materials. In this study we explore the effect of replacing the electronic $5d$ buffer element Re with variable valency by the main group element Sb with fixed valency. X-ray diffraction reveals ${\mathrm{Sr}}_{2}{\mathrm{FeRe}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}{\mathrm{O}}_{6}$ $(0lxl0.9)$ to crystallize without antisite disorder in the tetragonally distorted perovskite structure (space group $I4∕mmm$). The ferrimagnetic behavior of the parent compound ${\mathrm{Sr}}_{2}{\mathrm{FeReO}}_{6}$ changes to antiferromagnetic upon Sb subst…
The Maya Blue Pigment
2011
Abstract Maya blue is an artificial pigment fabricated by the Maya in the early first millennium AD. The pigment was used in Prehispanic times from the southern Maya region to most Mesoamerican cultures. Maya blue is made from indigotin mixed with palygorskite. This light blue pigment is surprisingly resistant to degradation in adverse environmental conditions, as it is unaffected by the attack of acids, alkalis, oxidants, reducing agents and organic solvents. Scientists do not fully agree in the mechanism of this organoclay interaction. The fabrication and distribution of this pigment probably required a sophisticated technical and commercial infrastructure. This chapter reviews the presen…
Maya Blue as a nanostructured polyfunctional hybrid organic–inorganic material: the need to change paradigms
2009
Maya Blue, an ancient nanostructured organic–inorganic hybrid material resulting from the attachment of indigo, a natural dye, to a phyllosilicate clay, palygorskite, has received considerable attention of late. Despite intensive research, several aspects remain unsolved, in particular the nature of the indigo–palygorskite association. Recent results suggest that the Maya Blue pigment is a complex system in which different topological isomers of various indigoid molecules attached to the palygorskite matrix coexist.
ChemInform Abstract: Maya Blue as a Nanostructured Polyfunctional Hybrid Organic-Inorganic Material: The Need to Change Paradigms.
2010
Maya Blue, an ancient nanostructured organic–inorganic hybrid material resulting from the attachment of indigo, a natural dye, to a phyllosilicate clay, palygorskite, has received considerable attention of late. Despite intensive research, several aspects remain unsolved, in particular the nature of the indigo–palygorskite association. Recent results suggest that the Maya Blue pigment is a complex system in which different topological isomers of various indigoid molecules attached to the palygorskite matrix coexist.
Modelling of defects and surfaces in perovskite ferroelectrics
2003
The results of electronic structure calculations for different terminations of SrTiO 3 (100) and (110) perovskite thin films are discussed. These calculations are based on the ab initio Hartree-Fock (HF) method and Density Functional Theory (DFT). Results are compared with previous ab initio plane-wave LDA and classical Shell Model (SM) calculations. Calculated considerable increase of the Ti-O chemical bond covalency nearby the surface is confirmed by experimental data. Our quantum chemical calculations performed by means of the intermediate neglect of differential overlap (INDO) method confirm the existence of self-trapped electrons in KNbO 3 , KTaO 3 and BaTiO 3 crystals. The relevant la…
Semi-empirical defect calculations for the perovskite KNbO3
2000
A new parametrization of the classical shell model for the cubic phase of the perovskite KNbO3 has been derived and used to calculate the structural, elastic and dielectric properties of this material. Using this parametrization, the defect formation and migration energies, as well as atomic displacements, have been calculated. In parallel, the quantum mechanical method of the intermediate neglect of the differential overlap (INDO) has been applied to the same problem. The migration energies for the O vacancy obtained by these quite different methods are reasonably close (0.68 eV and 0.79 eV, respectively) and also agree with the only experimental estimate available of approximately 1 eV. A…
Quantum chemical modelling of point defects in KNbO3 perovskite crystals
2000
Abstract We present results of semi-empirical quantum chemical calculations for several perovskite KNb x Ta 1−x O 3 (KTN) solid solutions, as well as point intrinsic defects – F centers and hole polarons bound to K vacancy – in KNbO 3 . Method of the intermediate neglect of the differential overlap (INDO) was combined with typically 320-atom supercells and atomic geometry optimization. Analysis of the optimized atomic and electronic structure has clearly demonstrated that several nearest Nb atoms substituting for Ta in KTaO 3 – unlike Ta impurities in KNbO 3 – reveal the self-ordering effect, which probably triggers the ferroelectricity observed in KTN. We predict co-existence of one-site (…